Expression of chloroplastic and cytosolic glutamine synthetase in barley leaves after cold-sensitive blocking of beta-carotene synthesis by amitrole or mutation.

d3-Amino-1,2,4-triazole (amitrole) provided to germinating barley at 20 degrees C in the light led to bleached seedling leaves and photodynamic destruction of chloroplast structure, whereas normal greening and chloroplast ultrastructure was obtained when the seedlings developed in the presence of amitrole in the light at 30 degrees C. Mass spectrometric analysis of the extractable herbicide demonstrated the same content of amitrole in leaves developed at 20 and 30 degrees C. A very similar temperature-sensitive syndrome is characteristic for the nuclear gene mutant tigrina-o(34) in barley. Amitrole and the mutation were shown to inhibit the cyclization of lycopene, leading to severe deficiencies in beta-carotene and its xanthophyll derivative lutein. Besides accumulation of lycopene, also its precursors phytoene, phytofluene and xi-carotene accumulated. Inhibition of carotenoid biosynthesis by amitrole and the mutation at 20 degrees C in the light led to a strong reduction of both transcript and protein levels for chloroplastic glutamine synthetase (GS(2)) while transcript amount and protein of the cytosolic isoenzyme (GS(1)) were unaffected. At 30 degrees C increased levels of mRNA for the chloroplastic isoform GS(2) were observed in wild type, mutant and amitrole-treated seedlings, but protein levels remained unchanged. Turnover rates of the GS(2) protein were the same at 20 and 30 degrees C. This extensive translational control of chloroplastic 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 chloroplastic glutamine synthetase (GS(2)) at 30 degrees C in the presence of amitrole or the tigrina-o(34) mutation might be due to two alternative pathways of ionone ring formation using either lycopene or neurosporene as substrates for cyclization.