DOWNREGULATION OF ARABINOGALACTAN PROTEINS DURING SPERMATOGENESIS IN THE MOSS PHYSCOMITRIUM PATENS

Seedless plants utilize flagellated sperm cells for reproduction that develop using a series of cell walls resulting in naked motile cells. Arabinogalactan Proteins (AGPs) have been shown to play an important role in the maturation of sperm in ferns by an unknown mechanism. We sought to identify AGPs expressed in the spermatids of the moss Physcomitrium patens to identify this mechanism because it is amenable to genetic manipulation. We tracked the expression of 121 putative AGP-encoding genes across three time points of development with RNAseq and quantified total AGPs to compare to the fern Ceratopteris richardii. Unexpectedly, AGP genes and proteins were significantly downregulated in P. patens. BURP domain-containing genes, which are expressed in pollen of angiosperms, were highly upregulated and may serve similar roles to the AGPs of ferns. This study shows that the fern cell walls do not share as significant of a need for AGPs of developing sperm in bryophytes and this may be related to the number of flagella found in the respective lineages.