Divergent transport mechanisms for pyrimidine nucleosides in petunia pollen.

Petunia hybrida pollen exhibits divergent transport mechanisms for pyrimidine nucleosides. Uridine and cytidine show all the properties of being actively transported, a nucleoside transport mechanism not hitherto reported in plant cells. Contrasting with this, thymidine transport has the properties of a nonactive, carrier-mediated system. Reasons for these different mechanisms are considered to lie in the high demand for uridine and cytidine, obtained perhaps from stylar tissue, for the biosynthetic reactions of the pollen tube, while thymidine demand is lower due to the absence of DNA replication in germinating Petunia pollen.