Pollen Tube Growth in Incompatible Pistils and Style-grafted Pistils of ‘Satonishiki’ Sweet Cherry

Prunus fruit trees of the Rosaceae family exhibit S-RNase-based gametophytic self-incompatibility (GSI), which enables pistils to reject self-pollen by suppressing pollen tube elongation. In other plant species with S-RNase-based GSI, it has been shown that this suppression consists of two steps: first, slowing down of pollen tube elongation in the middle part of the style and second, complete arrest involving programmed cell death. To characterize the suppression pattern of incompatible pollen tubes in Prunus, we observed pollen tube elongation of ‘Satonishiki’ sweet cherry (Prunus avium) in ‘Satonishiki’ and ‘Rainier’ pistils on agar plates and ‘Satonishiki’ and ‘Rainier’ pollen tube growth in pistils on ‘Satonishiki’ cut branches. Incompatible selfed pollen tubes delayed penetration into the stigma in both experiments. Observation of pollen tubes in pistils on agar plates was difficult 24 h after pollination (HAP) due to wilting of the styles, while observing them on cut branches was possible up to 72 HAP. In the pistils on cut branches, ‘Satonishiki’ pollen tubes barely elongated in self pistils from 32 to 48 HAP when compatible ‘Rainier’ pollen tubes reached the base of a style, but resumed growth after 48 HAP and reached the base of the style. An RNase activity staining indicated that S-RNase was inactive 48 HAP. Finally, we observed pollen tube elongation in the style-grafted pistils on the cut branches. ‘Satonishiki’ pollen tube elongation was accelerated when the upper one-third of the self-pollinated styles was grafted onto compatible ‘Taishonishiki’ pistils. These results indicated that the suppression of incompatible pollen tube elongation in P. avium was consistently reversible. We discuss the suppression mechanism of incompatible pollen tube growth and the possibility of artificial control of Prunus self-incompatibility.