Regulatory Involvement of Plastids in the Development of Peroxisomal Enzymes in the Cotyledons of Mustard ( Sinapis alba L.) Seedlings

Abstract

Summary The developmental changes of marker enzymes of the glyoxysomal function [isocitrate lyase (ICL) and malate synthase (MS)] and the leaf-peroxisomal function [glycolate oxidase (GO) and hydroxypyruvate reductase (HPR)] of the peroxisomes were investigated. White light suppressed the activities of ICL and MS while the activities of GO and HPR were increased. The stimulation of GO and HPR is mediated by continuous far-red light absorbed by phytochrome, whereas photosynthetic pigments are probably responsible for the photoinhibition of ICL and MS. Acetate and glucose had no regulatory effects on the levels of these enzymes. The same result was obtained if CO2-fixation was inhibited by withholding CO2, or if photorespiration was inhibited by withholding O2, in the light. However, Norflurazon (NF), an inhibitor of carotenoid synthesis leading to photooxidative destruction of chloroplasts in the light, prevented the light-mediated increase of GO and HPR while the photoinhibition of ICL and MS was alleviated. The same differential effects could be obtained by inhibiting plastid protein synthesis with Chloramphenicol, Lincomycin, Rifampicin, and a heat treatment (34 °C) destroying plastid ribosomes. Inhibition of cytoplasmic protein synthesis with Cycloheximide completely prevented the formation of all four enzymes. Inhibition of photosynthesis by Dichlorophenyl-dimethylurea (DMCU) in green cotyledons did not specifically influence the light-dependent changes of enzyme levels. NF reduced the light-mediated association of peroxisomes to plastids but had no detectable effect on the fine-structure of peroxisomes, even if located in close contact with photodamaged chloroplasts. From these data it is concluded that (i) the peroxisomal enzyme levels are not controlled by metabolites of fat degradation, photosynthesis, or photorespiration, (ii) a functional photosynthetic apparatus is not required for a normal development of both groups of peroxisomal enzymes in the light, (iii) plastids exert control over the accumulation of these cytoplasmic enzymes which can be disturbed by eliminating plastid protein synthesis. Thus, phytochrome-mediated photomorphogenesis of peroxisomes is regulated by a signal originating in the plastid either independently of, or as a consequence of, phytochrome-mediated plastid development.