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.
Highlights
Membrane trafficking and cytoskeletal dynamics are important cellular processes that drive tip growth in root hairs
A forward genetics approach enabled us to identify two Arabidopsis mutants with wavy root hair growth but with no other obvious phenotype. The phenotype of these mutants was reminiscent of root hairs treated with microtubule-disrupting drugs (Bibikova et al, 1999), prompting us to hypothesize that the genetic lesions in these mutants might encode proteins that are important for cytoskeletal regulation
Map-based cloning of our other wavy root hair mutant revealed a disruption in AGD1, a gene encoding an ADP-ribosylation factor (ARF)-GTPase-activating proteins (GAPs) (Vernoud et al, 2003)
Summary
Membrane trafficking and cytoskeletal dynamics are important cellular processes that drive tip growth in root hairs. Whereas a change in growth direction in pollen tubes occurs partly in response to chemotropic factors from the stigma (Kim et al, 2003), root hairs maintain an endogenous polarity that guides the tip to grow straight and away from the primary root body (Bibikova et al, 1997) In both cases, calcium appears to play an important role in maintaining tip growth, since artificially relocalizing the tip-focused cytoplasmic calcium gradient using caged calcium ionophores resulted in a change in tip growth direction that often followed the new calcium gradient (Malho and Trewavas, 1996; Bibikova et al, 1997). Whereas conversion of the inactive to the active GTPase is facilitated by guanine nucleotide exchange factors (GEFs), inactivation occurs through the action of GTPase-activating proteins (GAPs), which stimulate the hydrolysis of GTP to GDP (Bos et al, 2007)
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