Functional Genomics of Salt and Drought Stress Tolerance in the Temperate Crop Apple (Malus domestica)

Apple (Malus domestica) is an important economic fruit crop of the temperate regions of world. Apple productivity is known to be affected by several biotic and abiotic stresses. Among these, water scarcity and soil salinity significantly impact the physiological and metabolic processes of apple, leading to economic losses. Apple plants employ intricate physiological responses to combat drought and salt stress which are orchestrated by diverse endogenous molecular regulatory mechanisms. Modern ‘-omics’ analyses have unraveled the roles of various transcription factors in restoring cellular homeostasis and alleviating the adverse effects of drought and salinity stress on apple plants. Important functions of various miRNAs have recently been studied in the post-transcriptional regulation of gene expression under both stresses. Several protein-mediated regulatory networks underlying drought and salt stress adaptation responses in apple have lately been deciphered. All these regulons ultimately induce the biosynthesis and accumulation of protective compounds for mitigating the negative effects of drought and salinity stress on apple growth. This review coherently highlights a bunch of candidate genes involved in regulating drought and salinity stress in apple and is an exemplification of our present understanding of how apple plants respond to these stresses. The functions of these genes can further be carefully exploited for developing apple varieties with anticipated levels of drought and salt stress tolerance.