1. Paul de Figueiredo
  2. Marty Dickman  Is a corresponding author
  1. Texas A&M University, United States

The Irish potato famine was responsible for more than one million deaths and the emigration of one million people from Europe in the 1840s (Andrivon, 1996). Today, the microbe that caused the famine, an oomycete called Phytophthora infestans, continues to cause serious outbreaks of disease in potato crops. Traditional control measures, such as fungicides and breeding for resistance, often have only marginal success in combating the disease, especially when the climate favors the growth and development of P. infestans (Fry and Goodwin, 1997). Now, in eLife, Sophien Kamoun, Tolga Bozkurt and colleagues – including Yasin Dagdas and Khaoula Belhaj as joint first authors – reveal how one of the proteins produced by P. infestans manipulates host plant cells to weaken their defenses (Dagdas et al., 2016).

It is well established that plant pathogens secrete proteins and small molecules – collectively known as effectors – that can interfere with plant defenses and make it easier for pathogens to infect and spread (Djamei et al., 2011; de Wit et al., 2009; Rovenich et al., 2014; Gawehns et al., 2014). However, as part of an ongoing arms race between plants and pathogens, some effectors are recognized by proteins in the host plant, which triggers immune responses that act to contain the infection. Relatively little is known about how effectors interfere with plant defenses. In particular, the identities of the plant molecules that are targeted by the effectors, and details of how the effectors are transported into plant cells, remain unclear.

The success of P. infestans as a pathogen is largely due to its ability to secrete hundreds of different effectors. Now, Dagdas, Belhaj et al. – who are based at the Sainsbury Laboratory, the John Innes Centre and Imperial College – report how they carried out a screen for plant molecules that interact with effectors from P. infestans (Dagdas et al., 2016). The experiments were carried out in the leaves of tobacco, which is a commonly used plant model, and show that an effector called PexRD54 targets a process called autophagy in plant cells.

Autophagy is a complex “self-eating” process that occurs when plant and other eukaryotic cells experience certain stresses – for example, due to a shortage of nutrients or a change in environmental conditions. During autophagy, cell material is broken down to supply the building blocks needed to maintain essential processes (Li and Vierstra, 2009). More recently, autophagy has been implicated in a variety of other situations, including restricting the growth and spread of invading microbes. A growing body of evidence suggests that autophagy plays a dual role both in promoting the survival of cells and in triggering cell death.

During autophagy, cell materials are sequestered by structures called autophagosomes and then delivered to acidic cell compartments where the material is degraded and recycled. In addition to supporting the bulk degradation of cell materials, it was recently shown that autophagy allows the selective removal of cellular components that are damaged or no longer needed. In selective autophagy, the sequestered material is loaded into autophagosomes by specific interactions between receptor proteins and specific autophagy proteins, such as the ATG8 proteins (Stolz et al., 2014, Lamb et al., 2013).

Dagdas, Belhaj et al. found that PexRD54 interferes with the activity of a potato cargo receptor called Joka2. PexRD54 out-competes Joka2 to bind to an ATG8 protein and stimulate the formation of an autophagosome in the plant cell (Figure 1). In doing so, the oomycete cleverly reduces the loading of specific types of cargo into autophagosomes and thus limits the plant defense response.

Phytophthora infestans interferes with the immune responses of potato plants.

Spores of P. infestans land on the leaves of potato plants and germinate (top middle). The growing fungus enters the leaves and spreads around the plant, leading to disease (top right). Proteins called effectors are released from the pathogen and some are taken into the cells of the host plant (bottom left). These effectors (purple ovals) interact with host factors (green squares) to promote the progression of the disease. Dagdas, Belhaj et al. found that a P. infestans effector called PexRD54 (purple oval; bottom right) out-competes a plant cargo receptor known as Joka2 (green square) on the surface of a membrane structure called a phagophore, which eventually becomes an autophagosome. In this way, PexRD54 prevents the loading of cargo proteins into autophagosomes and inhibits plant defenses.

The reported observations expand upon studies of mammalian pathogens that also harbor effectors that interfere with autophagy (Table 1). Taken together, this work provides a template for future investigations into the ways in which effectors subvert host plant defenses. However, a number of interesting questions remain unanswered. For example, how do cargo receptors work? How are they regulated? What is the nature of the cargo in the autophagosomes and how does it regulate immune responses? In addition, our understanding of the mechanisms that control selective autophagy remain incomplete. How is the selectivity regulated, and what other cell mechanisms might be subverted by effectors? Phytophthora diseases can have devastating effects, but as this study illustrates, they can also illuminate and advance our understanding of fundamental cellular processes.

Table 1

Mammalian pathogens that express proteins that interfere with host autophagosome biogenesis or function.

DomainPathogenHostEffectorActivityRefs
VirusHIV virushumanNef1Inhibits host autophagyCampbell et al., 2015
CMV virushumanTrs1Inhibits host autophagyChaumorcel et al., 2012
Dengue virusmammalNS4AUpregulation of autophagyMcLean et al., 2011
BacteriaLegionellamammalRavZCleaves an Atg8 protein from pre-autophagosomesChoy et al., 2012; Horenkamp et al., 2015
CoxiellamammalCig2Disrupts interactions between acidic compartments and host autophagosomesNewton et al., 2014
SalmonellamammalSseLInhibits selective autophagy of cytosolic aggregatesMesquita et al., 2012
Anaplasma phagocytophilummammalAts-1Hijacks a pathway that activates autophagy to promote its growth inside cellsNiu et al., 2012
Vibrio parahemolyticusmammalVopQCreates pores in acidic compartments in host cellsSreelatha et al., 2013
EukaryotePhytophthoraplantPexRD54Inappropriately activates the formation of autophagosomesDagdas et al., 2016

References

Article and author information

Author details

  1. Paul de Figueiredo

    Norman Borlaug Institute, Department of Veterinary Pathobiology and Department of Microbial Pathogenesis and Immunology, Texas A&M University, College Station, United States
    Competing interests
    The authors declare that no competing interests exist.
  2. Marty Dickman

    Norman Borlaug Institute and the Department of Plant Pathology and Microbiology, Texas A&M University, College Station, United States
    For correspondence
    mbdickman@tamu.edu
    Competing interests
    The authors declare that no competing interests exist.

Publication history

  1. Version of Record published:

Copyright

© 2016, de Figueiredo et al.

This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.

Metrics

  • 2,717
    views
  • 481
    downloads
  • 3
    citations

Views, downloads and citations are aggregated across all versions of this paper published by eLife.

Download links

A two-part list of links to download the article, or parts of the article, in various formats.

Downloads (link to download the article as PDF)

Open citations (links to open the citations from this article in various online reference manager services)

Cite this article (links to download the citations from this article in formats compatible with various reference manager tools)

  1. Paul de Figueiredo
  2. Marty Dickman
(2016)
Plant Disease: Autophagy under attack
eLife 5:e14447.
https://doi.org/10.7554/eLife.14447

Further reading

    1. Biochemistry and Chemical Biology
    2. Microbiology and Infectious Disease
    Lina Antenucci, Salla Virtanen ... Perttu Permi
    Research Article

    Orchestrated action of peptidoglycan (PG) synthetases and hydrolases is vital for bacterial growth and viability. Although the function of several PG synthetases and hydrolases is well understood, the function, regulation, and mechanism of action of PG hydrolases characterised as lysostaphin-like endopeptidases have remained elusive. Many of these M23 family members can hydrolyse glycyl-glycine peptide bonds and show lytic activity against Staphylococcus aureus whose PG contains a pentaglycine bridge, but their exact substrate specificity and hydrolysed bonds are still vaguely determined. In this work, we have employed NMR spectroscopy to study both the substrate specificity and the bond cleavage of the bactericide lysostaphin and the S. aureus PG hydrolase LytM. Yet, we provide substrate-level evidence for the functional role of these enzymes. Indeed, our results show that the substrate specificities of these structurally highly homologous enzymes are similar, but unlike observed earlier both LytM and lysostaphin prefer the D-Ala-Gly cross-linked part of mature peptidoglycan. However, we show that while lysostaphin is genuinely a glycyl-glycine hydrolase, LytM can also act as a D-alanyl-glycine endopeptidase.

    1. Microbiology and Infectious Disease
    Lianhua Qin, Junfang Xu ... Haipeng Liu
    Research Article

    Deeper understanding of the crosstalk between host cells and Mycobacterium tuberculosis (Mtb) provides crucial guidelines for the rational design of novel intervention strategies against tuberculosis (TB). Mycobacteria possess a unique complex cell wall with arabinogalactan (AG) as a critical component. AG has been identified as a virulence factor of Mtb which is recognized by host galectin-9. Here, we demonstrate that galectin-9 directly inhibited mycobacterial growth through AG-binding property of carbohydrate-recognition domain 2. Furthermore, IgG antibodies with AG specificity were detected in the serum of TB patients. Based on the interaction between galectin-9 and AG, we developed a monoclonal antibody (mAb) screening assay and identified AG-specific mAbs which profoundly inhibit Mtb growth. Mechanistically, proteomic profiling and morphological characterizations revealed that AG-specific mAbs regulate AG biosynthesis, thereby inducing cell wall swelling. Thus, direct AG-binding by galectin-9 or antibodies contributes to protection against TB. Our findings pave the way for the rational design of novel immunotherapeutic strategies for TB control.