Trends in Plant Science
Gametophytic self-incompatibility inhibits pollen tube growth using different mechanisms
Section snippets
S-RNase-based self-incompatibility
Several families possess an S-RNase-based SI system (Table 1). In SI pollination, pollen germinates and grows normally on the stigma, and is only inhibited once it has grown some distance through the transmitting tract of the style. Analysis of stylar proteins led to the cloning of the pistil S-gene from Nicotiana alata some time ago, and this allowed the identification and cloning of many related S-haplotypes from members of the Solanaceae, and other species 3, 4.
Sequence data revealed that
Another type of gametophytic self-incompatibility system exists
A gametophytic system of SI control is also found in the field poppy, Papaver rhoeas. However, molecular and biochemical analysis has revealed that the stigmatic S gene and the mechanisms involved in pollen inhibition differ dramatically from that found in the Solanaceae.
Stigmatic S proteins and a postulated pollen receptor
The stigmatic S proteins from Papaver are small (∼15 kDa) secreted proteins. Several Papaver stigmatic S-haplotypes have been cloned [4]. Particular residues located in hydrophilic surface loops are crucial for the recognition of S1 pollen 26, 27. They are believed to act as signalling ligands because the recombinant protein triggers intracellular responses in incompatible pollen (see later). It is proposed that the stigmatic S proteins interact with the allelic product of the pollen S gene.
Conclusions: contrasting gametophytic self-incompatibility systems
It is clear that there is little in common between the two GSI systems. One uses a rather slow method: by uptake of RNase, to slow down incompatible pollen growth by stealth. The other uses a receptor–ligand interaction to trigger a Ca2+-signalling network to target several mechanisms that apparently act as ‘belt-and-braces’ to make sure pollen tubes are rapidly arrested and then ‘finished off’. It is intriguing that Papaver has such an apparently complex SI mechanism given that it is one of
Acknowledgements
Work in our laboratories is funded by the Biotechnology and Biological Sciences Research Council (BBSRC).
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