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Root hair tip growth provides a unique model system for the study of plant cell polarity. Transgenic plants expressing constitutively active (CA) forms of ROP (Rho-of-plants) GTPases have been shown to cause the disruption of root hair polarity likely as a result of the alteration of actin filaments (AF) and microtubules (MT) organization. Towards understanding the mechanism by which ROP controls the cytoskeletal organization during root hair tip growth, we have screened for  CA-rop2  suppressors or enhancers using CA1-1, a transgenic line that expresses  CA-rop2  and shows only mild disruption of tip growth. Here, we report the characterization of a  CA-rop2  enhancer ( cae1-1  CA1-1) that exhibits bulbous root hairs. The  cae1-1  mutation on its own caused a waving and branching root hair phenotype.  CAE1  encodes the root hair growth-related, ARM domain-containing kinesin-like protein MRH2 (and thus  cae1-1  was renamed to  mrh2-3 ). Cortical MT displayed fragmentation and random orientation in  mrh2  root hairs. Consistently, the MT-stabilizing drug taxol could partially rescue the wavy root hair phenotype of  mrh2-3 , and the MT-depolymerizing drug Oryzalin slightly enhanced the root hair tip growth defect in CA1-1. Interestingly, the addition of the actin-depolymerizing drug Latrunculin B further enhanced the Oryzalin effect. This indicates that the cross-talk of MT and AF organization is important for the  mrh2-3  CA1-1 phenotype. Although we did not observe an apparent effect of the  MRH2  mutation in AF organization, we found that  mrh2-3  root hair growth was more sensitive to Latrunculin B. Moreover, an ARM domain-containing MRH2 fragment could bind to the polymerized actin  in vitro . Therefore, our genetic analyses, together with cell biological and pharmacological evidence, suggest that the plant-specific kinesin-related protein MRH2 is an important component that controls MT organization and is likely involved in the ROP2 GTPase-controlled coordination of AF and MT during polarized growth of root hairs.

A Mutation in MRH2 Kinesin Enhances the Root Hair Tip Growth Defect Caused by Constitutively Activated ROP2 Small GTPase in Arabidopsis