Transcriptomics reveal how circadian regulation contributes to starch hyperaccumulation in Platymonas helgolandica along with glucose

Background: Platymonas helgolandica var. Tsingtaoensis ( P. helgolandica ) is a marine microalga. It can produce a large amount of starch, especially amylose, with addition of carbon source and specific circadian rhythm. The mechanism behind this phenomenon is still unclear. Analysis of this mechanism can help to develop P. helgolandica into a new green bioengineering chassis organism. Result We explained how circadian rhythm and glucose affect the rate of starch accumulation and starch structure in P. helgolandica based on the transcriptome. The glucose inhibited the photosynthetic system of P. helgolandica , while the circadian rhythm can alleviate the inhibition. Circadian rhythm induced the up-regulation of Embden-Meyerhof-Parnas (EMP) pathway and pentose phosphate pathway (PPP) in P. helgolandica , but had little effect on the tricarboxylic acid cycle (TCA cycle). PPP pathway provides Ribulose-1,5-bisphosphate (RuBP), which may be beneficial for dark reactions and nucleotide synthesis. And PPP pathway provides Nicotinamide adenine dinucleotide phosphate (NADPH), which facilitates energy substance synthesis. This will further upregulate the starch metabolic pathway. The transcript level of the key gene ADP-Glucose pyrophosphorylase (AGPase) is mainly regulated by glucose. The granule-bound starch synthase ( gbss ), a key gene for amylose synthesis, is mainly influenced by circadian rhythm. In general, the increase of starch synthesis and amylose ratio requires both glucose addition and circadian rhythm. Conclusion We report the first referenced transcriptome of P. helgolandica . Differences between transcripts reveal how circadian rhythm and glucose addition affected the rate of starch synthesis and structural variation. It provides a reference for an in-depth study of starch synthesis in green algae.