ARABIDOPSIS GLS MUTANTS AND DISTINCT FD-GOGAT GENES - IMPLICATIONS FOR PHOTORESPIRATION AND PRIMARY NITROGEN ASSIMILATION

Ferredoxin-dependent glutamate synthase (M-GOGAT) plays a major role i n photorespiration in Arabidopsis, as has been determined by the chara cterization of mutants deficient in Fd-GOGAT enzyme activity (gls), De spite genetic evidence for a single Fd-GOGAT locus and gene, we discov ered that Arabidopsis contains two expressed genes for Fd-GOGAT (GLU1 and GLU2). Physical and genetic mapping of the gls1 locus and GLU gene s indicates that GLU1 is linked to the gls1 locus, whereas GLU2 maps t o a different chromosome. Contrasting patterns of GLU1 and GLU2 expres sion explain why a mutation in only one of the two genes for Fd-GOGAT leads to a photorespiratory phenotype in the gls1 mutants. GLU1 mRNA w as expressed at the highest levels in leaves, and its mRNA levels were specifically induced by light or sucrose. In contrast, GLU2 mRNA was expressed at lower constitutive levels in leaves and preferentially ac cumulated in roots. Although these results suggest a major role for GL U1 in photorespiration, the sucrose induction of GLU1 mRNA in leaves a lso suggests a role in primary nitrogen assimilation. This possibility is supported by the finding that chlorophyll levels of a gls mutant a re significantly lower than those of the wild type when grown under co nditions that suppress photorespiration, Both the mutant analysis and gene regulation studies suggest that GLU1 plays a major role in photor espiration and also plays a role in primary nitrogen assimilation in l eaves, whereas the GLU2 gene may play a major role in primary nitrogen assimilation in roots.