Directional cell expansion--turning toward actin.

Significant recent progress toward understanding directional expansion in diffusely growing plant cells concerns actin. Tools for imaging actin, including both live-cell reporters and fixation protocols, have been improved. Proteins that interact with actin have been identified and their functions probed biochemically and genetically. Specifically, members of the actin-related protein2/3 (ARP2/3) complex and the Wiskott-Aldrich syndrome Verprolin-homologous (WAVE) complex have been identified. These proteins have salient functions in shaping trichomes and leaf pavement cells. Additionally, two targets of a rho-of-plants (ROP) G-protein have been discovered that exert opposing regulatory action on actin and microtubules, a pathway that appears to be responsible for establishing the undulating shapes of pavement cells. Finally, several mutants of the fragile fiber class have revealed a link between actin organization, cell wall synthesis, and phosphoinositol signaling.