Exploration of the Regulatory Pathways and Key Genes Involved in the Response to Saline–Alkali Stress in <i>Betula platyphylla</i> via RNA-Seq Analysis

The pH of saline–alkali soil is high because of carbonate salts, and the deleterious effects of saline–alkali soil on the growth of plants are greater than those of saline soil. Few studies have examined the saline–alkali tolerance of <i>Betula platyphylla</i> at the molecular level. To clarify the regulatory mechanism underlying saline–alkali tolerance in <i>B. platyphylla</i>, RNA sequencing analysis of <i>B. platyphylla</i> seedlings treated with NaHCO₃ was conducted. Differences in gene expression in the roots of <i>B. platyphylla</i> seedlings under saline–alkali stress (induced via NaHCO₃) for 3 h and 6 h were characterized, and a total of 595 and 607 alkali stress-responsive genes were identified, respectively. Most differentially expressed genes were involved in stress, signal transduction, secondary metabolic process, regulation of jasmonic acid, and the abiotic stimulus signaling pathway. The single nucleotide polymorphism loci in the differentially expressed genes were associated with the alkaline-salt tolerance in birch germplasm. In addition, birch plants overexpressing <i>WRKY70</i> and <i>NAC9</i> were obtained using the <i>A. tumefaciens</i>-mediated transient transformation method, and these two genes were found to play key roles in saline–alkali tolerance. Additional study revealed that <i>WRKY70</i> and <i>NAC9</i> can increase resistance to saline–alkali stress by enhancing reactive oxygen species scavenging and inhibiting cell death in birch plants. The results of this study enhance our understanding of the saline–alkali stress tolerance of <i>B. platyphylla</i> at the molecular level, and provide several key genes that could be used in the breeding of birch plants in the future.