ACTIN2 Is Essential for Bulge Site Selection and Tip Growth during Root Hair Development of Arabidopsis

Abstract

Root hairs develop as long extensions from root epidermal cells. After the formation of an initial bulge at the distal end of the epidermal cell, the root hair structure elongates by tip growth. Because root hairs are not surrounded by other cells, root hair formation provides an excellent system for studying the highly complex process of plant cell growth. Pharmacological experiments with actin filament-interfering drugs have provided evidence that the actin cytoskeleton is an important factor in the establishment of cell polarity and in the maintenance of the tip growth machinery at the apex of the growing root hair. However, there has been no genetic evidence to directly support this assumption. We have isolated an Arabidopsis mutant, deformed root hairs 1 (der1), that is impaired in root hair development. The DER1 locus was cloned by map-based cloning and encodes ACTIN2 (ACT2), a major actin of the vegetative tissue. The three der1 alleles develop the mutant phenotype to different degrees and are all missense mutations, thus providing the means to study the effect of partially functional ACT2. The detailed characterization of the der1 phenotypes revealed that ACT2 is not only involved in root hair tip growth, but is also required for correct selection of the bulge site on the epidermal cell. Thus, the der1 mutants are useful tools to better understand the function of the actin cytoskeleton in the process of root hair formation.Cell division, growth, and differentiation are basic processes underlying plant development. Plant cell growth is a complex process, as the cells are surrounded by a cell wall that limits enlargement of the cell. Therefore, plant cell growth requires a wellcoordinated and tightly controlled expansion of the protoplast and the cell wall. Root hairs provide an excellent system for studying the process of cell growth. The root hair structure is not surrounded by other cells that would limit its expansion. As a consequence, aberrations in root hair development are readily observable. Root epidermal cells in Arabidopsis develop into root hair-forming cells (trichoblasts) or non-root hair-forming cells (atrichoblasts), depending on positional cues relative to the underlying cortical cells.

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

“…The actin cytoskeleton also is essential for maintaining tip-focused growth. Actin-disrupting drugs inhibited or delocalized root hair growth (Bibikova et al, 1999;Ketelaar et al, 2003), consistent with reports showing that knockouts to the vegetative ACT2 gene in Arabidopsis have distorted root hair morphology (Ringli et al, 2002). Moreover, altering the expression of genes encoding proteins that affect actin turnover, such as profilin, actin-depolymering factor, formin, actininteracting protein, and cyclase-associated protein, disrupted normal root hair development (Ramachandran et al, 2000;Dong et al, 2001;Yi et al, 2005;Deeks et al, 2007;Ketelaar et al, 2007).…”

Whole-Genome Comparison of Leucine-Rich Repeat Extensins in Arabidopsis and Rice. A Conserved Family of Cell Wall Proteins Form a Vegetative and a Reproductive Clade,

“…It was proposed that the initial separation between vegetative and reproductive actins was contemporary with and perhaps contingent upon the invention of new developmental pathways involved in the formation of vegetative organs from reproductive ones (Meagher et al, 1999). Similar to actins (Ringli et al, 2002), LRXs are possibly involved in cell morphogenesis, as demonstrated for AtLRX1 (Baumberger et al, 2001). Thus, the same evolutionary mechanisms might have driven the specialization of LRX genes into reproductive and vegetative forms.…”

Haustorial Hairs Are Specialized Root Hairs That Support Parasitism in the Facultative Parasitic Plant Phtheirospermum japonicum

“…Other genes, like HHD2, regulate root hair elongation through the regulation of various cellular components. Such genes include COW1, MYA2, RHD2, EXPA7, RHS2, RSL2, and ACTIN2 (Cho and Cosgrove, 2002;Ringli et al, 2002;Foreman et al, 2003;Böhme et al, 2004;Peremyslov et al, 2008;Won et al, 2009;Yi et al, 2010). For example, a widely characterized cellular component involved in root hair elongation is ROS, which is produced by NADPH activity.…”