DOI: 10.1093/jxb/erg015 Adhesion and guidance in compatible pollination

The mechanisms of compatible pollination are less studied than those of incompatible pollination and yet most of the angiosperms show self-compatibility. From the release of pollen from anthers to the pene-tration of the micropyle by the pollen tube tip, there are numerous steps where the interaction between pollen and the pistil can be regulated. Recent studies have documented some diverse ways in which pollen tubes carrying sperm cells are guided to the ovules through the pistil extracellular matrices of the trans-mitting tract. What is still missing is an understand-ing of pollen tube cell biology in vivo. A recent ®nding supports the role of the synergids in the cru-cial guidance cue for the pollen tube tip at the micro-pyle, but experimental evidence for other `guidepost' cells in the pistil is still lacking. The fact that the pol-len tube must ®rst travel through the matrices of the stigma and style before it can respond to the cue from the ovule makes it likely that there is a hier-archy of signalling events in pollen±pistil interactions starting at the stigma and ending at the micropyle. On the pistil side, several model systems have been used in the discovery of molecules implicated in either physical or chemical guidance. In lily, which has a hollow style, adhesion molecules (pectin and SCA) are implicated in guidance. SCA alone is also capable of inducing pollen chemotropism in an in vitro assay, suggesting that this peptide plays a dual role in lily pollination: chemotactic in the stigma and haptotactic (adhesion mediated) in the style