Analysis of Pollen Behavior on Brassica Self-Incompatibility

Self-incompatibility (SI) is a system to prevent self-fertilization and maintain genetic diversity in flowering plants. In Brassicaceae, SI is sporophytically controlled by a single multiallelic locus named S-locus, and the number of S-alleles is estimated to be more than 100 in nature. Once compatible pollen grain is captured on stigmatic papilla, fertilization occurs through several sequential events including pollen adhesion, foot formation, pollen hydration, germination, pollen tube elongation, and penetration. In contrast, on incompatible pollination, it is considered that one of these events is inhibited therefore fertilization is unsuccessful. However, which events to be inhibited and factors involved in this inhibition remain unclear. In this study, we analyzed time-lapse imaging of pollen behaviors on pollination to reveal morphological characteristics of SI response. 

In the pollen grain diameter, the velocity and amount of pollen hydration varied between self- and cross-pollination, or individuals, and we classified the pollen behaviors into six patterns. All six patterns were observed in a stigma of each line but their ratios were varied on each line. Evaluation of the degree of SI/SC phonotype and the pollen viability revealed that germinated pollen on self-pollination didn’t penetrate into stigma, suggesting that pollen behavior on self-pollination is related to self-incompatibility. 

In conclusion, pollen behaviors in SI response showed various patterns such as hydration, dehydration and germination, and SI response is not always unidirectional. This suggests that SI pathway should be controlled by multistep inhibition of some events on pollination.