Flavonoid remodeling and functional characterization of CrHPPD during salt stress response in Carex rigescens

Soil salinity severely limits plant growth and productivity. Carex rigescens, a low-maintenance turfgrass species native to China, exhibits remarkable tolerance to abiotic stresses. Previous studies have highlighted the importance of the phenylalanine metabolic pathway in salt stress defense in C. rigescens; however, the specific mechanisms remain poorly understood. To elucidate the downstream metabolic and molecular components of this pathway, we analyzed flavonoid metabolism in two contrasting C. rigescens varieties-salt-sensitive 'Lvping No. 1' and salt-tolerant 'Lvping No. 2'-and functionally characterized the 4-hydroxyphenylpyruvate dioxygenase (CrHPPD) gene. Salt stress altered the abundance of several flavonoid metabolites, including 2'-hydroxygenistein, genistin, kaempferol, taxifolin, myricetin, and eriodictyol, identifying them as candidate salt-responsive metabolites. Naringenin and apigenin showed genotype- and tissue-dependent abundance patterns between the two varieties. We cloned CrHPPD, characterized its encoded protein, and found that CrHPPD-GFP displayed a cell periphery-associated fluorescence pattern in transient expression assays, although precise localization requires marker-based validation. CrHPPD expression was induced by NaCl and ABA, whereas PEG treatment elicited a weaker and more transient response. Furthermore, overexpression of CrHPPD in Arabidopsis thaliana enhanced germination rate, root length, catalase activity, and chlorophyll retention under salt stress, with the chlorophyll effect being most evident in OE6. Collectively, these findings reveal genotype- and tissue-specific flavonoid remodeling in C. rigescens under salt stress and demonstrate that CrHPPD positively contributes to salt tolerance when overexpressed in Arabidopsis. These results suggest that flavonoid metabolism and the HPPD-associated homogentisate/tocopherol antioxidant branch may represent two stress-responsive components of phenylalanine/tyrosine-derived metabolism, although their direct mechanistic connection requires further validation.