miRNAs are involved in regulating the formation of recovery tissues in virus infected Nicotiana tabacum

MiRNAs play an important role in regulating plant growth and immune response. Mosaic diseases are recognized as the most important plant diseases in the world, and mosaic symptoms are recovery tissues formed by plants against virus infection. However, the mechanism of the formation of mosaic symptoms remains elusive. In this study, two typical mosaic systems consisting of Nicotiana tabacum -cucumber mosaic virus (CMV) and N. tabacum -tobacco mosaic virus (TMV) were used to investigate the relevance of miRNAs to the appearance of mosaic symptoms. The results of miRNA-seq showed that there were significant differences in miRNA abundance between dark green tissues and chlorotic tissues in mosaic leaves caused by the infection of CMV or TMV. Compared with healthy tissues, miRNA expression was significantly increased in chlorotic tissues, but slightly increased in dark green tissues. Three miRNAs, namely miR1919, miR390a, and miR6157, were identified to be strongly up-regulated in chlorotic tissues of both mosaic systems. Results of overexpressing or silencing of the three miRNAs proved that they were related to chlorophyll synthesis, auxin response, and small GTPase-mediated immunity pathway, which were corresponding to the phenotype, physiological parameters and susceptibility of the chlorotic tissues in mosaic leaves. Besides, the newly identified novel-miRNA48, novel-miRNA96 and novel-miRNA103 may also be involved in this formation of mosaic symptoms. Taken together, our results demonstrated that miR1919, miR390a and miR6157 are involved in the formation of mosaic symptoms and plant antiviral responses, providing new insight into the role of miRNAs in the formation of recovery tissue and plant immunity.