The role of Ca2+ and Ca2+ channels in the gametophytic self-incompatibility of Pyrus pyrifolia

- self-S-RNase selectively blocks the activity of Ca2+ channels at apical pollen tube.
- Ca2+ conductance was suppressed by self-S-RNase.
- self-S-RNase disrupts the Ca2+ gradient prior to degrading RNA in the pollen tube.

In S-RNase-based self-incompatibility, S-RNase was previously thought to function as a selective RNase that inhibits pollen whose S-haplotype matches that in the pistil. In this study, we showed that S-RNase has a distinct effect on the regulation of Ca2+-permeable channel activity in the apical pollen tube in Pyrus pyrifolia. While non-self S-RNase has no effect, self S-RNase decreases the activity of Ca2+ channels and disrupts the Ca2+ gradient at the tip of the growing pollen tube during the gametophytic self-incompatibility (GSI) response. Extracellular Ca2+ influx was suppressed 5 min after self S-RNase treatment, and self-pollen tube growth was reduced at 50 min after self S-RNase treatment. In the self-incompatible response, the expression of Ca2+-related genes was inhibited before RNA degradation. Therefore, self S-RNase suppresses Ca2+ influx prior to arresting pollen tube growth via RNA degradation.

A model for the arrest of pollen-tube growth by self-S-RNase in Pyrus pyrifolia. Pollen and stigma started to mutually recognize as soon as pollen dropped on the stigma. If the pollen was nonself-pollen after recognizing, the Ca2+ channel activity was not affected and maintained the normal Ca2+ gradient at apical pollen tube. Vesicles fuse with the plasma membrane of the apical pollen tube forming more plasma membrane. At same time, the precursor substances of the cell wall are released, resulting in pollen tube elongation [1]. The S-RNase was discharged extracellular (A). However, Ca2+ channels at tip of pollen tube was decreased by self-S-RNase. The Ca2+ gradient was destroyed. The vesicle opened within pollen tube. The S-RNase further degraded RNA as cytotoxin (B). As a result, pollen tube growth slows and arrests.[1] A.H. de Win, E.S. Pierson, J. Derksen, Rational analyses of organelle trajectories in tobacco pollen tubes reveal characteristics of the actomyosin cytoskeleton, Biophys. J., 76 (1999) 1648-1658.143