Plants detect pathogens by surface-localized receptors. Few such receptors are known. The coreceptor BRI1-ASSOCIATED KINASE 1 (BAK1) is a frequent member of activated receptor complexes. The proteomics strategy described here uses BAK1 as molecular bait to identify potential receptors that are specifically activated by pathogen components. We demonstrate this approach by identifying Nicotiana benthamiana RECEPTOR-LIKE PROTEIN REQUIRED FOR CSP22 RESPONSIVENESS (NbCSPR). We show that NbCSPR is required for immune responses initiated by the bacterial cold shock protein, confers age-dependent immunity against bacteria, and restricts the transformation of N. benthamiana cells by Agrobacterium. Manipulation of this gene will provide new options for disease control and genetic transformation of crop species.
Recent studies have shown that in addition to the transcriptional circadian clock, many organisms, including Arabidopsis, have a circadian redox rhythm driven by the organism’s metabolic activities1, 2, 3. It has been hypothesized that the redox rhythm is linked to the circadian clock, but the mechanism and the biological significance of this link have only begun to be investigated4, 5, 6, 7. Here we report that the master immune regulator NPR1 (non-expressor of pathogenesis-related gene 1) of Arabidopsis is a sensor of the plant’s redox state and regulates transcription of core circadian clock genes even in the absence of pathogen challenge. Surprisingly, acute perturbation in the redox status triggered by the immune signal salicylic acid does not compromise the circadian clock but rather leads to its reinforcement. Mathematical modelling and subsequent experiments show that NPR1 reinforces the circadian clock without changing the period by regulating both the morning and the evening clock genes. This balanced network architecture helps plants gate their immune responses towards the morning and minimize costs on growth at night. Our study demonstrates how a sensitive redox rhythm interacts with a robust circadian clock to ensure proper responsiveness to environmental stimuli without compromising fitness of the organism.
Programmed cell death is characterized by a cascade of tightly controlled events that culminate in the orchestrated death of the cell. In multicellular organisms autophagy and apoptosis are recognized as two principal means by which these genetically determined cell deaths occur. During plant-microbe interactions cell death programs can mediate both resistant and susceptible events. Via oxalic acid (OA), the necrotrophic phytopathogen Sclerotinia sclerotiorum hijacks host pathways and induces cell death in host plant tissue resulting in hallmark apoptotic features in a time and dose dependent manner. OA-deficient mutants are non-pathogenic and trigger a restricted cell death phenotype in the host that unexpectedly exhibits markers associated with the plant hypersensitive response including callose deposition and a pronounced oxidative burst, suggesting the plant can recognize and in this case respond, defensively. The details of this plant directed restrictive cell death associated with OA deficient mutants is the focus of this work. Using a combination of electron and fluorescence microscopy, chemical effectors and reverse genetics, we show that this restricted cell death is autophagic. Inhibition of autophagy rescued the non-pathogenic mutant phenotype. These findings indicate that autophagy is a defense response in this necrotrophic fungus/plant interaction and suggest a novel function associated with OA; namely, the suppression of autophagy. These data suggest that not all cell deaths are equivalent, and though programmed cell death occurs in both situations, the outcome is predicated on who is in control of the cell death machinery. Based on our data, we suggest that it is not cell death per se that dictates the outcome of certain plant-microbe interactions, but the manner by which cell death occurs that is crucial.
Pathogens target proteins involved in autophagy to inhibit immune responses in plants.
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).
Chloroplast stromules are induced during plant immune responsesPro-PCD signals such as SA and H2O2 induce stromulesStromules form dynamic connections with nucleus during immune responsesConstitutively induced stromules enhance PCD during plant immune responses
Inter-organellar communication is vital for successful innate immune responses that confer defense against pathogens. However, little is known about how chloroplasts, which are a major production site of pro-defense molecules, communicate and coordinate with other organelles during defense. Here we show that chloroplasts send out dynamic tubular extensions called stromules during innate immunity or exogenous application of the pro-defense signals, hydrogen peroxide (H2O2) and salicylic acid. Interestingly, numerous stromules surround nuclei during defense response, and these connections correlate with an accumulation of chloroplast-localized NRIP1 defense protein and H2O2 in the nucleus. Furthermore, silencing and knockout of chloroplast unusual positioning 1 (CHUP1) that encodes a chloroplast outer envelope protein constitutively induces stromules in the absence of pathogen infection and enhances programmed cell death. These results support a model in which stromules aid in the amplification and/or transport of pro-defense signals into the nucleus and other subcellular compartments during immunity.
Flagellin perception in Arabidopsis is through recognition of its highly conserved N-terminal epitope (flg22) by flagellin-sensitive 2 (FLS2). Flg22 binding induces FLS2 heteromerization with BRI1-associated kinase 1 (BAK1) and their reciprocal activation followed by plant immunity. Here, we report the crystal structure of FLS2 and BAK1 ectodomains complexed with flg22 at 3.06 angstroms. A conserved and a nonconserved site from the inner surface of the FLS2 solenoid recognize the C- and N-terminal segment of flg22, respectively, without oligomerization or conformational changes in the FLS2 ectodomain. Besides directly interacting with FLS2, BAK1 acts as a coreceptor by recognizing the C terminus of the FLS2-bound flg22. Our data reveal the molecular mechanisms underlying FLS2-BAK1 complex recognition of flg22 and provide insight into the immune receptor complex activation.
Eukaryotic cells can use the autophagy pathway to defend against microbes that gain access to the cytosol or reside in pathogen-modified vacuoles. It remains unclear if pathogens have evolved specific mechanisms to manipulate autophagy. Here, we found that the intracellular pathogen Legionella pneumophila could interfere with autophagy using the bacterial effector protein RavZ to directly uncouple Atg8 proteins attached to phosphatidylethanolamine on autophagosome membranes. RavZ hydrolyzed the amide bond between the carboxyl-terminal glycine residue and an adjacent aromatic residue in Atg8 proteins, producing an Atg8 protein that could not be reconjugated by Atg7 and Atg3. Thus, intracellular pathogens can inhibit autophagy by irreversibly inactivating Atg8 proteins during infection.
I really wonder whether there are effectors from plant pathogens that target autophagy?! Would be nice to research...
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