Ionomic and metabolomic analyses reveal the resistance response mechanism to saline-alkali stress in Malus halliana seedlings

Saline-alkali stress is a major abiotic stress limiting plant growth. The selection of saline-alkali-tolerant rootstock is an effective strategy to reduce salinization-alkalization influence in apple production. M. halliana is a highly saline-alkali-resistant apple rootstock in northwestern China. However, few metabolic response studies have been conducted on this species. In plants under saline-alkali stress, the uptake of K, Mg and Zn in M. halliana leaves were inhibited, whereas the absorption of Fe2+, Cu2+ or Mn2+ were increased. Metabolic analysis revealed 140 differentially expressed metabolites, which were mainly involved in alkaloid biosynthesis, phenylalanine biosynthesis, ATP-binding cassette (ABC) transporters, and mineral absorption. Especially, the expression of sucrose, amino acids, alkaloids, flavonoids and carotenoids were significantly upregulated under saline-alkali stress. qRT-PCR analysis demonstrated that NHX8 and ZTP1 involved in Na+ and Fe2+ transport were upregulated, while Alai, MRS2-4 and ZTP29 involved in K+, Mg2+ and Zn2+ transport were down-regulated, respectively. ANT, ATP2A, CALM and SOS2 are involved in Ca2+ signal transduction, and ABCB1, ABCC10 and NatA are key transporters that maintain ionic homeostasis. M. halliana regulates Na+/K+ homeostasis by mediating Ca2+ signalling and ABC transporters. The accumulation of metabolites contributes to improving the saline-alkali resistance of M. halliana because of the scavenging of ROS. An increase in pheophorbide a content in porphyrin and chlorophyll metabolism leads to leaf senescence in M. halliana leaves, which contributes to a reduction in stress-induced injury. These findings provide important insights into the saline-alkali tolerance mechanism in apple, which also provides an important starting point for future research.