Evaluation of resistance resources and analysis of drought resistance mechanisms in apple rootstock hybrid progenies

Drought is a major environmental problem that limits apple (Malus domestica Borkh.) production in the northwest region of China. Rootstocks play an important role in improving drought resistance in apple trees. However, breeding programs to develop new drought-resistant rootstocks remain scarce. In this study, we evaluated drought resistance in Malus prunifolia (denoted as QZ), R3 (M. domestica), and their hybrid progenies (m2, m5, m13, k3, k5 and k15). The principal component analysis revealed that their drought resistance capabilities could be ranked as follows: k15 > k5 > k3 > QZ > m5 > m13 > m2 > R3. Compared with the drought-sensitive rootstock progeny m2, the drought-resistant rootstock progeny k15 exhibited less drought-related damage and higher antioxidant enzyme activity when under drought stress. Transcriptomics analysis showed that more stress-responsive genes were expressed in k15 under drought stress than in m2. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses revealed that carbohydrate metabolism and starch and sucrose metabolism were more active in k15 than in m2. In addition, k15 increased starch degradation by upregulating two genes encoding β-amylase (BAM), leading to the accumulation of higher levels of soluble sugars than m2 under drought stress. Overall, our results revealed that k15 sustained normal growth under drought stress by enhancing reactive oxygen species scavenging and elevating soluble sugar content. This study deepens our understanding of how apple rootstock resources respond to drought stress and provides insights that will help breed new drought-resistant rootstocks adapted to arid regions. One-sentence summary The hybrid progeny k15 of apple rootstocks enhanced the drought tolerance by improving its antioxidant capacity and osmotic adjustment ability.