A Novel Breeding Target for Salt-Tolerant Maize: ZmEXPA3 Overexpression Enhances Growth of Maize Under Both Non-Stressed and Salt Stress Conditions Through Cell-Wall Architecture Alteration

Expansins contribute to maize tolerance to salt stress, but the molecular mechanisms by which they function under high-salinity conditions remain poorly understood. In this research, the α-expansin gene ZmEXPA3 was characterized. We obtained overexpression transgenic lines in maize and determined physiological and biochemical indices to elucidate its molecular role in salt stress. Our results confirmed that ZmEXPA3 functioned as a positive salt tolerance regulator and was potentially regulated by abscisic acid (ABA) and methyl jasmonate (MeJA). ZmEXPA3 located to the cytoplasm and cell wall. Overexpression of ZmEXPA3 achieved thicker cell wall and bigger cell size and thereby promoted biomass accumulation. The ZmEXPA3-OE lines showed a marked reduction in malondialdehyde (MDA) and H2O2 accumulation compared to the WT under salt stress. Overexpression of ZmEXPA3 elevated the enzyme activity of peroxidase (POD) and superoxide dismutase (SOD) and proline accumulation and decreased the Na+/K+ ratio in roots. Transcriptome and Gene Ontology (GO) enrichment analysis of ZmEXPA3-OE lines and WT showed that many differentially expressed genes (DEGs) were enriched in cell-wall-related terms, plant hormone response, osmotic stress response, salt stress response, oxidoreductase activity, etc. Changes in these processes may be the primary reasons why ZmEXPA3 overexpression promotes growth and salt tolerance.