Pathogenic bacteria invade plant tissues and proliferate in the extracellular space. Plants have evolved the immune system to recognize and limit the growth of pathogens. Despite substantial progress ...in the study of plant immunity, the mechanism by which plants limit pathogen growth remains unclear. Here, we show that lignin accumulates in Arabidopsis leaves in response to incompatible interactions with bacterial pathogens in a manner dependent on Casparian strip membrane domain protein (CASP)‐like proteins (CASPLs). CASPs are known to be the organizers of the lignin‐based Casparian strip, which functions as a diffusion barrier in roots. The spread of invading avirulent pathogens is prevented by spatial restriction, which is disturbed by defects in lignin deposition. Moreover, the motility of pathogenic bacteria is negatively affected by lignin accumulation. These results suggest that the lignin‐deposited structure functions as a physical barrier similar to the Casparian strip, trapping pathogens and thereby terminating their growth.
Synopsis
Plants employ a multilayered immune system, but the exact mechanisms of how plants restrict pathogen growth remain unclear. In this study, the phenolic polymer and cell wall component lignin is shown to form a mechanical barrier against avirulent pathogens, thereby conferring disease resistance in plants.
Lignification is induced during incompatible plant‐pathogen interactions in Arabidopsis.
Lignin spatially restricts and encompasses bacteria in the extracellular space
Lignin deposition enhances disease resistance.
Casparian strip organizer proteins CASPL1D1 and CASPL4D1 are required for pathogen‐induced lignification.
Lignin deposition is required for innate immune defense during incompatible plant‐pathogen interactions in a manner dependent on Casparian strip organizer proteins.
Hypersensitive response (HR) is a form of programmed cell death (PCD) and the primary immune response that prevents pathogen invasion in plants. Here, we show that a microRNA miR164 and its target ...gene NAC4 (At5g07680), encoding a NAC transcription factor, play essential roles in the regulation of HR PCD in Arabidopsis thaliana.
Cell death symptoms were noticeably enhanced in NAC4-overexpressing (35S:NAC4) and mir164 mutant plants in response to avirulent bacterial pathogens. NAC4 expression was induced by pathogen infection and negatively regulated by miR164 expression.
NAC4-binding DNA sequences were determined by in vitro binding site selection using random oligonucleotide sequences. Microarray, chromatin immunoprecipitation and quantitative real time polymerase chain reaction (qRT-PCR) analyses, followed by cell death assays in protoplasts, led to the identification of NAC4 target genes LURP1, WRKY40 and WRKY54, which act as negative regulators of cell death.
Our results suggest that NAC4 promotes hypersensitive cell death by suppressing its target genes and this immune process is fine-tuned by the negative action of miR164.
A central component of the plant defense response to pathogens is the hypersensitive response (HR), a form of programmed cell death (PCD). Rapid and localized induction of HR PCD ensures that ...pathogen invasion is prevented. Autophagy has been implicated in the regulation of HR cell death, but the functional relationship between autophagy and HR PCD and the regulation of these processes during the plant immune response remain controversial. Here, we show that a small GTP-binding protein, RabG3b, plays a positive role in autophagy and promotes HR cell death in response to avirulent bacterial pathogens in Arabidopsis (Arabidopsis thaliana). Transgenic plants overexpressing a constitutively active RabG3b (RabG3bCA) displayed accelerated, unrestricted HR PCD within 1 d of infection, in contrast to the autophagy-defective atg5-1 mutant, which gradually developed chlorotic cell death through uninfected sites over several days. Microscopic analyses showed the accumulation of autophagic structures during HR cell death in RabG3bCA cells. Our results suggest that RabG3b contributes to HR cell death via the activation of autophagy, which plays a positive role in plant immunity-triggered HR PCD.
The gaseous plant hormone ethylene is a key signaling molecule regulating plant growth, development, and defense against pathogens. Octadecanoid-responsive arabidopsis 59 (ORA59) is an ethylene ...response factor (ERF) transcription factor and has been suggested to integrate ethylene and jasmonic acid signaling and regulate resistance to necrotrophic pathogens. Here we screened for ORA59 interactors using the yeast two-hybrid system to elucidate the molecular function of ORA59. This led to the identification of RELATED TO AP2.3 (RAP2.3), another ERF transcription factor belonging to the group VII ERF family. In binding assays, ORA59 and RAP2.3 interacted in the nucleus and showed ethylene-dependent nuclear localization. ORA59 played a positive role in ethylene-regulated responses, including the triple response, featured by short, thick hypocotyl and root, and exaggerated apical hook in dark-grown seedlings, and resistance to the necrotrophic pathogen
, as shown by the increased and decreased ethylene sensitivity and disease resistance in ORA59-overexpressing (
) and null mutant (
) plants, respectively. In genetic crosses,
crossed lines lost ORA59-mediated positive effects and behaved like
mutant. These results suggest that ORA59 physically interacts with RAP2.3 and that this interaction is important for the regulatory roles of ORA59 in ethylene responses.
The evolutionary plant-pathogen arms race has equipped plants with the immune system that can defend against pathogens. Pattern-triggered immunity and effector-triggered immunity are two major ...branches of innate immunity that share immune responses, including oxidative bursts, transcriptional reprogramming, and cell wall modifications such as lignin deposition. In a previous study, we reported that lignin rapidly accumulates in pathogen-infected Arabidopsis leaves and acts as a mechanical barrier, spatially restricting pathogens and cell death. Lignin deposition into the cell wall is a three-step process: monolignol biosynthesis, transport, and polymerization. While monolignol biosynthesis and polymerization are relatively well understood, the mechanism of monolignol transport remains unclear. In this study, we show that macroautophagy/autophagy modulates pathogen-induced lignin formation. Lignification and other immune responses were impaired in autophagy-defective atg (autophagy-related) mutants. In microscopy analyses, monolignols formed punctate structures in response to pathogen infection and colocalized with autophagic vesicles. Furthermore, autophagic activity and lignin accumulation were both enhanced in dnd1 (defense, no death 1) mutant with elevated disease resistance but no cell death and crossing dnd1-1 with atg mutants resulted in a lignin deficit, further supporting that lignin formation requires autophagy. Collectively, these findings demonstrate that lignification, particularly monolignol transport, is achieved through autophagic membrane trafficking in plant immunity.Abbreviations: ABC transporter: ATP-binding cassette transporter; ACD2/AT4G37000: accelerated cell death 2; ATG: autophagy-related; C3′H/AT2G40890: p-coumaroyl shikimate 3-hydroxylase; C4H/AT2G30490: cinnamate 4-hydroxylase; CA: coniferyl alcohol; CaMV: cauliflower mosaic virus; CASP: Casparian strip membrane domain protein; CASPL: CASP-like protein; CBB: Coomassie Brilliant Blue; CCoAOMT1/AT4G34050: caffeoyl-CoA O-methyltransferase 1; CCR1/AT1G15950: cinnamoyl-CoA reductase 1; CFU: colony-forming unit; COMT1/AT5G54160: caffeic acid O-methyltransferase 1; Con A: concanamycin A; DMAC: dimethylaminocoumarin; DND1/AT5G15410: defense, no death 1; CNGC2: cyclic nucleotide-gated channel 2; ER: endoplasmic reticulum; ESB1/AT2G28670/DIR10: enhanced suberin 1; ETI: effector-triggered immunity; EV: extracellular vesicle; F5H/AT4G36220: ferulate-5-hydroxylase; Fluo-3 AM: Fluo-3 acetoxymethyl ester; GFP: green fluorescent protein; HCT/AT5G48930: p-hydroxycinnamoyl-CoA:quinate/shikimate p-hydroxycinnamoyltransferase; HR: hypersensitive response; LAC: laccase; LTG: LysoTracker Green; LSD1/AT4G200380: lesion stimulating disease 1; PAL1/AT2G37040: phenylalanine ammonia-lyase 1; PAMP: pathogen-associated molecular patterns; PCD: programmed cell death; PE: phosphatidylethanolamine; PRX: peroxidase; Pst DC3000: Pseudomonas syringe pv. tomato DC3000; PTI: pattern-triggered immunity; SA: salicylic acid; SD: standard deviation; SID2/AT1G7410: SA induction-deficient 2; UGT: UDP-glucosyltransferase; UPLC: ultraperformance liquid chromatography; UPS: unconventional protein secretion; V-ATPase: vacuolar-type H
+
-translocating ATPase
Lignin, a major component of the secondary cell wall, is important for plant growth and development. Moreover, lignin plays a pivotal role in plant innate immunity. Lignin is readily deposited upon ...pathogen infection and functions as a physical barrier that limits the spread of pathogens. In this study, we show that an Arabidopsis MYB transcription factor MYB15 is required for the activation of lignin biosynthesis genes such as
PAL
,
C4H
,
4CL
,
HCT
,
C3′H
,
COMT
, and
CAD
, and consequently lignin formation during effector-triggered immune responses. Upon challenge with the avirulent bacterial pathogen
Pst
DC3000 (
AvrRpm1
), lignin deposition and disease resistance were reduced in
myb15
mutant plants. Furthermore, whereas invading pathogens, together with hypersensitive cell death, were restricted to the infection site in wild-type leaves, they spread beyond the infected area in
myb15
mutants. The exogenous supply of the lignin monomer coniferyl alcohol restored lignin production and rescued immune defects in
myb15
plants. These results demonstrate that regulation at the transcriptional level is key to pathogen-induced lignification and that MYB15 plays a central role in this process.
Receptor-like kinases are important signaling components that regulate a variety of cellular processes. In this study, an Arabidopsis cDNA microarray analysis led to the identification of the ...cysteine-rich receptor-like kinase CRK36 responsive to the necrotrophic fungal pathogen,
. To determine the function of
in plant immunity, T-DNA-insertion knockdown (
) and overexpressing (
) plants were prepared.
plants exhibited increased hypersensitive cell death and ROS burst in response to avirulent pathogens. Treatment with a typical pathogen-associated molecular pattern, flg22, markedly induced pattern-triggered immune responses, notably stomatal defense, in
plants. The immune responses were weakened in
plants. Protein-protein interaction assays revealed the
association of CRK36, FLS2, and BIK1. CRK36 enhanced flg22-triggered BIK1 phosphorylation, which showed defects with Cys mutations in the DUF26 motifs of CRK36. Disruption of
and
genes further impaired CRK36-mediated stomatal defense. We propose that CRK36, together with BIK1 and NADPH oxidases, may form a positive activation loop that enhances ROS burst and leads to the promotion of stomatal immunity.
Autophagy is a homeostatic degradation and recycling process that is also involved in defense against microbial pathogens and in certain forms of cellular suicide. Autophagy has been proposed to ...negatively regulate plant immunity-associated cell death related to the hypersensitive response (HR), as older autophagy-deficient mutants are unable to contain this type of cell death 5 to 10 d after infection. Such spreading cell death was found to require NPR1 (nonexpressor of PR genes 1), but surprisingly did not occur in younger atg mutants. In contrast, we find that npr1 mutants are not impaired in rapid programmed cell death activation upon pathogen recognition. Furthermore, our molecular evidence suggests that the NPR1-dependent spreading cell death in older atg mutants may originate from an inability to cope with excessive accumulation of ubiquitinated proteins and ER stress which derive from salicylic acid (SA)-dependent signaling (e.g., systemic acquired resistance). We also demonstrate that both senescence and immunity-related cell death seen in older atg mutants can be recapitulated in younger atg mutants primed with ER stress. We therefore propose that the reduction in SA signaling caused by npr1 loss-of-function is sufficient to alleviate the stress levels accumulated during aging in autophagy deficient cells which would otherwise become insurmountable and lead to uncontrolled cell death.
The tracheary elements (TEs) of the xylem serve as the water-conducting vessels of the plant vascular system. To achieve this, TEs undergo secondary cell wall thickening and cell death, during which ...the cell contents are completely removed. Cell death of TEs is a typical example of developmental programmed cell death that has been suggested to be autophagic. However, little evidence of autophagy in TE differentiation has been provided. The present study demonstrates that the small GTP binding protein RabG3b plays a role in TE differentiation through its function in autophagy. Differentiating wild type TE cells were found to undergo autophagy in an Arabidopsis culture system. Both autophagy and TE formation were significantly stimulated by overexpression of a constitutively active mutant (RabG3bCA), and were inhibited in transgenic plants overexpressing a dominant negative mutant (RabG3bDN) or RabG3b RNAi (RabG3bRNAi), a brassinosteroid insensitive mutant bri1-301, and an autophagy mutant atg5-1. Taken together, our results suggest that autophagy occurs during TE differentiation, and that RabG3b, as a component of autophagy, regulates TE differentiation.
Plant cell growth and stress signaling require Ca²⁺ influx through plasma membrane transport proteins that are regulated by reactive oxygen species. In root cell growth, adaptation to salinity ...stress, and stomatal closure, such proteins operate downstream of the plasma membrane NADPH oxidases that produce extracellular Superoxide anion, a reactive oxygen species that is readily converted to extracellular hydrogen peroxide and hydroxyl radicals, OH˙ In root cells, extracellular OH˙ activates a plasma membrane Ca²⁺ -permeable conductance that permits Ca²⁺ influx. In Arabidopsis thaliana, distribution of this conductance resembles that of annexin1 (ANN1). Annexins are membrane binding proteins that can form Ca²⁺ -permeable conductances in vitro. Here, the Arabidopsis loss-of-function mutant for annexin1 (Atann1) was found to lack the root hair and epidermal OH˙-activated Ca²⁺ -and K⁺ -permeable conductance. This manifests in both impaired root cell growth and ability to elevate root cell cytosolic free Ca²⁺ in response to OH˙. An OH˙-activated Ca²⁺ conductance is reconstituted by recombinant ANN1 in planar lipid bilayers. ANN1 therefore presents as a novel Ca²⁺-permeable transporter providing a molecular link between reactive oxygen species and cytosolic Ca²⁺ in plants.
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