Genome-wide identification of the pectin methylesterase (PME) gene family in watermelon (Citrullus lanatus) and the role in the regulation of fruit softening

The pectin methylesterase (PME) genes are key genes that regulate the hydrolysis and structural reorganization of pectin components in fruits and play indispensable roles in the structure and composition of plant cell walls. Therefore, elucidating the changes that occur in ClPME during the critical period of fruit development in watermelon is highly important for studying the regulatory mechanism of ClPME -mediated fruit softening in watermelon. In conjunction with the Arabidopsis thaliana PME protein structural domains, 72 members of the ClPME gene family were jointly identified from the whole genome of watermelon and analyzed for the physicochemical properties and phylogenetic relationships of their encoded proteins. Based on the transcriptome data, the expression patterns of ClPME in watermelon fruits at different developmental periods were analyzed. An enzyme activity assay revealed that the PME enzyme activities significantly differed among watermelons of different fruit hardness varieties. qRT‒PCR was used to analyze the relative expression of key ClPME genes during the critical period of fruit development. After transient silencing of the homologous genes of key ClPME genes in tomato, significant differential changes in tomato fruit hardness occurred. These genes were investigated for their possible role in watermelon fruit development. Moreover, the expression patterns of the key ClPME genes in response to different phytohormone sprays were different, suggesting that the mechanisms of watermelon fruit hardness changes in response to different phytohormone sprays. These findings provide a theoretical basis for elucidating the role and molecular genetic mechanism of the ClPME gene family in watermelon fruit hardness changes.