Endogenous hormones improve the salt tolerance of maize (Zea mays L.) by inducing root architecture and ion balance optimizations

Abstract The growth and development of maize is affected in a crucial way by the salinity of the soil it grows in. Therefore, if we want to improve the salt tolerance of maize, it is of paramount importance to understand how it responds to salt stress. To explore how maize adapts to a saline environment, we chose one salt‐tolerant maize variety, Jingnongyu 658 (JNY658), and one salt‐sensitive variety, Yunyu7 (YY7), and treated the seedlings of both varieties with a 100 mM NaCl solution. After harvesting them, we analysed the adaptive responses to salt stress with respect to various characteristics of the root architecture and water and nutrient acquisition. We found that when subjected to salt stress, both varieties exhibited a reduction in biomass, root activity, 15 N and H 2 18 O uptake amount, lateral root branching density, lateral root length, total root length, root surface area and root volume, where the YY7 suffered a greater reduction than the JNY658. Salt stress also induced a decreased absorption of K + , passively promoted the absorption of Na + and gave rise to an increased Na + /K + ratio in the roots of both varieties. It should be noted that JNY658 exposed to salt stress developed a lower Na + /K + ratio than YY7. Compared with the control group, the expression of ZmSOS1, ZmSOS2 and ZmNHX genes in the roots of JNY658 subjected to salt stress was significantly upregulated, which enabled them to maintain a lower Na + content. In both varieties, the salicylic acid (SA) and gibberellin 19 acid (GA 19 ) content increased significantly, but the auxin (IAA), jasmonic acid (JA) and trans‐zeatin (TZ) content was reduced. In addition, JNY658 in salt stress significantly increased their abscisic acid (ABA) content, due to the increased expression of its key biosynthesis genes, NCED. Compared with JNY658, the reductions in IAA, JA and TZ content in YY7 were greater, while the increases in ABA and SA content were smaller. The varieties in the ABA, JA, SA and IAA of roots subjected to salt stress significantly or extremely significantly correlated with the lateral root branching density, lateral root length, root surface area, Na + content, K + content and Na + /K + ratio. We conclude that the salt‐tolerant maize variety mitigated the toxic effect of salinity on maize root growth and development by regulating the hormone content and enhancing its expression of stress‐responsive genes while maintaining ion homeostasis and promoting water and nutrient acquisition, which ultimately resulted in a less pronounced decrease in plant biomass.