Salicylic acid acts upstream of nitric oxide in elevated carbon dioxide-induced flavonoid biosynthesis in tea plant (Camellia sinensis L.)

Abstract

Flavonoids are the most abundant polyphenols in tea (Camellia sinensis L.) with diverse medicinal values and stress adaptive roles. Flavonoid biosynthesis is largely influenced by environmental factors, including high atmospheric CO2. Although accumulation of certain flavonoids in tea leaves can be triggered by high CO2, the underlying mechanisms remain far from being substantiated. Despite the involvement of salicylic acid (SA) and nitric oxide (NO) in plant responses to biotic and abiotic stressors under high CO2, their roles in high CO2-induced plant secondary metabolism remain largely unknown, particularly in tea plants. This study revealed that the exposure of tea plants to high CO2 levels (550 and 800 μmol mol−1) rapidly increased the concentrations of flavonoid, SA and NO. Exogenous SA and sodium nitroprusside (a NO donor) increased flavonoid concentration only at ambient CO2 (400 μmol mol−1) but not at elevated CO2. Furthermore, suppression of endogenous SA by its biosynthesis inhibitor paclobutrazol and scavenging of NO by cPTIO (4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) reduced flavonoid concentration and phenylalanine ammonia-lyase activity under elevated CO2. While the scavenging of NO showed no effect on endogenous SA levels, SA deficiency attenuated elevated CO2-induced NO accumulation, suggesting that SA acts upstream of NO in elevated CO2-induced flavonoid biosynthesis. Notably, NO levels in SA-suppressed plants were several times greater than that of cPTIO-treated plants, indicating that SA-independent NO production might have a contribution to total NO accumulation under elevated CO2. This study unraveled the functional hierarchy and relationships of SA and NO in elevated CO2-induced flavonoid biosynthesis in tea leaves.