Point mutation of a plastidic invertase inhibits development of the photosynthetic apparatus and enhances nitrate assimilation in sugar-treated Arabidopsis seedlings

Abstract

Because the photosynthetic apparatus contains a massive amount of nitrogen in plants, the regulation of its development by sugar signals is important to the maintenance of the carbonnitrogen balance. In this study we isolated an Arabidopsis mutant (sicy-192) whose cotyledon greening was inhibited by treatments with sugars such as sucrose, glucose, and fructose. In the mutant, the gene encoding plastidic alkaline/neutral invertase (INV-E) was point-mutated at codon 294, with Tyr substituted for Cys (C294Y). Interestingly, the greening of cotyledons in the knock-out INV-E lines was not inhibited by treatment with the sugars. In addition, the knock-out INV-E lines expressing an INV-E:C294Y or INV-E:C294A gene had the same phenotype as sicy-192 mutants, whereas the lines expressing a wild-type INV-E gene had the same phenotype as wild-type plants. A recombinant INV-E:C294Y protein had the same enzymatic activity as a recombinant INV-E protein, suggesting that the Cys-294 residue of INV-E is important for its functions in the chloroplasts. On treatment with sucrose, the expression of photosynthesis-related genes was weaker in seedlings of mutant plants than wild-type seedlings, whereas the activity of nitrate reductase was stronger in the mutant plants than wild-type plants. These findings suggest that Cys-294 of INV-E is associated with the development of the photosynthetic apparatus and the assimilation of nitrogen in Arabidopsis seedlings to control the ratio of sucrose content to hexose content. © 2010 by The American Society for Biochemistry and Molecular Biology, Inc.