UI operates at the intergeneric level between Brassica and Arabidopsis when Arabidopsis species act as the pollen donor and the reciprocal crosses are compatible. Interestingly the strength of incompatibility was found to be greater in B. oleracea x A. thaliana crosses than that of A. lyrata x A. thaliana crosses, suggesting evolutionary distance between species influences the effectiveness of UI. Transmission electron microscopy analyses of UI pollinations within Arabidopsis revealed that secretory activity of the stigmatic papilla cell is not shut down, as is the case for SI, but in common with SI, is accompanied by autophagy-like activity, but is markedly more severe. Thus A. lyrata stigmatic papillae displayed a stress-like incompatibility response when incompatible A. thaliana pollen was applied. Our data has reinforced the consensus that the S-locus is not involved in UI in the Brassicaceae, but that SI and UI possibly share a universal downstream pollen-rejection pathway. Pollen coat ‘swap’ experiments were also carried out that demonstrated that factors carried by the Brassica pollen coat play an important role in the interspecific pollen-stigma interaction. Normally fully incompatible crosses of B. oleracea x A. thaliana were modulated by the presence of Brassica pollen coat such that A. thaliana pollen was able to hydrate and germinate on the B. oleracea stigma. Notably, these ‘compatibility factors’ appear to operate, and potentially be shared, across these genera, as the Brassica pollen coat could also overcome Arabidopsis UI by permitting A. thaliana pollen tube penetration of A. lyrata stigmatic papillae. A hypothetical model is proposed to explain UI based on rapidly evolving breeding systems where both compatibility and incompatibility pathways operate and interact with one another in the stigma. The model also accounts for UI in wider crosses where incompatibility is largely a product of the inability of pollen to activate the ‘compatibility system’ rather than activation of the ‘incompatibility system’. In an attempt to uncover pollen factors that may play a role in interspecific pollination relationships proteomic profiling of pollen coat from Arabidopsis thaliana, Arabidopsis lyrata and Brassica oleracea was carried out. This study revealed a strikingly large number of small cysteine-rich proteins (CRPs) that have not been previously identified as pollen coat components. Some CRPs shared by all three species were found to be under positive selection, thus these make good candidates for factors that could operate in interspecific pollen recognition. The richness of the pollen coat proteome data sets obtained as part of this and related studies demonstrates the complexity of the pollen coat in the Brassicaceae and provides a source of targets that can be explored for their potential roles in plant reproduction.
|Date of Award||13 Nov 2018|
|Supervisor||James Doughty (Supervisor) & Roderick Scott (Supervisor)|