ACTIN2 is essential for bulge site selection and tip growth during root hair development of Arabidopsis

Institute of Plant Biology, University of Zurich, 8008 Zurich, Switzerland.
Plant physiology (Impact Factor: 6.84). 09/2002; 129(4):1464-72. DOI: 10.1104/pp.005777
Source: PubMed


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.

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Available from: Beat Keller, Jun 19, 2014
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    • "Vesicular trafficking is essential during root hair site selection, determination, and subsequent hair outgrowth (Richter et al., 2011). It is modulated by several structural changes of the actin cytoskeleton (Ringli et al., 2002), cell wall composition (Park et al., 2011), and by accumulation of structural sterols in the plasma membrane (Ovečka et al., 2010). Early endosomes visualized by GFP-RabA1d and YFP-VTI12 markers showed higher area fraction (cell occupation) than late endosomes. "
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    ABSTRACT: The dynamic localization of endosomal compartments labeled with targeted fluorescent protein tags is routinely followed by time lapse fluorescence microscopy approaches and single particle tracking algorithms. In this way trajectories of individual endosomes can be mapped and linked to physiological processes as cell growth. However, other aspects of dynamic behavior including endosomal interactions are difficult to follow in this manner. Therefore we characterized the localization and dynamic properties of early and late endosomes throughout the entire course of root hair formation by means of spinning disc time lapse imaging and post-acquisition automated multitracking and quantitative analysis. Our results show differential motile behavior of early and late endosomes and interactions of late endosomes that may be specified to particular root hair domains. Detailed data analysis revealed a particular transient interaction between late endosomes – termed herein as dancing-endosomes – which is not concluding to vesicular fusion. Endosomes preferentially located in the root hair tip interacted as dancing-endosomes and traveled short distances during this interaction. Finally, sizes of early and late endosomes were addressed by means of super-resolution structured illumination microscopy (SIM) to corroborate measurements on the spinning disc. This is a first study providing quantitative microscopic data on dynamic spatio-temporal interactions of endosomes during root hair tip growth.
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    • "High levels of ROP2 are able to rescue the fer phenotype, but they cannot restore auxin 332 responses (Duan et al., 2010;Huang et al., 2013). This set of data suggests that FER is required 333 for auxin-induced root hair growth and that it functions upstream of ROP signaling to trigger 334 root hair cell expansion (Figure 1C,Ringli et al., 2002;Preuss et al., 2006;Park and Nebenfuhr, 2013;Larson et al., 2014)methodology (Libault et al., 2010). Besides its outstanding contribution to tissue patterning 358 (Schiefelbein et al., 2009;Wang et al., 2010;Bruex et al., 2012), the root epidermis hasVelasquez et al., 2011;Ischebeck et al., 2013;Löfke et al., 2013;Ichikawa et 362 al., 2014;Kiefer et al., 2015;Löfke et al., 2015). "
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    ABSTRACT: Auxin is a crucial growth regulator in plants. However, a comprehensive understanding of how auxin induces cell expansion is perplexing, because auxin acts in a concentration- and cell-type dependent manner. Consequently, it is desirable to focus on certain cell types to exemplify the underlying growth mechanisms. On the other hand, plant tissues display supracellular growth (beyond the level of single cells) and, hence, other cell types might compromise the growth of a certain tissue. Tip-growing cells do not display neighbour-induced growth constrains and, therefore, are a valuable source of information for growth controlling mechanisms. Here we focus on auxin-induced cellular elongation in root hairs, exposing a mechanistic view of plant growth regulation. We highlight a complex interplay between auxin metabolism and transport, steering root hair development in response to internal and external triggers. Auxin signalling modules and downstream cascades of transcription factors define a developmental program, which appears rate-limiting for cellular growth. With this knowledge in mind, the root hair cell is a very suitable model system to dissect cellular effectors required for cellular expansion.
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    • "However, genetic analysis indicates that the two subclasses of vegetative actins have different function in multicellular development. Lack of the most abundant ACT2 gives rise to stunted and bulbous root hairs, but normal branching of leaf trichomes (Gilliland et al., 2002; Ringli et al., 2002; Kandasamy et al., 2009). In contrast, the other strongly expressed actin, ACT7, was suggested to affect seed germination, root growth and trichome branching, but not root hair development (Kandasamy et al., 2001, 2009; Gilliland et al., 2003). "
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