Arabidopsis thaliana Rop GTPases are localized to tips of root hairs and control polar growth

Abstract

Plants contain a novel unique subfamily of Rho GTPases, vital components of cellular signalling networks. Here we report a general role for some members of this family in polarized plant growth processes. We show that Arabidopsis AtRop4 and AtRop6 encode functional GTPases with similar intrinsic GTP hydrolysis rates. We localized AtRop proteins in root meristem cells to the cross-wall and cell plate membranes. Polar localization of AtRops in trichoblasts specifies the growth sites for emerging root hairs. These sites were visible before budding and elongation of the Arabidopsis root hair when AtRops accumulated at their tips. Expression of constitutively active AtRop4 and AtRop6 mutant proteins in root hairs of transgenic Arabidopsis plants abolished polarized growth and delocalized the tip-focused Ca2+ gradient. Polar localization of AtRops was inhibited by brefeldin A, but not by other drugs such as latrunculin B, cytochalasin D or caffeine. Our results demonstrate a general function of AtRop GTPases in tip growth and in polar diffuse growth.

Functional analysis of barley RAC/ROP G‐protein family members in susceptibility to the powdery mildew fungus

“…The actin cytoskeleton can also be modi®ed by RAC/ROP proteins via the phosphoinositol pathway (Kost et al, 1999). Another RAC/ROP-dependent mechanism during pollen tube and root hair growth is the formation of a tip-focused Ca 2 accumulation that is needed for polar growth (Camacho and Malho, 2003;Fu et al, 2001;Molendijk et al, 2001). Calcium ions are also able to regulate the activity of the MLO protein via calmodulin (Kim et al, 2002), which links back to susceptibility in the barley±Bgh interaction.…”

“…The function of RAC/ROPs has been linked to rearrangement of both the cytoskeleton and subcellular Ca 2 gradients during polar pollen tube and root hair growth (e.g. Camacho and Malho, 2003;Gu et al, 2003;Jones et al, 2002;Molendijk et al, 2001). Intracellular Ca 2 , like the cytoskeleton, plays roles in both cellular accessibility and defence to fungal pathogens (Blume et al, 2000;Kim et al, 2002;Xu and Heath, 1998).…”

Directional cell expansion — turning toward actin

“…Previously, it has been established in plants that homologs of rho-GTPases, called rho-of-plants (ROP), participate in organizing both cortical actin and microtubules ( [39,40]; Xu and Scheres, this issue). This year, Fu et al [41 ] identified two CRIBmotif-containing ROP target proteins, RIC1 and RIC4, that mediate microtubule and actin organization via counteracting pathways.…”

A Class I ADP-Ribosylation Factor GTPase-Activating Protein Is Critical for Maintaining Directional Root Hair Growth in Arabidopsis

“…ROPs and Rabs have been shown to localize to the tips of root hairs and pollen tubes (Molendijk et al, 2001;Jones et al, 2002;Preuss et al, 2004), and consistent with its proposed function in polarized growth, tip localization of Rab4Ab was only observed in growing root hairs but not in mature root hairs that had ceased growing or root hair mutants with defective tip growth (Preuss et al, 2004). Furthermore, constitutively active or dominant negative ROPs altered tip growth, resulting in the formation of bulbous tips in pollen tubes (Kost et al, 1999;Li et al, 1999) and root hairs with wavy growth, multiple tips, and partially swollen tips (Molendijk et al, 2001;Jones et al, 2002;Bloch et al, 2005). Disrupting ROP GTPase function by the expression of constitutively active or dominant negative ROP has been shown to affect apical F-actin dynamics, calcium signaling, and ROS production.…”