Overexpression of watermelon m6A methyltransferase ClMTB enhances drought tolerance in tobacco by mitigating oxidative stress and photosynthesis inhibition and modulating stress-responsive gene expression

N6-methyladenosine (m6A) in RNA is a very important post-transcriptional modification mechanism in eukaryotes. It has been reported to have important regulatory roles in some stress responses in model plants, but there has been no research regarding m6A modifications in watermelon. In this study, we cloned and characterized m6A methyltransferase, ClMTB (mRNA adenosine methylase B, METTL14 human homolog protein) in watermelon. ClMTB expression could be weakly induced by drought stress as determined by the quantitative real-time PCR (qRT-PCR) and Promoter::GUS analyses. ClMTB over-expressed in tobacco plants increased drought tolerance via enhancing reactive oxygen species (ROS) scavenging system and alleviating photosynthesis inhibition under drought. Transcriptome profiles indicated the multiple hormone and stress-responsive genes were specifically induced in over-expressed ClMTB plants under drought conditions. These results suggest that ClMTB-mediated m6A modification serves as a positive regulatory factor of drought tolerance. This study is the first one to provide an understanding of the specific roles of ClMTB in watermelon adaptation to drought stress, and may also provide important insights into the signaling pathway mediated by m6A modification in response to stress conditions.