Pollen coatings - Chimaeric genetics and new functions

Pollen coatings have long been assumed to play a pivotal role in pollen-stigma interactions, but until now little clear evidence supporting such a function has been available. Recently, however, the use of isolated coatings of Brassica sp. in experiments in vivo has unequivocally demonstrated that the pollen coat layer is responsible for activation of the stigmatic surface, and that it contains the male determinant of the self-incompatibility system. Surprisingly, molecular analysis of the Brassica pollen coat reveals this layer to include both sporophytic and gametophytic components, the latter including a family of small highly-charged proteins which interact with stigmatic molecules known to be encoded by the S(incompatibility)-locus. Most recently, work on Brassica and Arabidopsis suggests that the adhesive function of the coating is more complex than suspected and involves both stigmatic factors and the exine surface itself. Despite this new insight into the genetics and function of pollen coatings, the mechanisms by which components of these layers are formed in the tapetum and translocated to the pollen grain surface remain far from clear.