Root Hair Formation: F-Actin-Dependent Tip Growth Is Initiated by Local Assembly of Profilin-Supported F-Actin Meshworks Accumulated within Expansin-Enriched Bulges

Abstract

Plant root hair formation is initiated when specialized elongating root epidermis cells (trichoblasts) assemble distinct domains at the plasma membrane/cell wall cell periphery complexes facing the root surface. These localities show accumulation of expansin and progressively transform into tip-growing root hair apices. Experimentation showed that trichoblasts made devoid of microtubules (MTs) were unaffected in root hair formation, whereas those depleted of F-actin by the G-actin sequestering agent latrunculin B had their root hair formation blocked after the bulge formation stage. In accordance with this, MTs are naturally depleted from early outgrowing bulges in which dense F-actin meshworks accumulate. These F-actin caps remain associated with tips of emerging and growing root hairs. Constitutive expression of the GFP-mouse talin fusion protein in transgenic Arabidopsis, which visualizes all classes of F-actin in a noninvasive mode, allowed in vivo confirmation of the presence of distinct F-actin meshworks within outgrowing bulges and at tips of young root hairs. Profilin accumulates, at both the protein and the mRNA levels, within F-actin-enriched bulges and at tips of emerging hairs. ER-based calreticulin and HDEL proteins also accumulate within outgrowing bulges and remain enriched at tips of emerging hairs. All this suggests that installation of the actin-based tip growth machinery takes place only after expansin-associated bulge formation and requires assembly of profilin-supported dynamic F-actin meshworks.

ADF Proteins Are Involved in the Control of Flowering and Regulate F-Actin Organization, Cell Expansion, and Organ Growth in Arabidopsis

“…Similar images of the actin cytoskeleton and the shape of Arabidopsis root hairs can be found in the literature (Miller et al, 1998;Baluska et al, 2000). It has been demonstrated that the longitudinally oriented actin cables in root hairs are essential for the elongation of these cells.…”

Root Hair-Specific EXPANSIN A7 Is Required for Root Hair Elongation in Arabidopsis

“…For example, expansins require acidic pH levels for optimum wall-loosening activity (Cosgrove, 2000) and, prior to hair bulge formation, sites of hair emergence exhibit local acidification (Bibikova et al, 1998). Moreover, expansin proteins localize at the root hair tip (Baluška et al, 2000) in Arabidopsis, and exogenous application of purified expansin proteins will cause the Arabidopsis root hair tip to burst . Previously, we identified two Arabidopsis expansin genes (AtEXPA7 and AtEX-PA18) that contain RHE consensus motifs in their promoter regions ( Cho and Cosgrove, 2002;Kim et al, 2006).…”

Unstable F-Actin Specifies the Area and Microtubule Direction of Cell Expansion in Arabidopsis Root Hairs

“…There is debate about the presence of filamentous actin in the apical area of growing root hairs. The expression of green fluorescent protein (GFP) fused to the actin binding site of animal talin (Kost et al, 1998), which binds to actin filaments, was studied in Arabidopsis root hairs (Balu ka et al, 2000). These authors observed fluorescence in the apex of growing root hairs.…”