Review Genome-wide analysis of the apple PLD gene family and a functional characterization of MdPLD17 in drought tolerance

Apple (Malus domestica Borkh.) is one of the most economically important fruit crops and is widely cultivated globally. However, apple trees are subject to drought stress, which affects their growth, quality, and yield. Phospholipase D (PLD) is the main phospholipid hydrolase in plants and plays a critical role in the regulation of growth and stress resistance. In this study, we identified 17 PLD genes in the apple genome and found them to be unevenly distributed across 11 apple chromosomes. We also determined their phylogenetic relationships and chromosomal locations. Gene structure analysis showed that the number of introns in MdPLD genes varied from 2 to 19, suggesting highly variable functions. We identified multiple stress-related cis-elements in promoter regions 2 kb upstream of MdPLD genes, and the expression of most MdPLD genes was altered under drought stress based on RNA-seq data. Comprehensive qRT-PCR analysis showed that MdPLD17 was highly up-regulated in response to drought stress. When MdPLD17 was overexpressed in apple callus, the transgenic lines showed higher drought tolerance than the wild types (WT). MdPLD17-OE callus exhibited higher fresh weight, a lower accumulation of MDA, H2O2, O-2(-), and less electrolyte leakage, suggesting that MdPLD17 plays a positive regulatory role in drought tolerance in apple. These findings provide evidence that MdPLD family members play an important role in drought tolerance and MdPLD17 can potentially be used as a favorable target gene in genetic applications for improving drought tolerance in apple.