Unraveling the distinct germination processes of sporopollenin-based pollen grains and spores through morphological analyses upon natural nano-architectonics process

Abstract

The outermost exine capsules of many pollen and spore grains are composed of a chemically inert yet mechanically robust sporopollenin biopolymer. These dynamically expansible and foldable capsules have great potential as renewable functional biomaterials with industrial applications. However, the mechanical and morphological variations in the shape, size, robustness, and apertural strength of the exine capsules across taxa of angiosperms and cryptogamic plants remains poorly understood. Thus, in this study, we unraveled the abortive microgel transformation of spores inspired by their germination mechanism, being compared with eudicot-based pollen microgels. After chemical treatments, significant mechanical degradation of exine was clearly observed for the Camellia pollen, whereas crosslinking density and modulus of spore exine remained almost constant. The significant volume expansion of Camellia pollen was observed akin to sunflower pollen; in contrast, the spores ballooned showed limited volume changes under equal levels of turgor pressure. Furthermore, spore underwent marked changes in volume when their aperture sutures were softened and ruptured, which are prerequisites for spore germination. Therefore, this study disentangled mechanical and morphological origins of biochemical pathways of pollen and spore germination, and germination-like hydration and desiccation, which will give clues about selection of pollen and spore species for potential biomaterial applications.