Title : A sporopollenin definition for the genomics age
Abstract:
Sporopollenin (SP) is the main polymeric component of the outer walls of embryophyte spores and pollen. Resistance to acetolysis had been the sole criterion for the identification of SP. This imprecise definition resulted in claims for the existence of SP in algae and misguided research on the evolution of SP and its roles during terrestrialization. ASCL (anther-specific chalcone synthase-like) and four other enzymes in the polyketide pathway are involved in the biosynthesis of SP. In this study, I propose that all plants with ASCL also possess the other two genes in the polyketide pathway and that all three enzymes (ACOS, ASCL, and TKPR) are embryophyte-specific. This would suggest that the three enzymes co-selected in the embryophyte lineage. I discovered that similarly to ASCL, ACOS and TKPR are also specific to embryophytes and not found in algae. Hence, we propose to expand the definition of SP to include ACOS and TKPR as follows: “Sporopollenin is a chemically resistant complex heteropolymer present in the outer walls of spores and pollen grains and is composed partly of hydroxylated polyketides derived from the conserved polyketide pathway, which involves CYP703, CYP704, ACOS, ASCL, and TKPR.”
