The Plant Journal 23 June 2021 | doi.org/10.1111/tpj.15380
Saraswati Nayar * and Thangavel G [*Corresponding author]
MADS-box transcription factors (TFs) have not been functionally delineated in microalgae till now. In this study, the role of CsubMADS1 from microalga Coccomyxa subellipsoidea C-169 has been explored. Unlike Type II MADS-box proteins of seed plants with MADS, Intervening, K-box and C domains, CsubMADS1 only has a MADS and Intervening domain. It forms a group with MADS TFs from algae in the phylogenetic tree within the Type II MIKCC clade. CsubMADS1 is expressed strongly in the lag phase of growth. CsubMADS1 monomer does not have a specific localization in the nucleus, and it forms homodimers to localize exclusively in the nucleus. The monomer has two nuclear localization signals (NLS), N terminal NLS and internal NLS. The internal NLS is functional, and the homodimer requires two NLSs for specific nuclear localization. Overexpression (OX) of CsubMADS1 slows down the growth of the culture and creates giant polyploid multinucleate cells, resembling autospore mother cells. This implies that the release of autospores from autospore mother cells may be delayed. Thus, in wild-type (WT) cells, CsubMADS1 may be playing a crucial role in slowing down growth during the lag phase. Due to starvation in two-month-old colonies on solid media, the WT colonies produce mucilage, whereas OX colonies produce significantly less mucilage. Thus, CsubMADS1 also plays a role in negatively regulating stress-induced mucilage production and probably has a role in stress tolerance during the lag phase. Taken together, CsubMADS1 is a key transcription factor involved in the development and stress tolerance of this polar microalga.