PerspectivePlant Science

Pollen Gets More Complex

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Science  05 Nov 2010:
Vol. 330, Issue 6005, pp. 767-768
DOI: 10.1126/science.1198346

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To prevent inbreeding, many flowering plants exhibit self-incompatibility (SI), a system for recognizing and rejecting their own pollen (1). In the Solanaceae, Rosaceae, and Plantaginaceae, the self-incompatibility trait is controlled by two physically linked S loci: the S-RNase gene representing the female S factor in the pistil and the S-locus F-box (SLF) gene representing the male S factor in the pollen (24) Both of these genes have many forms or alleles of S-RNase and SLF; a set of S-RNase and SLF alleles are inherited together as a single unit and are referred to as an S haplotype. The prevailing model for this system (see the figure) was that self pollen rejection occurs when self S-RNases enter the growing pollen tube and exert a cytotoxic effect, causing growth arrest. In contrast, SLF is required in compatible pollen tubes to block the action of non-self S-RNases, thus allowing for continued pollen tube growth and subsequent fertilization of the ovule. SLF is proposed to promote destruction of non-self S-RNases by the 26S-proteasome degradation pathway (24). However, there were some puzzling aspects of SLF that did not fully fit what would be expected for a pollen S factor, from both functional and evolutionary viewpoints (5). On page 796 of this issue, Kubo et al. (6) reveal that at least three related SLF genes encode pollen S factors in Petunia plants, and that these factors work collaboratively to prevent the pollen from being neutralized by non-self female cells. Their results represent a major shift in understanding how this SI system operates.