Plants encode at least two ancient and divergent classes of actin,reproductive and vegetative, and each class produces several subclassesof actin isovariants. To gain insight into the functional significanceof the actin isovariants, we generated transgenicArabidopsis lines that expressed a reproductive actin,ACT1, under the control of the regulatory sequences of a vegetativeactin gene, ACT2. In the wild-type plants, ACT1 ispredominantly expressed in the mature pollen, growing pollen tubes, andovules, whereas ACT2 is constitutively and strongly expressed in allvegetative tissues and organs, but not in pollen. Misexpression of ACT1in vegetative tissues causes dwarfing of plants and altered morphologyof most organs, and the effects are in direct proportion to proteinexpression levels. Similar overexpression of ACT2 has little effect.Immunolocalization of actin in leaf cells from transgenic plants withhighest levels of ACT1 protein revealed massive polymerization,bundling, and reorganization of actin filaments. This phenomenonsuggests that misexpression of ACT1 isovariant in vegetative tissuesaffects the dynamics of actin and actin-associated proteins, in turndisrupting the organization of actin cytoskeleton and normaldevelopment of plants.
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