The Role of the Pollen Grain Coating in Pollination and Self-incompatibility in Brassica oleracea

Abstract

The presence of viscous coatings on the exine surface of many entomophiious pollen grains is well established and aspects of their synthesis (Heslop-Harrison, 1968; Dickinson and Lewis, 1973b; Dickinson, 1973) and enzymic properties (Knox and Heslop-Harrison, 1970; Vithange and Knox, 1976) have been reported. In some species, however, there is strong circumstantial evidence that these coatings are involved in systems of self-incompatibility (SI) (for review, see Dickinson, 1990). The SI mechanisms of flowering plants fall naturally into two groups; those in which pollen compatibility is gametophytically controlled by the haploid grain nucleus (e.g., Nicotiana sp.) and the other where it is sporophytically determined by the genes of the mother plant (e.g., Brassica sp.). There are a number of features of the latter type of SI which point to the incompatibility (S) allele product of the pollen being carried in the exine coating. First, the coat is derived from the tapetal cells (Dickinson and Lewis, 1973b) wh ch are part of the sporophyte; second, this type of SI acts at the stigma surface, rather than deep in the style, suggesting that some superficial component might be involved. Third, a number of recent molecular studies also indicate a role for this layer in SI; for example, Guilluy et al. (1991) have demonstrated that PCR generated probes derived from female S-allele sequences hybridise with anther messenger RNA, while Nasrallah and Nasrallah (1989) have reported the presence of a low abundance transcript in the maturing anther with some homology with SLG sequences. Further, in recent and highly significant work, it has been shown that promotor sequences directing the expression of the female S-alleles in Brassica oleracea will not only induce the expression of reporter and other genes in the anther tapetum when transformed into other members of the Cruciferae (Toriyama et al., 1991), but will also cause expression in the mature pollen of Nicotiana (Thorsness et al., 1991), a species with gametophytic SI, where S-locus specific sequences would be expected to be expressed by the haploid genome of the pollen.