Ultrastructural and transcriptome changes of free-living sporangial
filaments in Pyropia yezoensis affected by light and culture
density
Abstract
In the life cycle of Pyropia yezoensis, sporangial filaments
connect conchocelis and thallus, but the mechanisms of maturation and
conchospore release of sporangial filaments are poorly understood. We
found that the morphological change from vegetative growth form (hollow
cells) to reproductive form (bipartite cells), and the release of
conchospores from bipartite cells were all closely correlated with
culture density and light intensity. Bipartite cells formed at low
density (50–1,000 fragments/mL) and when stimulated by high light
levels (40–100 µmol photons m−2
s−1), but conchospore release was inhibited at such
light intensities. At high densities (5,000–10,000 fragments/mL),
sporangial filaments retained the hollow cell morphology and rarely
formed bipartite cells. Ultrastructural observation showed that the
degradation of autophagosome-like structures in vacuoles caused the
typical hollow form. Transcriptome analysis indicated that adaptive
responses to environmental changes, mainly autophagy, endocytosis and
phosphatidylinositol metabolism, caused the morphological transformation
of free-living sporangial filaments. Meanwhile, the extensive promotion
of energy accumulation under high light levels promoted vegetative
growth of sporangial filaments, and thus inhibited conchospore release
from bipartite cells. These results provide a theoretical basis for
maturation of sporangial filaments and release of conchospores in
P. yezoensis and other related species.