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

  • 2,725
    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
    Eva Herdering, Tristan Reif-Trauttmansdorff ... Ruth Anne Schmitz
    Research Article

    Glutamine synthetases (GS) are central enzymes essential for the nitrogen metabolism across all domains of life. Consequently, they have been extensively studied for more than half a century. Based on the ATP-dependent ammonium assimilation generating glutamine, GS expression and activity are strictly regulated in all organisms. In the methanogenic archaeon Methanosarcina mazei, it has been shown that the metabolite 2-oxoglutarate (2-OG) directly induces the GS activity. Besides, modulation of the activity by interaction with small proteins (GlnK1 and sP26) has been reported. Here, we show that the strong activation of M. mazei GS (GlnA1) by 2-OG is based on the 2-OG dependent dodecamer assembly of GlnA1 by using mass photometry (MP) and single particle cryo-electron microscopy (cryo-EM) analysis of purified strep-tagged GlnA1. The dodecamer assembly from dimers occurred without any detectable intermediate oligomeric state and was not affected in the presence of GlnK1. The 2.39 Å cryo-EM structure of the dodecameric complex in the presence of 12.5 mM 2-OG demonstrated that 2-OG is binding between two monomers. Thereby, 2-OG appears to induce the dodecameric assembly in a cooperative way. Furthermore, the active site is primed by an allosteric interaction cascade caused by 2-OG-binding towards an adaption of an open active state conformation. In the presence of additional glutamine, strong feedback inhibition of GS activity was observed. Since glutamine dependent disassembly of the dodecamer was excluded by MP, feedback inhibition most likely relies on the binding of glutamine to the catalytic site. Based on our findings, we propose that under nitrogen limitation the induction of M. mazei GS into a catalytically active dodecamer is not affected by GlnK1 and crucially depends on the presence of 2-OG.

    1. Microbiology and Infectious Disease
    Yue Sun, Jingwei Li ... Xin Deng
    Research Article

    The model Gram-negative plant pathogen Pseudomonas syringae utilises hundreds of transcription factors (TFs) to regulate its functional processes, including virulence and metabolic pathways that control its ability to infect host plants. Although the molecular mechanisms of regulators have been studied for decades, a comprehensive understanding of genome-wide TFs in Psph 1448A remains limited. Here, we investigated the binding characteristics of 170 of 301 annotated TFs through chromatin immunoprecipitation sequencing (ChIP-seq). Fifty-four TFs, 62 TFs, and 147 TFs were identified in top-level, middle-level, and bottom-level, reflecting multiple higher-order network structures and direction of information flow. More than 40,000 TF pairs were classified into 13 three-node submodules which revealed the regulatory diversity of TFs in Psph 1448A regulatory network. We found that bottom-level TFs performed high co-associated scores to their target genes. Functional categories of TFs at three levels encompassed various regulatory pathways. Three and 25 master TFs were identified to involve in virulence and metabolic regulation, respectively. Evolutionary analysis and topological modularity network revealed functional variability and various conservation of TFs in P. syringae (Psph 1448A, Pst DC3000, Pss B728a, and Psa C48). Overall, our findings demonstrated a global transcriptional regulatory network of genome-wide TFs in Psph 1448A. This knowledge can advance the development of effective treatment and prevention strategies for related infectious diseases.