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

Abstract

SUMMARY The exocyst complex is an octameric evolutionarily conserved tethering complex engaged in the regulation of polarized secretion in eukaryotic cells. Here, we focus on the systematic comparison of two isoforms of the SEC15 exocyst subunit, SEC15a and SEC15b. We infer that 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. Although 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, a 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 a non‐redundant role for the SEC15b isoform there. In pollen tubes, SEC15b localizes to distinct cytoplasmic structures. Reciprocally to this, SEC15a also functions in the sporophyte, where it accumulates at plasmodesmata. Importantly, although overexpressed SEC15a could fully complement the sec15b phenotypic deviations in the sporophyte, the pollen‐specific overexpression of SEC15b was unable to fully compensate for the loss of SEC15a function in pollen. We conclude that the SEC15a and SEC15b isoforms evolved in seed plants, with SEC15a functioning mostly in pollen and SEC15b functioning mostly in the sporophyte.

Exocyst functions in plants: secretion and autophagy

“…Are core subunits of embryophyte exocyst functionally conserved [102] and how do they contribute to the overall land plant evolution as indicated e.g. for SEC15a versus SEC15b in seed plants or EXO70A1 vs. EXO70A2 in Angiosperms [103,104]?…”

“…NBR1 (receptor for selective autophagy) accumulates during Pseudomonas syringae infection (i.e. is not degraded) and participates in the defense responses [103]. During Phytophtora infestants infection, host plant NBR1/ ATG8-positive authophagosomes are targeted to the haustorium-plant interface to limit pathogen haustoria growth [106] and, in Blumeria graminis infection, EXO70B2 participates in papillae and encasement formation [109].…”

SEC1A and SEC6 synergistically regulate pollen tube polar growth

“…For example, immunofluorescence assays showed that SEC6 and SEC8 were localized at the tip of the tobacco pollen tube (Hála et al, 2008). SEC8 and SEC15a were localized at the pollen tube tip and plasma membrane in Arabidopsis (Bloch et al, 2016; Batystová et al, 2022). However, the localization of SEC5 in the pollen tube has not been reported thus far.…”