Actin and Myosin VIII in Developing Root Apex Cells

Abstract

Root apices represent an ideal model object for studies on plant cell growth and development. We have exploited this opportunity for detailed analysis of the actin-based cytoskeleton in cells of various root tissues throughout their cellular development. During mitosis, cells re-distribute their actin filaments (AFs) and myosin VIII molecules from the cytoplasm to the cell periphery where they accumulate at putative AF-organizing centres (AFOCs) facing the spindle poles. Postmitotic root cap columella cells differentiate first into gravity-sensing statocytes which are unique among postmitotic root cells due to the lack of any distinct cables of AFs. Later, statocytes, as well as peripheral cap cells, transform into secretory cells equipped with dense AF networks distributed throughout their cytoplasm. They retain abundant AFs after being shed from the root. Intriguingly, however, all root cap cells lack myosin VIII at their periphery. By contrast, all postmitotic cells of the root body, as they traverse the transition zone, show myosin VIII localized at their periphery. Myosin VIII localizes especially at the plasmodesmata in the non-growing cross walls. In cells of the transition zone, unique AF bundles develop which are proposed to participate in the onset of rapid cell elongation. These AF bundles are initiated at the nuclear peripheries and are organized via myosin VIII-enriched cross-walls, these two sites obviously act as the major AFOCs of postmitotic root-body cells. Treatment of roots with latrunculin B reveals that dynamic AFs are essential for both vacuome-driven cell elongation and root hair formation. In the transition zone and elongation region, cells of the inner cortex localize plant myosin VIII molecules abundantly at their pit-fields. These distinctive subcellular sites, like cross-walls and root hair apices, represent powerful AFOCs capable of organizing abundant AFs.