The Role of Radical Burst in Plant Defense Responses to Necrotrophic Fungi

Necrotrophic fungi, being the largest class of fungal plant pathogens, pose a serious economic problem to crop production. Nitric oxide （NO） is an essential regulatory molecule in plant immunity in synergy with reactive oxygen species （ROS）. Most experimental data available on the roles of NO and ROS during plant-pathogen interactions are from studies of infections by potential biotrophic pathogens, including bacteria and viruses. However, there are several arguments about the role of ROS in defense responses during plants and necrotrophic pathogens interaction and little is known about the role of NO as a counterpart of ROS in disease resistance to necrotrophic pathogens. This review focuses on the recent knowledge about the role of oxidative burst in plant defense response to necrotrophic fungi.

Repression of the Auxin Response Pathway Increases Arabidopsis Susceptibility to Necrotrophic Fungi

In plants, resistance to necrotrophic pathogens depends on the interplay between different hormone systems, such as those regulated by salicylic acid （SA）, jasmonic acid （JA）, ethylene, and abscisic acid. Repression of auxin signaling by the SA pathway was recently shown to contribute to antibacterial resistance. Here, we demonstrate that Arabidopsis auxin signaling mutants axrl, axr2, and axr6 that have defects in the auxin-stimulated SCF （Skpl-Cullin- F-box） ubiquitination pathway exhibit increased susceptibility to the necrotrophic fungi Plectosphaerella cucumerina and Botrytis cinerea. Also, stabilization of the auxin transcriptional repressor AXR3 that is normally targeted for removal by the SCF-ubiquitin/proteasome machinery occurs upon P. cucumerina infection. Pharmacological inhibition of auxin transport or proteasome function each compromise necrotroph resistance of wild-type plants to a similar extent as in non-treated auxin response mutants. These results suggest that auxin signaling is important for resistance to the necrotro- phic fungi P. cucumerina and B. cinerea. SGTlb （one of two Arabidopsis SGT1 genes encoding HSP90/HSC70 co-chaperones） promotes the functions of SCF E3-ubiquitin ligase complexes in auxin and JA responses and resistance conditioned by certain Resistance （R） genes to biotrophic pathogens. We find that sgtlb mutants are as resistant to P. cucumerina as wild-type plants. Conversely, auxin/SCF signaling mutants are uncompromised in RPP4-triggered resistance to the obligate biotrophic oomycete, Hyaloperonospora parasitica. Thus, the predominant action of SGTlb in R gene-conditioned resis- tance to oomycetes appears to be at a site other than assisting SCF E3-ubiquitin ligases. However, genetic additivity of sgtlb axrl double mutants in susceptibility to H. parasitica suggests that SCF-mediated ubiquitination contributes to lim- iting biotrophic pathogen colonization once plant-pathogen compatibility is established.

Insights into the role of jasmonic acid-mediated defenses against necrotrophic and biotrophic fungal pathogens

Jasmonic acid （JA） is a natural hormone regulator involved in development, responses against woundingand pathogen attack. Upon perception of pathogens, JA is synthesized and mediates a signaling cascade initiating various defense responses. Traditionally, necrotrophic fungi have been shown to be the primary activators of JA- dependent defenses through the JA-receptor, COIl. Conversely, plants infected with biotrophic fungi have classically been associated with suppressing JA-mediated responses. However, recent evidence has shown that certain biotrophic fungal species also trigger activation of JA-mediated responses and mutants deficient in JA signaling show an increase in susceptibility to certain biotrophic fungal pathogens. These findings suggest a new role for JA in defense against fungal biotrophs. This review will focus on recent research advancing our knowledge of JA-dependant responses involved in defense against both biotrophic and necrotrophic fungi.

SUMOylation Inhibition Mediated by Disruption of SUMO El-E2 Interactions Confers Plant Susceptibility to Necrotrophic Fungal Pathogens

Protein modification by SUMO modulates essential biological processes in eukaryotes. SUMOylation is facilitated by sequential action of the El-activating, E2-conjugating, and E3-1igase enzymes. In plants, SUMO regulates plant development and stress responses, which are key determinants in agricultural productivity. To generate additional tools for advancing our knowledge about the SUMO biology, we have developed a strategy for inhibiting in vivo SUMO conjugation based on disruption of SUMO El-E2 interactions through expression of E1 SAE2uFDct domain. Targeted mutagenesis and phylogenetic analyses revealed that this inhibition involves a short motif in SAE2uFDct highly divergent across kingdoms. Transgenic plants expressing the SAE2UFDCt domain displayed dose-dependent inhibition of SUMO conjugation, and have revealed the existence of a post-transcriptional mechanism that regulates SUMO E2 conjugating enzyme levels. Interestingly, these transgenic plants displayed increased suscep- tibility to necrotrophic fungal infections by Botrytis cinerea and Plectosphaerella cucumerina. Early after fungal inoculation, host SUMO conjugation was post-transcriptionally downregulated, suggesting that targeting SUMOylation machinery could constitute a novel mechanism for fungal pathogenicity. These findings support the role of SUMOylation as a mechanism involved in plant protection from environmental stresses. In addition, the strategy for inhibiting SUMO conjugation in vivo described in this study might be applicable in important crop plants and other non-plant organisms regardless of their genetic complexity.

A patatin-like protein synergistically regulated by jasmonate and ethylene signaling pathways plays a negative role in Nicotiana attenuata resistance to Alternaria alternata

Although patatin was initially identified as a major storage protein in potato tubers, patatin-like proteins(PLPs) have been recently reported to be widely present in many plant species and shown to be involved in plant-pathogen interactions. However, it is not clear whether PLPs are involved in Nicotiana attenuata resistance against the necrotrophic fungal pathogen, Alternaria alternata. In this study we identified a NaPLP gene, whose expression was highly elicited by A. alternata inoculation. Silencing NaPLP enhanced N. attenuata resistance to A. alternata, which was associated with higher induction levels of JA and ethylene biosynthetic genes, NaACS1, NaACO1 and NaLOX3. The induction of NaPLP expression by the fungus was abolished in JA-deficient plants and significantly reduced in ethylene-insensitive plants. In addition, NaPLP transcripts were highly induced by exogenous treatment with either methyl jasmonate(MeJA) or ethephon. Co-treatment with MeJA and ethephon led to a much higher induction level of NaPLP transcripts, and this synergistic induction was largely dependent on endogenous JA and ethylene signaling pathways. Thus, we conclude that the NaPLP gene is elicited by A. alternata via JA and ethylene signaling pathways in a synergistic way; however, unlike other JA-and ethylene-induced defense genes,NaPLP negatively affects plant resistance to the fungus likely by suppressing JA and ethylene biosynthetic gene expression.

Induction of Extracellular Lytic Enzymes by <i>Fusarium solani</i>

Fusarium solani is a necrotrophic parasitic fungus that causes wilt in some plants, causing severe economic losses in some areas of the country. The objective of this work was to analyze the induction of extracellular lytic enzymes produced by a strain of F. solani, isolated from a culture of tomato, in Villa de Arista, S.L.P. México. Polygalacturonase activity has a greater induction time at 10 days, and the xylanase has two times higher activity at 8 and 13 days of incubation at 28?C. Also, the xylanase activities A and B were very stable at 4?C. After 7 days of incubation, it has an activity of 100% and 96%, respectively, while polygalacturonase retains 61% of its initial activity. Both activities are better induced with glutamate and urea as nitrogen sources respectively, and both exhibit an initial pH optimum of 5.5. Finally, we didn’t find cellulase activity in the analyzing conditions.

Plant triterpenoid saponins function as susceptibility factors to promote the pathogenicity of Botrytis cinerea

The gray mold fungus Botrytis cinerea is a necrotrophic pathogen that causes diseases in hundreds of plant species,including high-value crops.Its polyxenous nature and pathogenic success are due to its ability to perceive host signals in its favor.In this study,we found that laticifer cells of Euphorbia lathyris are a source of susceptibility factors required by B.cinerea to cause disease.Consequently,poor-in-latex(pil)mutants,which lack laticifer cells,show full resistance to this pathogen,whereas lot-of-latex mutants,which produce more laticifer cells,are hypersusceptible.These S factors are triterpenoid saponins,which are widely distributed natural products of vast structural diversity.The downregulation of laticifer-specific oxydosqualene cyclase genes,which encode the first committed step enzymes for triterpene and,therefore,saponin biosynthesis,conferred disease resistance to B.cinerea.Likewise,the Medicago truncatula Iha-1 mutant,compromised in triterpenoid saponin biosynthesis,showed enhanced resistance.Interestingly,the application of different purified triterpenoid saponins pharmacologically complemented the diseaseresistant phenotype ofpil and hla-1 mutants and enhanced disease susceptibility in different plant species.We found that triterpenoid saponins function as plant cues that signal transcriptional reprogramming in B.cinerea,leading to a change in its growth habit and infection strategy,culminating in the abundant formation of infection cushions,the multicellular appressoria apparatus dedicated to plant penetration and biomass destruction in B.cinerea.Taken together,these results provide an explanation for how plant triterpenoid saponins function as disease susceptibility factors to promote B.cinerea pathogenicity.

A pair of nuclear factor Y transcription factors act as positive regulators in jasmonate signaling and disease resistance in Arabidopsis

The plant hormone jasmonate(JA)regulates plant growth and immunity by orchestrating a genome-wide transcriptional reprogramming.In the resting stage,JASMONATE-ZIM DOMAIN(JAZ)proteins act as main repressors to regulate the expression of JA-responsive genes in the JA signaling pathway.However,the mechanisms underlying de-repression of JA-responsive genes in response to JA treatment remain elusive.Here,we report two nuclear factor Y transcription factors NF-YB2 and NF-YB3(thereafter YB2 and YB3)play key roles in such de-repression in Arabidopsis.YB2 and YB3 function redundantly and positively regulate plant resistance against the necrotrophic pathogen Botrytis cinerea,which are specially required for transcriptional activation of a set of JA-responsive genes following inoculation.Furthermore,YB2 and YB3 modulated their expression through direct occupancy and interaction with histone demethylase Ref6 to remove repressive histone modifications.Moreover,YB2 and YB3 physically interacted with JAZ repressors and negatively modulated their abundance,which in turn attenuated the inhibition of JAZ proteins on the transcription of JA-responsive genes,thereby activating JA response and promoting disease resistance.Overall,our study reveals the positive regulators of YB2 and YB3 in JA signaling by positively regulating transcription of JA-responsive genes and negatively modulating the abundance of JAZ proteins.

Receptor-like kinases and receptor-like proteins： keys to pathogen recognition and defense signaling in plant innate immunity

Plants have evolved multiple layers of defense against various pathogens in the environment. Receptor-like kinases/proteins （RLKs/RLPs） are on the front lines of the battle between plants and pathogens since they are present at the plasma membrane and perceive signature molecules from either the invading pathogen or damaged plant tissue. With a few notable exceptions, most RLKs/RLPs are positive regulators of plant innate immunity. In this review, we summarize recently discovered RLKs/RLPs that are involved in plant defense responses against various classes of pathogens, We also describe what is currently known about the mechanisms of RLK-mediated initiation of signaling via protein-protein interactions and phosphorylation.

Inhibition of Botrytis cinerea and control of gray mold on table grapes by calcium propionate

Objectives:The gray mold fungus Botrytis cinerea(B.cinerea)infects a wide range of crops before and after harvest,causing huge losses worldwide.Inhibition mechanisms of B.cinerea in vitro and in plants by calcium propionate(CP),generally recognized as a safe substanee,are described in this study.Materials and methods:Wild-type and transgenic mutant strains of B.cinerea were used in the study to evaluate the effects of CP on fun gal growth and developme nt in vitro.Plant materials in cludi ng tomato leaves and table grapes were tested for con trolling efficie ncy of CP agai nst gray mold deterioration in vivo.Results:Mycelial growth of B.cinerea was inhibited by CP in a dose-dependent manner with occasional disruption of hyphal tips,causing cellular collapse and efflux of cell contents.Staining with fluorescein diacetate and propidium iodide indicated that CP decreased fungal cell viability.Inhibition efficiency of CP against B.cinerea was enhaneed by reducing pH.In contrast,the veil mutant,which exhibited deficiency in acid production,was more resista nt to CP,suggesti ng that inhibition of B.cinerea by CP is enhanced by the acidification ability of the fungus itself.Additionally,CP inhibited infection cushion development by germlings of B.cinerea.Infection assays with tomato leaves and table grapes showed that CP inhibited decay development in both host tissues.Moreover,application of CP on grapes 3 days prior to harvest could contribute to management of deterioration caused by spontaneous fungal diseases during storage.Conclusion:CP can suppress hyphal growth,inhibit infection cushion development,and reduce the virulence of B.cinerea.CP is thus promising for practical management of gray mold in fruit crops and merits further evaluation.