Identification of ZmP5CS Gene Family and Functional Analysis of ZmP5CS4 Under Salt Tolerance in Maize

Saline-alkali land represents an important reserve of arable resources in China, and exploiting its agricultural potential is crucial for ensuring food security. In maize (Zea mays L.), which is moderately sensitive to salt stress, proline serves as a key osmoprotectant, and Δ¹-pyrroline-5-carboxylate synthetase (P5CS), the rate-limiting enzyme in its biosynthesis, plays a vital role in plant stress responses. In this study, the maize ZmP5CS gene family was systematically identified and characterized through comprehensive bioinformatics analyses. Four ZmP5CS homologs were identified, most of which were predicted to localize to chloroplasts. Phylogenetic analysis classified these genes into four major clades. Among them, ZmP5CS4 (GRMZM2G028535) expression was significantly upregulated under salt stress. Association analysis using a natural population of 278 inbred lines revealed that nine SNPs significantly associated with relative P5CS enzyme activity were located within ZmP5CS4. Haplotype analysis further identified a superior haplotype, HapA, carried by 14 inbred lines. Under salt stress, lines carried by HapA exhibited higher P5CS enzyme activity, greater proline accumulation, lower standard evaluation scores, and slightly enhanced salt tolerance compared to lines carried by HapB. Functional validation via transgenic approaches demonstrated that ZmP5CS4 overexpression significantly increased proline content and plant survival under salt stress, whereas knockout of this gene led to heightened salt sensitivity. Collectively, this study elucidates the structure and function of the maize ZmP5CS gene family, establishes the critical role of ZmP5CS4 in the salt stress response, and provides both a theoretical foundation and a candidate gene resource for improving salt tolerance in maize breeding programs.