RESUMO
The plant-specific calcium binding protein CAS (calcium sensor) has been localized in chloroplast thylakoid membranes of vascular plants and green algae. To elucidate the function of CAS in Chlamydomonas reinhardtii, we generated and analyzed eight independent CAS knockdown C. reinhardtii lines (cas-kd). Upon transfer to high-light (HL) growth conditions, cas-kd lines were unable to properly induce the expression of LHCSR3 protein that is crucial for nonphotochemical quenching. Prolonged exposure to HL revealed a severe light sensitivity of cas-kd lines and caused diminished activity and recovery of photosystem II (PSII). Remarkably, the induction of LHCSR3, the growth of cas-kd lines under HL, and the performance of PSII were fully rescued by increasing the calcium concentration in the growth media. Moreover, perturbing cellular Ca(2+) homeostasis by application of the calmodulin antagonist W7 or the G-protein activator mastoparan impaired the induction of LHCSR3 expression in a concentration-dependent manner. Our findings demonstrate that CAS and Ca(2+) are critically involved in the regulation of the HL response and particularly in the control of LHCSR3 expression.
Assuntos
Adaptação Fisiológica/efeitos da radiação , Cálcio/farmacologia , Chlamydomonas reinhardtii/fisiologia , Cloroplastos/metabolismo , Luz , Proteínas de Plantas/metabolismo , Cálcio/metabolismo , Calmodulina/antagonistas & inibidores , Chlamydomonas reinhardtii/genética , Chlamydomonas reinhardtii/efeitos da radiação , Clorofila/metabolismo , Clorofila/efeitos da radiação , Cloroplastos/efeitos da radiação , Regulação para Baixo/fisiologia , Inibidores Enzimáticos/farmacologia , Regulação da Expressão Gênica de Plantas/fisiologia , Regulação da Expressão Gênica de Plantas/efeitos da radiação , Peptídeos e Proteínas de Sinalização Intercelular , Complexos de Proteínas Captadores de Luz/genética , Complexos de Proteínas Captadores de Luz/metabolismo , Peptídeos/farmacologia , Fenótipo , Fotossíntese/fisiologia , Fotossíntese/efeitos da radiação , Complexo de Proteína do Fotossistema II/metabolismo , Complexo de Proteína do Fotossistema II/efeitos da radiação , Proteínas de Plantas/genética , Proteômica , Deleção de Sequência , Transdução de Sinais/fisiologia , Transdução de Sinais/efeitos da radiação , Sulfonamidas/farmacologia , Tilacoides/metabolismo , Tilacoides/efeitos da radiação , Venenos de Vespas/farmacologiaRESUMO
* Guard cell movements are regulated by environmental cues including, for example, elevations in extracellular Ca(2+) concentration. Here, the subcellular localization and physiological function of the Ca(2+)-sensing receptor (CAS) protein was investigated. * CAS protein localization was ascertained by microscopic analyses of green fluorescent protein (GFP) fusion proteins and biochemical fractionation assays. Comparative guard cell movement investigations were performed in wild-type and cas loss-of-function mutant lines of Arabidopsis thaliana. Cytoplasmic Ca(2+) dynamics were addressed in plants expressing the yellow cameleon reporter protein YC3.6. * This study identified CAS as a chloroplast-localized protein that is crucial for proper stomatal regulation in response to elevations of external Ca(2+). CAS fulfils this role through modulation of the cytoplasmic Ca(2+) concentration. * This work reveals a novel role of the chloroplast in cellular Ca(2+) signal transduction.