ACTIN-CYTOSKELETAL CONTROL OF GRAVITY SENSING AND GRAVITY-ORIENTED TIP GROWTH

Gravity-sensing and gravity-oriented polarized growth of characean rhizoids and protonemata are dependent on the actin cytoskeleton. The multiple functions and dynamic nature of the actin cytoskeleton are conferred by the concerted action of a variety of actin-binding proteins. Monomer actin-binding profilin, actin-depolymerizing factor (ADF) and spectrin-like proteins concentrate in a central prominent spot in the apex of both cell types, where they colocalize with a dense, spherical actin array and a unique aggregate of endoplasmic reticulum (ER), which represents the structural center of the Spitzenkorper1. Spectrin-like proteins are suggested to be involved in stabilizing the ER aggregate by forming crosslinks between ER membranes and actin filaments. The apical actin filaments radially assemble from the center of the Spitzenkorper, where actin polymerization might be under local control of ADF and profilin. Distinct actin filaments extend into the outermost tip and form a dense meshwork in the apical and subapical region, before they are incorporated into thick bundles that generate rotational cytoplasmic streaming in the basal region of both cells. The actomyosin system not only mediates the transport of secretory vesicles to the growing tip and controls the incorporation pattern of cell wall material, but also coordinates the tip-focused distribution pattern of calcium channels in the apical membrane which establish the tip-high calcium gradient, the prerequisite for exocytosis. Microgravity experiments have added much to our understanding that both cell types use an efficient actomyosin-based system to control and correct the position of their statoliths and to direct sedimenting statoliths to confined graviperception sites at the plasma membrane. Actin´s involvement in the graviresponses is more indirect. The upward growth of negatively gravitropic protonemata was shown to be preceded by a statolith-induced relocalization the calcium gradient to the upper flank that does not occur in positively gravitropic (downward growing) rhizoids, in which statolith sedimentation is followed by differential flank growth. Combining these results, it is evident that the spatiotemporal control of actin polymerization and dynamic actin remodeling is fundamental for the process of gravity sensing and gravityoriented polarized growth in characean rhizoids and protonemata.