Whole-organ analysis of calcium behaviour in the developing pistil of olive (Olea europaea L.) as a tool for the determination of key events in sexual plant reproduction.

Background: The pistil is a place where multiple interactions between cells of different types, origin, and function occur. Ca(2+) is one of the key signal molecules in plants and animals. Despite the numerous studies on Ca(2+) signalling during pollen-pistil interactions, which constitute one of the main topics of plant physiology, studies on Ca(2+) dynamics in the pistil during flower formation are scarce. The purpose of this study was to analyze the contents and in situ localization of Ca(2+) at the whole-organ level in the pistil of olive during the whole course of flower development.

Results: The obtained results showed significant changes in Ca(2+) levels and distribution during olive pistil development. In the flower buds, the lowest levels of detectable Ca(2+) were observed. As flower development proceeded, the Ca(2+) amount in the pistil successively increased and reached the highest levels just after anther dehiscence. When the anthers and petals fell down a dramatic but not complete drop in calcium contents occurred in all pistil parts. In situ Ca(2+) localization showed a gradual accumulation on the stigma, and further expansion toward the style and the ovary after anther dehiscence. At the post-anthesis phase, the Ca(2+) signal on the stigmatic surface decreased, but in the ovary a specific accumulation of calcium was observed only in one of the four ovules. Ultrastructural localization confirmed the presence of Ca(2+) in the intracellular matrix and in the exudate secreted by stigmatic papillae.

Conclusions: This is the first report to analyze calcium in the olive pistil during its development. According to our results in situ calcium localization by Fluo-3 AM injection is an effective tool to follow the pistil maturity degree and the spatial organization of calcium-dependent events of sexual reproduction occurring in developing pistil of angiosperms. The progressive increase of the Ca(2+) pool during olive pistil development shown by us reflects the degree of pistil maturity. Ca(2+) distribution at flower anthesis reflects the spatio-functional relationship of calcium with pollen-stigma interaction, progamic phase, fertilization and stigma senescence.