Nitrogen stress regulates gene expression of enzymes in the flavonoid biosynthetic pathway of tomato

Abstract

Nitrogen deficiency is one stress factor that enhances anthocyanin content in many plants, but mechanisms underlying that response are poorly understood. Increases in flavonoids during nitrogen starvation could be due to increased deamination of phenylalanine. Under such conditions the ammonia would be recycled for protein synthesis and the cinnamic acid can be converted to flavonoids, but no regulatory processes that would enhance anthocyanin synthesis over other flavonoid formation has been reported. In tomato (Lycopersicon esculentum) leaves, anthocyanins and one flavonol consistently increased 2- to 3-fold under N-deficient conditions, while total non-anthocyanin flavonoids increased only 14%. Two components responsible for these increases were identified by proton nuclear magnetic resonance, mass spectroscopy, and gas chromatography as petunidin, an anthocyanidin, and quercetin-3-O-glucoside, a flavonol conjugate. RNA gel analyses of leaf tissue showed that steady-state mRNA levels for chalcone synthase (CHS) and dihydroflavonol-4-reductase (DFR), increased under N stress. In contrast, steady-state mRNA levels of a chalcone isomerase-homologous band (CHI) decreased in the same tissue. Three different anthocyanin-deficient mutants tested under N-stress produced changes in steady-state mRNA levels of CHS, DFR, and CHI comparable to wildtype plants. These results indicate that in addition to increasing flavonoid content N stress also produces differential effects on expression of genes encoding flavonoid biosynthetic enzymes.