IDENTIFICATION OF POLLEN COMPONENTS REGULATING POLLINATION-SPECIFIC RESPONSES IN THE STIGMATIC PAPILLAE OF BRASSICA-OLERACEA

When components of the mature pollen grain of Brassica oleracea are ap plied to the cuticularised surfaces of the stigmatic papilla, a number of changes can take place in the architecture of the subjacent cell w all. Treatment of the papillar surface with isolated pollen coating ev okes a rapid and extensive expansion of the outer of the two stigmatic wall layers. This response occurs within 4 h but is restricted to tho se regions where no callose is formed and where the coating exhibits a characteristic increase in electron opacity. This study establishes t hat the coating alone is responsible for initiating the expansion of t he outer wall, which is considered to be an essential step preceding p enetration of the stigma surface by the pollen tube. Vesicle-like incl usions, some staining intensely, occasionally occur in regions of the wall expanded by isolated coating, sometimes fusing to form a subcutic ular 'boundary layer'. These structures are not observed under compati ble grains in vivo and their presence is regarded as artefactual. Howe ver, close examination of the plasma membrane at sites beneath areas o f expanded cell wall reveals membrane-bound structures resembling the vesicles being generated by the cytoplasm and moving into the apoplast . These data indicate that signals from the coating initiate the loose ning of the wall matrix by stimulating an unusual form of localized se cretion which appears to be an essential prerequisite for stigmatic pe netration. Isolated pollen coating contaminated with fragments of the grain itself engender a very different response, including the formati on of densely-staining vesicles accompanied by extensive accumulations of callose; wall expansion never occurs under these circumstances. Se lf-pollinations in B. oleracea are frequently accompanied by the synth esis of stigmatic callose, the presence of which has, in the past, bee n interpreted as forming a structural barrier to incompatible pollen t ubes. However, callose elicited both by self-pollinations and coating supplemented by killed grains is independent of protein synthesis and occurs in the presence of the phosphatase inhibitor, okadaic acid. Sin ce both these inhibitors also overcome self-incompatibility (SI) in B. oleracea, these data strongly suggest that callose synthesis is unrel ated to the operation of SI, and it is proposed that callose, when for med, is elicited by molecules released from the necrotic pollen protop last. Interestingly, isolated pollen coating of self- and cross-genoty pes does not readily elicit callose. The significance of these changes in the stigmatic wall is discussed in the perspective of current view s on pollination and self-incompatibility in Brassica.