In many plant species, when pollen grains (containing the male gametes) land on the pistil (female reproductive organ) of flowers from the same plant or genetically identical plants, the self-incompatibility (SI) response is triggered and growth of "self" pollen is prevented, resulting in death of germinating pollen tubes before fertilisation can occur (Newbigin and Vierstra 2003). This mechanism maintains genetic diversity in plant populations by preventing inbreeding.
The first plant Arm protein to be discovered, Brassica Arm-repeat containing 1 (ARC1), a U-box/Arm protein, interacts with S-locus receptor kinases (SRKs) (Gu et al. 1998). Activation of transmembrane SRKs by pollen proteins triggers the stigmatic SI response in Brassica (Newbigin and Vierstra 2003) and ARC1 is expressed specifically in the stigma. Antisense inhibition of ARC1 reduces SI, allowing "self" pollen to grow, suggesting that ARC1 positively regulates SI (Stone et al. 1999). Interestingly, ARC1 is phosphorylated by SRKs; this phosphorylation is required for ARC1 binding to SRKs in vitro (Gu et al. 1998). ARC1 functions as an E3 ubiquitin ligase in vitro, and probably also in vivo, as ARC1 protein co-localises with the proteasome in a U-box-dependent manner when SRK is active (Stone et al. 2003). Interestingly, "self" pollen is able to germinate and grow on pistils treated with proteasome inhibitors. Together these data suggest that ARC1-mediated SI requires active proteasomal degradation in the stigma (Stone et al. 2003). Thus, during SI, active SRK activates ARC1, possibly by phosphorylation. ARC1 then probably interacts both with E2 ubiquitin ligases (via its U-box) and with target proteins (via the Arm repeats) that become ubiquitinated and degraded by the proteasome (Fig. 2). This allows the rejection and death of non-compatible "self" pollen.
Interestingly, a novel Arabidopsis U-box/Arm protein, AtPUB8, modifies the SI response in self-incompatible Arabidopsis species (Liu et al. 2007). At-PUB8 is not the closest Arabidopsis relative of ARC1 (see Sect. 4.2), suggesting that more than one U-box/Arm protein may regulate cell death during self-incompatibility in the Brassicaceae. Liu et al. (2007) also propose additional, non-SI functions for AtPUB8.
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