EDR1 physically interacts with MKK4/MKK5 and negatively regulates a MAP kinase cascade to modulate plant innate immunity

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.

A close-up of regulatory networks and signaling pathways of MKK5 in biotic and abiotic stresses

Mitogen-activated protein Kinase Kinase 5 (MKK5) is a central hub in the complex phosphorylation chain reaction of the Mitogen-activated protein kinases (MAPK) cascade, regulating plant responses to biotic and abiotic stresses. This review manuscript aims to provide a comprehensive analysis of the regulatory mechanism of the MKK5 involved in stress adaptation. This review will delve into the intricate post-transcriptional and post-translational modifications of the MKK5, discussing how they affect its expression, activity, and subcellular localization in response to stress signals. We also discuss the integration of the MKK5 into complex signaling pathways, orchestrating plant immunity against pathogens and its modulating role in regulating abiotic stresses, such as: drought, cold, heat, and salinity, through the phytohormonal signaling pathways. Furthermore, we highlight potential applications of the MKK5 for engineering stress-resilient crops and provide future perspectives that may pave the way for future studies. This review manuscript aims to provide valuable insights into the mechanisms underlying MKK5 regulation, bridge the gap from numerous previous findings, and offer a firm base in the knowledge of MKK5, its regulating roles, and its involvement in environmental stress regulation.

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.

Genome-Wide Association Studies on Chinese Wheat Cultivars Reveal a Novel Fusarium Crown Rot Resistance Quantitative Trait Locus on Chromosome 3BL

Fusarium crown rot (FCR), primarily caused by Fusarium pseudograminearum, has emerged as a new threat to wheat production and quality in North China. Genetic enhancement of wheat resistance to FCR remains the most effective approach for disease control. In this study, we phenotyped 435 Chinese wheat cultivars through FCR inoculation at the seedling stage in a greenhouse. Our findings revealed that only approximately 10.8% of the wheat germplasms displayed moderate or high resistance to FCR. A genome-wide association study (GWAS) using high-density 660K SNP led to the discovery of a novel quantitative trait locus on the long arm of chromosome 3B, designated as Qfcr.hebau-3BL. A total of 12 significantly associated SNPs were closely clustered within a 1.05 Mb physical interval. SNP-based molecular markers were developed to facilitate the practical application of Qfcr.hebau-3BL. Among the five candidate FCR resistance genes within the Qfcr.hebau-3BL, we focused on TraesCS3B02G307700, which encodes a protein kinase, due to its expression pattern. Functional validation revealed two transcripts, TaSTK1.1 and TaSTK1.2, with opposing roles in plant resistance to fungal disease. These findings provide insights into the genetic basis of FCR resistance in wheat and offer valuable resources for breeding resistant varieties.

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.

Receptor-like kinases OsRLK902-1 and OsRLK902-2 form immune complexes with OsRLCK185 to regulate rice blast resistance

Receptor-like kinases (RLKs) are major regulators of the plant immune response and play important roles in the perception and transmission of immune signals. RECEPTOR LIKE KINASE 902 (RLK902) is at the key node in leucine-rich repeat receptor-like kinase interaction networks and positively regulates resistance to the bacterial pathogen Pseudomonas syringae in Arabidopsis. However, the function of RLK902 in fungal disease resistance remains obscure. In this study, we found that the expression levels of OsRLK902-1 and OsRLK902-2, encoding two orthologues of RLK902 in rice, were induced by Magnaporthe oryzae, chitin, and flg22 treatment. osrlk902-1 and osrlk902-2 knockout mutants displayed enhanced susceptibility to M. oryzae. Interestingly, the osrlk902-1 rlk902-2 double mutant exhibited similar disease susceptibility, hydrogen peroxide production, and callose deposition to the two single mutants. Further investigation showed that OsRLK902-1 interacts with and stabilizes OsRLK902-2. The two OsRLKs form a complex with OsRLCK185, a key regulator in chitin-triggered immunity, and stabilize it. Taken together, our data demonstrate that OsRLK902-1 and OsRLK902-2, as well as OsRLCK185 function together in regulating disease resistance to M. oryzae in rice.

Defense signaling pathways in resistance to plant viruses: Crosstalk and finger pointing

Resistance to infection by plant viruses involves proteins encoded by plant resistance (R) genes, viz., nucleotide-binding leucine-rich repeats (NLRs), immune receptors. These sensor NLRs are activated either directly or indirectly by viral protein effectors, in effector-triggered immunity, leading to induction of defense signaling pathways, resulting in the synthesis of numerous downstream plant effector molecules that inhibit different stages of the infection cycle, as well as the induction of cell death responses mediated by helper NLRs. Early events in this process involve recognition of the activation of the R gene response by various chaperones and the transport of these complexes to the sites of subsequent events. These events include activation of several kinase cascade pathways, and the syntheses of two master transcriptional regulators, EDS1 and NPR1, as well as the phytohormones salicylic acid, jasmonic acid, and ethylene. The phytohormones, which transit from a primed, resting states to active states, regulate the remainder of the defense signaling pathways, both directly and by crosstalk with each other. This regulation results in the turnover of various suppressors of downstream events and the synthesis of various transcription factors that cooperate and/or compete to induce or suppress transcription of either other regulatory proteins, or plant effector molecules. This network of interactions results in the production of defense effectors acting alone or together with cell death in the infected region, with or without the further activation of non-specific, long-distance resistance. Here, we review the current state of knowledge regarding these processes and the components of the local responses, their interactions, regulation, and crosstalk.

Dephosphorylation of the MAP kinases MPK6 and MPK3 fine-tunes responses to wounding and herbivory in Arabidopsis

The Arabidopsis MAP Kinases (MAPKs) MPK6 and MPK3 and orthologs in other plants function as major stress signaling hubs. MAPKs are activated by phosphorylation and are negatively regulated by MAPK-inactivating phosphatases (MIPPs), which alter the intensity and duration of MAPK signaling via dephosphorylation. Unlike in other plant species, jasmonic acid (JA) accumulation in Arabidopsis is apparently not MPK6- and MPK3-dependent, so their role in JA-mediated defenses against herbivorous insects is unclear. Here we explore whether changes in MPK6/3 phosphorylation kinetics in Arabidopsis MIPP mutants lead to changes in hormone synthesis and resistance against herbivores. The MIPPs MKP1, DsPTP1, PP2C5, and AP2C1 have been implicated in responses to infection, drought, and osmotic stress, which all impinge on JA-mediated defenses. In loss-of-function mutants, we found that the four MIPPs alter wound-induced MPK6/3 phosphorylation kinetics and affect the accumulation of the defense hormones JA, abscisic acid, and salicylic acid, as compared to wild type plants (Col-0). Moreover, MPK6/3 misregulation in MIPP or MAPK mutant plants resulted in slight changes in the resistance to Trichoplusia ni and Spodoptera exigua larvae as compared to Col-0. Our data indicate that MPK6/3 and the four MIPPs moderately contribute to wound signaling and defense against herbivorous insects in Arabidopsis.

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.

A receptor-like kinase SlFERL mediates immune responses of tomato to Botrytis cinerea by recognizing BcPG1 and fine-tuning MAPK signaling

FERONIA (FER) is a receptor-like kinase showing versatile functions during plant growth, development, and responses to environmental stimuli. However, its functions during the interaction between fruit and necrotrophic fungal pathogens are still unclear. Combining reverse genetic approaches, physiological assays, co-immunoprecipitation, protein phosphorylation identification, and site-directed mutagenesis, we reported a tomato FER homolog SlFERL (Solanum lycopersicum FERONIA Like) involved in the immune responses to Botrytis cinerea invasion. The results indicated that SlFERL extracellular domain recognized and interacted with the secreted virulence protein BcPG1 from B. cinerea, further revealed that SlFERL triggered downstream signaling by phosphorylating SlMAP3K18 at Thr45, Ser49, Ser76, and Ser135. Moreover, we verified that SlMAP2K2 and SlMAP2K4 synergistically contributed to immune response of tomato to B. cinerea, in which SlFERL-SlMAP3K18 module substantially modulated protein level and/or kinase activity of SlMAP2K2/SlMAP2K4. These findings reveal a new pattern-triggered immune pathway, indicating that SlFERL participates in the immune responses to B. cinerea invasion via recognizing BcPG1 and fine-tuning MAPK signaling.

Wheat powdery mildew resistance: from gene identification to immunity deployment

Powdery mildew is one of the most devastating diseases on wheat and is caused by the obligate biotrophic phytopathogen Blumeria graminis f. sp. tritici (Bgt). Due to the complexity of the large genome of wheat and its close relatives, the identification of powdery mildew resistance genes had been hampered for a long time until recent progress in large-scale sequencing, genomics, and rapid gene isolation techniques. Here, we describe and summarize the current advances in wheat powdery mildew resistance, emphasizing the most recent discoveries about the identification of genes conferring powdery mildew resistance and the similarity, diversity and molecular function of those genes. Multilayered resistance to powdery mildew in wheat could be used for counteracting Bgt, including durable, broad spectrum but partial resistance, as well as race-specific and mostly complete resistance mediated by nucleotide-binding and leucine rich repeat domain (NLR) proteins. In addition to the above mentioned layers, manipulation of susceptibility (S) and negative regulator genes may represent another layer that can be used for durable and broad-spectrum resistance in wheat. We propose that it is promising to develop effective and durable strategies to combat powdery mildew in wheat by simultaneous deployment of multilayered immunity.

IbInvInh2, a novel invertase inhibitor in sweet potato, regulates starch content through post-translational regulation of vacuolar invertase IbβFRUCT2

As a key enzyme in the starch and sugar metabolic pathways in sweet potato (Ipomoea batatas (L.) Lam.), the vacuolar invertase (EC 3.2.1.26) IbβFRUCT2 is involved in partitioning and modulating the starch and sugar components of the storage root. However, the post-translational regulation of its invertase activity remains unclear. In this study, we identified three invertase inhibitors, IbInvInh1, IbInvInh2, and IbInvInh3, as potential interaction partners of IbβFRUCT2. All were found to act as vacuolar invertase inhibitors (VIFs) and belonged to the plant invertase/pectin methyl esterase inhibitor superfamily. Among the three VIFs, IbInvInh2 is a novel VIF in sweet potato and was confirmed to be an inhibitor of IbβFRUCT2. The N-terminal domain of IbβFRUCT2 and the Thr39 and Leu198 sites of IbInvInh2 were predicted to be engaged in their interactions. The transgenic expression of IbInvInh2 in Arabidopsis thaliana plants reduced the starch content of leaves, while its expression in the Ibβfruct2-expressing Arabidopsis plants increased the starch content of leaves, suggesting that the post-translational inhibition of IbβFRUCT2 activity by IbInvInh2 contributes to the regulation of the plant starch content. Taken together, our findings reveal a novel VIF in sweet potato and provide insights into the potential regulatory roles of the VIFs and invertase-VIF interaction in starch metabolism. These insights lay the foundation for using VIFs to improve the starch properties of crops.

VqMAPK3/VqMAPK6, VqWRKY33, and VqNSTS3 constitute a regulatory node in enhancing resistance to powdery mildew in grapevine

Grapevine powdery mildew is caused by Erysiphe necator, which seriously harms grape production in the world. Stilbene synthase makes phytoalexins that contribute to the resistance of grapevine against powdery mildew. A novel VqNSTS3 was identified and cloned from Chinese wild Vitis quinquangularis accession Danfeng-2. The novel VqNSTS3 was transferred into susceptible 'Thompson Seedless' by Agrobacterium-mediated transformation. The transgenic plants showed resistance to the disease and activated other resistance-related genes. VqNSTS3 expression in grapevine is regulated by VqWRKY33, and which binds to TTGACC in the VqNSTS3 promoter. Furthermore, VqWRKY33 was phosphorylated by VqMAPK3/VqMAPK6 and thus led to enhanced signal transduction and increased VqNSTS3 expression. ProVqNSTS3::VqNSTS3-GFP of transgenic VqNSTS3 in Arabidopsis thaliana was observed to move to and wrap the pathogen's haustoria and block invasion by Golovinomyces cichoracearum. These results demonstrate that stilbene accumulation of novel VqNSTS3 of the Chinese wild Vitis quinquangularis accession Danfeng-2 prevented pathogen invasion and enhanced resistance to powdery mildew. Therefore, VqNSTS3 can be used in generating powdery mildew-resistant grapevines.

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.

Soybean GmSAUL1, a Bona Fide U-Box E3 Ligase, Negatively Regulates Immunity Likely through Repressing the Activation of GmMPK3

E3 ubiquitin ligases play important roles in plant immunity, but their role in soybean has not been investigated previously. Here, we used Bean pod mottle virus (BPMV)-mediated virus-induced gene silencing (VIGS) to investigate the function of GmSAUL1 (Senescence-Associated E3 Ubiquitin Ligase 1) homologs in soybean. When two closely related SAUL1 homologs were silenced simultaneously, the soybean plants displayed autoimmune phenotypes, which were significantly alleviated by high temperature, suggesting that GmSAUL1a/1b might be guarded by an R protein. Interestingly, silencing GmSAUL1a/1b resulted in the decreased activation of GmMPK6, but increased activation of GmMPK3 in response to flg22, suggesting that the activation of GmMPK3 is most likely responsible for the activated immunity observed in the GmSAUL1a/1b-silenced plants. Furthermore, we provided evidence that GmSAUL1a is a bona fide E3 ligase. Collectively, our results indicated that GmSAUL1 plays a negative role in regulating cell death and immunity in soybean.

MAPKKKs in Plants: Multidimensional Regulators of Plant Growth and Stress Responses

Mitogen-activated protein kinase kinase kinase (MAPKKK, MAP3K) is located upstream of the mitogen-activated protein kinase (MAPK) cascade pathway and is responsible for receiving and transmitting external signals to the downstream MAPKKs. Although a large number of MAP3K genes play important roles in plant growth and development, and response to abiotic and biotic stresses, only a few members' functions and cascade signaling pathways have been clarified, and the downstream MAPKKs and MAPKs of most MAP3Ks are still unknown. As more and more signaling pathways are discovered, the function and regulatory mechanism of MAP3K genes will become clearer. In this paper, the MAP3K genes in plants were classified and the members and basic characteristics of each subfamily of MAP3K were briefly described. Moreover, the roles of plant MAP3Ks in regulating plant growth and development and stress (abiotic and biotic) responses are described in detail. In addition, the roles of MAP3Ks involved in plant hormones signal transduction pathway were briefly introduced, and the future research focus was prospected.

Field Pea (Pisum sativum) Germplasm Screening for Seedling Ascochyta Blight Resistance and Genome-Wide Association Studies Reveal Loci Associated with Resistance to Peyronellaea pinodes and Ascochyta koolunga

Ascochyta blight is a damaging disease that affects the stems, leaves, and pods of field pea (Pisum sativum) and impacts yield and grain quality. In Australia, field pea Ascochyta blight is primarily caused by the necrotrophic fungal species Peyronellaea pinodes and Ascochyta koolunga. In this study, we screened 1,276 Pisum spp. germplasm accessions in seedling disease assays with a mix of three isolates of P. pinodes and 641 accessions with three mixed isolates of A. koolunga (513 accessions were screened with both species). A selection of three P. sativum accessions with low disease scores for either pathogen, or in some cases both, were crossed with Australian field pea varieties PBA Gunyah and PBA Oura, and recombinant inbred line populations were made. Populations at the F3:4 and F4:5 generation were phenotyped for their disease response to P. pinodes and A. koolunga, and genotypes were determined using the diversity arrays technology genotyping method. Marker-trait associations were identified using a genome-wide association study approach. Trait-associated loci were mapped to the published P. sativum genome assembly, and candidate resistance gene analogues were identified in the corresponding genomic regions. One locus on chromosome 2 (LG1) was associated with resistance to P. pinodes, and the 8 Mb genomic region contains 156 genes, two of which are serine/threonine protein kinases, putatively contributing to the resistance trait. A second locus on chromosome 5 (LG3) was associated with resistance to A. koolunga, and the 35 Mb region contains 488 genes, of which five are potential candidate resistance genes, including protein kinases, a mitogen-activated protein kinase, and an ethylene-responsive protein kinase homolog.

Auxin promotes hypocotyl elongation by enhancing BZR1 nuclear accumulation in Arabidopsis

Auxin and brassinosteroids (BRs) are two major growth-promoting phytohormones that shape hypocotyl elongation; however, the cross-talk between auxin and BR in this process is not fully understood. In this study, we found that auxin-induced hypocotyl elongation is dependent on brassinazole-resistant 1 (BZR1), a core BR signaling component. Auxin promotes BZR1 nuclear accumulation in hypocotyl cells, a process dependent on mitogen-activated protein kinase 3 (MPK3) and MPK6, which are both activated by auxin and whose encoding genes are highly expressed in hypocotyls. We determined that MPK3/MPK6 phosphorylate and reduce the protein stability of general regulatory factor 4 (GRF4), a member of the 14-3-3 family of proteins that retain BZR1 in the cytoplasm. In summary, this study reveals the molecular mechanism by which auxin promotes hypocotyl elongation by enhancing BZR1 nuclear accumulation via MPK3/MPK6-regulated GRF4 protein stability.

The Role of Ubiquitination in Plant Immunity: Fine-Tuning Immune Signaling and Beyond

Ubiquitination is an essential posttranslational modification and plays a crucial role in regulating plant immunity by modulating protein activity, stability, abundance and interaction. Recently, major breakthroughs have been made in understanding the mechanisms associated with the regulation of immune signaling by ubiquitination. In this mini review, we highlight the recent advances in the role of ubiquitination in fine-tuning the resistance activated by plant pattern recognition receptors (PRRs) and intracellular nucleotide-binding site and leucine-rich repeat domain receptors (NLRs). We also discuss current understanding of the positive regulation of plant immunity by ubiquitination, including the modification of immune negative regulators and of the guardee proteins monitored by NLRs.

Negative Interactions Balance Growth and Defense in Plants Confronted with Herbivores or Pathogens

Plants have evolved a series of defensive mechanisms against pathogens and herbivores, but the defense response always leads to decreases in growth or reproduction, which has serious implications for agricultural production. Growth and defense are negatively regulated not only through metabolic consumption but also through the antagonism of different phytohormones, such as jasmonic acid (JA) and salicylic acid (SA). Meanwhile, plants can limit the expression of defensive metabolites to reduce the costs of defense by producing constitutive defenses such as glandular trichomes or latex and accumulating specific metabolites, determining the activation of plant defense or the maintenance of plant growth. Interestingly, plant defense pathways might be prepared in advance which may be transmitted to descendants. Plants can also use external organisms to protect themselves, thus minimizing the costs of defense. In addition, plant relatives exhibit cooperation to deal with pathogens and herbivores, which is also a way to regulate growth and defense.

The RECEPTOR-LIKE PROTEIN53 immune complex associates with LLG1 to positively regulate plant immunity

Pattern recognition receptors (PRRs) sense ligands in pattern-triggered immunity (PTI). Plant PRRs include numerous receptor-like proteins (RLPs), but many RLPs remain functionally uncharacterized. Here, we examine an Arabidopsis thaliana RLP, RLP53, which positively regulates immune signaling. Our forward genetic screen for suppressors of enhanced disease resistance1 (edr1) identified a point mutation in RLP53 that fully suppresses disease resistance and mildew-induced cell death in edr1 mutants. The rlp53 mutants showed enhanced susceptibility to virulent pathogens, including fungi, oomycetes, and bacteria, indicating that RLP53 is important for plant immunity. The ectodomain of RLP53 contains leucine-rich repeat (LRR) motifs. RLP53 constitutively associates with the LRR receptor-like kinase SUPPRESSOR OF BRASSINOSTEROID INSENSITIVE1-ASSOCIATED KINASE (BAK1)-INTERACTING RECEPTOR KINASE1 (SOBIR1) and interacts with the co-receptor BAK1 in a pathogen-induced manner. The double mutation sobir1-12 bak1-5 suppresses edr1-mediated disease resistance, suggesting that EDR1 negatively regulates PTI modulated by the RLP53-SOBIR1-BAK1 complex. Moreover, the glycosylphosphatidylinositol (GPI)-anchored protein LORELEI-LIKE GPI-ANCHORED PROTEIN1 (LLG1) interacts with RLP53 and mediates RLP53 accumulation in the plasma membrane. We thus uncovered the role of a novel RLP and its associated immune complex in plant defense responses and revealed a potential new mechanism underlying regulation of RLP immune function by a GPI-anchored protein.

A serine-rich effector from the stripe rust pathogen targets a Raf-like kinase to suppress host immunity

Puccinia striiformis f. sp. tritici (Pst) is an important obligate pathogen in wheat (Triticum aestivum L.) and secretes effectors into plant cells to promote infection. Identifying host targets of effector proteins and clarifying their roles in pathogen infection is essential for understanding pathogen virulence. In this study, we identified a serine-rich effector, Pst27791, from Pst that suppresses cell death in Nicotiana benthamiana. Stable overexpression of Pst27791 in wheat suppressed reactive oxygen species accumulation and the salicylic acid-dependent defense response. Transgenic wheat expressing the RNA interference construct of Pst27791 exhibited high resistance to Pst virulent isolate CYR31, indicating its importance in pathogenesis. Pst27791 interacting with wheat rapidly accelerated fibrosarcoma (Raf)-like kinase TaRaf46 in yeast and in planta. Knocking down TaRaf46 expression in wheat attenuated Pst infection and increased wheat immunity. The overexpression of TaRaf46 decreased wheat resistance to Pst and repressed MAPK activation in wheat. Pst27791 may stabilize TaRaf46 through the inhibition of proteasome-mediated degradation in N. benthamiana. The ability of Pst27791 to enhance Pst colonization was compromised when TaRaf46 was silenced, suggesting that the virulence of Pst27791 is mediated by TaRaf46. Overall, these results indicate that Raf-like kinase TaRaf46 is exploited by the Pst effector as a negative regulator of plant immunity to promote infection in wheat.

Silencing GmBIR1 in Soybean Results in Activated Defense Responses

Receptor-like kinases (RLKs) are a large group of pattern recognition receptors (PRRs) and play a critical role in recognizing pathogens, transducing defense signals, and mediating the activation of immune defense responses. Although extensively studied in the model plant Arabidopsis, studies of RLKs in crops, including soybean, are limited. When a BAK1-interacting receptor-like kinase (BIR1) homolog (referred to as GmBIR1 hereafter) was silenced by the BPMV (Bean pod mottle virus)-induced gene silencing (BPMV-VIGS), it resulted in phenotypes that were reminiscent of constitutively activated defense responses, including a significantly stunted stature with observable cell death on the leaves of the silenced plants. In addition, both SA and H2O2 were over-accumulated in the leaves of the GmBIR1-silenced plants. Consistent with this autoimmune phenotype, GmBIR1-silenced plants exhibited significantly enhanced resistance to both Pseudomonas syringae pv. glycinea (Psg) and Soybean mosaic virus (SMV), two different types of pathogens, compared to the vector control plants. Together, our results indicated that GmBIR1 is a negative regulator of immunity in soybean and the function of BIR1 homologs is conserved in different plant species.

TuRLK1, a leucine-rich repeat receptor-like kinase, is indispensable for stripe rust resistance of YrU1 and confers broad resistance to multiple pathogens

BACKGROUND

YrU1 is a nucleotide-binding site (NBS) and leucine-rich repeat (LRR) protein (NLR), with additional ankyrin-repeat and WRKY domains and confers effective resistance to stripe rust fungus Puccinia striiformis f. sp. Tritici (Pst). YrU1 was positionally cloned in the progenitor species of the A genome of bread wheat, Tricicum urartu, recently. However, the molecular mechanism and components involved in YrU1-mediated resistance are not clear.

RESULTS

In this study, we found that the transcript level of TuRLK1, which encodes a novel leucine-rich repeat receptor-like kinase, was up-regulated after inoculation with Pst in the presence of YrU1, through RNA-seq analysis in T. urartu accession PI428309. TuRLK1 contained only a small number of LRR motifs, and was localized in the plasma-membrane. Transient expression of TuRLK1 induced hypersensitive cell death response in N. benthamiana leaves. Silencing of TuRLK1, using barley stripe mosaic virus (BSMV)-induced gene silencing (VIGS) system in PI428309 that contains YrU1, compromised the resistance against stripe rust caused by Pst CY33, indicating that TuRLK1 was required for YrU1-activated plant immunity. Furthermore, overexpression of TuRLK1 could enhance powdery mildew resistance in bread wheat and Arabidopsis thaliana after inoculating with the corresponding pathogens.

CONCLUSIONS

Our study indicates that TuRLK1 is required for immune response mediated by the unique NLR protein YrU1, and likely plays an important role in disease resistance to other pathogens.

Dichotomy of the BSL phosphatase signaling spatially regulates MAPK components in stomatal fate determination

MAPK signaling modules play crucial roles in regulating numerous biological processes in all eukaryotic cells. How MAPK signaling specificity and strength are tightly controlled remains a major challenging question. In Arabidopsis stomatal development, the MAPKK Kinase YODA (YDA) functions at the cell periphery to inhibit stomatal production by activating MAPK 3 and 6 (MPK3/6) that directly phosphorylate stomatal fate-determining transcription factors for degradation in the nucleus. Recently, we demonstrated that BSL1, one of the four BSL protein phosphatases, localizes to the cell cortex to activate YDA, elevating MPK3/6 activity to suppress stomatal formation. Here, we showed that at the plasma membrane, all four members of BSL proteins contribute to the YDA activation. However, in the nucleus, specific BSL members (BSL2, BSL3, and BSU1) directly deactivate MPK6 to counteract the linear MAPK pathway, thereby promoting stomatal formation. Thus, the pivotal MAPK signaling in stomatal fate determination is spatially modulated by a signaling dichotomy of the BSL protein phosphatases in Arabidopsis, providing a prominent example of how MAPK activities are integrated and specified by signaling compartmentalization at the subcellular level.

BRASSINOSTEROID-SIGNALING KINASE1 modulates MAP KINASE15 phosphorylation to confer powdery mildew resistance in Arabidopsis

Perception of pathogen-associated molecular patterns (PAMPs) by plant cell surface-localized pattern-recognition receptors (PRRs) triggers the first line of plant innate immunity. In Arabidopsis thaliana, the receptor-like cytoplasmic kinase BRASSINOSTEROID-SIGNALING KINASE1 (BSK1) physically associates with PRR FLAGELLIN SENSING2 and plays an important role in defense against multiple pathogens. However, how BSK1 transduces signals to activate downstream immune responses remains elusive. Previously, through whole-genome phosphorylation analysis using mass spectrometry, we showed that phosphorylation of the mitogen-activated protein kinase (MAPK) MPK15 was affected in the bsk1 mutant compared with the wild-type plants. Here, we demonstrated that MPK15 is important for powdery mildew fungal resistance. PAMPs and fungal pathogens significantly induced the phosphorylation of MPK15 Ser-511, a key phosphorylation site critical for the functions of MPK15 in powdery mildew resistance. BSK1 physically associates with MPK15 and is required for basal and pathogen-induced MPK15 Ser-511 phosphorylation, which contributes to BSK1-mediated fungal resistance. Taken together, our data identified MPK15 as a player in plant defense against powdery mildew fungi and showed that BSK1 promotes fungal resistance in part by enhancing MPK15 Ser-511 phosphorylation. These results uncovered a mechanism of BSK1-mediated disease resistance and provided new insight into the role of MAPK phosphorylation in plant immunity.

MdMAPKKK1 Regulates Apple Resistance to Botryosphaeria dothidea by Interacting with MdBSK1

Plant MAPK cascade performs a critical role in the regulation of plant immunity and disease resistance. Although the function of MAPK cascade in immunity regulation is partially conserved between different species, the mechanism varies in different host and pathogen combinations. To date, the MAPK cascade function of woody plants in the regulation of disease resistance has seldom been reported. Here, we present evidence to show that apple MdMAPKKK1 performed an important role in the regulation of apple resistance to Botryosphaeria dothidea, the causal agent of apple ring rot. B. dothidea infection leads to enhanced MdMAPKKK1 expression and MAPK cascade activation, indicating that the MAPK cascade is involved in the defense against B. dothidea. MdMAPKKK1 overexpression-induced pathogen-independent cell death. MdMAPKKK1 silencing decreases the resistance of apple calli and fruits to B. dothidea. Further analysis indicates that MdMAPKKK1 can bind MdBSK1 and is likely phosphorylated by it. The MdBSK1-mediated phosphorylation of MdMAPKKK1 is important for resistance to B. dothidea. These results collectively indicate that apple resistance to B. dothidea is regulated by the interaction between MAPKKK1 and MdBSK1.

Regulation of Arabidopsis Matrix Metalloproteinases by Mitogen-Activated Protein Kinases and Their Function in Leaf Senescence

Leaf senescence is a developmentally programmed cell death process that is influenced by a variety of endogenous signals and environmental factors. Here, we report that MPK3 and MPK6, two Arabidopsis mitogen-activated protein kinases (MAPKs or MPKs), and their two upstream MAPK kinases (MAPKKs or MKKs), MKK4 and MKK5, are key regulators of leaf senescence. Weak induction of constitutively active MAPKKs driven by steroid-inducible promoter, which activates endogenous MPK3 and MPK6, induces leaf senescence. This gain-of-function phenotype requires functional endogenous MPK3 and MPK6. Furthermore, loss of function of both MKK4 and MKK5 delays leaf senescence. Expression profiling leads to the identification of matrix metalloproteinases (MMPs), a family of zinc- and calcium-dependent endopeptidases, as the downstream target genes of MPK3/MPK6 cascade. MPK3/MPK6 activation-triggered leaf senescence is associated with rapid and strong induction of At3-MMP and At2-MMP. Expression of Arabidopsis MMP genes is strongly induced during leaf senescence, qualifying them as senescence-associated genes (SAGs). In addition, either constitutive or inducible overexpression of At3-MMP is sufficient to trigger leaf senescence. Based on these findings, we conclude that MPK3/MPK6 MAPK cascade and MMP target genes further downstream are involved in regulating leaf senescence in Arabidopsis.

MPK3 and MPK6 control salicylic acid signaling by up-regulating NLR receptors during pattern- and effector-triggered immunity

Arabidopsis thaliana mitogen-activated protein kinases 3 and 6 (MPK3/6) are activated transiently during pathogen-associated molecular pattern-triggered immunity (PTI) and durably during effector-triggered immunity (ETI). The functional differences between these two kinds of activation kinetics and how they coordinate the two layers of plant immunity remain poorly understood. Here, by suppressor analyses, we demonstrate that ETI-mediating nucleotide-binding domain leucine-rich repeat receptors (NLRs) and the NLR signaling components NDR1 and EDS1 can promote the salicylic acid sector of defense downstream of MPK3 activity. Moreover, we provide evidence that both sustained and transient MPK3/6 activities positively control the expression of several NLR genes, including AT3G04220 and AT4G11170. We further show that NDR1 and EDS1 contribute to the up-regulation of these two NLRs in both an ETI and a PTI context. Remarkably, whereas in ETI MPK3/6 activities are dependent on NDR1 and EDS1, they are not in PTI, suggesting crucial differences in the two signaling pathways. Finally, we demonstrate that expression of the NLR AT3G04220 is sufficient to induce expression of defense genes from the salicylic acid branch. Overall, this study expands our knowledge of MPK3/6 functions during immunity and provides new insights into the intricate interplay of PTI and ETI.

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.

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.

Transcriptome and Metabolome Analyses Reveal Molecular Responses of Two Pepper (Capsicum annuum L.) Cultivars to Cold Stress

Low temperature is a significant factor affecting field-grown pepper. The molecular mechanisms behind peppers' response to cold stress remain unknown. Transcriptomic and metabolomic analyses were used to investigate the responses of two pepper cultivars, XS (cold-sensitive) and GZ (cold-resistant), to cold stress; these were screened from 45 pepper materials. In this study, compared with the control group (0 h), we identified 10,931 differentially expressed genes (DEGs) in XS and GZ, 657 differentially expressed metabolites (DEMs) in the positive ion mode, and 390 DEMs in the negative ion mode. Most DEGs were involved in amino acid biosynthesis, plant hormone signal transduction, and the mitogen-activated protein kinase (MAPK) signaling pathway. Furthermore, metabolomic analysis revealed that the content of free polyamines (PAs), plant hormones, and osmolytes, mainly contained increased putrescine, spermine, spermidine, abscisic acid (ABA), jasmonic acid (JA), raffinose, and proline, in response to cold stress. Importantly, the regulation of the ICE (inducer of CBF expression)-CBF (C repeat binding factors)-COR (cold regulated) pathway by Ca²⁺ signaling, MAPK signaling, and reactive oxygen species (ROS) signaling plays a key role in regulating responses of peppers to cold stress. Above all, the results of the present study provide important insights into the response of peppers to cold stress, which will reveal the potential molecular mechanisms and contribute to pepper screening and breeding in the future.

Ubiquitination of Receptorsomes, Frontline of Plant Immunity

Sessile plants are constantly exposed to myriads of unfavorable invading organisms with different lifestyles. To survive, plants have evolved plasma membrane-resident pattern recognition receptors (PRRs) and intracellular nucleotide-binding domain leucine-rich repeat receptors (NLRs) to initiate sophisticated downstream immune responses. Ubiquitination serves as one of the most important and prevalent posttranslational modifications (PTMs) to fine-tune plant immune responses. Over the last decade, remarkable progress has been made in delineating the critical roles of ubiquitination in plant immunity. In this review, we highlight recent advances in the understanding of ubiquitination in the modulation of plant immunity, with a particular focus on ubiquitination in the regulation of receptorsomes, and discuss how ubiquitination and other PTMs act in concert to ensure rapid, proper, and robust immune responses.

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.

MYC2 influences salicylic acid biosynthesis and defense against bacterial pathogens in Arabidopsis thaliana

Arabidopsis MYC2 is a basic helix-loop-helix transcription factor that works both as a negative and positive regulator of light and multiple hormonal signaling pathways, including jasmonic acid and abscisic acid. Recent studies have suggested the role of MYC2 as a negative regulator of salicylic acid (SA)-mediated defense against bacterial pathogens. By using myc2 mutant and constitutively MYC2-expressing plants, we further show that MYC2 also positively influences SA-mediated defense; whereas, myc2 mutant plants are resistant to virulent pathogens only, MYC2 over-expressing plants are hyper-resistant to multiple virulent and avirulent strains of bacterial pathogens. MYC2 promotes pathogen-induced callose deposition, SA biosynthesis, expression of PR1 gene, and SA-responsiveness. Using bacterially produced MYC2 protein in electrophoretic mobility shift assay (EMSA), we have shown that MYC2 binds to the promoter of several important defense regulators, including PEPR1, MKK4, RIN4, and the second intron of ICS1. MYC2 positively regulates the expression of RIN4, MKK4, and ICS1; however, it negatively regulates the expression of PEPR1. Pathogen inoculation enhances MYC2 association at ICS1 intron and RIN4 promoter. Mutations of MYC2 binding site at ICS1 intron or RIN4 promoter abolish the associated GUS reporter expression. Hyper-resistance of MYC2 over-expressing plants is largely light-dependent, which is in agreement with the role of MYC2 in SA biosynthesis. The results altogether demonstrate that MYC2 possesses dual regulatory roles in SA biosynthesis, SA signaling, pattern-triggered immunity (PTI), and effector-triggered immunity (ETI) in Arabidopsis.

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.

MKK4/5-MPK3/6 Cascade Regulates Agrobacterium-Mediated Transformation by Modulating Plant Immunity in Arabidopsis

Agrobacterium tumefaciens is a specialized plant pathogen that causes crown gall disease and is commonly used for Agrobacterium-mediated transformation. As a pathogen, Agrobacterium triggers plant immunity, which affects transformation. However, the signaling components and pathways in plant immunity to Agrobacterium remain elusive. We demonstrate that two Arabidopsis mitogen-activated protein kinase kinases (MAPKKs) MKK4/MKK5 and their downstream mitogen-activated protein kinases (MAPKs) MPK3/MPK6 play major roles in both Agrobacterium-triggered immunity and Agrobacterium-mediated transformation. Agrobacteria induce MPK3/MPK6 activity and the expression of plant defense response genes at a very early stage. This process is dependent on the MKK4/MKK5 function. The loss of the function of MKK4 and MKK5 or their downstream MPK3 and MPK6 abolishes plant immunity to agrobacteria and increases transformation frequency, whereas the activation of MKK4 and MKK5 enhances plant immunity and represses transformation. Global transcriptome analysis indicates that agrobacteria induce various plant defense pathways, including reactive oxygen species (ROS) production, ethylene (ET), and salicylic acid- (SA-) mediated defense responses, and that MKK4/MKK5 is essential for the induction of these pathways. The activation of MKK4 and MKK5 promotes ROS production and cell death during agrobacteria infection. Based on these results, we propose that the MKK4/5-MPK3/6 cascade is an essential signaling pathway regulating Agrobacterium-mediated transformation through the modulation of Agrobacterium-triggered plant immunity.

The TIR-NBS protein TN13 associates with the CC-NBS-LRR resistance protein RPS5 and contributes to RPS5-triggered immunity in Arabidopsis

Nucleotide-binding site (NBS)-leucine-rich repeat (LRR) domain receptor (NLR) proteins play important roles in plant innate immunity by recognizing pathogen effectors. The Toll/interleukin-1 receptor (TIR)-NBS (TN) proteins belong to a subtype of the atypical NLRs, but their function in plant immunity is poorly understood. The well-characterized Arabidopsis thaliana typical coiled-coil (CC)-NBS-LRR (CNL) protein Resistance to Pseudomonas syringae 5 (RPS5) is activated after recognizing the Pseudomonas syringae type III effector AvrPphB. To explore whether the truncated TN proteins function in CNL-mediated immune signaling, we examined the interactions between the Arabidopsis TN proteins and RPS5, and found that TN13 and TN21 interacted with RPS5. However, only TN13, but not TN21, was involved in the resistance to P. syringae pv. tomato (Pto) strain DC3000 carrying avrPphB, encoding the cognate effector recognized by RPS5. Moreover, the regulation of Pto DC3000 avrPphB resistance by TN13 appeared to be specific, as loss of function of TN13 did not compromise resistance to Pto DC3000 hrcC^- or Pto DC3000 avrRpt2. In addition, we demonstrated that the CC and NBS domains of RPS5 play essential roles in the interaction between TN13 and RPS5. Taken together, our results uncover a direct functional link between TN13 and RPS5, suggesting that TN13 acts as a partner in modulating RPS5-activated immune signaling, which constitutes a previously unknown mechanism for TN-mediated regulation of plant immunity.

A phylogenetic study of the members of the MAPK and MEK families across Viridiplantae

Protein phosphorylation is regulated by the activity of enzymes generically known as kinases. One of those kinases is Mitogen-Activated Protein Kinases (MAPK), which operate through a phosphorylation cascade conformed by members from three related protein kinase families namely MAPK kinase kinase (MEKK), MAPK kinase (MEK), and MAPK; these three acts hierarchically. Establishing the evolution of these proteins in the plant kingdom is an interesting but complicated task because the current MAPK, MAPKK, and MAPKKK subfamilies arose from duplications and subsequent sub-functionalization during the early stage of the emergence of Viridiplantae. Here, an in silico genomic analysis was performed on 18 different plant species, which resulted in the identification of 96 genes not previously annotated as components of the MAPK (70) and MEK (26) families. Interestingly, a deeper analysis of the sequences encoded by such genes revealed the existence of putative domains not previously described as signatures of MAPK and MEK kinases. Additionally, our analysis also suggests the presence of conserved activation motifs besides the canonical TEY and TDY domains, which characterize the MAPK family.

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

Mitogen-activated protein kinase (MAPK) cascades regulate a myriad of plant biological processes, including disease resistance. Plant genomes encode a large number of MAPK kinase kinases (MAPKKKs) that can be divided into two subfamilies, namely MEKK-like kinases and Raf-like kinases. Thus far, the functions of MEKK-like MAPKKKs have been relatively well characterized, but the roles of Raf-like MAPKKKs in plant MAPK cascades remain less understood. Here, we report the role of OsEDR1, a Raf-like MAPKKK, in the regulation of the MAPK cascade in rice response to the bacterial pathogen Xanthomonas oryzae pv. oryzicola (Xoc). We found that OsEDR1 inhibits OsMPKK10.2 (a MAPK kinase) activity through physical interaction. Upon Xoc infection, OsMPKK10.2 is phosphorylated at S304 to activate OsMPK6 (a MAPK). Interestingly, activated OsMPK6 phosphorylates OsEDR1 at S861, which destabilizes OsEDR1 and thus releases the inhibition of OsMPKK10.2, leading to increased OsMPKK10.2 activity and enhanced resistance of rice plants to Xoc. Taken together, these results provide new insights into the functions of Raf-like kinases in the regulation of the MAPK cascade in plant immunity.

Exogenous phytohormone application and transcriptome analysis of Mikania micrantha provides insights for a potential control strategy

Effective and complete control of the invasive weed Mikania micrantha is required to avoid increasing damages. We exogenously applied indole 3-acetic acid (IAA), gibberellin (GA), and N-(2-Chloro-4-pyridyl)-N'-phenylurea (CPPU), and their combinations i.e. IAA + CPPU (IC), GA + CPPU (GC), and GA + IAA + CPPU (GIC), at 5, 10, 25, 50, and 75 ppm against distilled water as a control (CK), to examine their effects on the weed. The increasing concentrations of these hormones when applied alone or in combination were fatal to M. micrantha and led towards the death of inflorescences and/or florets. CPPU and GIC were found as the most effective phytohormones. Transcriptome analysis revealed differential regulation of genes in auxin, cytokinin, gibberellin and abscisic acid signaling pathways, suggesting their role in the prohibition of axillary bud differentiation. Collectively, CPPU and GIC at a high concentration (75 ppm) could be used as a control measure to protect forests and other lands from the invasion of M. micrantha.

Update on the Roles of Rice MAPK Cascades

The mitogen-activated protein kinase (MAPK) cascades have been validated playing critical roles in diverse aspects of plant biology, from growth and developmental regulation, biotic and abiotic stress responses, to phytohormone signal transduction or responses. A classical MAPK cascade consists of a MAPK kinase kinase (MAPKKK), a MAPK kinase (MAPKK), and a MAPK. From the 75 MAPKKKs, eight MAPKKs, and 15 MAPKs of rice, a number of them have been functionally deciphered. Here, we update recent advances in knowledge of the roles of rice MAPK cascades, including their components and complicated action modes, their diversified functions controlling rice growth and developmental responses, coordinating resistance to biotic and abiotic stress, and conducting phytohormone signal transduction. Moreover, we summarize several complete MAPK cascades that harbor OsMAPKKK-OsMAPKK-OsMAPK, their interaction with different upstream components and their phosphorylation of diverse downstream substrates to fulfill their multiple roles. Furthermore, we state a comparison of networks of rice MAPK cascades from signal transduction crosstalk to the precise selection of downstream substrates. Additionally, we discuss putative concerns for elucidating the underlying molecular mechanisms and molecular functions of rice MAPK cascades in the future.

Arabidopsis E3 ligase KEG associates with and ubiquitinates MKK4 and MKK5 to regulate plant immunity

Mitogen-activated protein kinase (MAPK) cascades are highly conserved signaling modules that regulate plant immune responses. The Arabidopsis thaliana Raf-like MAPK kinase kinase ENHANCED DISEASE RESISTANCE1 (EDR1) is a key negative regulator of plant immunity that affects the protein levels of MKK4 and MKK5, two important MAPK cascade members, but the underlying mechanism is poorly understood. Here, genome-wide phosphorylation analysis demonstrated that the E3 ligase KEEP ON GOING (KEG) is phosphorylated in the edr1 mutant but not the wild type, suggesting that EDR1 negatively affects KEG phosphorylation. The identified phosphorylation sites in KEG appear to be important for its accumulation. The keg-4 mutant, a previously identified edr1 suppressor, enhances susceptibility to the powdery mildew pathogen Golovinomyces cichoracearum. In addition, MKK4 and MKK5 protein levels are reduced in the keg-4 mutant. Furthermore, we demonstrate that MKK4 and MKK5 associate with full-length KEG, but not with truncated KEG-RK or KEG-RKA, and that KEG ubiquitinates and mediates the degradation of MKK4 and MKK5. Taken together, these results indicate that MKK4 and MKK5 protein levels are regulated by KEG via ubiquitination, uncovering a mechanism by which plants fine-tune immune responses by regulating the homeostasis of key MAPK cascade members via ubiquitination and degradation.

Genome-wide analysis of mitogen-activated protein (MAP) kinase gene family expression in response to biotic and abiotic stresses in sugarcane

To systematically analyze mitogen-activated protein (MAP) kinase gene families and their expression profiles in sugarcane (Saccharum spp. hybrids; Sh) under diverse biotic and abiotic stresses, we identified 15 ShMAPKs, 6 ShMAPKKs and 16 ShMAPKKKs genes in the sugarcane cultivar R570 genome. These were also confirmed in one S. spontaneum genome and two transcriptome datasets of sugarcane trigged by Acidovorax avenae subsp. avenae (Aaa) and Xanthomonas albilineans (Xa) infections. Phylogenetic analysis revealed that four subgroups were present in each ShMAPK and ShMAPKK family and three sub-families (RAF, MEKK and ZIK) presented in the ShMAPKKK family. Conserved protein motif and gene structure analyses supported the evolutionary relationships of the three families inferred from the phylogenetic analysis. All of the ShMAPK, ShMAPKK and ShMAPKKK genes identified in Saccharum spp. R570 were distributed on chromosomes 1-7 and 9-10. RNA-seq and qRT-PCR analyses indicated that ShMAPK07 and ShMAPKKK02 were defense-responsive genes in sugarcane challenged by both Aaa and Xa stimuli, while some genes were upregulated specifically by Aaa and Xa infection. Additionally, ShMAPK05 acted as a negative regulator under drought and salinity stress, but served as a positive regulator under salicylic acid (SA) treatment. ShMAPK07 plays a positive role under drought stress, but a negative role under SA treatment. ShMAPKKK01 was negatively modulated by both salinity stress and SA treatment, whereas ShMAPKKK06 was positively regulated by both of the two stress stimuli. Our results suggest that members of MAPK cascade gene families regulate adverse stress responses through multiple signal transduction pathways in sugarcane.

An Aphid-Secreted Salivary Protease Activates Plant Defense in Phloem

Recent studies have reported that aphids facilitate their colonization of host plants by secreting salivary proteins into host tissues during their initial probing and feeding. Some of these salivary proteins elicit plant defenses, but the molecular and biochemical mechanisms that underlie the activation of phloem-localized resistance remain poorly understood. The aphid Myzus persicae, which is a generalized phloem-sucking pest, encompasses a number of lineages that are associated with and adapted to specific host plant species. The current study found that a cysteine protease Cathepsin B3 (CathB3), and the associated gene CathB3, was upregulated in the salivary glands and saliva of aphids from a non-tobacco-adapted (NTA) aphid lineage, when compared to those of a tobacco-adapted lineage. Furthermore, the knockdown of CathB3 improved the performance of NTA lineages on tobacco, and the propeptide domain of CathB3 was found to bind to tobacco cytoplasmic kinase ENHANCED DISEASE RESISTANCE 1-like (EDR1-like), which triggers the accumulation of reactive oxygen species in tobacco phloem, thereby suppressing both phloem feeding and colonization by NTA lineages. These findings reveal a novel function for a cathepsin-type protease in aphid saliva that elicits effective host plant defenses and warranted the theory of host specialization for generalist aphids.

RGF1-RGI1, a Peptide-Receptor Complex, Regulates Arabidopsis Root Meristem Development via a MAPK Signaling Cascade

Root growth is maintained by the continuous division of cells in the apical meristem. ROOT MERISTEM GROWTH FACTOR 1 (RGF1) is a critical peptide hormone regulating root stem cell niche maintenance. Previous studies discovered that five closely related leucine-rich repeat receptor-like protein kinases (LRR-RLKs), named RGF1 INSENSITIVES (RGIs) or RGF1 RECEPTORS (RGFRs), are able to perceive the RGF1 signal and redundantly control root stem cell niche maintenance. RGF1 regulates root meristem activity mainly via two downstream transcription factors, PLETHORA 1 (PLT1) and PLT2. Regulatory proteins connecting cell surface RGF1-RGI1 and nuclear PLTs, however, were not identified. Here, we report that the mitogen-activated protein (MAP) kinase kinase 4 (MKK4) and MAP kinase 3 (MPK3) were co-immunoprecipitated with RGI1-FLAG after Arabidopsis seedlings were treated with RGF1. Genetic and biochemical assays confirmed that MKK4 and MKK5, and their downstream targets MPK3 and MPK6, are essential RGI-dependent regulators of root meristem development. In addition, we found that the MKK4/MKK5-MPK3/MPK6 module functions downstream of YDA, a MAPKKK. Our results demonstrate that RGF1-RGI1 regulate the expression of PLT1/PLT2 via a YDA-MKK4/MKK5-MPK3/MPK6 signaling cascade.

The YDA-MKK4/MKK5-MPK3/MPK6 Cascade Functions Downstream of the RGF1-RGI Ligand-Receptor Pair in Regulating Mitotic Activity in Root Apical Meristem

The mitotic activity of root apical meristem (RAM) is critical to primary root growth and development. Previous studies have identified the roles of ROOT GROWTH FACTOR 1 (RGF1), a peptide ligand, and its receptors, RGF1 INSENSITIVEs (RGIs), a clade of five leucine-rich-repeat receptor-like kinases, in promoting cell division in the RAM, which determines the primary root length. However, the downstream signaling components remain elusive. In this study, we identify a complete mitogen-activated protein kinase (MAPK or MPK) cascade, composed of YDA, MKK4/MKK5, and MPK3/MPK6, that functions downstream of the RGF1-RGI ligand-receptor pair. Similar to the rgi1/2/3/4/5 quintuple mutant, loss-of-function mutants of MPK3 and MPK6, MKK4 and MKK5, or YDA show a short-root phenotype, which is associated with reduced mitotic activity and lower expression of PLETHORA 1 (PLT1)/PLT2 in the RAM. Furthermore, MPK3/MPK6 activation in response to exogenous RGF1 treatment is impaired in the rgi1/2/3/4/5 quintuple, yda single, and mkk4 mkk5 double mutants. Epistatic analyses demonstrated that the expression of constitutively active MKK4, MKK5, or YDA driven by the RGI2 promoter can rescue the short-root phenotype of the rgi1/2/3/4/5 mutant. Taken together, these results suggest that the YDA-MKK4/MKK5-MPK3/MPK6 cascade functions downstream of the RGF1-RGI ligand-receptor pair and upstream of PLT1/PLT2 to modulate the stem cell population and primary root growth in Arabidopsis.

Co-regulation of indole glucosinolates and camalexin biosynthesis by CPK5/CPK6 and MPK3/MPK6 signaling pathways

Secondary plant metabolites, represented by indole glucosinolates (IGS) and camalexin, play important roles in Arabidopsis immunity. Previously, we demonstrated the importance of MPK3 and MPK6, two closely related MAPKs, in regulating Botrytis cinerea (Bc)-induced IGS and camalexin biosynthesis. Here we report that CPK5 and CPK6, two redundant calcium-dependent protein kinases (CPKs), are also involved in regulating the biosynthesis of these secondary metabolites. The loss-of-function of both CPK5 and CPK6 compromises plant resistance to Bc. Expression profiling of CPK5-VK transgenic plants, in which a truncated constitutively active CPK5 is driven by a steroid-inducible promoter, revealed that biosynthetic genes of both IGS and camalexin pathways are coordinately upregulated after the induction of CPK5-VK, leading to high-level accumulation of camalexin and 4-methoxyindole-3-yl-methylglucosinolate (4MI3G). Induction of camalexin and 4MI3G, as well as the genes in their biosynthesis pathways, is greatly compromised in cpk5 cpk6 mutant in response to Bc. In a conditional cpk5 cpk6 mpk3 mpk6 quadruple mutant, Bc resistance and induction of IGS and camalexin are further reduced in comparison to either cpk5 cpk6 or conditional mpk3 mpk6 double mutant, suggesting that both CPK5/CPK6 and MPK3/MPK6 signaling pathways contribute to promote the biosynthesis of 4MI3G and camalexin in defense against Bc.

Receptor-Like Protein Kinases Function Upstream of MAPKs in Regulating Plant Development

Mitogen-activated protein kinases (MAPKs) are a group of protein kinase broadly involved in various signal pathways in eukaryotes. In plants, MAPK cascades regulate growth, development, stress responses and immunity by perceiving signals from the upstream regulators and transmitting the phosphorylation signals to the downstream signaling components. To reveal the interactions between MAPK cascades and their upstream regulators is important for understanding the functional mechanisms of MAPKs in the life span of higher plants. Typical receptor-like protein kinases (RLKs) are plasma membrane-located to perceive endogenous or exogenous signal molecules in regulating plant growth, development and immunity. MAPK cascades bridge the extracellular signals and intracellular transcription factors in many RLK-mediated signaling pathways. This review focuses on the current findings that RLKs regulate plant development through MAPK cascades and discusses questions that are worth investigating in the near future.