A role for ABIL3 in plant cell morphogenesis.

Actin nucleation facilitated by the ARP2/3 complex plays a central role in plant cell shape development. The molecular characterization of the distorted class of trichome mutants has recently revealed the SCAR/WAVE complex as an essential upstream activator of ARP2/3 function in plants. The SCAR/WAVE complex is conserved from animals to plants and, generally, is composed of the five subunits SCAR/WAVE, PIR121, NAP125, BRICK and ABI. In plants, four of the five subunits have been shown to participate in trichome and pavement morphogenesis. Plant ABI-like proteins (ABIL), however, which constitute a small four-member protein family in Arabidopsis thaliana, have not been characterized functionally, so far. Here we demonstrate that microRNA knock-down of the ABIL3 gene leads to a distorted trichome phenotype reminiscent of ARP2/3 mutant phenotypes and consistent with a crucial role of the ABIL3 protein in an ARP2/3-activating SCAR/WAVE complex. In contrast to ARP2/3 mutants, however, the ABIL3 knock-down stimulated cell elongation in the root, indicating distinct functions of the ABIL3 protein in different tissues. Furthermore, we provide evidence that ABIL3 associates with microtubules in vivo, opening up the intriguing possibility that ABI-like proteins have a function in linking SCAR/WAVE-dependent actin nucleation with organization of the microtubule cytoskeleton.