Unraveling male sterility in Paullinia coriacea (Sapindaceae): the combined roles of callose and stomata

Context Studies on the reproductive structure of Paullinia mainly focus on the gynoecium, fruit development, and the sexual system. Paullinia coriacea has unisexual flowers, but their sterility phenotypes have not been addressed. Aims To anatomically examine the flowers of P. coriacea and compare the functionality of the reproductive whorls in pistillate and staminate flowers, so as to better understand how male and female sterility are expressed in this species. Methods Flower buds and flowers were collected, fixed, and processed using standard procedures for light and scanning electron microscopy. Key results The anthers of the stamens in staminate flowers contain pollen grains at the binucleate microgametophyte stage. In contrast, pistillate flowers mainly have indehiscent staminodes that do not release their collapsed pollen grains. The staminode anther wall lacks stomata and wall thickenings in some endothecial cells. During microsporogenesis, the callose cell wall shows structural defects. The gynoecia of staminate flowers include an undeveloped stigma and style, collapsed ovary wall tissues, and ovules whose cells do not have a clear identity. Conclusions Male sterility in P. coriacea pistillate flowers results from failures in anther dehiscence, related to anther wall structure, and pollen grain collapse caused by defective callose deposition during microsporogenesis. Female sterility in staminate flowers is associated with the collapse of the ovary and ovule tissues. Implications These findings suggest that different mechanisms and their combinations lead to sterility phenotypes within the family and even the tribe.