Comparative Transcriptomic Profiling Reveals Divergent Drought-Response Mechanisms Between Resistant and Susceptible Apple Genotype Roots

Drought stress caused a significant threat to apple growth and production. Although there is an increasing studies concerning the molecular mechanism in apple response to drought, most of these studies focus on the leaves. However, the roots is the first organism to sense the drought signal and play important role in drought response. The molecular mechanisms underlying the apple roots in response to drought needs to be further explored. In this study, we conducted a comparative transcriptomic analysis of roots from drought-resistant (Malus prunifolia) and drought-susceptible (Malus hupehensis) apple genotypes under different soil water contents with 60% (control), 30% (mild drought), 10% (moderate drought), and 5% (severe drought). The further Mapman pathways analysis showed that Malus prunifolia exhibited more rapid activation of abscisic acid (ABA) biosynthesis (NCED, PYL) and signaling pathway, as well as the induction of transcription factors (NAC, WRKY, MYB) compare to Malus prunifolia under mild and moderate drought treatments. This might be one of the reasons why Malus prunifolia exhibits greater drought resistance. Furthermore, weighted gene co-expression network analysis (WGCNA) was adopted for the identification of core drought-responsive genes. Notably, three hub genes, ubiquitin-conjugating enzyme 32 (UBC32), basic leucine-zipper 4 (bZIP4), and highly ABA-induced PP2C gene 2 (HAI2), were selected from the different modules, suggesting their vital roles in drought response. Taken together, our results gain insights into the global expression alterations in drought-resistance and susceptible germplasms under different drought conditions and identify some key genes involved in drought response, which is helpful for drought-resistant apple breeding in future.