Sesuvium portulacastrum SpC3H Enhances Salt Tolerance of Arabidopsis thaliana by Regulating Lignin Synthesis and Scavenging Reactive Oxygen Species

Lignin constitutes a fundamental component of plant defense mechanisms against environmental stressors. 4-coumarate 3-hydroxylase (C3H) serves as a pivotal enzyme in lignin biosynthesis. However, its role in the halophyte <i>Sesuvium portulacastrum</i> remains uncharacterized. In this study, the <i>SpC3H</i> gene was cloned, and subsequent sequence alignment and phylogenetic analyses revealed the highest similarity (57.14%) with BvC3H from <i>Beta vulgaris</i>, exhibiting the closest evolutionary relationship with <i>Beta vulgaris</i> and <i>Spinacia oleracea</i> C3H protein. Quantitative real-time polymerase chain reaction demonstrated that <i>SpC3H</i> expression was markedly upregulated in both roots and leaves of <i>S. portulacastrum</i> under 800 mM NaCl treatment. Root expression peaked at 48 h (25.3-fold), whereas leaves displayed dual expression maxima at 12 h (7.9-fold) and 72 h (10.7-fold). Subcellular localization assays confirmed cytoplasmic distribution. Heterologous expression in <i>Arabidopsis thaliana</i> indicated that transgenic lines exhibited enhanced growth performance, higher fresh weight, and elevated lignin contents relative to wild-type plants under salt stress, accompanied by reduced reactive oxygen species (ROS) accumulation and lower relative electrical conductivity. Furthermore, activities of superoxide dismutase and peroxidase, together with expression of lignin biosynthesis-associated and antioxidant enzyme genes, were markedly elevated. Collectively, these findings establish that SpC3H confers salt tolerance by promoting lignin biosynthesis and activating antioxidant defenses to eliminate ROS, thereby providing a theoretical foundation for genetic improvement of plant salt tolerance.