Traditionally, plant organs are divided into organs with indeterminate growth such as shoots, roots and vascular cambia, whose growth is maintained by meristems, groups of pluripotent cells, and organs with determinate growth such as leaves or floral organs. Fossil evidence indicates that seed plant leaves evolved from ancestral shoot systems, and further, the dichotomous morphology of early seed plant leaves suggests growth via a persistent apical meristem (reviewed in [Kenrick and Crane, 1997; Floyd and Bowman, 2010]). However, as anatomical features typical of apical or vascular meristems are not present in leaves, whether developing leaves grow from a localized meristem has been debated for nearly a century (Foster, 1936; Hagemann and Gleissberg, 1996).

In one of the first detailed examinations of development at the plant shoot apex Caspar Wolff described the leaf lamina arising from the margins of Brassica 'capitata' (cabbage) leaves (Wolff, 1759). Subsequently, Avery suggested that early lamina growth of Nicotiana tabaccum was initiated by a row of subepidermal initial cells located at the upper-lower (adaxial-abaxial) leaf boundary that he termed the 'marginal meristem' (Avery, 1933). However, it had already been noted that later protracted growth in leaves occurs in tissues that are not marginal, but rather within the developing lamina in a region described as a 'plate meristem' (Schüepp, 1918, 1926). Thus, early views of leaf development were perceived to consist of two growth phases (Foster, 1936). An early ephemeral phase of cell divisions without cell expansion produces the characteristic 6–10 cell layers of the leaf thickness via submarginal periclinal cell divisions and epidermal anticlinal divisions. This is followed by a later prolonged growth phase where the bulk of two-dimensional lamina growth is produced via a plate meristem in which cell divisions are predominantly anticlinal. Analyses of leaf development in the middle of the 20th century sought to identify patterns of submarginal cell divisions to identify initial cells, but the patterns of cell division were highly variable between species casting doubt on the presence of specific initials (Foster, 1936).

More recently, examination of mitotic indices during leaf development revealed that a higher rate of cell division is observed in submarginal (i.e. plate meristem) regions of the leaf as compared to the margins (Maksymowych and Erickson, 1960; Fuchs, 1966; Thomasson, 1970; Dubuc-Lebreux and Sattler, 1981; Jéune, 1981). Furthermore, sector analysis of leaf development in several eudicot species, including N. tabacum, revealed that most clonal sectors were located between the midrib and the margin, with only a minority extending all the way to the margin (Dulieu, 1968; Poethig and Sussex, 1985; Dolan and Poethig, 1998), indicating that leaves do not grow from the margins sensu stricto, and calling into question the concept of the leaf marginal meristem. However, noting the overall lack of organized cell division patterns in plants, Hagemann and Gleissberg argued that the defining features of meristems are their organogenetic potential and cytohistological state rather than specific cell division patterns. Thus, in their view the marginal meristem (or ‘blastozone’, as they refer to it) is responsible for primary morphogenetic events, e.g. lamina initiation, but is used up early in leaf development, with most lamina expansion occurring during a later leaf differentiation phase (Hagemann and Gleissberg, 1996). Although this is a compelling model, evidence for this interpretation has been mainly observational and circumstantial.

Here we show that removal of multiple growth suppressing transcriptional factors results in indeterminate growth of the margins of all lateral organs, coupled with sustained organogenesis and activity of gene modules shared amongst other plant meristems. Our finding supports the presence of a specific leaf meristem, and conforms to the view stemming from the fossil record that recruitment of suppressors of meristematic activity was critical in seed plant leaf evolution and development.

In Arabidopsis, leaf morphogenesis is initiated at the flanks of the shoot apical meristem (SAM) where leaf primordia develop as flattened lamina with defined abaxial, adaxial and marginal cell types (Tsukaya, 2013). Lamina development requires the juxtaposition of abaxial/adaxial polarity factors, including adaxial class III HD-Zip and abaxial KANADI transcription factors. These lie on either side of a narrow middle domain expressing the WUSCHEL RELATED HOMEOBOX (WOX) genes, PRESSED FLOWER (PRS) and WOX1, and together promote organ growth and differentiation (Nakata et al., 2012; Wang et al., 2011; Eshed et al., 2004). In Arabidopsis leaf development, expression of growth genes rapidly diminishes distally but can persist proximally (Donnelly et al., 1999; Nath et al., 2003). This proximo-distal differentiation gradient is regulated by CIN-TCP transcription factors (Nath et al., 2003). A reduction of five CIN-TCPs targeted by the endogenous microRNA, miR319a (also known as miR-JAW) results in delayed basipetal progression of a mitotic arrest front and increased cell proliferation particularly at leaf margins, producing crinkly and serrated leaves (Efroni et al., 2008; Ori et al., 2007; Palatnik et al., 2003). Increased distal leaf growth and serrations are also observed when the activities of the four NGA transcription factors are reduced (Figure 1—figure supplement 1) (Trigueros et al., 2009; Alvarez et al., 2009). The NGAs and CIN-TCPs are co-expressed at many stages of leaf development, exemplified by the distal expression of TCP3, TCP4, NGA1 and NGA4 in young leaves (Figure 1—figure supplement 2) and in contrast to the reported expression of miR319 at the leaf base (Obayashi et al., 2009; Nag et al., 2009). This, together with similarities in their loss-of-function phenotypes, suggests shared roles in leaf development. To investigate functional redundancy, we introduced a constitutive expression construct of miR319a (35S:miR319) targeting the five CIN-TCP genes (Palatnik et al., 2003) into a quadruple NGA mutant (nga1,2,3,4) that lacks NGA activities.

Strikingly, simultaneous reduction in expression of these nine genes resulted in continuous de novo formation of tissue at the margins of all lateral organs including cotyledons, leaves and floral organs (Figure 1A–B, Figure 1—figure supplement 3). Indistinguishable phenotypes were observed in plants constitutively expressing both miR319a and the previously characterized artificial miRNA amiR-NGA (Alvarez et al., 2009), facilitating easier and more extensive characterization of the indeterminate growth phenotype. 35S:miR319a/35S:amiR-NGA plants grow more slowly, are later flowering, and their leaf margins harbor proliferative cell populations unlike those of 35S:amiR-NGA and 35S:miR319a singly transgenic plants (Figure 1C–G, Figure 1—figure supplement 3–8). Application of stain to 35S:miR319a/35S:amiR-NGA leaf margins indicates continued proliferation at the leaf margin, with the marker displaced sub-marginally over time (Figure 1D1–D2, Figure 1—figure supplement 3). In 35S:miR319a nga1,2,3,4 or 35S:miR319a/35S:amiR-NGA plants, the entirety of the older leaf margin consists of small densely packed cells lacking chlorophyll, rather than the large, elongate cells characteristic of wild-type leaf margins (Figure 1C–I, Figure 1—figure supplement 5–8). Sections of leaf primordia and differentiating leaves suggest that the six-cell-layered blade organization of young wild-type leaf primordia is maintained at 35S:miR319a/35S:amiR-NGA leaf margins ([Nakata et al., 2012]; Figure 1E–G, Figure 1—figure supplement 8).

Figure 1 with 15 supplements see all

Download asset Open asset

Figure 1—source data 1

Mean size of the leaf in wild-type and 35S:amiR-NGA plants, corresponding to the data shown in Figure 1—figure supplement 1C.

https://doi.org/10.7554/eLife.15023.003

Download elife-15023-fig1-data1-v3.xlsx

Figure 1—source data 2

Mean size of the palisade mesophyll cells in wild-type and 35S:amiR-NGA plants, corresponding to the data discussed in the legend to Figure 1—figure supplement 1D and E.

https://doi.org/10.7554/eLife.15023.004

Download elife-15023-fig1-data2-v3.xlsx

Figure 1—source data 3

CYCB1;1:GUS expression in distal wild-type and 35S:miR-NGA, correspnding to the data shown in Figure 1—figure supplement 1F–I.

https://doi.org/10.7554/eLife.15023.005

Download elife-15023-fig1-data3-v3.jpg

Figure 1—source data 4

Effects on expression of different CIN-TCP and NGATHA family members and possible off targets in amiR-NGA and miR319a overexpressing plants- Figure 1—figure supplement 3.

https://doi.org/10.7554/eLife.15023.006

Download elife-15023-fig1-data4-v3.xlsx

Figure 1—source data 5

Differences in flowering time among wild-type, 35S:amiR-NGA, 35S:miR319a and 35S:amiR-NGA/35S:miR319a plants, corresponding to the data shown in Figure 1—figure supplement 4D.

https://doi.org/10.7554/eLife.15023.007

Download elife-15023-fig1-data5-v3.xlsx

The digital differentiation index (DDI) assesses relative leaf maturity from global gene expression profiles (Efroni et al., 2008). The index of dissected, older 35S:miR319a/35S:amiR-NGA leaf margins clearly matches that of initiating primordia (Figure 1J). This result is further supported by the expression of markers that highlight continued cell division, distinguishing epidermal nuclei (ATML1:H2B-mYFP), epidermal plasma membrane (ATML1:mCitrine-RCI2A), general cell division (CYCB1;1:GFP), initiating trichomes (highlighted by GL2:GFP) and stomatal lineage proliferation (TMM:GUS-GFP) (Figure 1K,L, Figure 1—figure supplement 7), which demonstrate ongoing leaf-primordium-like activity at the leaf margins. In initiating wild-type leaves, auxin flux, marked by PINFORMED1 (PIN1) expression, converges at the distal tip and at serrations, where it inwardly canalizes leaf vascular development, before becoming restricted to proximal margins of older leaves (Bilsborough et al., 2011; Scarpella et al., 2006). Compared to wild-type leaves, in both 35S:amiR-NGA and 35S:miR319a individual knockdown leaves auxin flux persists longer at distal leaf margins. Strikingly, in the 35S:miR319a/35S:amiR-NGA combined knockdown leaves, auxin flux continues around the entire leaf margin (Figure 1O, Figure 1—figure supplement 9). Auxin canalization and ongoing de novo vasculature morphogenesis at these margins is marked by expression of the provascular makers ATHB8 and MONOPTEROS (MP) (Figure 1M–N, Figure 1—figure supplements 7 and 10). Paralleling marginal auxin flux, the organ marginal markers PRS and WOX1 are transiently expressed in initiating wild-type leaves before becoming proximally restricted. When NGA or CIN-TCP activities are reduced, PRS and WOX1 distal expression persists in older leaves whereas in the combined loss in 35S:miR319a/35S:amiR-NGA leaves PRS and WOX1 expression occurs in an uninterrupted marginal band, again suggesting that these margins retain meristematic properties equivalent to initiating leaf primordia (Figure 1P, Figure 1—figure supplements 11 and 12).

That expression of miR319a-amiR-NGA under control of the PRS regulatory sequences results in indeterminate margins confirms that marginal loss of NGA and CIN-TCP activity is sufficient to allow the maintenance of these meristematic characteristics (Figure 1—figure supplement 13). Notably the lamina away from the margins of PRS>>miR319a-amiR-NGA is thinner and more wild-type in appearance than that of 35S:miR319a/35S:amiR-NGA leaves suggesting that the broader, non-marginal expression of the NGAs and CIN-TCPs may reflect an activity in regulating cell expansion that remains functional in PRS>>miR319a-amiR-NGA leaves (Figure 1—figure supplement 8 and Figure 1—figure supplement 13).

The extended maintenance of primordium identity was also observed in the cotyledons of 35S:miR319a nga1,2,3,4 or 35S:miR319a/35S:amiR-NGA plants, which continuously produce tissue with leaf characteristics including stellate trichome formation (Figure 1A, Figure 1—figure supplement 14). Changes in the expression pattern of the cell division marker CYCB1;1:GFP are apparent in the distal embryonic cotyledons while the respective expression of ATML1:H2B-mYFP and MONOPTEROS demonstrates the absence of the normal, marginal cell differentiation program and ectopic production of provascular strands implying an active marginal meristem similar to that observed in leaves (Figure 1—figure supplement 15). Notably there was no evidence for impaired dormancy of 35S:miR319a nga1,2,3,4 or 35S:miR319a/35S:amiR-NGA seed suggesting that the seed-based program of imposed dormancy was as effective on this cotyledon marginal meristem as on the embryonic shoot and root meristems. After germination, cotyledons of 35S:miR319a nga1,2,3,4 or 35S:miR319a/35S:amiR-NGA seedlings continue growth and express growth markers unlike wild-type (Figure 1—figure supplements 14 and 15). The floral organs of NGA and CIN-TCP compromised plants also exhibit prolonged marginal growth (Figure 1—figure supplement 3). Hence NGA and CIN-TCP redundantly suppress marginal growth in all aerial lateral organs.

Ongoing growth from the organ margin may be a consequence of ectopic activation of a SAM program. We surveyed the expression of genes that are expressed in the SAM but not in leaves of Arabidopsis, and no evidence was found for the expression of meristem genes including SHOOT MERISTEMLESS (STM), WUSCHEL (WUS) and CLAVATA1/3 (CLV1/3) in indeterminate leaf margins of 35S:miR319a/35S:amiR-NGA plants (Figure 2A–E). In agreement, the 35S:miR319a-amiR-NGA transgene conferred indeterminate growth of cotyledon and/or leaf margins in stm-11 knat6-1 bp-9 triple and wus-1 single mutants where SAM activity is respectively lost or disrupted (Figure 2F–L). Thus, continued marginal growth in 35S:miR319a-amiR-NGA double knockdown leaves is not a consequence of secondarily acquiring characteristics of the indeterminate SAM, as for example in YABBY-compromised mutants (Sarojam et al., 2010).

Figure 2 with 1 supplement see all

Download asset Open asset

The NAC transcription factors CUP-SHAPED COTYLEDON2 (CUC2) and CUC3 are regulators of leaf margin shape in Arabidopsis and other angiosperm species, and ectopic activation of CUC genes promotes adventitious shoot formation (Blein et al., 2008; Aichinger et al., 2012; Hibara et al., 2003), suggesting that deregulation of CUC genes may account for the indeterminate growth phenotype. However, we found that the cotyledon and/or leaf margins continue to grow in cuc2 cuc3 and cuc1 cuc2 mutants expressing the 35S:miR319a-amiR-NGA transgene. This indicates that continued margin growth is independent of CUC-mediated marginal elaboration (Figure 2M–Q, Figure 2—figure supplement 1).

Since lamina growth is an outcome of an interaction between adaxial and abaxial factors and involves the marginal leaf WOX genes (Nakata et al., 2012; Eshed et al., 2004), the role of polarity factors and WOX genes in maintaining continued marginal growth was investigated. The respective adaxial, marginal and abaxial genes PHABULOSA (PHB), PRS and KANADI1 (KAN1) are expressed in young, wild-type leaf primordia before diminishing in a basipetal fashion (Figure 3A,C,E,G–H, Figure 3—figure supplement 1). At the margins of 35S:miR319a/35S:amiR-NGA leaves, PHB, PRS and KAN1 gene expression continues indefinitely, with spatial relationships maintained, implying that in older leaves with reduced CIN-TCP and NGA activities, the collective interplay among these genes is sustained as established in initiating wild-type leaf primordia (Figure 3A–H, Figure 1—figure supplement 11, Figure 3—figure supplement 1). To test whether adaxial/abaxial tissue polarity and associated WOX activities are required for marginal leaf growth we examined the effects of mutations in these genes on indeterminate marginal growth. Semi-dominant PHB alleles produce two leaf types on the same plant: partially radialized leaves with distal lamina and completely radialized (adaxialized) leaves (Figure 3I). In 35S:miR319a/35S:amiR-NGA phb-1d/+ plants, leaves with distal lamina exhibited ectopic marginal growth while radialized leaves did not, demonstrating that ongoing marginal growth first requires the juxtaposition of polarity factors (Figure 3J, Figure 3—figure supplement 2). PRS and WOX1 redundantly promote growth as an output of the abaxial/adaxial polarity program (Nakata et al., 2012). The combined loss of NGA and CIN-TCP activities in 35S:miR319a/35S:amiR-NGA plants results in both PRS and WOX1 expression occurring as an uninterrupted marginal band in older leaves (Figure 1P, Figure 1—figure supplements 11 and 12). Notably, prs wox1 double mutants suppressed the indeterminate marginal growth in 35S:miR319a/35S:amiR-NGA plants (Figure 3K–L, Figure 3—figure supplement 3). Hence the ongoing leaf margin growth is dependent on both the polarity program and the leaf-specific WOX genes.

Figure 3 with 3 supplements see all

Download asset Open asset

To further characterize the relationships between the different leaf domains, we investigated weak polarity mutant backgrounds where ectopic sites of adaxial/abaxial juxtaposition lead to outgrowths, which have marginal identity, from the leaf lamina (Nakata et al., 2012; Wang et al., 2011; Eshed et al., 2004). The abaxial surfaces of developing kan1 kan2 mutant leaves exhibit ectopic expression of PIN1, PRS and NGA1 (Figure 3M–P). Reducing both NGA and CIN-TCP activity in the kan1 kan2 background results in a striking proliferation of leaf tissue from the abaxial surface (Figure 3Q, Figure 3—figure supplement 2). Similarly, reducing NGA and CIN-TCP activities in mutants of the adaxial factor ASYMMETRIC LEAVES2 (AS2), where patches of ectopic, adaxial PRS expression are observed, resulted in adaxial lamina proliferation (Figure 3R–U, Figure 3—figure supplement 2). A shift in the marginal program with corresponding lamina outgrowths can also be achieved through direct manipulation of WOX1 expression, such as ectopic abaxial expression of WOX1 in FIL:WOX1 plants (Figure 4A–C) (Nakata et al., 2012). Here, as in kan1 kan2 mutant leaves, we detected PIN1 and NGA1 expression in the abaxial outgrowths.

Figure 4

Download asset Open asset

The indeterminate cell proliferation and patterning of the leaf margin in 35S:miR319a/35S:amiR-NGA plants suggests it is self-organizing, a property of meristems, consistent with results demonstrating positive and negative feedbacks between PRS/WOX1 and adaxial/abaxial polarity factors (Nakata and Okada, 2012). The lack of marginal growth in radialized phb-1d/+ organs and its ectopic placement at discrete positions of the lamina when the adaxial/abaxial patterning is compromised argues for a major role of the polarity factors in marginal positioning of a leaf meristem that requires the intervening activity of WOX genes. In turn, a negative feedback loop between the marginally restricted meristem and NGA/CIN-TCP activities may lead to the ephemeral nature of this meristem. In agreement, leaves of FIL:WOX1 that are likely relieved from such feedback regulation, maintained a highly meristematic nature and failed to differentiate and expand when NGA and CIN-TCP activities were jointly reduced (Figure 4D,E).

The observation that loss of NGAs and CIN-TCPs results in indeterminate leaf margins suggests that the early wild-type leaf primordium has a meristem that acts during a brief developmental window and that is gradually restricted spatially (Figure 4F–G). This interpretation is consistent with classical morphological and anatomical studies in which the definition of the marginal meristem was extended to include the entire meristematic leaf primordium at very early stages of leaf development (Hagemann, 1970). If a leaf primoridum is damaged or bifurcated at this stage, nearly complete regeneration of normal leaf morphology is possible (Goebel, 1902; Figdor, 1906; Snow and Snow, 1941; Sachs, 1969). Subsequently, as the meristematic regions become restricted to the margins or portions of the margins, damage or bifurcation of the leaf primordium results in progressively more limited regenerative capacity (Snow and Snow, 1941; Sachs, 1969; Figdor, 1926). Arabidopsis leaves are argued to possess a basal meristem that remains transiently active after leaf initiation before transitioning to petiole development — a process regulated by the BLADE-ON-PETIOLE (BOP) genes (Hepworth et al., 2005; Ichihashi et al., 2011; Kuchen et al., 2012; Laux et al., 1996). Our observations are consistent with early distal expression of CIN-TCP and NGA genes repressing the meristem distally, but the lack of early proximal expression allows marginal persistence of the leaf meristem at the leaf base, as reflected by PRS expression dynamics and leaf marginal cell differentiation along the proximo-distal axis (Nakata et al., 2012) (Figure 1P, Figure 1—figure supplements 6 and 8).

Whereas Arabidopsis leaves differentiate from tip to base, leaf differentiation in some other angiosperm species can proceed from base to tip (Trécul 1853; Ikeuchi et al., 2013). We thus speculate that variations in lateral organ growth within an individual and among species reflect differential maintenance of meristem activity along the marginal and proximo-distal axes. Remarkable diversity in leaf shape can arise from growth variation along the margin including leaf lobing. Lobe formation in many species relies on leaf-specific activity of class 1 KNOX (KNOX1) genes, which the simple leaves of Arabidopsis lack (Piazza et al., 2010). However, lobes can be mimicked by ectopic KNOX1 expression in Arabidopsis leaves. The radialized leaves of phb-1d/+ plants, a prs wox1 background and NGA1 over-expression, all suppresses the KNOX1-induced lobing phenotype, indicating that an active marginal meristem is a prerequisite to respond to KNOX1 activity (Figure 3—figure supplement 3). Thus modulation in the marginal restriction of meristem activities can contribute to leaf shape diversity.

Cotyledons and floral organs are viewed as modified leaves. In Arabidopsis, lack of a basally restricted meristem may distinguish them from leaves in their response to reduced CIN-TCPs and NGA activity. In these organs, additional growth is confined to the distal region whereas in leaves the entire margin is affected (Figure 1A–B, Figure 1—figure supplements 3, 14 and 15). The observation that leaf tissue grows from cotyledon tips suggests a brief activity of a marginal meristem in cotyledons. Prolongation of the marginal meristem activity likely uncouples growth from the embryonic cotyledon program, and therefore, cotyledons continue to grow the same way as leaves.

How can our observations of a potential continuing meristematic activity at leaf margins be reconciled with classical concepts of marginal and plate meristems in leaves and with the denial of their existence based on mitotic indices and sector analyses? Seed plant leaves evolved from ancestral shoot systems; thus, the shoot apical meristem (SAM) may provide an analogy, or perhaps homology (Floyd and Bowman, 2010). The seed plant SAM exhibits two distinct organizational features. Firstly, SAMs feature a tunica-corpus structure in which cell divisions in the tunica are almost exclusively anticlinal (Schmidt, 1924). Secondly, the seed plant SAM exhibits cytohistological zonation that is correlated with functional zonation (Foster, 1938). The central zone (CZ) exhibits low rates of mitoses and acts to supply cells to the peripheral zone (PZ) and rib zone (RZ) where mitotic activity is high, and organogenesis occurs (Steeves and Sussex, 1989). Consistent with these patterns of cell division, cell lineage analyses of the SAM reveals that the majority of sectors observed do not extend to include the SAM, but rather are presumed to originate in derivatives of the peripheral/rib zones (Dulieu, 1969; Jegla and Sussex, 1989; Furner and Pumfrey, 1992).

As with SAMs, leaf meristems can also be interpreted to consist of distinct organizational zones. Regions of low and high mitotic activity correspond to the classically defined ‘marginal’ and ‘plate’ meristems (Foster, 1936; Avery, 1933; Schüepp, 1926; Maksymowych and Wochok, 1969; Maksymowych and Erickson, 1960; Fuchs, 1966; Thomasson, 1970; Dubuc-Lebreux and Sattler, 1981; Jéune, 1981). Consistent with these mitotic indices, cell lineage analyses reveal that the majority of sectors produced in developing leaves are derived from regions internal to the margins (Dulieu, 1968; Poethig and Sussex, 1985; Dolan and Poethig, 1998). While marginal activity of the leaf meristem in wild-type Arabidopsis may be brief, we show here that when extended, cells generated at the margins are displaced towards the center of the leaf, displaying a maturation gradient, similar to the PZ and RZ cells displaced from the CZ of the SAM.

The CZ of the SAM is characterized by the expression of a WOX gene, WUSCHEL (Mayer et al., 1998). Loss-of-function WUS alleles generate a functional SAM, but the CZ fails to be maintained, leading to the eventual depletion of cells in the active PZ and RZ (Laux et al., 1996). Similarly, the leaf meristem exhibits WOX gene expression, whose function is required for continued leaf growth, but leaves can initiate and grow for a while without marginal WOX expression (Nakata et al., 2012; Vandenbussche et al., 2009). The SAM features a tunica-corpus structure in which cell divisions in the tunica are almost exclusively anticlinal ([Schmidt, 1924] and others). As with the SAM, the leaf marginal domain is also organized into epidermal and sub-epidermal layers. Analysis of periclinal chimeras revealed that the epidermal layers of the leaf are clonally related, whereas the mesophyll and vascular bundles are derived from subepidermal layers ([Foster, 1936; Avery, 1933; Baur, 1909] and references therein). The lack of differentiation of leaf marginal cells in 35S:miR319a/35S:amiR-NGA plants is consistent with these cells remaining meristematic.

Our results are largely consistent with classical views of leaf development — that the leaf primordium is broadly meristematic at its inception, and that meristematic potential is subsequently restricted to the marginal regions (Foster, 1936; Hagemann and Gleissberg, 1996; Jéune, 1981; Sachs, 1969). In our view, the marginal and plate meristems represent two zones of a leaf meristem, analogous, or perhaps homologous, to the central and peripheral zones of the SAM.

We suggest that the marginal restriction of the leaf meristem is in part maintained and guided by the same adaxial and abaxial factors that function in shoot and cambial meristems, and all three meristems are maintained by the activity of different WOX paralogs, suggesting the repeated use of a molecular module (Figure 4F) (Aichinger et al., 2012). Sharing of genetic modules implies either common descent or co-option of modules to pattern novel structures. Since seed plant leaves evolved from ancestral shoot systems, common descent is plausible. In this scenario, the leaf meristem module has been modified from an ancestral shoot meristem module to include the leaf-specific WOX1 and PRS paralogs (Lin et al., 2013; Nardmann and Werr, 2013) that arose in a common ancestor of seed plants. Additional regulators such as the YABBY genes, which are instrumental in lamina growth and restrict activity of SAM factors (Sarojam et al., 2010), and later acting factors limiting leaf meristem activity (i.e., CIN-TCP and NGA) were integrated into the leaf program. Growth suppressors modulating leaf meristem activity were recruited from genes of both ancient and recent origins — CIN-TCP genes are present in all land plants (Navaud et al., 2007) whereas NGA genes evolved recently, perhaps within seed plants (Alvarez et al., 2009). Thus the leaf marginal meristem genetic program may have been derived via elaboration of an ancestral shoot program, reflecting the derivation of the leaf from a modified shoot. The identification of such genetic framework provides a unification of how the entire seed plant shoot system is built from apical, vascular, cambial, and leaf meristems that are mechanistically similar. The evolution of seed plant leaves from an ancestral shoot system can be interpreted as evolving via the recruitment of regulatory mechanisms to suppress the morphogenetic potential of the leaf meristem.

1. 1. Alvarez JP
   2. Furumizu C
   3. Eshed Y
   4. Bowman J

   (2016) Leaf margins of indeterminate Arabidopsis leaves

   Publicly available at the NCBI Gene Expression Omnibus (accession no: GSE78693).

   https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE78693

1. 1. Aichinger E
   2. Kornet N
   3. Friedrich T
   4. Laux T

   (2012) Plant stem cell niches

   Annual Review of Plant Biology 63:615–636.

   https://doi.org/10.1146/annurev-arplant-042811-105555

   • PubMed
   • Google Scholar
2. 1. Alvarez JP
   2. Goldshmidt A
   3. Efroni I
   4. Bowman JL
   5. Eshed Y

   (2009) The NGATHA distal organ development genes are essential for style specification in Arabidopsis

   The Plant Cell 21:1373–1393.

   https://doi.org/10.1105/tpc.109.065482

   • PubMed
   • Google Scholar
3. 1. Alvarez JP
   2. Pekker I
   3. Goldshmidt A
   4. Blum E
   5. Amsellem Z
   6. Eshed Y

   (2006) Endogenous and synthetic microRNAs stimulate simultaneous, efficient, and localized regulation of multiple targets in diverse species

   The Plant Cell 18:1134–1151.

   https://doi.org/10.1105/tpc.105.040725

   • PubMed
   • Google Scholar
4. 1. Avery GS

   (1933) Structure and Development of the Tobacco Leaf

   American Journal of Botany 20:565–592.

   https://doi.org/10.2307/2436259

   • Google Scholar
5. 1. Baur E

   (1909) Das Wesen und die Erblichkeitsverhältnisse der "Varietates albomarginatae hort.“ von Pelargonium zonale

   Zeitschrift Für Induktive Abstammungs- Und Vererbungslehre 1:330–351.

   https://doi.org/10.1007/bf01990603

   • Google Scholar
6. 1. Bilsborough GD
   2. Runions A
   3. Barkoulas M
   4. Jenkins HW
   5. Hasson A
   6. Galinha C
   7. Laufs P
   8. Hay A
   9. Prusinkiewicz P
   10. Tsiantis M

   (2011) Model for the regulation of Arabidopsis thaliana leaf margin development

   PNAS 108:3424–3429.

   https://doi.org/10.1073/pnas.1015162108

   • PubMed
   • Google Scholar
7. 1. Blein T
   2. Pulido A
   3. Vialette-Guiraud A
   4. Nikovics K
   5. Morin H
   6. Hay A
   7. Johansen IE
   8. Tsiantis M
   9. Laufs P

   (2008) A conserved molecular framework for compound leaf development

   Science 322:1835–1839.

   https://doi.org/10.1126/science.1166168

   • PubMed
   • Google Scholar
8. 1. Dolan L
   2. Poethig R

   (1998) Clonal analysis of leaf development in cotton

   American Journal of Botany 85:315–321.

   https://doi.org/10.2307/2446322

   • PubMed
   • Google Scholar
9. 1. Donnelly PM
   2. Bonetta D
   3. Tsukaya H
   4. Dengler RE
   5. Dengler NG

   (1999) Cell cycling and cell enlargement in developing leaves of Arabidopsis

   Developmental Biology 215:407–419.

   https://doi.org/10.1006/dbio.1999.9443

   • PubMed
   • Google Scholar
10. 1. Dubuc-Lebreux M
    2. Sattler R

    (1981)

    Développement des organes foliacés chez Nicotiana tabacum et le probleme des Méristémes marginaux

    Phytomorphology 30:17–32.

    • Google Scholar
11. 1. Dulieu H

    (1968)

    Emploi Des chimeres chlorophylliennes pout l'etude de l'ontogenése foliaire

    Bulletin Scientifique De Bourgogne 25:13–72.

    • Google Scholar
12. 1. Dulieu H

    (1969)

    Mutations somatiques chlorophylliennes induites et ontogénie caulinaire

    Bulletin Scientifique De Bourgogne 26:18–102.

    • Google Scholar
13. 1. Efroni I
    2. Blum E
    3. Goldshmidt A
    4. Eshed Y

    (2008) A protracted and dynamic maturation schedule underlies Arabidopsis leaf development

    The Plant Cell Online 20:2293–-2306.

    https://doi.org/10.1105/tpc.107.057521

    • PubMed
    • Google Scholar
14. 1. Eshed Y
    2. Izhaki A
    3. Baum SF
    4. Floyd SK
    5. Bowman JL

    (2004) Asymmetric leaf development and blade expansion in Arabidopsis are mediated by KANADI and YABBY activities

    Development 131:2997–3006.

    https://doi.org/10.1242/dev.01186

    • PubMed
    • Google Scholar
15. 1. Figdor W

    (1906)

    Über Regeneration der Blattspreite bei Scolopendrium Scolopendrium

    Berichte Der Deutschen Botanischen Gesellschaft 24:13–16.

    • Google Scholar
16. 1. Figdor W

    (1926) Über das Restitutionsvermögen der Blätter von Bryophyllum calycinum Salisb

    Planta 2:424–428.

    https://doi.org/10.1007/BF01916356

    • Google Scholar
17. 1. Floyd SK
    2. Bowman JL

    (2010) Gene expression patterns in seed plant shoot meristems and leaves: homoplasy or homology?

    Journal of Plant Research 123:43–55.

    https://doi.org/10.1007/s10265-009-0256-2

    • Google Scholar
18. 1. Foster AS

    (1936) Leaf differentiation in angiosperms

    The Botanical Review 2:349–372.

    https://doi.org/10.1007/BF02870152

    • Google Scholar
19. 1. Foster AS

    (1938) Structure and Growth of the Shoot Apex in Ginkgo Biloba

    Bulletin of the Torrey Botanical Club 65:531–556.

    https://doi.org/10.2307/2480793

    • Google Scholar
20. 1. Friml J
    2. Vieten A
    3. Sauer M
    4. Weijers D
    5. Schwarz H
    6. Hamann T
    7. Offringa R
    8. Jürgens G

    (2003) Efflux-dependent auxin gradients establish the apical-basal axis of Arabidopsis

    Nature 426:147–153.

    https://doi.org/10.1038/nature02085

    • PubMed
    • Google Scholar
21. 1. Fuchs C

    (1966)

    Observations sur l'extension en largeur Du limbe foliaire Du Lupinus albus L

    Comptes Rendus De l'AcadéMie Des Sciences SéRies D 263:1212–1215.

    • Google Scholar
22. 1. Furner I
    2. Pumfrey J

    (1992)

    Cell fate in the shoot apical meristem of Arabidopsis thaliana

    Development 115:755–764.

    • Google Scholar
23. 1. Furumizu C
    2. Alvarez JP
    3. Sakakibara K
    4. Bowman JL

    (2015) Antagonistic roles for KNOX1 and KNOX2 genes in patterning the land plant body plan following an ancient gene duplication

    PLoS Genetics 11:e1004980.

    https://doi.org/10.1371/journal.pgen.1004980

    • PubMed
    • Google Scholar
24. 1. Goebel K

    (1902)

    Ueber Regeneration im Pflanzenreich

    Biologisches Centralblatt 22:385–397.

    • Google Scholar
25. 1. Hagemann W
    2. Gleissberg S

    (1996) Organogenetic capacity of leaves: The significance of marginal blastozones in angiosperms

    Plant Systematics and Evolution 199:121–152.

    https://doi.org/10.1007/BF00984901

    • Google Scholar
26. 1. Hagemann W

    (1970)

    Studien zur Entwicklungsgeschichte der Angiospermenblfitter. Ein Beitrag zur Klärung ihres Gestaltungsprinzips

    Botanische Jahrbücher Fur Systematik 90:297–413.

    • Google Scholar
27. 1. Hawker NP
    2. Bowman JL

    (2004) Roles for Class III HD-Zip and KANADI genes in Arabidopsis root development

    Plant Physiology 135:2261–2270.

    https://doi.org/10.1104/pp.104.040196

    • PubMed
    • Google Scholar
28. 1. Hepworth SR
    2. Zhang Y
    3. McKim S
    4. Li X
    5. Haughn GW

    (2005) BLADE-ON-PETIOLE-dependent signaling controls leaf and floral patterning in Arabidopsis

    The Plant Cell Online 17:1434–1448.

    https://doi.org/10.1105/tpc.104.030536

    • PubMed
    • Google Scholar
29. 1. Hibara K
    2. Takada S
    3. Tasaka M

    (2003) CUC1 gene activates the expression of SAM-related genes to induce adventitious shoot formation

    The Plant Journal 36:687–696.

    https://doi.org/10.1046/j.1365-313X.2003.01911.x

    • PubMed
    • Google Scholar
30. 1. Hibara K
    2. Karim MR
    3. Takada S
    4. Taoka K
    5. Furutani M
    6. Aida M
    7. Tasaka M

    (2006) Arabidopsis CUP-SHAPED COTYLEDON3 Regulates Postembryonic Shoot Meristem and Organ Boundary Formation

    The Plant Cell 18:2946–2957.

    https://doi.org/10.1105/tpc.106.045716

    • Google Scholar
31. 1. Ichihashi Y
    2. Kawade K
    3. Usami T
    4. Horiguchi G
    5. Takahashi T
    6. Tsukaya H

    (2011) Key proliferative activity in the junction between the leaf blade and leaf petiole of Arabidopsis

    Plant Physiology 157:1151–1162.

    https://doi.org/10.1104/pp.111.185066

    • PubMed
    • Google Scholar
32. 1. Ikeuchi M
    2. Tatematsu K
    3. Yamaguchi T
    4. Okada K
    5. Tsukaya H

    (2013) Precocious progression of tissue maturation instructs basipetal initiation of leaflets in Chelidonium majus subsp. asiaticum (Papaveraceae)

    American Journal of Botany 100:1116–1126.

    https://doi.org/10.3732/ajb.1200560

    • PubMed
    • Google Scholar
33. 1. Jegla DE
    2. Sussex IM

    (1989) Cell lineage patterns in the shoot meristem of the sunflower embryo in the dry seed

    Developmental Biology 131:215–225.

    https://doi.org/10.1016/S0012-1606(89)80053-3

    • PubMed
    • Google Scholar
34. 1. Jéune B

    (1981)

    Modèle empirique Du Développement des feuilles de Dicotylédones

    Bulletin Du Museum National d'Hisoirie Naturelle. Séction B, Adansonia, Botanique, Phytochimié 4:433–459.

    • Google Scholar
35. Book

    1. Kenrick P
    2. Crane P

    (1997)

    The Origin and Early Diversification of Land Plants: A Cladistic Study

    Washington, DC: Smithsonian Institution Press..

    • Google Scholar
36. 1. Kuchen EE
    2. Fox S
    3. de Reuille PB
    4. Kennaway R
    5. Bensmihen S
    6. Avondo J
    7. Calder GM
    8. Southam P
    9. Robinson S
    10. Bangham A
    11. Coen E

    (2012) Generation of leaf shape through early patterns of growth and tissue polarity

    Science 335:1092–1096.

    https://doi.org/10.1126/science.1214678

    • PubMed
    • Google Scholar
37. 1. Laux T
    2. Mayer KF
    3. Berger J
    4. Jürgens G

    (1996)

    The WUSCHEL gene is required for shoot and floral meristem integrity in Arabidopsis

    Development 122:87–96.

    • PubMed
    • Google Scholar
38. 1. Lee J-Y
    2. Colinas J
    3. Wang JY
    4. Mace D
    5. Ohler U
    6. Benfey PN

    (2006) Transcriptional and posttranscriptional regulation of transcription factor expression in Arabidopsis roots

    PNAS 103:6055–6060.

    https://doi.org/10.1073/pnas.0510607103

    • Google Scholar
39. 1. Lifschitz E
    2. Eviatar T
    3. Rozman A
    4. Shalit A
    5. Goldshmidt A
    6. Amsellem Z
    7. Alvarez JP
    8. Eshed Y

    (2006) The tomato FT ortholog triggers systemic signals that regulate growth and flowering and substitute for diverse environmental stimuli

    PNAS 103:6398–6403.

    https://doi.org/10.1073/pnas.0601620103

    • Google Scholar
40. 1. Lin H
    2. Niu L
    3. McHale NA
    4. Ohme-Takagi M
    5. Mysore KS
    6. Tadege M

    (2013) Evolutionarily conserved repressive activity of WOX proteins mediates leaf blade outgrowth and floral organ development in plants

    PNAS 110:366–371.

    https://doi.org/10.1073/pnas.1215376110

    • Google Scholar
41. 1. Maksymowych R
    2. Erickson RO

    (1960) Development of the Lamina in Xanthium italicum Represented by the Plastochron Index

    American Journal of Botany 47:451–459.

    https://doi.org/10.2307/2439558

    • Google Scholar
42. 1. Maksymowych R
    2. Wochok ZS

    (1969) Activity of Marginal and Plate Meristems During Leaf Development of Xanthium pennsylvanicum

    American Journal of Botany 56:26–30.

    https://doi.org/10.2307/2440391

    • Google Scholar
43. 1. Mayer KF
    2. Schoof H
    3. Haecker A
    4. Lenhard M
    5. Jürgens G
    6. Laux T

    (1998) Role of WUSCHEL in regulating stem cell fate in the Arabidopsis shoot meristem

    Cell 95:805–815.

    https://doi.org/10.1016/S0092-8674(00)81703-1

    • PubMed
    • Google Scholar
44. 1. Nadeau JA
    2. Sack FD

    (2002) Control of stomatal distribution on the Arabidopsis leaf surface

    Science 296:1697–1700.

    https://doi.org/10.1126/science.1069596

    • PubMed
    • Google Scholar
45. 1. Nag A
    2. King S
    3. Jack T

    (2009) miR319a targeting of TCP4 is critical for petal growth and development in Arabidopsis

    PNAS 106:22534–22539.

    https://doi.org/10.1073/pnas.0908718106

    • Google Scholar
46. 1. Nakata M
    2. Matsumoto N
    3. Tsugeki R
    4. Rikirsch E
    5. Laux T
    6. Okada K

    (2012) Roles of the middle domain-specific Wuschel-Related Homeobox genes in early development of leaves in Arabidopsis

    Plant Cell 24:519–535.

    https://doi.org/10.1105/tpc.111.092858

    • Google Scholar
47. 1. Nakata M
    2. Okada K

    (2012) The three-domain model: a new model for the early development of leaves in Arabidopsis thaliana

    Plant Signaling & Behavior 7:1423–1427.

    https://doi.org/10.4161/psb.21959

    • PubMed
    • Google Scholar
48. 1. Nardmann J
    2. Werr W

    (2013) Symplesiomorphies in the WUSCHEL clade suggest that the last common ancestor of seed plants contained at least four independent stem cell niches

    New Phytologist 199:1081–1092.

    https://doi.org/10.1111/nph.12343

    • PubMed
    • Google Scholar
49. 1. Nath U
    2. Crawford BC
    3. Carpenter R
    4. Coen E

    (2003) Genetic control of surface curvature

    Science 299:1404–1407.

    https://doi.org/10.1126/science.1079354

    • PubMed
    • Google Scholar
50. 1. Navaud O
    2. Dabos P
    3. Carnus E
    4. Tremousaygue D
    5. Hervé C

    (2007) TCP transcription factors predate the emergence of land plants

    Journal of Molecular Evolution 65:23–33.

    https://doi.org/10.1007/s00239-006-0174-z

    • PubMed
    • Google Scholar
51. 1. Obayashi T
    2. Hayashi S
    3. Saeki M
    4. Ohta H
    5. Kinoshita K

    (2009) ATTED-II provides coexpressed gene networks for Arabidopsis

    Nucleic Acids Research 37:D987–991.

    https://doi.org/10.1093/nar/gkn807

    • PubMed
    • Google Scholar
52. 1. Ori N
    2. Cohen AR
    3. Etzioni A
    4. Brand A
    5. Yanai O
    6. Shleizer S
    7. Menda N
    8. Amsellem Z
    9. Efroni I
    10. Pekker I
    11. Alvarez JP
    12. Blum E
    13. Zamir D
    14. Eshed Y

    (2007) Regulation of LANCEOLATE by miR319 is required for compound-leaf development in tomato

    Nature Genetics 39:787–791.

    https://doi.org/10.1038/ng2036

    • PubMed
    • Google Scholar
53. 1. Palatnik JF
    2. Allen E
    3. Wu X
    4. Schommer C
    5. Schwab R
    6. Carrington JC
    7. Weigel D

    (2003) Control of leaf morphogenesis by microRNAs

    Nature 425:257–263.

    https://doi.org/10.1038/nature01958

    • PubMed
    • Google Scholar
54. 1. Piazza P
    2. Bailey CD
    3. Cartolano M
    4. Krieger J
    5. Cao J
    6. Ossowski S
    7. Schneeberger K
    8. He F
    9. de Meaux J
    10. Hall N
    11. Macleod N
    12. Filatov D
    13. Hay A
    14. Tsiantis M

    (2010) Arabidopsis thaliana leaf form evolved via loss of KNOX expression in leaves in association with a selective sweep

    Current Biology 20:2223–2228.

    https://doi.org/10.1016/j.cub.2010.11.037

    • PubMed
    • Google Scholar
55. 1. Poethig RS
    2. Sussex IM

    (1985) The cellular parameters of leaf development in tobacco: a clonal analysis

    Planta 165:170–184.

    https://doi.org/10.1007/BF00395039

    • PubMed
    • Google Scholar
56. 1. Roeder AH
    2. Chickarmane V
    3. Cunha A
    4. Obara B
    5. Manjunath BS
    6. Meyerowitz EM

    (2010) Variability in the control of cell division underlies sepal epidermal patterning in Arabidopsis thaliana

    PLoS Biology 8:e1000367.

    https://doi.org/10.1371/journal.pbio.1000367

    • PubMed
    • Google Scholar
57. 1. Sachs T

    (1969)

    Regeneration experiments on the determination of the form of leaves

    Israel Journal Of Botany 18:21–30.

    • Google Scholar
58. 1. Sarojam R
    2. Sappl PG
    3. Goldshmidt A
    4. Efroni I
    5. Floyd SK
    6. Eshed Y
    7. Bowman JL

    (2010) Differentiating Arabidopsis shoots from leaves by combined YABBY activities

    The Plant Cell Online 22:2113–2130.

    https://doi.org/10.1105/tpc.110.075853

    • PubMed
    • Google Scholar
59. 1. Scarpella E
    2. Marcos D
    3. Friml J
    4. Berleth T

    (2006) Control of leaf vascular patterning by polar auxin transport

    Genes & Development 20:1015–1027.

    https://doi.org/10.1101/gad.1402406

    • PubMed
    • Google Scholar
60. 1. Schmid M
    2. Davison TS
    3. Henz SR
    4. Pape UJ
    5. Demar M
    6. Vingron M
    7. Schölkopf B
    8. Weigel D
    9. Lohmann JU

    (2005) A gene expression map of Arabidopsis thaliana development

    Nature Genetics 37:501–506.

    https://doi.org/10.1038/ng1543

    • PubMed
    • Google Scholar
61. 1. Schmidt A

    (1924)

    Histologische studien an phanerogamen vegertationspunkten

    Botanisches Archiv 8:345–404.

    • Google Scholar
62. 1. Schüepp O

    (1918)

    Zur Entwicklungsgeschichte Des Blattes Von Acer Pseudoplatanus L

    Vierteljahrsschrift Der Naturforschenden Gesellschaft in Zürich 63:99–105.

    • Google Scholar
63. Book

    1. Schüepp O

    (1926)

    Meristeme

    Berlin, Gebr. Borntraeger.

    • Google Scholar
64. 1. Shani E
    2. Burko Y
    3. Ben-Yaakov L
    4. Berger Y
    5. Amsellem Z
    6. Goldshmidt A
    7. Sharon E
    8. Ori N

    (2009) Stage-specific regulation of Solanum lycopersicum leaf maturation by class 1 KNOTTED1-LIKE HOMEOBOX proteins

    The Plant Cell 21:3078–3092.

    https://doi.org/10.1105/tpc.109.068148

    • PubMed
    • Google Scholar
65. 1. Snow M
    2. Snow R

    (1941)

    Regeneration of leaflets in lupins

    The New Phytologist 40:133–138.

    • Google Scholar
66. Book

    1. Steeves T
    2. Sussex I

    (1989)

    Patterns in Plant Development (2nd edn)

    Cambridge University Press.

    • Google Scholar
67. 1. Thomasson M

    (1970)

    Quelques observations sur la répartition des zones de croissance de la feuille Du Jasminium Nudiflorum Lindley

    Candollea 25:297–340.

    • Google Scholar
68. 1. Trigueros M
    2. Navarrete-Gomez M
    3. Sato S
    4. Christensen SK
    5. Pelaz S
    6. Weigel D
    7. Yanofsky MF
    8. Ferrandiz C

    (2009) The NGATHA genes direct style development in the arabidopsis gynoecium

    The Plant Cell Online 21:1394–1409.

    https://doi.org/10.1105/tpc.109.065508

    • Google Scholar
69. 1. Trécul A

    (1853)

    Mémoires sur la formation des feuilles

    Annales Des Sciences Naturelles-Zoologie Et Biologie Animale 20:235–314.

    • Google Scholar
70. 1. Tsukaya H

    (2013) Leaf development

    The Arabidopsis Book 11:e0163.

    https://doi.org/10.1199/tab.0163

    • PubMed
    • Google Scholar
71. 1. Vandenbussche M
    2. Horstman A
    3. Zethof J
    4. Koes R
    5. Rijpkema AS
    6. Gerats T

    (2009) Differential recruitment of WOX transcription factors for lateral development and organ fusion in petunia and arabidopsis

    The Plant Cell Online 21:2269–2283.

    https://doi.org/10.1105/tpc.109.065862

    • Google Scholar
72. 1. Wang W
    2. Xu B
    3. Wang H
    4. Li J
    5. Huang H
    6. Xu L

    (2011) YUCCA genes are expressed in response to leaf adaxial-abaxial juxtaposition and are required for leaf margin development

    Plant Physiology 157:1805–1819.

    https://doi.org/10.1104/pp.111.186395

    • PubMed
    • Google Scholar
73. Book

    1. Wolff C

    (1759)

    Theoria Generationis

    Halae ad Salam, Christ. Hendel.

    • Google Scholar

Author details

1. John Paul Alvarez

   School of Biological Sciences, Monash University, Melbourne, Australia

   Contribution

   JPA, Conception and design, Acquisition of data, Analysis and interpretation of data, Drafting or revising the article

   For correspondence

   john.alvarez@monash.edu

   Competing interests

   The authors declare that no competing interests exist.

2. Chihiro Furumizu

   School of Biological Sciences, Monash University, Melbourne, Australia

   Contribution

   CF, Conception and design, Acquisition of data, Analysis and interpretation of data, Drafting or revising the article

   Competing interests

   The authors declare that no competing interests exist.

3. Idan Efroni

   The Robert H. Smith Institute of Plant Sciences and Genetics in Agriculture, The Hebrew University of Jerusalem, Rehovot, Israel

   Contribution

   IE, Acquisition of data, Analysis and interpretation of data

   Competing interests

   The authors declare that no competing interests exist.

4. Yuval Eshed

   Department of Plant and environmental Sciences, Weizmann Institute of Science, Rehovot, Israel

   Contribution

   YE, Conception and design, Analysis and interpretation of data, Drafting or revising the article

   For correspondence

   yuval.eshed@weizmann.ac.il

   Competing interests

   The authors declare that no competing interests exist.

5. John L Bowman

   1. School of Biological Sciences, Monash University, Melbourne, Australia
   2. Department of Plant Biology, University of California, Davis, Davis, California, United States

   Contribution

   JLB, Conception and design, Analysis and interpretation of data, Drafting or revising the article

   For correspondence

   John.Bowman@monash.edu

   Competing interests

   The authors declare that no competing interests exist.

   "This ORCID iD identifies the author of this article:" 0000-0001-7347-3691

• 5,994

  Page views

• 1,548

  Downloads

• 116

  Citations

Article citation count generated by polling the highest count across the following sources: Scopus, Crossref, PubMed Central.