Transcriptomic Analysis Of The Functions Of BR,JA,and SA Involved In Maize Chlorotic Mottle Virus Infection

Maize(Zea mays L.)is one of the most widely cultivated crops in the world.However,maize lethal necrosis(MLN)has brought a devastating blow to maize production in recent years.As the essential pathogen of MLN,maize chlorotic mottle virus(MCMV)infection can cause chlorotic mottle and necrosis on leaves,which is a major potential threat to the development of the global maize industry.It is important to screening hormone pathways that play key roles in the interaction between maize and MCMV and identifying the role of these hormone pathways in the infection of MCMV,which can provide theoretical basis for understanding the pathogenic mechanism of MCMV and proposing antiviral strategies.To fully understand the immune responses of maize during MCMV infection,high throughput sequencing technology was used to compare the differences of gene expression between mock-inoculated(control group,CK)and MCMV-inoculated maize plants(experimental group,MCMV).We obtained about 9.5 G of clean reads from experimental and control samples,according to transcriptome sequencing,respectively.With the restrictive conditions,206 differentially expressed genes(DEGs)were identified significantly changed from 21,939 genes,86 of which were upregulated and 120 were downregulated in MCMV-inoculated group compared with that of mock-inoculated group.To better understand the function of DEGs and their associated metabolism pathways,GO functional enrichment analysis and KEGG pathway enrichment analysis were conducted.107 of the 806 GO terms were significantly enriched with p-value ? 0.05.KEGG pathway enrichment analysis of these DEGs resulted in 8 pathways with significant enrichment.The transcriptome data showed that brassinosteroid(BR)-associated genes were upregulated after MCMV infection.Exogenous 2,4-epibrassinolide(BL)or brassinazole(BRZ)applications indicated that BR pathway was involved in the susceptibility to MCMV infection.In addition,treatment of BR on maize induced the accumulation of nitric oxide(NO),and the changes of NO content played positive roles in the disease incidence of MCMV.Moreover,MCMV infection was delayed when the BL-treated plants were applied with NO scavenger,which suggested that BR induced the susceptibility of maize to MCMV infection in a NO-dependent manner.Taken together,our results demonstrate that BR pathway promotes the susceptibility of maize to MCMV in a NO-dependent manner.Compared with that of mock-inoculated plants,the expression of jasmonic acid(JA)synthesisrelated genes in SL1 of MCMV-inoculated plants was up-regulated according to transcriptome data.The results of RT-qPCR and high performance liquid chromatography(HPLC)also showed that the expression level of JA synthesis genes were up-regulated and JA content was increased in SL1 of MCMV-infected maize.In addition,we also found that exogenous JA application on leaves increased maize resistance to MCMV.Similarly,our results also showed that the expression level of salicylic acid(SA)synthesisrelated genes in SL1 of MCMV-inoculated plants was significantly higher than that of mock-inoculated plants.What's more,the content of SA in SL1 of MCMV-inoculated plants was higher than that of mockinoculated plants,and the resistance to MCMV was increased by foliar spraying of exogenous SA.Notably,when JA and SA were applied simultaneously,the induction degree of maize plants to MCMV resistance was weaker than that of single application of JA or SA,which indicated that JA and SA had obvious antagonism to MCMV resistance in maize.In addition,we also found that the expression of marker gene of SA pathway ZmPR1 a and SA synthesis gene family ZmPALs increased significantly after foliar spraying BL,which means BL might induce the activation of SA pathway during MCMV infection.Considering the responses of BR,JA and SA to MCMV infection and their influences on MCMV accumulation,we proposed a model: BR accumulation in maize leaves was induced after MCMV infection.On one hand,the increase of BR concentration promoted the accumulation of MCMV.On the other hand,high-level BR activated the antiviral hormone signal SA in plants.SA began to restrict viral replication when accumulating to a certain threshold.In this process,NO might play a key role.In addition,MCMV infection also induced JA accumulation in maize leaves,which could also induce maize resistance to MCMV like SA.However,there was obvious antagonism between SA and JA.Thses complex hormonal systems maintain the balance of plants and viruses,which also explains from another perspective that MCMV is a compatible pathogen of maize and MCMV infection alone only causes mild symptoms.This study provides a basis for the hormonal analysis of the pathogenesis of MCMV.