Fig. 6. A simple actin configuration-based model derived through observations of wild-type Arabidopsis cells depicts how the strategic placement of dense F-actin (red arrowheads/areas) can restrict local cell expansion, whereas vesicles are free to move out into the assimilation zone and produce expansion in other areas (green) where the F-actin meshwork has been loosened through the activity of the ARP2/3 complex (of which CROOKED is an essential component). (A-D) Major locations of dense actin in wild-type Arabidopsis cells. (A) Actin accumulations are seen at branch junctions and branch tips in trichome cells. The non-expanding, dense-actin filled tip is projected through the diffuse expansion growth that takes place in the flanks of the extending branches. Non-expansion of the absolute apex in trichomes produces a fine pointed tip. (B) Dense F-actin is found at lobe junctions and in different strategic locations in multi-lobed pavement cells. Contrary to other cell types, the lobes in pavement cells have to constantly adjust to the growth patterns of neighboring cells on all sides. Thus, they may show fluctuations in their dense actin localization, depending upon their position relative to other cells. (C) The elongate, cylindrical hypocotyl cells appear to have dense actin at their ends. Capping of the ends with dense actin reroutes vesicles into the flanking regions. This results is the formation of long cylindrical cells of varying length whose ends maintain a nearly constant diameter. (D) Side view of a blunt tipped growing root-hair cell. The positioning of dense actin bundles changes constantly in an elongating root hair (data not shown) though the general distance from the absolute tip remains almost the same. A dense-actin cap at the tip frequently indicates cessation of hair growth and may lead to swelling behind the actin-blocked tip region. Note that the formation of a pointed tip, as in trichomes, requires a dense actin cap, whereas a broad growing tip requires placement of dense actin at the sides in order to funnel vesicles into the tip region.

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