Pathogen-inducible OsMPKK10.2-OsMPK6 cascade phosphorylates the Raf-like kinase OsEDR1 and inhibits its scaffold function to promote rice disease resistance

Chimeric mutations in grapevine ENHANCED DISEASE RESISTANCE1 improve resistance to powdery mildew without growth penalty

Grapevine (Vitis vinifera L.) incurs severe quality degradation and yield loss from powdery mildew, a major fungal disease caused by Erysiphe necator. ENHANCED DISEASE RESISTANCE1 (EDR1), a Raf-like mitogen-activated protein kinase kinase kinase, negatively regulates defense responses against powdery mildew in Arabidopsis (Arabidopsis thaliana). However, little is known about the role of the putatively orthologous EDR1 gene in grapevine. In this study, we obtained grapevine VviEDR1-edited lines using CRISPR/Cas9. Plantlets containing homozygous and bi-allelic indels in VviEDR1 developed leaf lesions shortly after transplanting into the soil and died at the seedling stage. Transgenic plants expressing wild-type VviEDR1 and mutant Vviedr1 alleles as chimera (designated as VviEDR1-chi) developed normally and displayed enhanced resistance to powdery mildew. Interestingly, VviEDR1-chi plants maintained a spatiotemporally distinctive pattern of VviEDR1 mutagenesis: while almost no mutations were detected from terminal buds, ensuring normal function of the apical meristem, mutations occurred in young leaves and increased as leaves matured, resulting in resistance to powdery mildew. Further analysis showed that the resistance observed in VviEDR1-chi plants was associated with callose deposition, increased production of salicylic acid and ethylene, H2O2 production and accumulation, and host cell death. Surprisingly, no growth penalty was observed with VviEDR1-chi plants. Hence, this study demonstrated a role of VviEDR1 in the negative regulation of resistance to powdery mildew in grapevine and provided an avenue for engineering powdery mildew resistance in grapevine.

Gene-edited protein kinases and phosphatases in molecular plant breeding

Protein phosphorylation, the most common and essential post-translational modification, belongs to crucial regulatory mechanisms in plants, affecting their metabolism, intracellular transport, cytoarchitecture, cell division, growth, development, and interactions with the environment. Protein kinases and phosphatases, two important families of enzymes optimally regulating phosphorylation, have now become important targets for gene editing in crops. We review progress on gene-edited protein kinases and phosphatases in crops using clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9). We also provide guidance for computational prediction of alterations and/or changes in function, activity, and binding of protein kinases and phosphatases as consequences of CRISPR/Cas9-based gene editing with its possible application in modern crop molecular breeding towards sustainable agriculture.

OsMPK12 positively regulates rice blast resistance via OsEDC4-mediated transcriptional regulation of immune-related genes

Mitogen-activated protein kinase (MAPK) signalling cascades are functionally important signalling modules in eukaryotes. Transcriptome reprogramming of immune-related genes is a key process in plant immunity. Emerging evidence shows that plant MAPK cascade is associated with processing (P)-body components and contributes to transcriptome reprogramming of immune-related genes. However, it remains largely unknown how this process is regulated. Here, we show that OsMPK12, which is induced by Magnaporthe oryzae infection, positively regulates rice blast resistance. Further analysis revealed that OsMPK12 directly interacts with enhancer of mRNA decapping protein 4 (OsEDC4), a P-body-located protein, and recruits OsEDC4 to where OsMPK12 is enriched. Importantly, OsEDC4 directly interacts with two decapping complex members OsDCP1 and OsDCP2, indicating that OsEDC4 is a subunit of the mRNA decapping complex. Additionally, we found that OsEDC4 positively regulates rice blast resistance by regulating expression of immune-related genes and maintaining proper mRNA levels of some negatively-regulated genes. And OsMPK12 and OsEDC4 are also involved in rice growth and development regulation. Taken together, our data demonstrate that OsMPK12 positively regulates rice blast resistance via OsEDC4-mediated mRNA decay of immune-related genes, providing new insight into not only the new role of the MAPK signalling cascade, but also posttranscriptional regulation of immune-related genes.

OsMAPK6 phosphorylation and CLG1 ubiquitylation of GW6a non-additively enhance rice grain size through stabilization of the substrate

The chromatin modifier GRAIN WEIGHT 6a (GW6a) enhances rice grain size and yield. However, little is known about its gene network determining grain size. Here, we report that MITOGEN-ACTIVED PROTEIN KINASE 6 (OsMAPK6) and E3 ligase CHANG LI GENG 1 (CLG1) interact with and target GW6a for phosphorylation and ubiquitylation, respectively. Unexpectedly, however, in vitro and in vivo assays reveal that both of the two post-translational modifications stabilize GW6a. Furthermore, we uncover two major GW6a phosphorylation sites (serine142 and threonine186) targeted by OsMAPK6 serving an important role in modulating grain size. In addition, our genetic and molecular results suggest that the OsMAPK6-GW6a and CLG1-GW6a axes are crucial and operate in a non-additive manner to control grain size. Overall, our findings identify a previously unknown mechanism by which phosphorylation and ubiquitylation non-additively stabilize GW6a to enhance grain size, and reveal correlations and interactions of these posttranslational modifications during rice grain development.

Recent advances in understanding the role of two mitogen-activated protein kinase cascades in plant immunity

The mitogen-activated protein kinase (MAPK/MPK) cascade is an important intercellular signaling module that regulates plant growth, development, reproduction, and responses to biotic and abiotic stresses. A MAPK cascade usually consists of a MAPK kinase kinase (MAPKKK/MEKK), a MAPK kinase (MAPKK/MKK/MEK), and a MAPK. The well-characterized MAPK cascades in plant immunity to date are the MEKK1-MKK1/2-MPK4 cascade and the MAPKKK3/4/5-MKK4/5-MPK3/6 cascade. Recently, major breakthroughs have been made in understanding the molecular mechanisms associated with the regulation of immune signaling by both of these MAPK cascades. In this review, we highlight the most recent advances in understanding the role of both MAPK cascades in activating plant defense and in suppressing or fine-tuning immune signaling. We also discuss the molecular mechanisms by which plants stabilize and maintain the activation of MAPK cascades during immune signaling. Based on this review, we reveal the complexity and importance of the MEKK1-MKK1/2-MPK4 cascade and the MAPKKK3/4/5-MKK4/5-MPK3/6 cascade, which are tightly controlled by their interacting partners or substrates, in plant immunity.

The Raf-like MAPKKKs STY8, STY17, and STY46 negatively regulate Botrytis cinerea resistance by limiting MKK7 protein accumulation in Arabidopsis

Plant diseases, which seriously damage crop production, are in most cases caused by fungal pathogens. In this study, we found that the Raf-like MAPKKKs STY8 (SERINE/THREONINE/TYROSINE KINASE 8), STY17, and STY46 negatively regulate resistance to the fungal pathogen Botrytis cinerea through jasmonate response in Arabidopsis. Moreover, STY8/STY17/STY46 homologs negatively contribute to chitin signaling. We further identified MKK7 as the MAPKK component interacting with STY8/STY17/STY46 homologs. MKK7 positively contributes to resistance to B. cinerea and chitin signaling. Furthermore, we found that STY8/STY17/STY46 homologs negatively affect the accumulation of MKK7, in accordance with the opposite roles of MKK7 and STY8/STY17/STY46 homologs in defense against B. cinerea. These results provide new insights into the mechanisms precisely regulating plant immunity via Raf-like MAPKKKs.

OsMKK1 is a novel element that positively regulates the Xa21-mediated resistance response to Xanthomonas oryzae pv. oryzae in rice

KEY MESSAGE

OsMKK1, a MAPK gene, positively regulates rice Xa21-mediated resistance response and also plays roles in normal growth and development process of rice. The mitogen-activated protein kinase (MAPK) cascade was highly conserved among eukaryotes, which played crucial roles in plant responses to pathogen infection. Bacterial blight is the most devastating bacterial disease. Xa21 confers broad-spectrum resistance to Xanthomonas oryzae pv. Oryzae (Xoo). This study identified that the transcription level of OsMKK1 was up-regulated in resistant response against Xoo, thus overexpression (OsMKK1-OX) and RNA interference (OsMKK1-RNAi) transgenic rice lines under the background of Xa21 was constructed. Compared with recipient control plants 4021, the OsMKK1-OX lines significantly enhanced disease resistance to Xoo, on the contrary, the resistance of OsMKK1-RNAi lines was weakened, demonstrated that OsMKK1 played a positive role in Xa21-mediated disease resistance pathway. A number of pathogenesis-related proteins, including PR1A, PR2 and PR10A showed enhanced expression in OsMKK1-OX lines, supported that these PR genes may be regulated by OsMKK1 to participate in the defense responses. In addition, the agronomic traits of OsMKK1 transgenic plants were affected. Overall, these results revealed the role of OsMKK1 in Xa21-mediated resistance against Xoo and in the normal growth and development process in rice.

MITOGEN-ACTIVATED PROTEIN KINASE3 enhances disease resistance of edr1 mutants by phosphorylating MAPKKK5

Mitogen-activated protein kinase (MAPK/MPK) cascades are key signaling modules that regulate plant immunity. ENHANCED DISEASE RESISTANCE1 (EDR1) encodes a Raf-like MAPK kinase kinase (MAPKKK) that negatively regulates plant defense in Arabidopsis (Arabidopsis thaliana). The enhanced resistance of edr1 requires MAPK KINASE4 (MKK4), MKK5, and MPK3. Although the edr1 mutant displays higher MPK3/6 activation, the mechanism by which plants increase MAPK cascade activation remains elusive. Our previous study showed that MAPKKK5 is phosphorylated at the Ser-90 residue in edr1 mutants. In this study, we demonstrated that the enhanced disease resistance of edr1 required MAPKKK5. Phospho-dead MAPKKK5S90A partially impaired the resistance of edr1, and the expression of phospho-mimetic MAPKKK5S90D in mapkkk5-2 resulted in enhanced resistance to the powdery mildew Golovinomyces cichoracearum strain UCSC1 and the bacterial pathogen Pseudomonas syringae pv. tomato (Pto) strain DC3000. Thus, Ser-90 phosphorylation in MAPKKK5 appears to play a crucial role in disease resistance. However, MAPKKK5-triggered cell death was not suppressed by EDR1. Furthermore, activated MPK3 phosphorylated the N terminus of MAPKKK5, and Ser-90 was one of the phosphorylated sites. Ser-90 phosphorylation increased MAPKKK5 stability, and EDR1 might negatively regulate MAPK cascade activation by suppressing the MPK3-mediated feedback regulation of MAPKKK5. Taken together, these results indicate that MPK3 phosphorylates MAPKKK5 to enhance MAPK cascade activation and disease resistance in edr1 mutants.

Genome-Wide Characterization of the Maize (Zea mays L.) WRKY Transcription Factor Family and Their Responses to Ustilago maydis

Members of the WRKY transcription factor (TF) family are unique to plants and serve as important regulators of diverse physiological processes, including the ability of plants to manage biotic and abiotic stressors. However, the functions of specific WRKY family members in the context of maize responses to fungal pathogens remain poorly understood, particularly in response to Ustilago maydis (DC.) Corda (U. maydis), which is responsible for the devastating disease known as corn smut. A systematic bioinformatic approach was herein employed for the characterization of the maize WRKY TF family, leading to the identification of 120 ZmWRKY genes encoded on 10 chromosomes. Further structural and phylogenetic analyses of these TFs enabled their classification into seven different subgroups. Segmental duplication was established as a major driver of ZmWRKY family expansion in gene duplication analyses, while the Ka/Ks ratio suggested that these ZmWRKY genes had experienced strong purifying selection. When the transcriptional responses of these genes to pathogen inoculation were evaluated, seven U. maydis-inducible ZmWRKY genes were identified, as validated using a quantitative real-time PCR approach. All seven of these WKRY proteins were subsequently tested using a yeast one-hybrid assay approach, which revealed their ability to directly bind the ZmSWEET4b W-box element, thereby controlling the U. maydis-inducible upregulation of ZmSWEET4b. These results suggest that these WRKY TFs can control sugar transport in the context of fungal infection. Overall, these data offer novel insight into the evolution, transcriptional regulation, and functional characteristics of the maize WRKY family, providing a basis for future research aimed at exploring the mechanisms through which these TFs control host plant responses to common smut and other fungal pathogens.

Inhibitory effects of Bacillus vallismortis T27 against apple Valsa canker caused by Valsa mali

Apple Valsa canker caused by the pathogenic fungus Valsa mali, are one of the most destructive diseases of woody plants worldwide. One rhizosphere microbe strain, designated as T27 and subsequently identified as Bacillus vallismortis based on morphological and phylogenetic analyses, was studied as a potential biocontrol agent. Inoculation assay showed the B. vallismortis T27 suppressed the mycelial growth of V. mali with 81.33% antifungal effect on dual culture plates and caused hyphal deformities, wrinkles. The T27 fermentation broth significantly suppress the fungi's ability to acidify the surrounding environment. The addition of T27 cell-free supernatant (CFS) caused the pH of the fungal culture medium to increase from 3.60 to 5.10. B. vallismortis T27 showed the presence of Surfactin, IturinA and Bacilysin antimicrobial biosynthetic genes by the PCR assay. In addition, the B. vallismortis T27 was able to promote plant growth by producing siderophores and solubilizing phosphorus. The application of 2% fermentation broth of T27 resulted in a significant increase of 55.99% in the height of tomato plants and a 33.03% increase in the fresh weight of tomatoes. Under laboratory and field conditions, the B. vallismortis T27 exhibited strong antifungal activities on detached twigs and intact plants. The treatment of T27 resulted in a 35.9% reduction in lesion area on detached twigs. Furthermore, when applied to intact plants, T27 demonstrated a scar healing rate of 85.7%, surpassing the 77.8% observed in the treatment with tebuconazole. Comparative transcriptome analysis showed down-regulation of the genes associated with the fungal cell wall and cell membrane's synthesis and composition during V. mali treated with the B. vallismortis T27. In addition, gene transcription level analysis under treatment with B. vallismortis T27 revealed a significant increase in the expression levels of genes associated with diterpene biosynthesis, alanine, aspartic acid and glutamate metabolism, and plant hormone signaling in the apple, consistent with qRT-PCR and RNA-seq results. In this study, B. vallismortis T27 isolated from rhizosphere soil and identified as a novel biological control agent against apple Valsa canker. It exhibited effectively control over Valsa canker through multiple mechanisms, including disrupting the fungal cell membrane structure, altering the fungal growth environment, activating the plant MAPK pathway, and inducing upregulation of plant terpene biosynthetic genes. These findings highlight the potential of B. vallismortis T27 as a promising and multifaceted approach for managing apple Valsa canker.

Activated Expression of Rice DMR6-like Gene OsS3H Partially Explores the Susceptibility to Bacterial Leaf Streak Mediated by Knock-Out OsF3H04g

Downy Mildew Resistance 6-like (DMR6-like) genes are identified as salicylic acid (SA) hydroxylases and negative regulators of plant immunity. Previously, we identified two rice DMR6-like genes, OsF3H03g, and OsF3H04g, that act as susceptible targets of transcription activator-like effectors (TALEs) from Xanthomonas oryzae pv. oryzicola (Xoc), which causes bacterial leaf streak (BLS) in rice. Furthermore, all four homologs of rice DMR6-like proteins were identified to predominantly carry the enzyme activity of SA 5-hydroxylase (S5H), negatively regulate rice broad-spectrum resistance, and cause the loss of function of these OsDMR6s, leading to increased resistance to rice blast and bacterial blight (BB). Here, we curiously found that an OsF3H04g knock-out mutant created by T-DNA insertion, osf3h04g, was remarkedly susceptible to BLS and BB and showed an extreme reduction in SA content. OsF3H04g knock-out rice lines produced by gene-editing were mildly susceptible to BLS and reduced content of SA. To explore the susceptibility mechanism in OsF3H04g loss-of-function rice lines, transcriptome sequencing revealed that another homolog, OsS3H, had induced expression in the loss-of-function OsF3H04g rice lines. Furthermore, we confirmed that a great induction of OsS3H downstream and genomically adjacent to OsF3H04g in osf3h04g was primarily related to the inserted T-DNA carrying quadruple enhancer elements of 35S, while a slight induction was caused by an unknown mechanism in gene-editing lines. Then, we found that the overexpression of OsS3H increased rice susceptibility to BLS, while gene-editing mediated the loss-of-function OsS3H enhanced rice resistance to BLS. However, the knock-out of both OsF3H04g and OsS3H by gene-editing only neutralized rice resistance to BLS. Thus, we concluded that the knock-out of OsF3H04g activated the expression of the OsS3H, partially participating in the susceptibility to BLS in rice.

OsMKK6 Regulates Disease Resistance in Rice

Mitogen-activated protein kinase cascades play important roles in various biological programs in plants, including immune responses, but the underlying mechanisms remain elusive. Here, we identified the lesion mimic mutant rsr25 (rust spots rice 25) and determined that the mutant harbored a loss-of-function allele for OsMKK6 (MITOGEN-ACTIVATED KINASE KINASE 6). rsr25 developed reddish-brown spots on its leaves at the heading stage, as well as on husks. Compared to the wild type, the rsr25 mutant exhibited enhanced resistance to the fungal pathogen Magnaporthe oryzae (M. oryzae) and to the bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo). OsMKK6 interacted with OsMPK4 (MITOGEN-ACTIVATED KINASE 4) in vivo, and OsMKK6 phosphorylated OsMPK4 in vitro. The Osmpk4 mutant is also a lesion mimic mutant, with reddish-brown spots on its leaves and husks. Pathogen-related genes were significantly upregulated in Osmpk4, and this mutant exhibited enhanced resistance to M. oryzae compared to the wild type. Our results indicate that OsMKK6 and OsMPK4 form a cascade that regulates immune responses in rice.

Unraveling genomic regions and candidate genes for multiple disease resistance in upland cotton using meta-QTL analysis

Verticillium wilt (VW), Fusarium wilt (FW) and Root-knot nematode (RKN) are the main diseases affecting cotton production. However, many reported quantitative trait loci (QTLs) for cotton resistance have not been used for agricultural practices because of inconsistencies in the cotton genetic background. The integration of existing cotton genetic resources can facilitate the discovery of important genomic regions and candidate genes involved in disease resistance. Here, an improved and comprehensive meta-QTL analysis was conducted on 487 disease resistant QTLs from 31 studies in the last two decades. A consensus linkage map with genetic overall length of 3006.59 cM containing 8650 markers was constructed. A total of 28 Meta-QTLs (MQTLs) were discovered, among which nine MQTLs were identified as related to resistance to multiple diseases. Candidate genes were predicted based on public transcriptome data and enriched in pathways related to disease resistance. This study used a method based on the integration of Meta-QTL, known genes and transcriptomics to reveal major genomic regions and putative candidate genes for resistance to multiple diseases, providing a new basis for marker-assisted selection of high disease resistance in cotton breeding.

Phosphorylation and ubiquitination of OsWRKY31 are integral to OsMKK10-2-mediated defense responses in rice

Mitogen-activated protein kinase (MPK) cascades play vital roles in plant innate immunity, growth, and development. Here, we report that the rice (Oryza sativa) transcription factor gene OsWRKY31 is a key component in a MPK signaling pathway involved in plant disease resistance in rice. We found that the activation of OsMKK10-2 enhances resistance against the rice blast pathogen Magnaporthe oryzae and suppresses growth through an increase in jasmonic acid and salicylic acid accumulation and a decrease of indole-3-acetic acid levels. Knockout of OsWRKY31 compromises the defense responses mediated by OsMKK10-2. OsMKK10-2 and OsWRKY31 physically interact, and OsWRKY31 is phosphorylated by OsMPK3, OsMPK4, and OsMPK6. Phosphomimetic OsWRKY31 has elevated DNA-binding activity and confers enhanced resistance to M. oryzae. In addition, OsWRKY31 stability is regulated by phosphorylation and ubiquitination via RING-finger E3 ubiquitin ligases interacting with WRKY 1 (OsREIW1). Taken together, our findings indicate that modification of OsWRKY31 by phosphorylation and ubiquitination functions in the OsMKK10-2-mediated defense signaling pathway.

EDR1 associates with its homologs to synergistically regulate plant immunity in Arabidopsis

ENHANCED DISEASE RESISTANCE 1 (EDR1), a Raf-like mitogen-activated protein kinase (MAPK) kinase kinase (MAPKKK), is a negative regulator of resistance. There are three homologs, RAF3/4/5, of EDR1 in Arabidopsis. However, the roles of RAF3/4/5 in resistance and their functional link with EDR1 in plant immunity remain unclear. Here, we showed that the raf3/4/5 triple mutant displayed wild-type-like phenotypes to the powdery mildew pathogen Golovinomyces cichoracearum UCSC1 and the bacterial pathogen Pseudomonas syringae pv. tomato (Pto) DC3000. However, the edr1 raf3/4/5 quadruple mutant exhibited enhanced resistance to G. cichoracearum UCSC1 and Pto DC3000 compared to edr1. Consistently, MPK3/6 kinase activity was more highly activated in edr1 raf3/4/5 than that in edr1. Moreover, the enhanced resistance of edr1 raf3/4/5 required SALICYLIC ACID INDUCTION DEFICIENT 2 (SID2), an isochorismate synthase required for salicylic acid (SA) synthesis. Additionally, unlike EDR1, RAF3/4/5 weakly and indirectly associated with MKK4/5, and EDR1 was directly associated with RAF3/4/5. Taken together, these data indicate that EDR1 associates with RAF3/4/5, and they may function together to synergistically suppress MAPK cascades activation, which reveal the complexity and importance of Raf-like MAPKKKs in plant immunity regulation.

Comprehensive analysis of MAPK gene family in Populus trichocarpa and physiological characterization of PtMAPK3-1 in response to MeJA induction

Mitogen-activated protein kinases (MAPKs) play important roles in plant growth and development, as well as hormone and stress responses by signaling to eukaryotic cells, through MAPK cascade, the presence of various cues; thereby, regulating various responses. The MAPK cascade consists mainly of three gene families, MAPK, MAPKK, and MAPKKK, which activate downstream signaling pathways through sequential phosphorylation. Although the MAPK cascade gene family has been reported in several species, there is a lack of comprehensive analysis in poplar. We identified 21 MAPK genes, 11 MAPKK genes, and 104 MAPKKK genes in Populus trichocarpa. The phylogenetic classification was supported by conservative motif, gene structure and motif analysis. Whole genome duplication has an important role in the expansion of MAPK cascade genes. Analysis of promoter cis-elements and expression profiles indicates that MAPK cascade genes have important roles in plant growth and development, abiotic and biotic stresses, and phytohormone response. Expression profiling revealed a significant upregulation of PtMAPK3-1 expression in response to drought, salt and disease stresses. Poplar transiently overexpressing PtMAPK3-1 and treated with methyl jasmonic acid (MeJA) had higher catalase and peroxidase levels than non-overexpressing poplar. This work represents the first complete inventory of the MAPK cascade in P. trichocarpa, which reveals that PtMAPK3-1 is induced by the MeJA hormone and participates in the MeJA-induced enhancement of the antioxidant enzyme system.

Group IIc WRKY transcription factors regulate cotton resistance to Fusarium oxysporum by promoting GhMKK2-mediated flavonoid biosynthesis

WRKY transcription factors (TFs) are crucial regulators in response to pathogen infection. However, the regulatory mechanisms of WRKY TFs in response to Fusarium oxysporum f. sp. vasinfectum (Fov), the most devastating pathogen of cotton, remain unclear. Here, transcriptome sequencing indicated that the group IIc WRKY TF subfamily was the most important TF subfamily in response to Fov. Gain-of-function and loss-of-function analyses showed that group IIc WRKY TFs positively regulated cotton resistance to Fov. A series of chromatin immunoprecipitation sequencing, yeast one-hybrid assay and electrophoresis mobility shift assay experiments indicated that group IIc WRKY TFs directly bound to the promoter of GhMKK2 and regulated its expression. Importantly, a novel mitogen-activated protein kinase (MAPK) cascade composed of GhMKK2, GhNTF6 and GhMYC2 was identified. The functional analysis indicated that group IIc WRKY TFs induced the GhMKK2-GhNTF6 pathway to increase resistance to Fov by upregulating the GhMYC2-mediated expression of several flavonoid biosynthesis-related genes, which led to flavonoid accumulation. In conclusion, our study demonstrated a novel disease defense mechanism by which the WRKY-MAPK pathway promotes flavonoid biosynthesis to defend against pathogen infection. This pathway improves our understanding of the interaction mode between WRKY TFs and MAPK cascades in plant immunity and the vital role of plant flavonoids in pathogen defense.

Advances in the Genetic Basis and Molecular Mechanism of Lesion Mimic Formation in Rice

Plant lesion mutation usually refers to the phenomenon of cell death in green tissues before senescence in the absence of external stress, and such mutants also show enhanced resistance to some plant pathogens. The occurrence of lesion mimic mutants in rice is affected by gene mutation, reactive oxygen species accumulation, an uncontrolled programmed cell death system, and abiotic stress. At present, many lesion mimic mutants have been identified in rice, and some genes have been functionally analyzed. This study reviews the occurrence mechanism of lesion mimic mutants in rice. It analyzes the function of rice lesion mimic mutant genes to elucidate the molecular regulation pathways of rice lesion mimic mutants in regulating plant disease resistance.

A Single Amino Acid Substitution in MIL1 Leads to Activation of Programmed Cell Death and Defense Responses in Rice

Lesion mimic mutants are an ideal model system for elucidating the molecular mechanisms of programmed cell death and defense responses in rice. In this study, we identified a lesion mimic mutant termed miner infection like 1-1 (mil1-1). The mil1-1 exhibited lesions on the leaves during development, and the chloroplasts of mil1-1 leaves were disrupted. Reactive oxygen species were found to accumulate in mil1-1 leaves. Cell death and DNA fragmentation were observed in mil1-1 leaves, indicating that the cells in the spots of mil1-1 leaves experienced programmed cell death. Most agronomic traits decreased in mil1-1, suggesting that the growth retardation in mil1-1 caused reduced per-plant grain yield. However, the mutation of MIL1 activated the expression of pathogen response genes and enhanced resistance to bacterial blight. The MIL1 gene was cloned using the positional cloning approach. A missense mutation 751 bp downstream of ATG was found in mil1-1. The defects of mil1-1 were able to be rescued by delivering a wild-type MIL1 gene into mil1-1. MIL1 encoded hydroperoxide lyase 3 (OsHPL3), and the expression of OsHPL3 was induced via hormone and abiotic stresses. Our findings provide insights into the roles of MIL1 in regulating programmed cell death, development, yield, and defense responses in rice.

MAPKs trigger antiviral immunity by directly phosphorylating a rhabdovirus nucleoprotein in plants and insect vectors

Signaling by the evolutionarily conserved mitogen-activated protein kinase or extracellular signal-regulated kinase (MAPK/ERK) plays critical roles in converting extracellular stimuli into immune responses. However, whether MAPK/ERK signaling induces virus immunity by directly phosphorylating viral effectors remains largely unknown. Barley yellow striate mosaic virus (BYSMV) is an economically important plant cytorhabdovirus that is transmitted by the small brown planthopper (SBPH, Laodelphax striatellus) in a propagative manner. Here, we found that the barley (Hordeum vulgare) MAPK MPK3 (HvMPK3) and the planthopper ERK (LsERK) proteins interact with the BYSMV nucleoprotein (N) and directly phosphorylate N protein primarily on serine 290. The overexpression of HvMPK3 inhibited BYSMV infection, whereas barley plants treated with the MAPK pathway inhibitor U0126 displayed greater susceptibility to BYSMV. Moreover, knockdown of LsERK promoted virus infection in SBPHs. A phosphomimetic mutant of the N Ser290 (S290D) completely abolished virus infection because of impaired self-interaction of BYSMV N and formation of unstable N-RNA complexes. Altogether, our results demonstrate that the conserved MAPK and ERK directly phosphorylate the viral nucleoprotein to trigger immunity against cross-kingdom infection of BYSMV in host plants and its insect vectors.

Plant elicitor peptide signalling confers rice resistance to piercing-sucking insect herbivores and pathogens

Rice is a staple food crop worldwide, and its production is severely threatened by phloem-feeding insect herbivores, particularly the brown planthopper (BPH, Nilaparvata lugens), and destructive pathogens. Despite the identification of many BPH resistance genes, the molecular basis of rice resistance to BPH remains largely unclear. Here, we report that the plant elicitor peptide (Pep) signalling confers rice resistance to BPH. Both rice PEP RECEPTORs (PEPRs) and PRECURSORs of PEP (PROPEPs), particularly OsPROPEP3, were transcriptionally induced in leaf sheaths upon BPH infestation. Knockout of OsPEPRs impaired rice resistance to BPH, whereas exogenous application of OsPep3 improved the resistance. Hormone measurement and co-profiling of transcriptomics and metabolomics in OsPep3-treated rice leaf sheaths suggested potential contributions of jasmonic acid biosynthesis, lipid metabolism and phenylpropanoid metabolism to OsPep3-induced rice immunity. Moreover, OsPep3 elicitation also strengthened rice resistance to the fungal pathogen Magnaporthe oryzae and bacterial pathogen Xanthamonas oryzae pv. oryzae and provoked immune responses in wheat. Collectively, this work demonstrates a previously unappreciated importance of the Pep signalling in plants for combating piercing-sucking insect herbivores and promises exogenous application of OsPep3 as an eco-friendly immune stimulator in agriculture for crop protection against a broad spectrum of insect pests and pathogens.

Effect of Piriformospora indica-Induced Systemic Resistance and Basal Immunity Against Rhizoctonia cerealis and Fusarium graminearum in Wheat

Wheat is among the top 10 and most widely grown crops in the world. However, wheat is often infected with many soil-borne diseases, including sharp eyespot, mainly caused by the necrotrophic fungus Rhizoctonia cerealis, and Fusarium head blight (FHB), caused by Fusarium graminearum, resulting in reduced production. Piriformospora indica is a root endophytic fungus with a wide range of host plants, which increases their growth and tolerance to biotic and abiotic stresses. In this study, the capability of P. indica to protect wheat seedlings against R. cerealis and F. graminearum was investigated at the physiological, biochemical, and molecular levels. Our results showed that P. indica significantly reduced the disease progress on wheat caused by F. graminearum and R. cerealis in vivo, but not showed any antagonistic effect on F. graminearum and R. cerealis in vitro. Additionally, P. indica can induce systemic resistance by elevating H₂O₂ content, antioxidase activity, relative water content (RWC), and membrane stability index (MSI) compared to the plants only inoculated with F. graminearum or R. cerealis and control. RNA-seq suggested that transcriptome changes caused by F. graminearum were more severe than those caused by R. cerealis. The number of differentially expressed genes (DEGs) in the transcriptome can be reduced by the addition of P. indica: for F. graminearum reduced by 18% and for R. cerealis reduced 58%. The DEGs related to disease resistance, such as WRKY and MAPK, were upregulated by P. indica colonization. The data further revealed that the transcriptional resistance to F. graminearum and R. cerealis mediated by P. indica is quite different.

The Raf-like kinase Raf36 negatively regulates plant resistance against the oomycete pathogen Phytophthora parasitica by targeting MKK2

Oomycetes represent a unique group of plant pathogens that are phylogenetically distant from true fungi and cause significant crop losses and environmental damage. Understanding of the genetic basis of host plant susceptibility facilitates the development of novel disease resistance strategies. In this study, we report the identification of an Arabidopsis thaliana T-DNA mutant with enhanced resistance to Phytophthora parasitica with an insertion in the Raf-like mitogen-activated protein kinase kinase kinase gene Raf36. We generated additional raf36 mutants by CRISPR/Cas9 technology as well as Raf36 complementation and overexpression transformants, with consistent results of infection assays showing that Raf36 mediates Arabidopsis susceptibility to P. parasitica. Using a virus-induced gene silencing assay, we silenced Raf36 homologous genes in Nicotiana benthamiana and demonstrated by infection assays the conserved immune function of Raf36. Mutagenesis analyses indicated that the kinase activity of Raf36 is important for its immune function and interaction with MKK2, a MAPK kinase. By generating and analysing mkk2 mutants and MKK2 complementation and overexpression transformants, we found that MKK2 is a positive immune regulator in the response to P. parasitica infection. Furthermore, infection assay on mkk2 raf36 double mutant plants indicated that MKK2 is required for the raf36-conferred resistance to P. parasitica. Taken together, we identified a Raf-like kinase Raf36 as a novel plant susceptibility factor that functions upstream of MKK2 and directly targets it to negatively regulate plant resistance to P. parasitica.

Aquaporin OsPIP2;2 links the H2O2 signal and a membrane-anchored transcription factor to promote plant defense

To overcome pathogen infection, plants deploy a highly efficient innate immune system, which often uses hydrogen peroxide (H2O2), a versatile reactive oxygen species, to activate downstream defense responses. H2O2 is a potential substrate of aquaporins (AQPs), the membrane channels that facilitate the transport of small compounds across plasma membranes or organelle membranes. To date, however, the functional relationship between AQPs and H2O2 in plant immunity is largely undissected. Here, we report that the rice (Oryza sativa) AQP OsPIP2;2 transports pathogen-induced apoplastic H2O2 into the cytoplasm to intensify rice resistance against various pathogens. OsPIP2;2-transported H2O2 is required for microbial molecular pattern flg22 to activate the MAPK cascade and to induce the downstream defense responses. In response to flg22, OsPIP2;2 is phosphorylated at the serine residue S125, and therefore gains the ability to transport H2O2. Phosphorylated OsPIP2;2 also triggers the translocation of OsmaMYB, a membrane-anchored MYB transcription factor, into the plant cell nucleus to impart flg22-induced defense responses against pathogen infection. On the contrary, if OsPIP2;2 is not phosphorylated, OsmaMYB remains associated with the plasma membrane, and plant defense responses are no longer induced. These results suggest that OsPIP2;2 positively regulates plant innate immunity by mediating H2O2 transport into the plant cell and mediating the translocation of OsmaMYB from plasma membrane to nucleus.

Regulatory Mechanisms of Mitogen-Activated Protein Kinase Cascades in Plants: More than Sequential Phosphorylation

Mitogen-activated protein kinase (MAPK) cascades play crucial roles in almost all biological processes in plants. They transduce extracellular cues into cells, typically through linear and sequential phosphorylation and activation of members of the signaling cascades. However, accumulating data suggest various regulatory mechanisms of plant MAPK cascades in addition to the traditional phosphorylation pathway, in concert with their large numbers and coordinated roles in plant responses to complex ectocytic signals. Here, we highlight recent studies that describe the uncanonical mechanism of regulation of MAPK cascades, regarding the activation of each tier of the signaling cascades. More particularly, we discuss the unusual role for MAPK kinase kinases (MAPKKKs) in the regulation of MAPK cascades, as accumulating data suggest the non-MAPKKK function of many MAPKKKs. In addition, future work on the biochemical activation of MAPK members that needs attention will be discussed.

Tryptamine 5-Hydroxylase Is Required for Suppression of Cell Death and Uncontrolled Defense Activation in Rice

Lesion-mimic mutants are useful materials to dissect mechanisms controlling programmed cell death (PCD) and defense response in plants. Although dozens of lesion-mimic mutant genes have been identified in plants, the molecular mechanisms underlying PCD and defense response remain to be extensively elucidated. Here, we identified a rice lesion mimic mutant, named lesion mimic 42 (lm42), from an ethylmethylsulfone (EMS)-induced mutant population. The lm42 mutant displayed flame-red spots on the leaves and sheaths at the 3-leaf developmental stage and exhibited impaired photosynthetic capacity with decreased chlorophyll content and decomposed chloroplast thylakoids. The lesion development of lm42 was light- and temperature-dependent. We identified a single base mutation (T38A), changing a Leu to Gln, in the first exon of LOC_Os12g16720 (LM42), which encodes a tryptamine 5-hydroxylase, by map-based cloning. We carried out transgenic complementation to confirm that this mutation caused the lm42 phenotype. We further knocked out the LM42 gene by CRISPR/Cas9 to recreate the lm42 phenotype. LM42 is highly expressed in leaves, leaf sheaths and roots. Loss-of-function of LM42 activated expression of ROS-generating genes and inhibited expression of ROS-scavenging genes, leading to ROS accumulation and eventually cell death. Furthermore, its disruption induced expression of defense-response genes and enhanced host resistance to both fungal pathogen Magnaporthe oryzae and bacterial pathogen Xanthomonas oryzae pv. oryzae. Our transcriptomic data suggested that the way lm42 led to lesion-mimic was probably by affecting ribosome development. Overall, our results demonstrate that tryptamine 5-hydroxylase-coding gene LM42 is required for suppression of cell death and uncontrolled activation of defense responses in rice.

MAP kinase cascades in plant development and immune signaling

Mitogen-activated protein kinase (MAPK) cascades are important signaling modules regulating diverse biological processes. During the past 20 years, much progress has been made on the functions of MAPK cascades in plants. This review summarizes the roles of MAPKs, known MAPK substrates, and our current understanding of MAPK cascades in plant development and innate immunity. In addition, recent findings on the molecular links connecting surface receptors to MAPK cascades and the mechanisms underlying MAPK signaling specificity are also discussed.

Protein Elicitor EsxA Induces Resistance to Seedling Blight and PR Genes Differential Transcription in Rice

Protein elicitors can induce plant systemic resistance to pathogens. In an earlier study, we cloned an EsxA gene from the plant growth-promoting rhizobacterium Paenibacillus terrae NK3-4 and expressed it in Pichia pastoris. In addition to being important for the pathogenicity of animal pathogens, EsxA can also induce an immune response in animals. While, we found the exogenously expressed EsxA has the activity of elicitor, which can trigger hypersensitive response and reactive oxygen species burst in leaves as well as enhanced rice plant growth. The effects of EsxA on seedling blight (Fusarium oxysporum) resistance and gene transcription, including pathogenesis-related (PR) genes in rice were evaluated. The germination rate was 95.0% for seeds treated with EsxA and then inoculated with F. oxysporum, which was 2.8-times higher than that of F. oxysporum-infected control seeds that were not treated with EsxA (Con). The buds and roots of EsxA-treated seedlings were 2.4- and 15.9-times longer than those of Con seedlings. The plants and roots of seedlings dipped in an EsxA solution and then inoculated with F. oxysporum were longer than those of the Con seedlings. Theplant length, number of total roots, and number of white roots were respectively 23.2%, 1.74-times, and 7.42-times greater for the seedlings sprayed with EsxA and then inoculated with F. oxysporum than for the Con seedlings. The EsxA induction efficiency (spray treatment) on seedling blight resistance was 60.9%. The transcriptome analysis revealed 1137 and 239 rice genes with EsxA-induced up-regulated and down-regulated transcription levels, respectively. At 48 h after the EsxA treatment, the transcription of 611 and 160 genes was up-regulated and down-regulated, respectively, compared with the transcription levels for the untreated control at the same time-point. Many disease resistance-related PR genes had up-regulated transcription levels. The qPCR data were consistent with the transcriptome sequencing results. EsxA triggered rice ISR to seedling blight and gene differential transcription, including the up-regulated transcription of rice PR genes. These findings may be relevant for the use of EsxA as a protein elicitor to control plant diseases.

The rice Raf-like MAPKKK OsILA1 confers broad-spectrum resistance to bacterial blight by suppressing the OsMAPKK4-OsMAPK6 cascade

Mitogen-activated protein kinase kinase kinase (MAPKKK) are the first components of MAPK cascades, which play pivotal roles in signaling during plant development and physiological processes. The genome of rice encodes 75 MAPKKKs, of which 43 are Raf-like MAPKKKs. The functions and action modes of most of the Raf-like MAPKKKs, whether they function as bona fide MAPKKKs and which are their downstream MAPKKs, are largely unknown. Here, we identified the osmapkkk43 mutant, which conferred broad-spectrum resistance to Xanthomonas oryzae pv. oryzae (Xoo), the destructive bacterial pathogen of rice. Oryza sativa (Os)MAPKKK43 encoding a Raf-like MAPKKK was previously known as Increased Leaf Angle 1 (OsILA1). Genetic analysis indicated that OsILA1 functioned as a negative regulator and acted upstream of the OsMAPKK4-OsMAPK6 cascade in rice-Xoo interactions. Unlike classical MAPKKKs, OsILA1 mainly phosphorylated the threonine 34 site at the N-terminal domain of OsMAPKK4, which possibly influenced the stability of OsMAPKK4. The N-terminal domain of OsILA1 is required for its homodimer formation and its full phosphorylation capacity. Taken together, our findings reveal that OsILA1 acts as a negative regulator of the OsMAPKK4-OsMAPK6 cascade and is involved in rice-Xoo interactions.

OsVQ1 links rice immunity and flowering via interaction with a mitogen-activated protein kinase OsMPK6

Knocking out OsVQ1 in rice released OsMPK6 for activation and in turn promoted H₂O₂ accumulation, which repressed the expression of flowering-promoting genes, thus delaying rice flowering but enhancing disease resistance. The valine-glutamine (VQ) protein family, which contains the conserved motif FxxxVQxLTG ("x" represents any amino acid), plays a crucial role in plant growth and immunity along with mitogen-activated protein kinase (MAPK) cascades. However, only a few rice VQ proteins have been functionally characterized, and the roles of the MAPK-VQ module in rice biological processes are not fully understood. Here, we investigated the role of OsVQ1 in rice disease resistance and the control of flowering time. The OsVQ1-knock out (KO) mutants exhibited increased resistance to Xanthomonas oryzae pathovars, accumulated high levels of hydrogen peroxide (H₂O₂), and showed a late flowering phenotype under natural long-day conditions, while the OsVQ1-overexpressing plants showed phenotypes similar to that of the wild type. Further studies revealed that OsVQ1 physically interacted with and inhibited OsMPK6 activity. In addition, OsVQ1 expression was downregulated by the pathogen-induced OsMPKK10.2-OsMPK6-OsWRKY45 cascade, suggesting a feedback loop between OsVQ1 and OsMPK6. Moreover, the OsVQ1-KO/osmpk6 double-mutant exhibited increased susceptibility to X. oryzae infection and showed an early flowering phenotype, which may partially be attributed to the reduced accumulation of H₂O₂ and the consequent up-expression of flowering-promoting genes. These results suggested that the OsVQ1-OsMPK6 module was involved in rice immunity and flowering.

Two VQ Proteins are Substrates of the OsMPKK6-OsMPK4 Cascade in Rice Defense Against Bacterial Blight

BACKGROUND

The plant-specific valine-glutamine (VQ) protein family with the conserved motif FxxxVQxLTG reportedly functions with the mitogen-activated protein kinase (MAPK) in plant immunity. However, the roles of VQ proteins in MAPK-mediated resistance to disease in rice remain largely unknown.

RESULTS

In this study, two rice VQ proteins OsVQ14 and OsVQ32 were newly identified to function as the signaling components of a MAPK cascade, OsMPKK6-OsMPK4, to regulate rice resistance to Xanthomonas oryzae pv. oryzae (Xoo). Both OsVQ14 and OsVQ32 positively regulated rice resistance to Xoo. In vitro and in vivo studies revealed that OsVQ14 and OsVQ32 physically interacted with and were phosphorylated by OsMPK4. OsMPK4 was highly phosphorylated in transgenic plants overexpressing OsMPKK6, which showed enhanced resistance to Xoo. Meanwhile, phosphorylated OsVQ14 and OsVQ32 were also markedly accumulated in OsMPKK6-overexpressing transgenic plants.

CONCLUSIONS

We discovered that OsVQ14 and OsVQ32 functioned as substrates of the OsMPKK6-OsMPK4 cascade to enhance rice resistance to Xoo, thereby defining a more complete signal transduction pathway for induced defenses.