Commonalities between pollen/stigma and host/pathogen interactions: calcium accumulation during stigmatic penetration by Brassica oleracea pollen tubes

Parallels have been explored between the early stages of stigmatic penetration by pollen tubes and the infection of epidermal cells by fungal pathogens. In a striking resemblence to events following the infection of Hordeum sp. by Erysiphe graminis, X-ray microanalysis has revealed the accumulation of calcium at the stigmatic surface following pollinations in Brassica oleracea. X-ray mapping strongly indicates the calcium to be localised at the points at which either the pollen grain or its tube makes contact with the surface of the stigmatic papilla. No definitive measures were made of the concentration of calcium at these sites, but controls indicated the levels to be well in excess of those found in the cytosol. X-ray microanalysis at the pollen/stigma interface failed to detect the presence of silicon, an element frequently accumulated by epidermal cells in response to pathogenic challenge. The phenylpropanoid biosynthesis pathway is activated by many plant hosts following infection by fungal pathogens, and the accumulation of autofluorescent material in the stigma 24 h after contact with self pollen strongly indicates this pathway also to be activated after pollination. The timing of this response, however, suggests that phenolic products are unlikely to be involved in the rejection of self pollen. These data are discussed in the perspective of current views of defence systems present in angiosperm epidermal cells, and why these mechanisms fail to identify and reject incompatible pollen tubes.