Study On The Role Of Tomato Domains Rearranged Methyltransferase7 In Regulating Leaf Senescence

Dynamic DNA methylation is an important epigenetic mark associated with leaf development and senescence.DNA methylation is catalyzed by DNA methyltransferases,however,how these DNA methyltransferases affect leaf development and senescence is largely unknown.Tomato is one of the most important economic vegetable crops worldwide.Elucidating the essential role of DNA methylation in regulating tomato leaf senescence can enhance our understanding of such mechanisms and has potentials for development of novel strategies to delay senescence,as well as improve tomato quality.In this study,via whole genome bisulfite sequence(WGBS)and RNA-seq of tomato wild-type(WT)and Sldrm7-RNAi transgenic lines,the function and the involved biological process of SlDRM7 in leaf senescence were studied.The main results are as follows:(1)The tomato domains rearranged methyltransferase SlDRM7 silencing(Sldrm7-RNAi)lines showed pleiotropic developmental defects including dwarf plant,small and interveinal chlorosis leaves.Meanwhile,we found the chlorotic phenotype segregated in T1 progeny,displaying approx.2:1 ratio of chlorotic to normal green leaves among viable seedlings,and the seedlings with chlorotic phenotype(named drm7i)continued to segregate in T2 to T8 generations,while seedlings with normal green leaves(named drm7i＿ns)showed no phenotypic separation in T2 to T8.Via comparative analysis of physiological capacity and gene expression,we found that SlDRM7 silencing resulted in reduced chlorophyll content and photosynthetic capacity,as well as significantly enhanced expression of senescence related genes,suggesting that SlDRM7 might be an important epigenetic modification factor in mediating leaf senescence in tomato.Moreover,SlDRM7 was activated by natural aging-and dark-induced senescence,suggesting that SlDRM7 might act as a negative epigenetic regulator to inhibit leaf senescence.(2)WGBS analysis showed that compared with drm7i＿ns,drm7i had no significant effect on genome-wide methylation,but a total of 16,418 differential methylation regions(DMRs)were detected by comparative methylation analysis,associating with thousands of genes whose promoters or introns were differentially methylated.It is suggested that SlDRM7 silencing led to epigenetic changes in leaf genome,which may play an important role in leaf senescence.Combined analyses of WGBS and RNA-seq revealed that silencing of SlDRM7 caused alterations in both methylation levels and transcript levels of 289 genes,which are directly or indirectly involved in photosynthesis,chlorophyll synthesis,and starch degradation.The photosynthetic capacity decreased in drm7i,which is consistent with the reduced chlorophyll content and repression of genes involved in chlorophyll biosynthesis,photosystem,and photosynthesis.Furthermore,starch granules were highly accumulated in chloroplasts of drm7i and associated with repressed expression of genes in the starch degradation pathway.Collectively,these results demonstrate that SlDRM7 acts as an epi-regulator to modulate the expression of genes related to starch and chlorophyll metabolism,thereby affecting leaf chlorosis and senescence in tomatoes.Meanwhile,a self-feedback regulatory pathway is established by activating SlDRM7 expression to make a balance between vegetative growth and senescence.