Overlapping and divergent signaling pathways for ARK1 and AGD1 in the control of root hair polarity in Arabidopsis thaliana

Abstract

We previously showed that seedlings harboring mutations in genes encoding ARK1, an armadillo repeat-containing kinesin, or AGD1, a class 1 ARF-GAP have root hairs that exhibit wavy/spiral growth and two tips originating from one initiation site. These root hair defects were accompanied by bundling of endoplasmic microtubules and filamentous actin (F-actin) that extended to the extreme root hair apex. The similar phenotypes of arkl and agdl mutants suggest a tight coordination between the cytoskeleton and membrane trafficking in the control of root hair polarity. Indeed, cell biological and genetic studies of the agdl mutant provided evidence that AGD1's involvement in root hair development involves cross-talk among phosphoinositides (PIs), the actin cytoskeleton and other small GTPases such as ROP2 and RABA4b. Here we show that arkl root hairs mirror those of agdl with regard to altered targeting of ROP2 and RABA4b, as well as abnormal tonoplast organization. Furthermore, like agdl, enhanced root hair defects in double mutants in ARK1 and genes encoding a type B phosphatidylinositol-4-phosphate 5-kinase 3 (PIP5K3), a phosphatidylinositol-4-phosphate (PI-4P) phosphatase (RHD4), a phosphatidylinositol transfer protein (COW1), and a vegetative actin isoform (ACT2), were observed. However, root hair shape of some arkl double mutant combinations, particularly those with act2, pip5k3 and rhd4 (arkl act2, arkl pip5k3, arkl rhd4), differed in some respects from agdl act2, agdl pip5k3, and agdl rhd4. Taken together our results continue to point to commonalities between ARK1 and AGD1 in specifying root hair polarity, but that these two modulators of tip-growth can also regulate root hair development through divergent signaling routes with AGD1 acting predominantly during root hair initiation and ARK1 functioning primarily in sustained tip growth. © 2013 Yoo and Blancaflor.