bZIPs regulate laminarin metabolism via the circadian rhythms in diatom Phaeodactylum tricornutum

The basic region/leucine zipper motif (bZIP) transcription factor (TF) family is one of the most important gene families in plants, regulating the plant growth, development, reproduction, as well as stress responses. It was well known that some bZIP TFs in higher plants can regulate polysaccharide metabolism. However, bZIP TFs in microalgae and their function on the polysaccharide metabolism are still unknown. In this study, bioinformatic and molecular methods were carried out to analyze bZIP TFs in the diatom Phaeodactylum tricornutum (Pt). As a result, a total of twenty-two PtbZIP genes were identified and subsequently classified into seven distinct clades based on phylogenetic relationships, conserved motifs, and gene structures. The number of introns varied from zero to three among the PtbZIP genes. All PtbZIP genes were randomly distributed to 16 chromosomes, and each containing one or two PtbZIP domains. Among the 22 identified PtbZIP genes, PtbZIP12 and PtbZIP13 genes were identified as a putative pair of duplicated genes. Furthermore, it was observed that laminarin content increased during the day and decreased during the night. It was proposed that the 12 PtbZIPs showing the same circadian rhythms could positively affect the accumulation of laminarin, while the 10 PtbZIP genes with the opposite expression pattern might have a negative effect on the laminarin metabolism. Based on these findings, a regulatory mechanism for laminarin biosynthesis was proposed in P. tricornutum. The results of this study provide valuable information for further analysis of bZIP TFs functions and laminarin metabolism in microalgae, which may be useful for optimizing the mass production of laminarin.