OsPRMT6a-mediated arginine methylation of OsJAZ1 regulates jasmonate signaling and spikelet development in rice

Although both protein arginine methylation(PRMT)and jasmonate(JA)signaling are crucial for regulating plant development,the relationship between these processes in the control of spikelet development remains unclear.In this study,we used the CRISPR/Cas9 technology to generate two OsPRMT6a loss-of-function mutants that exhibit various abnormal spikelet structures.Interestingly,we found that OsPRMT6a can methylate arginine residues in JA signal repressors OsJAZ1 and OsJAZ7.We showed that arginine methylation of OsJAZ1 enhances the binding affinity of OsJAZ1 with the JA receptors OsCOI1a and OsCOI1b in the presence of JAs,thereby promoting the ubiquitination of OsJAZ1 by the SCF^(OsCOI1a/OsCOI1b) complex and degradation via the 26S proteasome.This process ultimately releases OsMYC2,a core transcriptional regulator in the JA signaling pathway,to activate or repress JA-responsive genes,thereby maintaining normal plant(spikelet)development.However,in the osprmt6a-1 mutant,reduced arginine methylation of OsJAZ1 impaires the interaction between OsJAZ1 and OsCOI1a/OsCOI1b in the presence of JAs.As a result,OsJAZ1 proteins become more stable,repressing JA responses,thus causing the formation of abnormal spikelet structures.Moreover,we discovered that JA signaling reduces the OsPRMT6a mRNA level in an OsMYC2-dependent manner,thereby establishing a negative feedback loop to balance JA signaling.We further found that OsPRMT6a-mediated arginine methylation of OsJAZ1 likely serves as a switch to tune JA signaling to maintain normal spikelet development under harsh environmental conditions such as high temperatures.Collectively,our study establishes a direct molecular link between arginine methylation and JA signaling in rice.

Roles of Plant Hormones and Their Interplay in Rice Immunity

ABSTRACT Plant hormones have been extensively studied for their importance in innate immunity particularly in the dicotyledonous model plant Arabidopsis thaliana. However, only in the last decade, plant hormones were demonstrated to play conserved and divergent roles in fine-tuning immune responses in rice （Oryza sativa L.）, a monocotyledonous model crop plant. Emerging evidence showed that salicylic acid （SA） plays a role in rice basal defense but is differentially required by rice pattern recognition receptor （PRR） and resistance （R） protein-mediated immunity, and its function is likely dependent on the signaling pathway rather than the change of endogenous levels. Jasmonate （JA） plays an important role in rice basal defense against bacterial and fungal infection and may be involved in the SA-mediated resistance. Ethylene （ET） can act as a positive or negative modulator of disease resistance, depending on the pathogen type and environmental conditions. Brassinosteroid （BR） signaling and abscisic acid （ABA） either promote or defend against infection of pathogens with distinct infection/colonization strategies. Auxin and gibberellin （GA） are generally thought of as negative regulators of innate immunity in rice. Moreover, GA interacts antagonistically with JA signaling in rice development and immunity through the DELLA protein as a master regulator of the two hormone pathways. In this review, we summarize the roles of plant hormones in rice immunity and discuss their interplay/crosstalk mechanisms and the complex regulatory network of plant hormone pathways in fine-tuning rice immunity and growth.

Molecular Basis of Disease Resistance and Perspectives on Breeding Strategies for Resistance Improvement in Crops

Crop diseases are major factors responsible for substantial yield losses worldwide,which affects global food security.The use of resistance(R)genes is an effective and sustainable approach to controlling crop diseases.Here,we review recent advances on R gene studies in the major crops and related wild species.Current understanding of the molecular mechanisms underlying R gene activation and signaling,and susceptibility(S)gene-mediated resistance in crops are summarized and discussed.Furthermore,we propose some new strategies for R gene discovery,how to balance resistance and yield,and how to generate crops with broad-spectrum disease resistance.With the rapid development of new genome-editing technologies and the availability of increasing crop genome sequences,the goal of breeding next-generation crops with durable resistance to pathogens is achievable,and will be a key step toward increasing crop production in a sustainable way.

Rice Interploidy Crosses Disrupt Epigenetic Regulation, Gene Expression, and Seed Development

Seed development in angiosperms requires a 2：1 maternal-to-paternal genome ratio （2m：lp） in the endo- sperm. When the ratio is disrupted, the seed development is impaired. Rice interploidy crosses result in endosperm failures, but the underlying molecular mechanisms remain unclear. Here, we report that the defective endosperm in rice interploidy crosses was associated with nonadditive expression of small RNAs and protein-coding genes. Interestingly, 24-nt small interfering RNAs were enriched in the 5＇ and 3＇ flanking sequences of nonadditively expressed genes in the interploidy crosses and were negatively associated with the expression of imprinted genes. Furthermore, some PRC2 family genes and DNA methylaUon-related genes including OsMETlb and OsCMT3a were upregulated in the 2x4 cross （polli- nating a diploid ＂mother＂ with a tetraploid ＂father＂） but repressed in the reciprocal cross. These different epigenetic effects could lead to precocious or delayed cellularization during endosperm development. Notably, many endosperm-preferred genes, including starch metabolic and storage protein genes during grain filling, were found to be associated with DNA methylation or H3K27me3, which are repressed in both 2×4 and 4 ×2 crosses. WUSCHEL homeobox2 （WOX2）-Iike （WOX2L）, an endosperm-preferred gene, was expressed specifically in the rice endosperm, in contrast to WOX2 expression in the Arabidopsis embryo. Disruption of WOX2L in transgenic rice by CRISPR/Cas9-mediated gene editing blocked starch and protein accumulation, resulting in seed abortion. In addition to gene repression, disrupting epigenetic process in the interploidy crosses also induced expression of stress-responsive genes. Thus, maintaining the 2m：lp genome ratio in the endosperm is essential for normal grain development in rice and other cereal crops.

Altered Disease Development in the eui Mutants and Eui Overexpressors Indicates that Gibberellins Negatively Regulate Rice Basal Disease Resistance

Gibberellins （GAs） form a group of important plant tetracyclic diterpenoid hormones that are involved in many aspects of plant growth and development. Emerging evidence implicates that GAs also play roles in stress responses. However, the role of GAs in biotic stress is largely unknown. Here, we report that knockout or overexpression of the Elongated uppermost internode （Eu~ gene encoding a GA deactivating enzyme compromises or increases, respectively, disease resistance to bacterial blight （Xanthomonas oryzae pv. oyrzae） and rice blast （Magnaporthe oryzae）. Exogenous application of GA3 and the inhibitor of GA synthesis （uniconazol） could increase disease susceptibility and resistance, respectively, to bacterial blight. Similarly, uniconazol restored disease resistance of the eui mutant and GA3 decreased disease resistance of the Eui overexpressors to bacterial blight. Therefore, the change of resistance attributes to GA levels. In consistency with this, the GA metabolism genes OsGA2Oox2 and OsGA20xl were down-regulated during pathogen challenge. We also found that PRla induction was enhanced but the SA level was decreased in the Eui overexpressor, while the JA level was reduced in the eui mutant. Together, our current study indicates that GAs play a negative role in rice basal disease resistance, with EUI as a positive modulator through regulating the level of bioactive GAs.