Programmed-cell-death hallmarks in incompatible pollen and papillar stigma cells of Olea europaea L. under free pollination

Programmed cell death (PCD) is a process that occurs both in animals and in plants and is an essential element in developmental processes. Pollination is a key factor in fruit production and self-incompatibility is one of the main limiting factors of this process. PCD has recently been put forward as a possible cause of pollen-growth arrest. As far as the olive is concerned, no data have been published concerning the mechanisms involved in hindering the growth of pollen tubes in incompatible pollen. Thus, we have studied olive pistils excised from freely pollinated flowers at different stages before and during the progamic phase using different cytochemical techniques, including trypan blue staining. To discover whether the elimination of incompatible pollen might be associated to PCD, we applied different tests to the excised pistils: (1) TUNEL assay; (2) DNA degradation analysis; (3) detection of caspase-3-like activity. Once we had determined that PCD was involved in pollen selection after free pollination, we conducted experiments after controlled pollination in pistils excised from flowers: (a) developing in the absence of pollen; (b) pollinated with sterile pollen that does not germinate; (c) self-pollinated; (d) pollinated with compatible pollen. Our results demonstrate that the growth of tubes in incompatible pollen is halted in the stylar area in a way that suggests the intervention of PCD. Furthermore, any pollen, even if sterile, seemed to accelerate PCD in papillar cells in the olive.