OASA1D-Mediated Tryptophan Enrichment Improves Redox and Ionic Homeostasis Under Salt Stress in Rice

Salinity restricts rice growth by disrupting cellular ion balance and promoting oxidative damage. Although exogenous melatonin can improve rice salt tolerance, whether expansion of the endogenous tryptophan pool enhances melatonin biosynthetic capacity and stress acclimation remains unclear. Here, we investigated a homozygous transgenic rice line constitutively expressing OASA1D, a feedback-insensitive D323N variant of the anthranilate synthase α-subunit OASA1. The OASA1D-expressing line exhibited strong resistance to 5-methyltryptophan and accumulated approximately twofold more tryptophan than wild-type plants in both shoots and roots under control and 150 mM NaCl conditions. The expanded tryptophan pool was accompanied by a 1.9–2.2-fold increase in endogenous melatonin and elevated expression of the melatonin biosynthetic genes OsTDC1, OsT5H, OsSNAT1, and OsASMT1. Under salt stress, OASA1D seedlings maintained greater shoot and root growth, biomass, and soil–plant analysis development (SPAD) values than wild-type seedlings. OASA1D also showed lower H2O2 and malondialdehyde accumulation and reduced electrolyte leakage, together with higher superoxide dismutase, catalase, and ascorbate peroxidase activities. Salt-induced expression of OsDREB2A, OsLEA3-1, OsP5CS1, OsWRKY45, OsHKT1;5, OsNHX1, and OsSOS1 was enhanced in OASA1D. Consistently, OASA1D shoots accumulated less Na+, retained more K+, and maintained a higher K+/Na+ ratio under salinity. Together, these results show that constitutive OASA1D expression expands the endogenous tryptophan pool and is associated with enhanced melatonin biosynthetic capacity, antioxidant defence, ionic homeostasis, and salt tolerance in rice.