RECOMBINANT STIGMATIC SELF-INCOMPATIBILITY-(S-) PROTEIN ELICITS A CA2+ TRANSIENT IN POLLEN OF PAPAVER-RHOEAS

Evidence for Ca2+ signalling in pollen during the self-incompatibility (SI) response in Papaver rhoeas L. has been presented previously. However, it was not known whether the S-protein alone could act as an elicitor of the response or whether the presence of other stigmatic components was required, since relatively crude stigmatic extracts had been used. The S-1 gene has since been cloned and its product expressed in Escherichia coli has been shown to exhibit biological activity. In this paper it is reported that the recombinant protein (S(1)e) elicits a transient rise in [Ca2+](i) in incompatible pollen. The Ca2+ signal appears indistinguishable from that elicited by S-gene products partially purified from plant extracts in terms of both its timing and spatial distribution. Pollen tube growth is arrested directly after the rise in [Ca2+](i). The results provide direct evidence that the S-protein alone acts as an elicitor which triggers the Ca2+ signal for the pollen SI response. In addition, it is now clear that the recombinant S-protein does not require several post-translational processing events which take place in the plant to act as an elicitor. With respect to the spatial distribution of the Ca2+ transient, data are presented which correlate the localized rise in intracellular Ca2+ ([Ca2+](i)) with the 'nuclear complex' and the endoplasmic reticulum which is associated with this region.