Abstract
Angiosperms reproduce throughout pollination, at which pollen grains move from anthers to stigma. After pollen capture, pollen hydration occurs throughout newly formed pollen foot (pollen adhesion) prior to germination1, whereas few reported in grass that an unusual phenomenon, called picolitre ‘pollen exudation’, occurs shortly after pollen capture2-5. However, the precise role(s) including its chemical composition are unknown. Here we show the exact dynamics of captured pollen grains during pollen exudation in rice. We found that the exudates originated from the grains ran down toward the receptive part to help the grains to self-position, causing ‘roly-poly toy’-like rocking motion to lead pollen adhesion. Furthermore, single-cell metabolomics6 suggests that high content of sugars, fatty acids, and redox-related metabolites site-specifically determined in the exudates likely participate in osmotic and molecular signaling in stigmatic apoplast, increasing the fluid viscosity. Altering the center of gravity in an object causes a rocking motion, and thus the observed pollen behavior suggests possible within-grain organelle rearrangements attributed to the metabolic changes induced at pollen capture. Therefore, we anticipate that pollen exudation plays a crucial role on the optimal pollen adhesion that enables rapid pollination in rice. This study also illustrates significance of the event in grass family.