The Microtubule-Associated Protein MAP18 Affects ROP2 GTPase Activity during Root Hair Growth

Abstract

Establishment and maintenance of the polar site are important for root hair tip growth. We previously reported that Arabidopsis (Arabidopsis thaliana) MICROTUBULE-ASSOCIATED PROTEIN18 (MAP18) functions in controlling the direction of pollen tube growth and root hair elongation. Additionally, the Rop GTPase ROP2 was reported as a positive regulator of both root hair initiation and tip growth in Arabidopsis. Both loss of function of ROP2 and knockdown of MAP18 lead to a decrease in root hair length, whereas overexpression of either MAP18 or ROP2 causes multiple tips or a branching hair phenotype. However, it is unclear whether MAP18 and ROP2 coordinately regulate root hair growth. In this study, we demonstrate that MAP18 and ROP2 interact genetically and functionally. MAP18 interacts physically with ROP2 in vitro and in vivo and preferentially binds to the inactive form of the ROP2 protein. MAP18 promotes ROP2 activity during root hair tip growth. Further investigation revealed that MAP18 competes with RhoGTPase GDP DISSOCIATION INHIBITOR1/SUPERCENTIPEDE1 for binding to ROP2, in turn affecting the localization of active ROP2 in the plasma membrane of the root hair tip. These results reveal a novel function of MAP18 in the regulation of ROP2 activation during root hair growth.

Interplay between Ions, the Cytoskeleton, and Cell Wall Properties during Tip Growth

“…Kang et al (2017) found that MAP18 and ROP2 are in the same genetic pathway and that they interact physically with MAP18, preferentially binding the inactive form of ROP2 and thereby promoting ROP2 activity. Kang et al (2017) went on to show that MAP18 competes with the ROP2 guanine dissociation inhibitor for binding to ROP2. From these and earlier studies (Wang et al, 2007;Zhu et al, 2013), Kang et al (2017) suggest that MAP18 functions through ROP2 to reinforce polarity and independently of ROP2 by modulating the cytoskeleton.…”

“…To investigate MAP18's function in root hairs, Kang et al (2017) tested for genetic and physical interactions between MAP18 and Rho/Rac of Plants2 (ROP2), a small GTPase considered to be a master regulator of tip growth in root hairs (Jones et al, 2002). Kang et al (2017) found that MAP18 and ROP2 are in the same genetic pathway and that they interact physically with MAP18, preferentially binding the inactive form of ROP2 and thereby promoting ROP2 activity. Kang et al (2017) went on to show that MAP18 competes with the ROP2 guanine dissociation inhibitor for binding to ROP2.…”

“…Interestingly, MAP18 localizes similarly in root hairs (Table II), and MAP18 knockdown plants have root hairs half the length of wild-type plants (Kang et al, 2017). To investigate MAP18's function in root hairs, Kang et al (2017) tested for genetic and physical interactions between MAP18 and Rho/Rac of Plants2 (ROP2), a small GTPase considered to be a master regulator of tip growth in root hairs (Jones et al, 2002).…”

“…Kang et al (2017) went on to show that MAP18 competes with the ROP2 guanine dissociation inhibitor for binding to ROP2. From these and earlier studies (Wang et al, 2007;Zhu et al, 2013), Kang et al (2017) suggest that MAP18 functions through ROP2 to reinforce polarity and independently of ROP2 by modulating the cytoskeleton.…”

ROP GTPases Structure-Function and Signaling Pathways

“…Interestingly, RHs developed in pluripetala (plp) mutant plants, which lack protein prenylation, suggesting that type II ROPs possibly also contribute to RH initiation (Running et al, 2004;Sorek et al, 2011;Chai et al, 2016). ROP2 activity also is regulated by Microtubule Associated Protein18, which interacts preferentially with GDP-bound ROP2 and competes with RhoGDI (Kang et al, 2017).…”