Actin cytoskeleton in plants: From transport networks to signaling networks

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Abstract
The plant actin cytoskeleton is characterized by a high diversity in regard to gene families, isoforms, and degree of polymerization. In addition to the most abundant F‐actin assemblies like filaments and their bundles, G‐actin obviously assembles in the form of actin oligomers composed of a few actin molecules which can be extensively cross‐linked into complex dynamic meshworks. The role of the actomyosin complex as a force generating system — based on principles operating as in muscle cells — is clearly established for long‐range mass transport in large algal cells and specialized cell types of higher plants. Extended F‐actin networks, mainly composed of F‐actin bundles, are the structural basis for this cytoplasmic streaming of high velocities On the other hand, evidence is accumulating that delicate meshworks built of short F‐actin oligomers are critical for events occurring at the plasma membrane, e.g., actin interventions into activities of ion channels and hormone carriers, signaling pathways based on phospholipids, and exo‐ and endocytotic processes. These unique F‐actin arrays, constructed by polymerization‐depolymerization processes propelled via synergistic actions of actin‐binding proteins such as profilin and actin depolymerizing factor (ADF)/cofilin are supposed to be engaged in diverse aspects of plant morphogenesis. Finally, rapid rearrangements of F‐actin meshworks interconnecting endocellular membranes turn out to be especially important for perception‐signaling purposes of plant cells, e.g., in association with guard cell movements, mechano‐ and gravity‐sensing, plant host–pathogen interactions, and wound‐healing. Microsc. Res. Tech. 47:135–154, 1999.