EXPRESSION OF CHLOROPLASTIC AND CYTOSOLIC GLUTAMINE-SYNTHETASE IN BARLEY LEAVES AFTER COLD-SENSITIVE BLOCKING OF BETA-CAROTENE SYNTHESIS BYAMITROLE OR MUTATION

d3-Amino-1,2,4-triazole (amitrole) provided to germinating barley at 2 0 degrees C in the light led to bleached seedling leaves and photodyna mic destruction of chloroplast structure, whereas normal greening and chloroplast ultrastructure was obtained when the seedlings developed i n the presence of amitrole in the light at 30 degrees C. Mass spectrom etric analysis of the extractable herbicide demonstrated the same cont ent of amitrole in leaves developed at 20 and 30 degrees C. A very sim ilar temperature-sensitive syndrome is characteristic for the nuclear gene mutant tigrina-o(34) in barley. Amitrole and the mutation were sh own to inhibit the cyclization of lycopene, leading to severe deficien cies in beta-carotene and its xanthophyll derivative lutein. Besides a ccumulation of lycopene, also its precursors phytoene, phytofluene and xi-carotene accumulated. Inhibition of carotenoid biosynthesis by ami trole and the mutation at 20 degrees C in the light led to a strong re duction of both transcript and protein levels for chloroplastic glutam ine synthetase (GS(2)) while transcript amount and protein of the cyto solic isoenzyme (GS(1)) were unaffected. At 30 degrees C increased lev els of mRNA for the chloroplastic isoform GS(2) were observed in wild type, mutant and amitrole-treated seedlings, but protein levels remain ed unchanged. Turnover rates of the GS(2) protein were the same at 20 and 30 degrees C. This extensive translational control of chloroplasti c GS(2) synthesis was also observed in a hear shock experiment, which revealed transiently increased mRNA levels for chloroplastic GS(2) but unchanged protein levels. Permissive synthesis of beta-carotene and c hloroplastic glutamine synthetase (GS(2)) at 30 degrees C in the prese nce of amitrole or the tigrina-o(34) mutation might be due to two alte rnative pathways of ionone ring formation using either lycopene or neu rosporene as substrates for cyclization.