Non-cell-autonomous regulation of crucifer self-incompatibility by Auxin Response Factor ARF3

In many angiosperms, outcrossing is enforced by genetic self-incompatibility (SI), which allows cells of the pistil to recognize and specifically inhibit "self" pollen. SI is often associated with increased stigma-anther separation, a morphological trait that promotes cross-pollen deposition on the stigma. However, the gene networks responsible for coordinate evolution of these complex outbreeding devices are not known. In self-incompatible members of the Brassicaceae (crucifers), the inhibition of "self"-pollen is triggered within the stigma epidermal cell by allele-specific interaction between two highly polymorphic proteins, the stigma-expressed S-locus receptor kinase (SRK) and its pollen coat-localized ligand, the S-locus cysteine-rich (SCR) protein. Using Arabidopsis thaliana plants that express SI as a result of transformation with a functional SRK-SCR gene pair, we identify Auxin Response Factor 3 (ARF3) as a mediator of cross-talk between SI signaling and pistil development. We show that ARF3, a regulator of pistil development that is expressed in the vascular tissue of the style, acts non-cell-autonomously to enhance the SI response and simultaneously down-regulate auxin responses in stigma epidermal cells, likely by regulating a mobile signal derived from the stylar vasculature. The inverse correlation we observed in stigma epidermal cells between the strength of SI and the levels of auxin inferred from activity of the auxin-responsive reporter DR5::GUS suggests that the dampening of auxin responses in the stigma epidermis promotes inhibition of "self" pollen in crucifer SI.