Physiological, transcriptomic and metabolomic responses of the marine diatom Phaeodactylum tricornutum to auxin

Auxin, an essential phytohormone, is widely used to promote plant growth and development. However, the effect of auxin on diatoms and its mechanism remain underexplored. Here, we studied the impact of auxin 3-indoleacetic acid (IAA) on the marine diatom Phaeodactylum tricornutum and the underlying molecular mechanisms. We found that 5 g L-1 of IAA promotes the growth of P. tricornutum in a time- dependent manner. Treatment with IAA resulted in significant changes in photosynthetic pigments and malondialdehyde, chlorophyll fluorescence, and antioxidant enzyme activities. In addition, transcriptomics showed that IAA exposure leads to upregulation of a large number of differential genes (DEGs) in carbon fixation and porphyrin metabolism, several of which were verified by qPCR. Furthermore, corresponding metabolites of these pathways were also validated by metabolomic analysis. Thus, IAA exhibits growth-promotion effect on the diatom mainly through increasing photosynthetic carbon sequestration and the expression of genes in porphyrin synthesis. Our results provide key data on the action mechanism of IAA in promoting the growth of a diatom.