SEC6 exocyst subunit contributes to multiple steps of growth and development of Physcomitrella ( Physcomitrium patens )

Abstract

Spatially directed cell division and expansion is important for plant growth and morphogenesis, and relies on the cooperation between the cytoskeleton and secretory pathway. The phylogenetically conserved octameric complex exocyst mediates exocytotic vesicle tethering at the plasma membrane. Unlike other exocyst subunits of land plants, the core exocyst subunit SEC6 exists as a single paralog in Physcomitrium patens and Arabidopsis thaliana genomes. Arabidopsis SEC6 (AtSEC6) loss‐of‐function (LOF) mutation causes male gametophytic lethality. Our attempts to inactivate the P. patens SEC6 gene (PpSEC6) using targeted gene replacement produced two independent partial LOF (“weak allele”) mutants via the PpSEC6 gene locus perturbation. These mutants exhibited the same pleiotropic developmental defects: protonema with dominant chloronema stage, diminished caulonemal filament elongation rate, and post‐initiation gametophore development failure. Mutant gametophore buds, mostly initiated from chloronema cells, exhibited disordered cell file organization and cross‐wall perforations, resulting in arrested development at the 8 –10 cell stage. Complementation of both sec6 moss mutant lines by both PpSEC6 and AtSEC6 cDNA rescued gametophore development, including sexual organ differentiation. However, regular sporophyte formation and viable spore production were recovered only by the expression of PpSEC6, whereas the AtSEC6 complementants were only exceptionally fertile, indicating moss‐specific SEC6 functions.