Rice MYC2 (OsMYC2) modulates light-dependent seedling phenotype, disease defence but not ABA signalling

The concerted function of a novel class of transcription factors, ZBFs, in light, jasmonate, and abscisic acid signaling pathways

Several classes of transcription factors have been investigated in light signaling pathways that bind to the light-responsive elements (LREs) present in the promoters of light regulatory genes for transcriptional regulation. Some of these transcription factors have been shown to bind to numerous promoters through genome-wide ChIP-on-chip (ChIP-chip) studies. Furthermore, through the integration of ChIP-seq and RNA-seq techniques, it has been demonstrated that a transcription factor modifies the expression of numerous genes with which it interacts. However, the mode of action of these transcription factors and their dependency on other regulators in the pathway has just started to be unraveled. In this review, we focus on a particular class of transcription factors, ZBFs (Z-box-binding factors), and their associated partners within the same or other classes of transcription factors and regulatory proteins during photomorphogenesis. Moreover, we have further made an attempt to summarize the crosstalk of these transcription factors with jasmonic acid-, abscisic acid-, and salicylic acid-mediated defense signaling pathways. This review offers an in-depth insight into the manner in which ZBFs and their interactors reshape cellular functions and plant behavior. The underlying principles not only contribute to a comprehensive understanding but also establish a framework for analyzing the interplay between early developmental events and hormone signaling, a regulation orchestrated by the ZBF family.

TkJAZs-TkMYC2-TkSRPP/REF Regulates the Biosynthesis of Natural Rubber in Taraxacum kok-saghyz

Taraxacum kok-saghyz (TKS) is a natural rubber (NR)-producing plant and a model plant for studying the biosynthesis of NR. Analyzing and studying the biosynthetic mechanism of NR is an important way to cultivate high-yield rubber TKS varieties. JAZ proteins, which belong to the Jasmonate ZIM domain family, function as negative regulators in the jasmonic acid (JA) signal transduction pathway. MYC2 is typically regarded as a regulatory factor for the target genes of JAZ proteins; JAZ proteins indirectly influence the gene expression regulated by MYC2 by modulating its activity. Theoretically, JAZ is expected to participate in growth, development, and responses to environmental cues related to rubber and biomass accumulation in TKS, all of which rely on the interaction between JAZ and MYC2. In this study, we identified 11 TkJAZs through homology searching of the TKS genomes and bioinformatics analyses. Subcellular localization, Y2H, and BiFC analysis demonstrate that TkJAZs and TkMYC2 are localized in the nucleus, with all TkJAZs and TkMYC2 showing nuclear colocalization interactions. Overexpression of TkMYC2 in TKS inhibited leaf development, promoted root growth, and simultaneously increased NR production. RNA-seq and qRT-PCR analysis revealed that the TkSRPP/REF genes exhibit varying degrees of upregulation compared to the wild type, upregulating the TkREF1 gene by 3.7-fold, suggesting that TkMYC2 regulates the synthesis of NR by modulating the TkSRPP/REF genes.

MKKK20 works as an upstream triple-kinase of MKK3-MPK6-MYC2 module in Arabidopsis seedling development

The mitogen-activated protein kinase (MAPK) cascade is involved in several signal transduction processes in eukaryotes. Here, we report a mechanistic function of MAP kinase kinase kinase 20 (MKKK20) in light signal transduction pathways. We show that MKKK20 acts as a negative regulator of photomorphogenic growth at various wavelengths of light. MKKK20 not only regulates the expression of light signaling pathway regulatory genes but also gets regulated by the same pathway genes. The atmyc2 mkkk20 double mutant analysis shows that MYC2 works downstream to MKKK20 in the regulation of photomorphogenic growth. MYC2 directly binds to the promoter of MKKK20 to modulate its expression. The protein-protein interaction study indicates that MKKK20 physically interacts with MYC2, and this interaction likely suppresses the MYC2-mediated promotion of MKKK20 expression. Further, the protein phosphorylation studies demonstrate that MKKK20 works as the upstream kinase of MKK3-MPK6-MYC2 module in photomorphogenesis.

Genome-wide analysis of basic helix-loop-helix genes in Dendrobium catenatum and functional characterization of DcMYC2 in jasmonate-mediated immunity to Sclerotium delphinii

Dendrobium catenatum, belonging to the Orchidaceae, is a precious Chinese herbal medicine. Sclerotium delphinii (P1) is a broad-spectrum fungal disease, which causes widespread loss in the near-wild cultivation of D. catenatum. Thus, resistance breeding of D. catenatum has become the key to solve this problem. The basic helix-loop-helix (bHLH) gene family is closely related to plant resistance to external stresses, but the related research in D. catenatum is not deep enough yet. Phylogenetic analysis showed that 108 DcbHLH genes could be divided into 23 subgroups. Promoter cis-acting elements revealed that DcbHLHs contain a large number of stress-related cis-acting elements. Transcriptome analysis of MeJA and P1 treatment manifested that exogenous MeJA can change the expression pattern of most bHLH genes, especially the IIIe subgroup, including inhibiting the expression of DcbHLH026 (MYC2a) and promoting the expression of DcbHLH027 (MYC2b). Subcellular localization indicated that they were located in the nucleus. Furthermore, exogenous MeJA treatment significantly delayed disease time and reduced lesion size after infection with P1. DcMYC2b-overexpression Arabidopsis lines showed significantly smaller lesions after being infected with P1 than the wild type, indicating that DcMYC2b functions as an important positive regulator in D. catenatum defense against P1. Our findings shed more insights into the critical role of the DcbHLH family in plants and the resistance breeding of D. catenatum.

Key Genes in the JAZ Signaling Pathway Are Up-Regulated Faster and More Abundantly in Caterpillar-Resistant Maize

Jasmonic acid (JA) and its derivatives, collectively known as jasmonates (JAs), are important signaling hormones for plant responses against chewing herbivores. In JA signaling networks, jasmonate ZIM-domain (JAZ) proteins are transcriptional repressors that regulate JA-modulated downstream herbivore defenses. JAZ repressors are widely presented in land plants, however, there is only limited information about the regulation/function of JAZ proteins in maize. In this study, we performed a comprehensive expression analysis of ZmJAZ genes with other selected genes in the jasmonate pathway in response to feeding by fall armyworm (Spodoptera frugiperda, FAW), mechanical wounding, and exogenous hormone treatments in two maize genotypes differing in FAW resistance. Results showed that transcript levels of JAZ genes and several key genes in JA-signaling and biosynthesis pathways were rapidly and abundantly expressed in both genotypes in response to these various treatments. However, there were key differences between the two genotypes in the expression of ZmJAZ1 and ZmCOI1a, these two genes were expressed significantly rapidly and abundantly in the resistant line which was tightly regulated by endogenous JA level upon feeding. For instance, transcript levels of ZmJAZ1 increase dramatically within 30 min of FAW-fed Mp708 but not Tx601, correlating with the JA accumulation. The results also demonstrated that wounding or JA treatment alone was not as effective as FAW feeding; this suggests that insect-derived factors are required for optimal defense responses.

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.

CRISPR/Cas9-Mediated SlMYC2 Mutagenesis Adverse to Tomato Plant Growth and MeJA-Induced Fruit Resistance to Botrytis cinerea

Methyl jasmonate (MeJA), a natural phytohormone, played a critical role not only in plant growth but also in plant defense response to biotic and abiotic stresses. MYC2, a basic helix-loop-helix transcription factor, is a master regulator in MeJA signaling pathway. In the present work, slmyc2 mutants were generated by the clustered regularly interspaced short palindromic repeats and associated Cas9 protein (CRISPR/Cas9) system to investigate the role of SlMYC2 in tomato plant growth and fruit disease resistance induced by exogenous MeJA. The results showed that slmyc2 mutants possessed a higher number of flowers and a lower fruit setting rate in comparison with wild-type plants. In addition, the fruit shape of slmyc2 mutant was prolate, while the control fruits were oblate. Knockout of SlMYC2 significantly decreased the activities of disease defensive and antioxidant enzymes, as well as the expression levels of pathogen-related (PR) genes (SlPR-1 and SlPR-STH2) and the key genes related to jasmonic acid (JA) biosynthesis and signaling pathway including allene oxide cyclase (SlAOC), lipoxygenase D (SlLOXD), SlMYC2, and coronatine insensitive 1 (SlCOI1), and consequently aggravated the disease symptoms. By contrast, the disease symptoms were largely reduced in MeJA-treated fruit that possessed higher activities of these enzymes and expression levels of genes. However, the induction effects of MeJA on fruit disease resistance and these enzymes' activities and genes' expressions were significantly attenuated by knockout of SlMYC2. Therefore, the results indicated that SlMYC2 played positive regulatory roles not only in the growth of tomato plants but also in MeJA-induced disease resistance and the antioxidant process in tomato fruits.

Jasmonates-the Master Regulator of Rice Development, Adaptation and Defense

Rice is one of the most important food crops worldwide, as well as the model plant in molecular studies on the cereals group. Many different biotic and abiotic agents often limit rice production and threaten food security. Understanding the molecular mechanism, by which the rice plant reacts and resists these constraints, is the key to improving rice production to meet the demand of an increasing population. The phytohormone jasmonic acid (JA) and related compounds, collectively called jasmonates, are key regulators in plant growth and development. They are also one of the central players in plant immunity against biotic attacks and adaptation to unfavorable environmental conditions. Here, we review the most recent knowledge about jasmonates signaling in the rice crop model. We highlight the functions of jasmonates signaling in many adaptive responses, and also in rice growth and development processes. We also draw special attention to different signaling modules that are controlled by jasmonates in rice.