NITROGEN STRESS REGULATES GENE-EXPRESSION OF ENZYMES IN THE FLAVONOIDBIOSYNTHETIC-PATHWAY OF TOMATO

Nitrogen deficiency is one stress factor that enhances anthocyanin con tent in many plants, but mechanisms underlying that response are poorl y understood. Increases in flavonoids during nitrogen starvation could be due to increased deamination of phenylalanine. Under such conditio ns the ammonia would be recycled for protein synthesis and the cinnami c acid can be converted to flavonoids, but no regulatory processes tha t would enhance anthocyanin synthesis over other flavonoid formation h as been reported. In tomato (Lycopersicon esculentum) leaves, anthocya nins and one flavonol consistently increased 2- to 3-fold under N-defi cient conditions, while total non-anthocyanin flavonoids increased onl y 14%. Two components responsible for these increases were identified by proton nuclear magnetic resonance, mass spectroscopy, and gas chrom atography as petunidin, an anthocyanidin, and quercetin-3-O-glucoside, a flavonol conjugate. RNA gel analyses of leaf tissue showed that ste ady-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 i n the same tissue. Three different anthocyanin-deficient mutants teste d under N-stress produced changes in steady-state mRNA levels of CHS, DFR, and CHI comparable to wildtype plants. These results indicate tha t in addition to increasing flavonoid content N stress also produces d ifferential effects on expression of genes encoding flavonoid biosynth etic enzymes.