Relationship between Tapetum Cells and Microspores Based on Protein Localization in Fraxinus angustifolia (Oleaceae) Pollen Grains

This study contributes to the knowledge of the traffic of substances between tapetum cells and microspores through the localization of pollen proteins during microsporogenesis, by using immunocytochemical techniques with TEM. Because the Oleaceae genera, including Olea and Fraxinus, share a very similar protein profile, an Ole e 1-like protein (which cross-reacts with the Olea europaea major allergen Ole e 1) was detected in Fraxinus angustifolia pollen grains. From the tetrad to mature-pollen grain stages, distinct labeling intensities were localized in the tapetal cells, anther locule, and orbicules as well as in the microspore and pollen cytoplasms and walls. The localization of this protein in the glycocalyx proves the special role of this layer in ectexine development. At exine deposition stages, the detection of immunogold particles in the anther locule and orbicules can be interpreted as showing an active transfer of these proteins from the tapetum cells to microspores. Moreover, at the bicellular stage, the diffusion of proteins from the pollen cytoplasm to the intine channels indicates their contribution to intine formation. These findings provide new evidence of the role of sporophytic and gametophytic tissues in pollen grain development. Moreover, the Ole e 1-like protein may have an important role in modifying pollen grain walls.