IMPLICATION OF THE PHYTOCHROME IN LIGHT REGULATION OF THE TOMATO GENE(S) ENCODING FERREDOXIN-DEPENDENT GLUTAMATE SYNTHASE

We have investigated the light regulation of the tomato gene(s) encodi ng ferredoxin-dependent glutamate synthase (Fd-GOGAT, EC 1.4.7.1). Fd- GOGAT transcripts, Fd-GOGAT polypeptides and Fd-GOGAT activity were pr esent in dark-grown tomato plants. The increase in Fd-GOGAT activity o bserved upon illumination of etiolated tomato seedlings was correlated with an increase in the abundance of Fd-GOGAT protein. We conclude, f rom these results, that the basic level of constitutive Fd-GOGAT gene expression in etiolated tomato leaves is stimulated by light, and that this light-stimulation is more likely to be due to de novo synthesis of additional Fd-GOGAT enzyme than to a light activation of the preexi sting enzyme in etiolated tissue. The photoreceptor phytochrome is inv olved in the light regulation of tomato Fd-GOGAT mRNA level as shown b y red light modulation of the Fd-GOGAT gene transcript pool. Moreover, following light exposure of etiolated plantlets, the increase in the amount of Fd-GOGAT transcripts appears to be at least three times high er than that of the corresponding protein. This result, therefore sugg ests that an additional post-transcriptional control might regulate th e final enzyme level.