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

Abstract

G. 1995. Expression of chloroplastic and cytosolic glutamine synthetase in barley leaves after cold-sensitive blocking of |3-carotene synthesis by amitrole or mutation. -Physiol. Plant. 94: 535-544.3-Amino-l,2,4-triazole (amitrole) provided to germinating barley at 20°C in tbe 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 al 30°C. Mass spectrometric analysis of the extractable herbicide demonstrated the same content of amitrole in leaves developed at 20 and 30°C. A very similar temperature-sensitive syndrome is characteristic for tbe nucJear gene mutant ligrina-o^ in barley. Amitrole and the mutation were shown to inhibit the cycliration of lycopene, leading to severe deficiencies in j3-carotene and its xanthophyll derivative lutein. Besides accumulation of lycopene. also its precursors phytoene, phytofluene and f-carotene accumulated. Inhibition of carotenoid biosynthesis by amitrole and the mutation at 20°C in the light led to a strong reduction of bcth transcript and protein levels for chloroplastic glutamine synthetase (GS;) while transcript amount and protein of the cytosolic isoenzyme (GS|) were unaffected. At 30°C increased levels of mRNA for the chloroplastic isoform GS, were observed in wild type, mutant and amitrole-treated seedlings, but protein levels remained unchanged. Turnover rates of the GS2 protein were the same at 20 and 30°C. This extensive translational control of chloroplastic GSj synthesis was also observed in a heat shock experiment, which revealed transiently increased mRNA levels for chloroplastic GS2 but unchanged protein levels.Permissive synthesis of /3-carotene and chloroplastic glutamine synthetase (GS,) at 30°C in the presence of amitrole or the tigrina-o''' mutation might be due to two alternative pathways of ionone ring formation using either lycopene or neurosporene as substrates for cyclization.

Amitrole treatment of etiolated barley seedlings leads to deregulation of tetrapyrrole synthesis and to reduced expression of Lhc and RbcS genes

“…All these treatments resulted in reduced levels of Lhc mRNA and RbcS mRNA in the light (Johanningmeier and Howell 1984;Johanningmeier 1988). As previously shown (Zito et al 1995) it can be excluded that AM directly inhibits cell transcription in general. Thus the present results with AM-treated barley seedlings show that MgProto/ MgProtoMe and both Lhc and RbcS mRNA levels are inversely correlated, and strengthen the hypothesis that these chlorophyll precursors, as a plastid signal, penetrate into the cytoplasm and activate a signalling pathway by interacting with binding protein(s) in the cytoplasm/nucleus (Kropat et al 2000).…”