Abstract

Photosynthetic organisms must acclimate to their light environment to optimize photosynthesis and minimize photo-oxidative damage. In plants, qE, a component of non-photochemical quenching (NPQ), is required for thermal dissipation of excess absorbed light energy. In C. reinhardtii, LHCSR3 was shown to be crucial for functional qE (Peers et al, 2009). Increased cyclic electron flow (CEF) around PS I is known to promote qE induction by formation of a trans-thylakoid proton gradient. Functional CEF depends on a reorganization of the major protein complexes in the thylakoid membrane, leading to the formation of a PS I -cytochrome b6f supercomplex (CEF-supercomplex) (Iwai et al, 2010; Terashima et al, 2012). There is evidence that that the chloroplast localized Ca2+ sensor protein (CAS) is crucial for effective photo-acclimation in C. reinhardtii by controlling the expression of LHCSR3 (Petroutsos et al, 2011) and required for expression of proteins essential for the carbon-concentrating mechanism (CCM) (Wang et al, 2016). Moreover CAS and Ca2+ are involved in regulation of CEF . Ca2+-dependent regulation of CEF was shown via a combined function of ANR1 and CAS , associated with each other in the CEF -supercomplex (Terashima et al, 2012). Further evidence for the importance of Ca2+ in the regulation of photosynthesis is given by the identification (Hohner et al, 2013) and characterization of a C. reinhardtii protein, which combines four EF-hands and a thioredoxin domain, designated as calredoxin (CRX). Functional data demonstrate that recombinant CRX possesses Ca2+-dependent thioredoxin activity, thereby linking Ca2+ with redox regulation (Hochmal et al, 2016). Down-regulation of calredoxin expression resulted in increased CEF , high ROS and elevated conductivity of the ATPase as well as in a strong repression of highlight-induced proteins, including CCM -induced proteins, a phenotype similar to a CAS -deficient mutant, suggesting a link between CAS and CRX .

References

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