Diversification of SEC15a and SEC15b isoforms of an exocyst subunit in seed plants is manifested in their specific roles in Arabidopsis sporophyte and male gametophyte

Batystová K., Synek L., Klejchová M., Janková Drdová E., Sabol P., Potocký M., Žárský V., Hála M.

Abstract: The exocyst complex is an octameric evolutionarily conserved tethering complex engaged in the regulation of polarized secretion in eukaryotic cells. Here, we focus on systematic comparison of two isoforms of the SEC15 exocyst subunit, SEC15a and SEC15b. We infer that the SEC15 gene duplication and diversification occurred in the common ancestor of seed plants (Spermatophytes). In Arabidopsis, SEC15a represents the main SEC15 isoform in the male gametophyte, and localizes to the pollen tube tip at the plasma membrane. While pollen tubes of sec15a mutants are impaired, sporophytes show no phenotypic deviations. Conversely, SEC15b is the dominant isoform in the sporophyte and localizes to the plasma membrane in root and leaf cells. Loss-of-function sec15b mutants exhibit retarded elongation of hypocotyls and root hairs, loss of apical dominance, dwarfed plant stature and reduced seed coat mucilage formation. Surprisingly, the sec15b mutants also exhibit compromised pollen tube elongation in vitro despite its very low expression in pollen, suggesting non-redundant role for the SEC15b isoform there. In pollen tubes, SEC15b localizes to distinct cytoplasmic structures. Reciprocally to this, SEC15a functions also in the sporophyte, where it accumulates at plasmodesmata. Importantly, although overexpressed SEC15a could fully complement the sec15b phenotypic deviations in sporophyte, pollen-specific overexpression of SEC15b was unable to fully compensate for the loss of SEC15a function in pollen. We conclude that SEC15a and SEC15b isoforms evolved in seed plants, whereby SEC15a functions mostly in pollen, whereas SEC15b mostly in the sporophyte.
DOI: 10.1111/tpj.15744
IEB authors: Edita Drdová, Michal Hála, Martin Potocký, Lukáš Synek, Viktor Žárský