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Actin polymerization drives polar growth in Arabidopsis root hair cells.

ABSTRACT: In plants, the actin cytoskeleton is a prime regulator of cell polarity, growth, and cytoplasmic streaming. Tip growth, as observed in root hairs, caulonema, and pollen tubes, is governed by many factors, including calcium gradients, exocytosis and endocytosis, reactive oxygen species, and the cytoskeleton. Several studies indicate that the polymerization of G-actin into F-actin also contributes to tip growth. The structure and function of F-actin within the apical dome is variable, ranging from a dense meshwork to sparse single filaments. The presence of multiple F-actin structures in the elongating apices of tip-growing cells suggests that this cytoskeletal array is tightly regulated. We recently reported that sublethal concentrations of fluorescently labeled cytochalasin could be used to visualize the distribution of microfilament plus ends using fluorescence microscopy, and found that the tip region of the growing root hair cells of a legume plant exhibits a clear response to the nodulation factors secreted by Rhizobium. (1) In this current work, we expanded our analysis using confocal microscopy and demonstrated the existence of highly dynamic fluorescent foci along Arabidopsis root hair cells. Furthermore, we show that the strongest fluorescence signal accumulates in the tip dome of the growing root hair and seems to be in close proximity to the apical plasma membrane. Based on these findings, we propose that actin polymerization within the dome of growing root hair cells regulates polar growth.

SUBMITTER: Vazquez LA

PROVIDER: S-EPMC4203500 | biostudies-literature | 2014

REPOSITORIES: biostudies-literature

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Actin polymerization drives polar growth in Arabidopsis root hair cells.

Vazquez Luis Alfredo Bañuelos LA Sanchez Rosana R Hernandez-Barrera Alejandra A Zepeda-Jazo Isaac I Sánchez Federico F Quinto Carmen C Torres Luis Cárdenas LC

Plant signaling & behavior 20140101 8

In plants, the actin cytoskeleton is a prime regulator of cell polarity, growth, and cytoplasmic streaming. Tip growth, as observed in root hairs, caulonema, and pollen tubes, is governed by many factors, including calcium gradients, exocytosis and endocytosis, reactive oxygen species, and the cytoskeleton. Several studies indicate that the polymerization of G-actin into F-actin also contributes to tip growth. The structure and function of F-actin within the apical dome is variable, ranging from ...[more]

PMID: 25763621

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Project description:Background and aimsSelected beneficial Pseudomonas spp. strains have the ability to influence root architecture in Arabidopsis thaliana by inhibiting primary root elongation and promoting lateral root and root hair formation. A crucial role for auxin in this long-term (1week), long-distance plant-microbe interaction has been demonstrated.MethodsArabidopsis seedlings were cultivated in vitro on vertical plates and inoculated with pathogenic strains Pseudomonas syringae pv. maculicola (Psm) and P. syringae pv. tomato DC3000 (Pst), as well as Agrobacterium tumefaciens (Atu) and Escherichia coli (Eco). Root hair lengths were measured after 24 and 48h of direct exposure to each bacterial strain. Several Arabidopsis mutants with impaired responses to pathogens, impaired ethylene perception and defects in the exocyst vesicle tethering complex that is involved in secretion were also analysed.Key resultsArabidopsis seedling roots infected with Psm or Pst responded similarly to when infected with plant growth-promoting rhizobacteria; root hair growth was stimulated and primary root growth was inhibited. Other plant- and soil-adapted bacteria induced similar root hair responses. The most compromised root hair growth stimulation response was found for the knockout mutants exo70A1 and ein2. The single immune pathways dependent on salicylic acid, jasmonic acid and PAD4 are not directly involved in root hair growth stimulation; however, in the mutual cross-talk with ethylene, they indirectly modify the extent of the stimulation of root hair growth. The Flg22 peptide does not initiate root hair stimulation as intact bacteria do, but pretreatment with Flg22 prior to Psm inoculation abolished root hair growth stimulation in an FLS2 receptor kinase-dependent manner. These early response phenomena are not associated with changes in auxin levels, as monitored with the pDR5::GUS auxin reporter.ConclusionsEarly stimulation of root hair growth is an effect of an unidentified component of living plant pathogenic bacteria. The root hair growth response is triggered in the range of hours after bacterial contact with roots and can be modulated by FLS2 signalling. Bacterial stimulation of root hair growth requires functional ethylene signalling and an efficient exocyst-dependent secretory machinery.

Project description:BackgroundDendrite morphogenesis plays an essential role in establishing the connectivity and receptive fields of neurons during the development of the nervous system. To generate the diverse morphologies of branched dendrites, neurons use external cues and cell surface receptors to coordinate intracellular cytoskeletal organization; however, the molecular mechanisms of how this signaling forms branched dendrites are not fully understood.MethodsWe performed in vivo time-lapse imaging of the PVD neuron in C. elegans in several mutants of actin regulatory proteins, such as the WAVE Regulatory Complex (WRC) and UNC-34 (homolog of Enabled/Vasodilator-stimulated phosphoprotein (Ena/VASP)). We examined the direct interaction between the WRC and UNC-34 and analyzed the localization of UNC-34 in vivo using transgenic worms expressing UNC-34 fused to GFP.ResultsWe identify a stereotyped sequence of morphological events during dendrite outgrowth in the PVD neuron in C. elegans. Specifically, local increases in width ("swellings") give rise to filopodia to facilitate a "rapid growth and pause" mode of growth. In unc-34 mutants, filopodia fail to form but swellings are intact. In WRC mutants, dendrite growth is largely absent, resulting from a lack of both swelling and filopodia formation. We also found that UNC-34 can directly bind to the WRC. Disrupting this binding by deleting the UNC-34 EVH1 domain prevented UNC-34 from localizing to swellings and dendrite tips, resulting in a stunted dendritic arbor and reduced filopodia outgrowth.ConclusionsWe propose that regulators of branched and linear F-actin cooperate to establish dendritic branches. By combining our work with existing literature, we propose that the dendrite guidance receptor DMA-1 recruits the WRC, which polymerizes branched F-actin to generate "swellings" on a mother dendrite. Then, WRC recruits the actin elongation factor UNC-34/Ena/VASP to initiate growth of a new dendritic branch from the swelling, with the help of the actin-binding protein UNC-115/abLIM. Extension of existing dendrites also proceeds via swelling formation at the dendrite tip followed by UNC-34-mediated outgrowth. Following dendrite initiation and extension, the stabilization of branches by guidance receptors further recruits WRC, resulting in an iterative process to build a complex dendritic arbor.

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Actin polymerization drives polar growth in Arabidopsis root hair cells.

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Actin polymerization drives polar growth in Arabidopsis root hair cells.

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