Bilin-Dependent Photoacclimation in Chlamydomonas reinhardtii

Abstract

In land plants, linear tetrapyrrole (bilin)-based phytochrome photosensors optimize photosynthetic light capture by mediating massive reprogramming of gene expression. But, surprisingly, many green algal genomes lack phytochrome genes. Studies of the heme oxygenase mutant (hmox1) of the green alga Chlamydomonas reinhardtii suggest that bilin biosynthesis in plastids is essential for proper regulation of a nuclear gene network implicated in oxygen detoxification during dark-to-light transitions. hmox1 cannot grow photoautotrophically and photoacclimates poorly to increased illumination. We show that these phenotypes are due to reduced accumulation of photosystem I (PSI) reaction centers, the PSI electron acceptors 59-monohydroxyphylloquinone and phylloquinone, and the loss of PSI and photosystem II antennae complexes during photoacclimation. The hmox1 mutant resembles chlorophyll biosynthesis mutants phenotypically, but can be rescued by exogenous biliverdin IXa, the bilin produced by HMOX1. This rescue is independent of photosynthesis and is strongly dependent on blue light. RNA-seq comparisons of hmox1, genetically complemented hmox1, and chemically rescued hmox1 reveal that tetrapyrrole biosynthesis and known photoreceptor and photosynthesis-related genes are not impacted in the hmox1 mutant at the transcript level. We propose that a bilin-based, bluelight-sensing system within plastids evolved together with a bilin-based retrograde signaling pathway to ensure that a robust photosynthetic apparatus is sustained in light-grown Chlamydomonas.

Single-Cell RNA Sequencing of Batch Chlamydomonas Cultures Reveals Heterogeneity in their Diurnal Cycle Phase

“…Transcriptome analysis in the green unicellular alga Chlamydomonas (Chlamydomonas reinhardtii) has proliferated since the genome was released in 2007 (Merchant et al, 2007). Since then, dozens of experiments have been conducted that aimed to describe the changes in gene expression in response to changes in nutrient availability such as nitrogen (Plumley and Schmidt, 1989;Blaby et al, 2013;Park et al, 2015), sulfur (González-Ballester et al, 2010), phosphorus (Bajhaiya et al, 2016), Chlamydomonas single cell RNA sequencing acetate (Bogaert et al, 2019) and essential metals (Urzica et al, 2012;Blaby-Haas et al, 2016;Merchant et al, 2006;Malasarn et al, 2013;Blaby-Haas and Merchant, 2012;Kropat et al, 2015), as well as changes that occur in response to light (Tilbrook et al, 2016) or across the diurnal cycle (Zones et al, 2015;Strenkert et al, 2019) and following chemical treatments (Blaby et al, 2015;Ma et al, 2020;Wittkopp et al, 2017). A common feature of the prior studies is the use of bulk RNA-seq obtained from the sequencing of RNA extracted from pools of cells, which is due to the technical necessity to meet the material requirements for library preparation.…”

GreenCut protein CPLD49 of Chlamydomonas reinhardtii associates with thylakoid membranes and is required for cytochrome b6f complex accumulation

“…Absorbance changes associated with P 700 oxidation–reduction were performed using a JTS10 spectrophotometer (Alric et al ., ; Wittkopp et al ., ). Cells were pelleted by centrifugation (1200 g , 5 min, 20°C) and resuspended to a concentration of 25–30 μg ml −1 Chl in fresh medium supplemented with 10% ficoll PM400 (GE Healthcare).…”

Characterization of Ferredoxin-Dependent Biliverdin Reductase PCYA1 Reveals the Dual Function in Retrograde Bilin Biosynthesis and Interaction With Light-Dependent Protochlorophyllide Oxidoreductase LPOR in Chlamydomonas reinhardtii

“…Based on characterization of the Chlamydomonas hmox1 mutant, two recent reports proposed a more ancient and possibly widely conserved function of bilins as biogenic retrograde signals essential for photoacclimation and functional chloroplast maintenance during diurnal transition from dark to light ( Duanmu et al, 2013 , 2017 ). It was also hypothesized that chlorochrome, a putative bilin-dependent blue-light photoreceptor residing in the chloroplast, is involved in regulation of the chlorophyll (Chl) biosynthesis and photosystem I (PSI) and light-harvesting complex I (LHCI) protein accumulation ( Wittkopp et al, 2017 ).…”