Fig. 2.
Biochemical model for compatible and incompatible pollinations and competitive interaction. (A) Incompatible pollination. An S2 pollen tube produces PiSLF2, and as it is growing in an S1S2 pistil, it takes up S1-RNase and S2-RNase from the style. PiSLF2 preferentially interacts with S1-RNase (a non-self S-RNase) to mediate its ubiquitination and degradation. S2-RNase (self S-RNase) is not affected and can exert its cytotoxic function in degrading pollen RNA. As a result, growth of the S2 pollen tube is arrested. (B) Compatible pollination. An S3 pollen tube produces PiSLF3 and, as it is growing in an S1S2 pistil, it takes up S1-RNase and S2-RNase from the style. PiSLF3 interacts strongly with S1-RNase and S2-RNase (non-self S-RNases), and mediates their ubiquitination and degradation. Thus, the S3 pollen tube is able to grow through the style to effect fertilization. (C) Competitive interaction. An S2S3 heteroallelic pollen tube produces both PiSLF2 and PiSLF3 and, as it is growing in an S2S3 pistil, it takes up S2-RNase and S3-RNase from the style. PiSLF2 preferentially interacts with S3-RNase to mediate its ubiquitination and degradation, and PiSLF3 preferentially interacts with S2-RNase to mediate its ubiquitination and degradation. As a result, the S2S3 pollen tube is able to grow through the style to effect fertilization.