Influence of Sugars on Blue Light-Induced Chloroplast Relocations

Abstract

The aim of this study was to investigate the influence of sugars on blue light-induced chloroplast movements. Sucrose and glucose inhibited chloroplast responses in the detached leaves of Arabidopsis thaliana and in Lemna trisulca fronds in a concentration and time-dependent manner. The prolonged exposure necessary for inhibition indicates that sugars may act via altered gene expression. Overexpression of phototropin2, a photoreceptor responsible for the strong blue light response of chloroplasts, counteracted the sugar effect. This may suggest that sugars modify some component(s) of the phototropin2-mediated signal transduction pathway. The expression of PHOT2 was not suppressed by sugars in wild type plants, it was even up-regulated by glucose. Impaired chloroplast movements were observed only in mature Arabidopsis plants. The mRNA of SAG12, a late senescence marker, was not detectable in the sugar-incubated leaves. The SAG13 mRNA level and its regulation by sugars were similar in wild type and PHOT2 overexpressor. Thus, the sugar insensitivity of 35S:PHOT2 chloroplast responses was not due to delayed senescence. The sugar-induced transduction pathway involved remains unclear. 3-O-methylglucose did not affect chloroplast movements suggesting the participation of a hexokinase-dependent pathway. Only the amplitude of avoidance response was reduced in gin2-1, a hexokinase1 null mutant. Probably other hexokinases, or glycolysis-associated signals play a role in the suppression of chloroplast responses.