Effects of High Light Stress on Carotenoid-Deficient Chloroplasts in Pisum sativum

Abstract

The effects of high light stress on chloroplast ultrastructure and protein and mRNA composition were investigated in carotenoid-deficient peas (Pisum sativum, L.). In low light, the thylakoid membrane polypeptide pattem was altered, with several prominent chlorophyll-binding proteins present in diminished amounts. This change was found to be reflected in the ultrastructural organization of intemal chloroplast membranes. In contrast to the normal grana stacking found in the controls, carotenoid-deficient plastids contained long, unstacked lamellae. Exposure to photooxidative light that resulted in destruction of >70% of chlorophyll did not lead to changes in total RNA and total cellular protein pattems. This treatment did lead to gross alterations in the chloroplast structure. Within 24 hours the plastid was seen as a swollen vesicle with only a few membrane remnants still present. Accumulation of five plastid-encoded mRNAs encoding a diverse array of photosynthetic proteins was found to be affected in different ways. While psaA mRNA was rapidly reduced by more than 75%, levels of psbF/E and atpB/E were reduced by 50%. psbA and petA mRNAs, on the other hand, appeared to be more resistant to photobleaching and remained relatively unchanged during 24 hours of high fluence-rate light treatment.Long-term exposure of a green plant to strong light can result in injury to the photosynthetic apparatus, the damage being caused in large part as a consequence of light-and oxygen-dependent destruction of the photosynthetic pigments. This bleaching is an extemal symptom of the intracellular photooxidative damage occurring within the chloroplast. In higher plants damage caused as a consequence of Chl photooxidation has been investigated in several plant species (3, 5, 7, 11-13, 18, 19) mainly in relation to the study of the effects of chlorosis-inducing herbicides. More recently, experiments with herbicides inhibiting carotenoid biosynthesis have provided a useful system to study this process with respect both to plastid damage and to its consequences for nuclear gene expression (26).We have previously characterized some of the damaging effects of photooxidative light on pea seedlings (Pisum sati-'This is Carnegie

The aba Mutant of Arabidopsis thaliana (L.) Heynh. Has Reduced Chlorophyll Fluorescence Yields and Reduced Thylakoid Stacking

“…The aba-i and aba-3 genotypes are less extreme in xanthophyll imbalance (44.7 and 40.0 ;ig of zeaxanthin g-l fresh weight, respectively [21]) and also showed less extreme alterations in chloroplast ultrastructure (Table I). Pea and wheat chloroplasts depleted of carotenoids by inhibitors of carotenoid biosynthesis also showed reduced thylakoid stacking (1,23 (19). CP29 also contains xanthophylls (9,25).…”

Regulation of Plastid Gene Expression during Photooxidative Stress

“…Recently, Sagar and Briggs (27) (19,20,22,23). When the plants are transferred to high light, rapid bleaching occurs, and there is a sharp decrease in the level of these mRNAs.…”

Carotenoids in photosynthesis: Protection of D1 degradation in the light

“…The psbA transcript for the D1 protein is most abundant in the chloroplast and the rate of its synthesis is quite high . At least over the first 4 h of illumination the amount of D1 m R N A is unaffected either in norfiurazontreated or control plants (Sagar and Briggs 1990). Therefore, we attribute the decreased D1 content in illuminated Scenedesmus depleted in carotenoids to light degradation and not to impaired synthesis.…”

“…The disadvantage of these experiments originates from the etiolated growth of higher plants in darkness. In the presence of norflurazon and light, differentiation of etioplasts to chloroplasts is prevented by photooxidation (Sagar and Briggs 1990). This can be overcome by working with green algae which build up a competent photosynthetic apparatus in the dark.…”