Morphological insights into the pollen–stigma interaction in Mimosa caesalpiniifolia Benth. (Leguminosae): exploring the lock-and-key mechanism

Mimosa caesalpiniifolia Benth. is a bee-pollinated legume tree with significant reproductive success which is widely distributed among Brazilian vegetations. Its reproductive success is attributed to an efficient pollen–stigma interaction. Thus, our objective was to understand the pollen–stigma interaction of this species by describing the morphology of the pollen dispersal unit and the stigma. We aimed to identify characteristics that confirm the “key-lock” interaction process, anticipated in species with clustered pollen dispersion, as found in individuals of the Mimoseae tribe. Anthers and stigmas were sampled from unpollinated and pollinated flowers and processed for surface, anatomical, histochemical and ultrastructural analyses. The embryo sacs and seeds produced were counted in the ovaries and pods, respectively. The pollen is dispersed as acalymmate polyads that are symmetric, with a circular outline in polar view and an elliptical outline in front view; the major diameter mesuares 10.9 µm and the minor diameter, 7.8 µm. Each polyad is formed of eight pollen grains that have an areolate-verrucate exine, along with starch grains and protein bodies as reserve substances. The stigma is semidry, punctiform, with a central hole, cuticularized, composed of cells that secrete proteins, polysaccharides, phenolics, and oil. The discovery of eight pollen grains per polyad, with only one polyad deposited on each stigma, along with the presence of eight ovules per ovary, indicates a key-lock mechanism between the stigma and polyad. This mechanism promotes the fertilization of all eight embryo sacs and results in the production of nearly the same quantity of seeds per pod.