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

Abstract

3‐Amino‐ 1,2,4‐triazole (amitrole) provided to germinating barley at 20°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°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 the nuclear gene mutant ligrina‐o34 in barley. Amitrole and the mutation were shown to inhibit the cyclization of lycopene, leading to severe deficiencies in β‐carotene and its xanthophyll derivative lutein. Besides accumulation of lycopene, also its precursors phytoene, phytofluene and ξ‐carotene accumulated. Inhibition of carotenoid biosynthesis by amitrole and the mutation at 20°C in the light led to a strong reduction of both transcript and protein levels for chloroplastic glutamine synthetase (GS2) while transcript amount and protein of the cytosolic isoenzyme (GS1) were unaffected. At 30°C increased levels of mRNA for the chloroplastic isoform GS2 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 GS2 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 β‐carotene and chloroplastic glutamine synthetase (GS2) at 30°C in the presence of amitrole or the tigrina‐o34 mutation might be due to two alternative pathways of ionone ring formation using either lycopene or neurosporene as substrates for cyclization.

Impaired carotenogenesis can affect organization and functionality of etioplast membranes

“…Previous research on AM‐treated barley plants has shown a partial recovery of carotenoid biosynthesis with an increase in growth temperature (Zito et al 1995; Agnolucci et al 1996; Rascio et al 1996), and the same temperature effect has been found in the barley lycopenic mutant tigrina‐o 34 (Casadoro et al 1983). In contrast, the temperature change did not affect the inhibition of carotenogenesis due to NF (La Rocca et al 1998; Dalla Vecchia et al 2001).…”

Comparative proteomics of a lycopene‐accumulating mutant reveals the important role of oxidative stress on carotenogenesis in sweet orange (Citrus sinensis [L.] osbeck)

“…The idea was supported by the fact that among the carotenoids, lycopene exhibits the highest physical quenching rate constant with singlet oxygen, and in humans the level is higher than that of β‐carotene 1, 2. The phenomena that elevated oxidative stress by amitrole treatment led to lycopene accumulation had been observed in maize 48, 49, and barley 50. Oxidative stress induced by cupric ions also activated lycopene biosynthesis in tomato fruits 51.…”