ROP/RAC GTPases are master regulators of cell polarity in plants, implicated in the regulation of diverse signaling cascades including cytoskeleton organization, vesicle trafficking, and Ca(2+) gradients [1-8]. The involvement of ROPs in differentiation processes is yet unknown. Here we show the identification of a novel ROP/RAC effector, designated interactor of constitutive active ROPs 1 (ICR1), that interacts with GTP-bound ROPs. ICR1 knockdown or silencing leads to cell deformation and loss of root stem-cell population. Ectopic expression of ICR1 phenocopies activated ROPs, inducing cell deformation of leaf-epidermis-pavement and root-hair cells [3, 5, 6, 9]. ICR1 is comprised of coiled-coil domains and forms complexes with itself and the exocyst vesicle-tethering complex subunit SEC3 [10-13]. The ICR1-SEC3 complexes can interact with ROPs in vivo. Plants overexpressing a ROP- and SEC3-noninteracting ICR1 mutant have a wild-type phenotype. Taken together, our results show that ICR1 is a scaffold-mediating formation of protein complexes that are required for cell polarity, linking ROP/RAC GTPases with vesicle trafficking and differentiation.

No abstract

Rho GTPases regulate the actin cytoskeleton, exocytosis, endocytosis, and other signaling cascades. Rhos are subdivided into four subfamilies designated Rho, Racs, Cdc42, and a plant-specific group designated RACs/Rops. This research demonstrates that ectopic expression of a constitutive active Arabidopsis RAC, AtRAC10, disrupts actin cytoskeleton organization and membrane cycling. We created transgenic plants expressing either wild-type or constitutive active INTRODUCTIONRho GTPases are molecular switches best known for regulating actin organization (Hall, 1998). Rhos are subdivided into four subfamilies designated Rho, Racs, Cdc42, and a plant-specific group designated RACs or Rops (Hall, 1998;Winge et al., 2000;Yang, 2002). In animal cells, Rhos, Racs, and Cdc42 differentially regulate the actin cytoskeleton (Hall, 1998). Similarly, plant RACs are shown to regulate actin organization (Fu et al., 2001Molendijk et al., 2001;Jones et al., 2002;Yang, 2002;Chen et al., 2003;Cheung et al., 2003).Rhos seem to regulate exocytosis and endocytosis events such as pinocytosis, endocytosis of clathrin-coated pits, and localization of the multiprotein vesicle-tethering complex, the exocyst (Ridley et al., 1992;Lamaze et al., 1996;Di Cesare et al., 2000;Donaldson and Jackson, 2000;Malecz et al., 2000;Guo et al., 2001;Etienne-Manneville and Hall, 2002). Vesicle transport can be divided into five major steps: budding from a source membrane, targeting of the vesicle to specific regions, priming, docking at the target membrane, and fusion of the vesicles with the target membrane (Pfeffer, 1994(Pfeffer, , 2001Jurgens and Geldner, 2002). In yeast, Cdc42 and Rho1 have been shown to regulate homotypic vesicle docking during vacuole formation in an actin-dependent manner (Eitzen et al., 2001(Eitzen et al., . 2002Muller et al., 2001;Eitzen, 2003), whereas Rho3 and Cdc42 regulate vesicle docking late in exocytosis during polar growth in budding yeast independent of their role in actin polarization (Adamo et al., 1999(Adamo et al., , 2001. In addition, it is well established that actin cytoskeleton function is crucial for endocytosis (Engqvist-Goldstein and Drubin, 2003).Like other members of the Ras superfamily of small GTPases, Rhos exist in either GTP-bound active state or GDP-bound inactive state. Rhos have an intrinsic GTPase activity that is enhanced via interaction with GTPase-activating proteins. Activation of the Rhos occurs via interaction with GDP/GTP exchange factors (GEFs). Conserved dominant mutations abolishing the GTPase activity render Rhos constitutive active. Other conserved mutations preventing the GDP/GTP exchange are thought to cause irreversible interactions between the mutant Rhos and GEFs, converting the former dominant negative mutants (Hall, 1998;Winge et al., 2000;Yang, 2002).The plant-specific Rho subfamily, designated either RACs or Rops, is subdivided into two major subgroups called type-I and type-II (Winge et al., 2000;Yang, 2002;Christensen et al., 2003). All type-I RACs are putatively prenylated, whe...

No abstract

In plants, auxin distribution and tissue patterning are coordinated via a feedback loop involving the auxin-regulated cell polarity factor ICR1 and the secretory machinery.

Patterning of the root xylem into protoxylem (PX) and metaxylem is regulated by auxin-cytokinin signaling and microRNA miR165a/166bmediated suppression of genes encoding Class III HOMEODOMAIN LEU-ZIPPER (HD-ZIPIII) proteins. We found that, in Arabidopsis, osmotic stress via core abscisic acid (ABA) signaling in meristematic endodermal cells induces differentiation of PX in radial and longitudinal axes in association with increased VND7 expression. Similarly, in tomato, ABA enhanced PX differentiation longitudinally and radially, indicating an evolutionarily conserved mechanism. ABA increased expression of miR165a/166b and reduced expression of the miR165a/166b repressor ZWILLE (ZLL) (also known as ARGONAUTE10), resulting in reduced levels of all five HD-ZIPIII RNAs. ABA treatments failed to induce additional PX files in a miR165a/166b-resistant PHB mutant, phb1-d, and in scr and shr mutants, in which miR165a/166b expression is strongly reduced. Thus, ABA regulates xylem patterning and maturation via miR165a/ 166b-regulated expression of HD-ZIPIII mRNAs and associated VND7 levels. In lateral root initials, ABA induced an increase in miR165a levels in endodermal precursors and inhibited their reduction in the future quiescent center specifically at preemergence stage. Hence, ABA-induced inhibition of lateral root is associated with reduced HD-ZIPIII levels.

No abstract

No abstract