Coordination between photorespiration and carbon concentrating mechanism in Chlamydomonas reinhardtii: transcript and protein changes during light-dark diurnal cycles and mixotrophy conditions

Abstract

Carbon concentrating mechanism (CCM) and photorespiration (PR) are interlinked and co-regulated in Chlamydomonas reinhardtii, but conditions where co-regulation alters are not sufficiently explored. Here, we uncover that PR gene transcripts, like CCM transcripts, are induced even in the dark when both processes are not active. Such diurnal cycles show that transcript levels peak in the middle of 12 h day, decline by early part of 12-h dark followed by their onset again at mid-dark. Interestingly, the onset in the mid-dark phase is sensitive to high CO, implying that the active carbon sensing mechanism operates even in the dark. The rhythmic alterations of both CCM and PR transcript levels are unlinked to circadian clock: the "free-running state" reveals no discernible rhythmicity in transcript changes. Only continuous light leads to high transcript levels but no detectable transcripts were observed in continuous dark. Asynchronous continuous light cultures, upon shifting to low from high CO exhibit only transient induction of PR transcripts/proteins while CCM transcript induction is stable, indicating the loss of co-regulation between PR and CCM gene transcription. Lastly, we also describe that both CCM and PR transcripts/proteins are induced in low CO even in mixotrophic cultures, but only in high light, the same being attenuated in high CO, implying that high light is a mandatory "trigger" for CCM and PR induction in low CO mixotrophy. Our study provides comprehensive analyses of conditions where CCM and PR were differently regulated, setting a paradigm for a detailed mechanistic probing of these responses.

The induction of pyrenoid synthesis by hyperoxia and its implications for the natural diversity of photosynthetic responses in Chlamydomonas

“…Further, the formation of these pyrenoids enhance the discrimination of CO 2 /O 2 , implying that H 2 O 2 induction of pyrenoids could convey performance advantages under hyperoxia. Consistent with this hypothesis, the induction of the CCM has been found to be coordinated with that of genes for photorespiratory enzymes, although the specific metabolic control of this co-regulation had remained unknown ( Tirumani et al, 2019 ). Interestingly, a separate RNA expression study ( Blaby et al, 2015 ) did not show strong induction of pyrenoid components by H 2 O 2 , but, importantly, was conducted on TAP-grown cells, which we found do not show H 2 O 2 -induced formation of the pyrenoid ( Appendix 1—figure 20 ).…”

“…It has been argued that mixotrophic conditions alter the relationship between the onset of photorespiration and the expression of the CCM ( Tirumani et al, 2019 ), and that photorespiration, hydrogen peroxide detoxification, and acetate assimilation (i.e. the glyoxolate cycle) are all localized in the peroxisomal microbodies ( Lauersen et al, 2016 ).…”

The Induction of Pyrenoid Synthesis by Hyperoxia and its Implications for the Natural Diversity of Photosynthetic Responses inChlamydomonas

“…Further, the formation of these pyrenoids enhance the discrimination of CO 2 /O 2 , implying that H 2 O 2 induction of pyrenoids could convey performance advantages under hyperoxia. Consistent with this hypothesis, the induction of the CCM has been found to be coordinated with that of genes for photorespiratory enzymes, though the specific metabolic control of this co-regulation had remained unknown (Tirumani et al, 2019). Interestingly, a separate RNA expression study (Blaby et al, 2015) did not show strong induction of pyrenoid components by H 2 O 2 , but, importantly, was conducted on TAP-grown cells, which we found do not show H 2 O 2 -induced formation of the pyrenoid (Supplementary Material Figure 17).…”

Chlamydomonas reinhardtii Alternates Peroxisomal Contents in Response to Trophic Conditions

“…Nevertheless, these observations suggest that the peroxisomal contents are altered by light conditions in C. reinhardtii . When C. reinhardtii is in the dark, photosynthesis and photorespiration are not activated, yet increases in expressions of mRNAs and proteins that upregulate photosynthesis and photorespiration are observed [ 41 ]. The alteration of the peroxisomal contents may contribute to balancing carbon flux by altering the FA metabolism in illuminated and dark conditions.…”