Signalling Systems Regulating Pollen-Pistil Interactions in the Crucifer family
In flowering plants, the process of fertilization begins with pollen landing on the stigma (at top of the pistil). This is followed by pollen germination and pollen tube growth down the pistil to fertilize the ovule. Our research is focussed on the members of the crucifer family (Brassica and Arabidopsis) where complex intracellular signalling takes place in the stigma to identify and accept compatible pollen (Figure 1) while rejecting self-incompatible pollen (Figure 2) and ignoring foreign pollen. In a search for factors that act early in the compatible pollen responses, we have identified an exocyst subunit, Exo70A1, as a critical component in the stigma to promote pollen hydration and pollen tube growth. Exo70A1 is thought to facilitate the delivery of secretory vesicles to the plasma membrane to release essential factors for the pollen grain, and we are currently exploring this model in more detail.
In contrast to the compatible pollen recognition system is the self-incompatibility system which prevents self-fertilization and inbreeding. In the self-incompatibility response, a stigma-specific S Receptor Kinase (SRK) is responsible for binding the SP11/SCR ligand from the self-incompatible pollen and activates a signal transduction pathway in the stigmatic papilla to reject this pollen. My research group has identified three proteins which are involved in this signalling pathway, THL1, THL2 and ARC1. THL1 and THL2 belong to the thioredoxin h family and act as negative regulators of SRK. ARC1 is an E3 ubiquitin ligase which is activated downstream of SRK and is proposed to target compatibility factors for ubiquitination and degradation of the 26S proteasome. These compatibility factors are normally required in the compatible pollen response, and ARC1’s inhibition of these factors causes pollen rejection. Exo70A1 is one compatibility factor that we have identified which can bind to ARC1 and is ubiquitinated by ARC1. We are currently exploring in more detail the mechanism by which ARC1 is able to inhibit Exo70A1 in vivo to cause pollen rejection.
Figure 1: Model of Cellular Responses to Compatible Pollen
Figure 2: Model of Cellular Responses to Self-Incompatible Pollen
Web Person: Daphne Goring
© 2007 Department of Cell & Systems Biology