Abstract
Photosystem II (PSII) reaction centre D1 protein of oxygenic phototrophs is pivotal for sustaining photosynthesis. Also, it is targeted by herbicides and herbicide-resistant weeds harbour single amino acid substitutions in D1. Conservation of D1 primary structure is seminal in the photosynthetic performance in many diverse species. In this study, we analysed built-in and environmentally-induced (high temperature and high photon fluency – HT/HL) phenotypes of two D1 mutants of Chlamydomonas reinhardtii with Ala250Arg (A250R) and Ser264Lys (S264K) substitutions. Both mutations differentially affected efficiency of electron transport and oxygen production. In addition, targeted metabolomics revealed that the mutants undergo specific differences in primary and secondary metabolism, namely, amino acids, organic acids, pigments, NAD, xanthophylls and carotenes. Levels of lutein, β-carotene and zeaxanthin were in sync with their corresponding gene transcripts in response to HT/HL stress treatment in the parental (IL) and A250R strains. D1 structure analysis indicated that, among other effects, remodelling of H-bond network at the QB site might underpin the observed phenotypes. Thus, the D1 protein, in addition to being pivotal for efficient photosynthesis, may have a moonlighting role in rewiring of specific metabolic pathways, possibly involving retrograde signalling.
Highlights
Oxygenic photosynthesis evolved at least 2.4 billion years ago, making oxygenic life possible on earth and becoming an essential energy resource
The A250R and S264K mutants differ in their sensitivity to triazine herbicides, A250R is highly sensitive while S264K is resistant[12,25,33]
The differences in total Chl content became evident during the early exponential growth phase (OD750 ~0.4), when A250R and S264K, respectively, accumulated 60% and 42% of the Chla + b per cell of the reference strain IL (Table S1), and maximized at the late growth phase (Figs 1A and S1B)
Summary
Oxygenic photosynthesis evolved at least 2.4 billion years ago, making oxygenic life possible on earth and becoming an essential energy resource. Photochemical reactions[6,7,8,9,10,11] It enabled analysis of the effects of amino acid substitutions in D1 on photosynthesis and response of such mutants to environmental extremes in Chlamydomonas[12,13,14,15]. Besides modification of herbicide sensitivity, site-specific mutations in the DE-stromal-loop of D1 often resulted in severe reduction of mutant photoautotrophic growth and photosynthetic performance, due mainly to impairment of the electron transfer between primary (QA) and secondary (QB) PSII quinones[20,21]. Studies with a number of biological systems harbouring specific D1 amino acid mutations have provided knowledge on D1 structural motifs that regulate PSII function, enable plant acclimation to extreme temperatures (as in thermophiles) and resilience to herbicide-resistant biotypes. The A250R and S264K mutants differ in their sensitivity to triazine herbicides, A250R is highly sensitive while S264K is resistant[12,25,33]
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