Comparative DNA Methylome of Phytoplasma Associated Retrograde Metamorphosis in Sesame (Sesamum indicum L.)

SIMPLE SUMMARY: Sesame (Sesamum indicum L.) is an important oilseed crop that is well known for its highly nutritious oil content. Due to its high oil content and nutritional properties, sesame is also known as the ‘queen of oilseeds’. Phytoplasma-associated diseases such as phyllody and little leaf are critical threats to sesame cultivation worldwide. Sesame phyllody is the leading biotic constraint drastically affecting sesame productivity and resulting in yield losses of up to 80% in major sesame-producing countries in the world. In this study, we explored the role of DNA methylation in phytoplasma infection. Whole Genome Bisulfite Sequencing and a Quantitative analysis of DNA methylation using real-time PCR (qAMP) revealed an alteration of methylation pattern upon phytoplasma infection. Few selected development and defense-related loci were either hypo- or hypermethylated. We hereby report the first methylome study in healthy and phytoplasma-infected sesame. Our study provides fundamental insights into the role of DNA methylation in the symptom development of phytoplasma infection in sesame plants. ABSTRACT: Phytoplasma-associated diseases such as phyllody and little leaf are critical threats to sesame cultivation worldwide. The mechanism of the dramatic conversion of flowers to leafy structures leading to yield losses and the drastic reduction in leaf size due to Phytoplasma infection remains yet to be identified. Cytosine methylation profiles of healthy and infected sesame plants studied using Whole Genome Bisulfite Sequencing (WGBS) and Quantitative analysis of DNA methylation with the real-time PCR (qAMP) technique revealed altered DNA methylation patterns upon infection. Phyllody was associated with global cytosine hypomethylation, though predominantly in the CHH (where H = A, T or C) context. Interestingly, comparable cytosine methylation levels were observed between healthy and little leaf-affected plant samples in CG, CHG and CHH contexts. Among the different genomic fractions, the highest number of differentially methylated Cytosines was found in the intergenic regions, followed by promoter, exonic and intronic regions in decreasing order. Further, most of the differentially methylated genes were hypomethylated and were mainly associated with development and defense-related processes. Loci for STOREKEEPER protein-like, a DNA-binding protein and PP2-B15, an F-Box protein, responsible for plugging sieve plates to maintain turgor pressure within the sieve tubes were found to be hypomethylated by WGBS, which was confirmed by methylation-dependent restriction digestion and qPCR. Likewise, serine/threonine-protein phosphatase-7 homolog, a positive regulator of cryptochrome signaling involved in hypocotyl and cotyledon growth and probable O-methyltransferase 3 locus were determined to be hypermethylated. Phytoplasma infection-associated global differential methylation as well as the defense and development-related loci reported here for the first time significantly elucidate the mechanism of phytoplasma-associated disease development.