Salicylic acid and broad spectrum of NBS-LRR family genes are involved in SMV-soybean interactions.

Soybean mosaic virus (SMV) is a severe pathogen reducing crop yield and seed quality of soybean. Although several resistance gene loci including Rsv1, Rsv3 and Rsv4 are identified in some soybean varieties, most of the soybean genes related to SMV infection are still not characterized. In order to reveal genome-wide gene expression profiles in response to SMV infection, we used transcriptome analysis to determine SMV-responsive genes in susceptible variety Hefeng25. Time course RNA-seq analysis at 1, 5 and 10 dpi identified many deregulated pathways and gene families. "Plant-pathogen interaction" pathway with KEGG No. of KO04626 was highly enriched and dozens of NBS-LRR family genes were significantly down-regulated at 5 dpi. qRT-PCR analyses were performed to verify expression patterns of these genes and most were in accordance with the RNA-seq data. As NBS-LRR family proteins are broadly involved in plant immunity responses, our results indicated the importance of this time point (5 dpi) for SMV-soybean interaction. Consistent with it, SMV titer was increased from 1 dpi to 10 dpi and peaked at 5 dpi. Expression of SA (salicylic acid) marker gene PR-1 was induced by SMV infection. Application of exogenous MeSA, an active form of SA, primed the plant resistant to virus infection and reduced SMV accumulation in soybean. Interestingly, MeSA treatment also significantly upregulated expressions of SMV-responsive NBS-LRR genes. Compared with susceptible line Hefeng25, endogenous SA level was higher and was consistently induced by SMV infection in resistant variety RV8143. Moreover, expressions of NBS-LRR family genes were up-regulated by SMV infection in RV8143, while they were down-regulated by SMV infection in Hefeng25. Our results implied that SA and NBS-LRR family genes were involved in SMV-soybean interaction. SMV could compromise soybean defense responses by repression of NBS-LRR family genes in Hefeng25, and SA was implicated in this interaction process.

Down-regulation of genes coding for core RNAi components and disease resistance proteins via corresponding microRNAs might be correlated with successful Soybean mosaic virus infection in soybean.

Plants protect themselves from virus infections by several different defence mechanisms. RNA interference (RNAi) is one prominent antiviral mechanism, which requires the participation of AGO (Argonaute) and Dicer/DCL (Dicer-like) proteins. Effector-triggered immunity (ETI) is an antiviral mechanism mediated by resistance (R) genes, most of which encode nucleotide-binding site-leucine-rich repeat (NBS-LRR) family proteins. MicroRNAs (miRNAs) play important regulatory roles in plants, including the regulation of host defences. Soybean mosaic virus (SMV) is the most common virus in soybean and, in this work, we identified dozens of SMV-responsive miRNAs by microarray analysis in an SMV-susceptible soybean line. Amongst the up-regulated miRNAs, miR168a, miR403a, miR162b and miR1515a predictively regulate the expression of AGO1, AGO2, DCL1 and DCL2, respectively, and miR1507a, miR1507c and miR482a putatively regulate the expression of several NBS-LRR family disease resistance genes. The regulation of target gene expression by these seven miRNAs was validated by both transient expression assays and RNA ligase-mediated rapid amplification of cDNA ends (RLM-RACE) experiments. Transcript levels for AGO1, DCL1, DCL2 and five NBS-LRR family genes were repressed at different time points after SMV infection, whereas the corresponding miRNA levels were up-regulated at these same time points. Furthermore, inhibition of miR1507a, miR1507c, miR482a, miR168a and miR1515a by short tandem target mimic (STTM) technology compromised SMV infection efficiency in soybean. Our results imply that SMV can counteract soybean defence responses by the down-regulation of several RNAi pathway genes and NBS-LRR family resistance genes via the induction of the accumulation of their corresponding miRNA levels.