Characterization and differentiation of pollen lipidomes and proteomes from different intrafloral stamens in heterantherous Senna bicapsularis

Numerous compounds in pollen can affect the foraging decision-making of bees. Clarification of phytochemical components and identification of key substances for bee foraging preference in pollen are essential steps for apiculture and developing a conservation strategy. Senna bicapsularis, a heterantherous plant that possesses three different stamen types in the same flower, among which bees forage selectively, provides us with an ideal research model for identification of potential substances of bee foraging preference. The lipid and protein compositions of pollen from the anthers of different stamens of S. bicapsularis were investigated and compared. The polyunsaturated fatty acids (PUFAs) and monounsaturated FAs (MUFAs) were highest among lipid molecules in pollen from short (S) stamens than from long (L) and medium (M) stamens. This result is consistent with the FA content measurement, showing the highest FAs and UFAs content in S pollen, especially α-linolenic acid. We inferred that α-linolenic acid might be one of the key substances for bee foraging preference in pollen. Moreover, proteomic analysis showed that several differentially expressed proteins involved in lipid biosynthesis were highly accumulated in S pollen, such as choline kinase 2, 3-oxoacyl-ACP synthase-like protein and choline/ethanolamine phosphotransferase 1, in line with the highest FA content of S pollen. Additionally, DEPs involved in 'starch and sucrose metabolism', 'phenylpropanoid biosynthesis' and 'cyanoamino acid metabolism' were more represented in S compared with L and M pollen. The study suggests that differences in proteomic and lipidomic profiling among the three different stamen types might affect foraging decision-making of bumblebees.

Structure and development of anthers and connective glands in two species of Stigmaphyllon (Malpighiaceae): are heteromorphic anthers related to division of labour?

Heteranthery has been largely associated with a division of labour between anthers. Most species of Stigmaphyllon (Malpighiaceae) present heteromorphic anthers and glandular connectives of different development; yet, the functional meaning of this condition has never been explored in the genus. The aims of this study were to provide a comparative description of the structure and development of anthers and their connective glands in S. bonariense and S. jatrophifolium and to assess the existence of division of functions. Natural populations were selected to collect flowers at different stages. Anthers were subjected to morpho-anatomical, histochemical and pollen viability studies. For both species, abundance of pollen grains and size of anther and their connective glands were estimated. Three types of stamens are recognized: stamen with small, intermediate and large anthers. Anthers of both species exhibit a similar glandular tissue in the connective, and the histochemical analysis revealed that it produce a mucilagous secretion. The pattern of anther wall development, stainability and release of pollen grains was identical among anther types. For both species, we observed a positive relationship between anther size and abundance of pollen grains, but an inverse relationship between area of anthers and size (area and thickness) of connective glands in small anthers vs. intermediate and large ones. Our results evidence a specialization of anthers related to division of labour between heteromorphic stamens in two species of Stigmaphyllon. Thus, one set of anthers produces large amount of pollen grains for pollination and another sets large quantities of mucilage, which would improve pollen transport (better adherence to pollinator body and dampness maintenance). Nevertheless, heteranthery in both Stigmaphyllon species would represent a transitional state towards the division of labour rather than a stable state.