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

Abstract

Membrane trafficking and cytoskeletal dynamics are important cellular processes that drive tip growth in root hairs. These processes interact with a multitude of signaling pathways that allow for the efficient transfer of information to specify the direction in which tip growth occurs. Here, we show that AGD1, a class I ADP ribosylation factor GTPase-activating protein, is important for maintaining straight growth in Arabidopsis (Arabidopsis thaliana) root hairs, since mutations in the AGD1 gene resulted in wavy root hair growth. Live cell imaging of growing agd1 root hairs revealed bundles of endoplasmic microtubules and actin filaments extending into the extreme tip. The wavy phenotype and pattern of cytoskeletal distribution in root hairs of agd1 partially resembled that of mutants in an armadillo repeat-containing kinesin (ARK1). Root hairs of double agd1 ark1 mutants were more severely deformed compared with single mutants. Organelle trafficking as revealed by a fluorescent Golgi marker was slightly inhibited, and Golgi stacks frequently protruded into the extreme root hair apex of agd1 mutants. Transient expression of green fluorescent protein-AGD1 in tobacco (Nicotiana tabacum) epidermal cells labeled punctate bodies that partially colocalized with the endocytic marker FM4-64, while ARK1-yellow fluorescent protein associated with microtubules. Brefeldin A rescued the phenotype of agd1, indicating that the altered activity of an AGD1-dependent ADP ribosylation factor contributes to the defective growth, organelle trafficking, and cytoskeletal organization of agd1 root hairs. We propose that AGD1, a regulator of membrane trafficking, and ARK1, a microtubule motor, are components of converging signaling pathways that affect cytoskeletal organization to specify growth orientation in Arabidopsis root hairs.

The Microtubule Plus-End Tracking Protein ARMADILLO-REPEAT KINESIN1 Promotes Microtubule Catastrophe in Arabidopsis

“…Because ARK1 is an uncharacterized kinesin belonging to an ungrouped plantspecific family of kinesins, its function cannot be predicted based on phylogeny (Reddy and Day, 2011;Zhu and Dixit, 2012). However, with previous evidence showing that ark1 mutants have root hairs with increased endoplasmic microtubule abundance and wavy/branched morphologies, it has been suggested that ARK1 specifically promotes microtubule depolymerization in root hairs (Sakai et al, 2008;Yoo et al, 2008).…”

“…Interestingly, previous studies using in vitro analyses have shown that the ARM domain of ARK1 is able to bind to actin (Yang et al, 2007) as well as the Never in Mitosis, Gene A (NIMA)-related kinase, NEK6 (Sakai et al, 2008). ARK1 is also thought to play a pivotal role in the tightly coordinated tip growth signaling pathway as a potential linker among the cytoskeletal, endomembrane, and GTPase signaling components (Yoo et al, 2008;Yoo and Blancaflor, 2013).…”

Regulation of membrane trafficking and organ separation by the NEVERSHED ARF-GAP protein

“…The ARF-GAP VASCULAR NETWORK DEFECTIVE3/SCARFACE (VAN3/SCF) and three closely related homologs -ARF-GAP DOMAIN1 (AGD1), AGD2 and AGD4 -together control vein patterning in developing leaves, potentially by affecting auxin signaling and transport (Koizumi et al, 2005;Sieburth et al, 2006). AGD1 also plays a key role in the orientation of root hair growth by influencing cytoskeletal organization (Yoo et al, 2008). ROOT AND POLLEN ARF-GAP (RPA) affects polarized tip growth of root hairs and of pollen tubes (Song et al, 2006).…”

The Folylpolyglutamate Synthetase Plastidial Isoform Is Required for Postembryonic Root Development in Arabidopsis

“…Components of these signaling and developmental networks include a dynamic cytoskeleton and vesicular trafficking system, both of which play pivotal roles in regulating membrane recycling and cell wall deposition to sustain normal root growth (Bernal et al, 2008;Yoo et al, 2008;Beck et al, 2010). Also part of the intricate developmental networks that govern root growth are a collection of transcription factors and an array of chemical messengers, including sugars, nutrients, amino acids, and most of the major plant hormones (Benková and Hejátko, 2009;Forde and Walch-Liu, 2009;Iyer-Pascuzzi et al, 2009).…”