LHCSR3 affects de-coupling and re-coupling of LHCII to PSII during state transitions in Chlamydomonas reinhardtii

Thomas Roach,Chae Sun Na

doi:10.1038/srep43145

Abstract

Photosynthetic organisms have to tolerate rapid changes in light intensity, which is facilitated by non-photochemical quenching (NPQ) and involves modification of energy transfer from light-harvesting complexes (LHC) to the photosystem reaction centres. NPQ includes dissipating excess light energy to heat (qE) and the reversible coupling of LHCII to photosystems (state transitions/qT), which are considered separate NPQ mechanisms. In the model alga Chlamydomonas reinhardtii the LHCSR3 protein has a well characterised role in qE. Here, it is shown in the npq4 mutant, deficient in LHCSR3, that energy coupling to photosystem II (PSII) more akin to qT is also disrupted, but no major differences in LHC phosphorylation or LHC compositions were found in comparison to wild-type cells. The qT of wild-type cells possessed two kinetically distinguishable phases, with LHCSR3 participating in the more rapid (<2 min) phase. This LHCSR3-mediated qT was sensitive to physiological levels of H2O2, which accelerated qE induction, revealing a way that may help C. reinhardtii tolerate a sudden increase in light intensity. Overall, a clear mechanistic overlap between qE and qT is shown.

Highlights

Promotes disulphide bridge formation inhibits LHC phosophorylation[21]
Chlorophyll excitation at 77 K leads to emission peaks centred at 685 and 715 nm, corresponding to PSII and PSI, respectively, thereby indicating the location of the mobile fraction of LHCII
Dark adapting pre high light-treated wild-type cells led to a decrease of fluorescence at 685 nm, typical of a transition to state II and a de-coupling of energy transfer away from PSII, which occurred in npq[4] (Fig. 2a)

Summary

Introduction

Promotes disulphide bridge formation inhibits LHC phosophorylation[21]. The Stt7-deficient C. reinhardtii (stt7) is not prone to photoinhibition[24], but the npq4stt[7,8,9] double mutant is more light-sensitive than npq[413]. Using npq[4], stt[7,8,9] and stt[7] (a non-leaky Stt7-kinase deficient mutant) it is shown that LHCSR3 is involved in a de-coupling and re-coupling of energy transfer to PSII during qT. This process was sensitive to H2O2 and, in agreement with an involvement of LHCSR3, the qT of npq[4] was much less H2O2-sensitive than wild-type cells. During high light the H2O2-sensitivity of transitioning to state I accelerated qE induction, enabling cells to adjust to high light quicker

Methods

Results

Conclusion