Thesis presented June 29, 2022

Abstract:
Starch is by far the major storage compound accumulated by plants and algae, one of the most abundant polysaccharides present on earth and principal source of dietary calories in the human and animal diet. Starch synthesis occurs during the day, using global outputs of photosynthesis and its degradation starts as night falls to sustain energy-demanding cellular functions. Little is known about the regulatory mechanisms governing starch metabolism in microalgae and current knowledge is limited to factors impacting starch accumulation under adverse environmental conditions such as nitrogen limitation. A link between light perception and starch accumulation has been suggested in the case of higher plants: Mutants devoid of Phytochrome have a reduced CO₂ uptake but over-accumulate daytime sucrose and starch while PHOT has been found to mediate starch degradation in guard cells in the light to energize stomatal opening. Light perception and metabolism have not been associated in microalgae. Here we present a detailed PHOT-dependent signalling cascade, linking blue light perception with starch accumulation in the green microalga Chlamydomonas reinhardtii:
Pathway 1: Blue light, via PHOT, represses the bHLH domain-containing transcription factor (TF) GAPr4, an activator of GAP1, (glyceraldehyde-3-phosphate dehydrogenase, involved in glycolysis and in photosynthetic CO₂ fixation). Both the PHOT knock-out mutant phot and GAP1 over-expressing lines accumulate high amounts of starch.
Pathway 2: Blue light, via PHOT, activates the RWP5 TF (belonging to the RWP-K family of TFs), a repressor of starch accumulation in a GAP1-independent manner.
Pathway 3: PHOT-dependent phosphorylation of the serine/threonine kinase Blue Starch Kinase 1 (BSK1; orthologue of the Arabidopsis HT1 kinase that controls stomatal movements in response to CO₂) mediates GAP1 and starch accumulation levels in a GAPr4-independent manner.

Our work advances the current understanding of how light signaling controls metabolism in microalgae. It also adds one more aspect in the multifaceted role of PHOT so far reported to control gametogenesis at low nitrogen, expression of genes encoding chlorophyll and carotenoid biosynthesis, the size of the eyespot and photoprotection.

Keywords:
signalling, Chlamydomonas reinhardtii, metabolism

On-line thesis.