One plant actin isovariant, ACT7, is induced by auxin and required for normal callus formation

During plant growth and development, the phytohormone auxin induces a wide array of changes that include cell division, cell expansion, cell different iation, and organ initiation. It has been suggested that the actin cytoskel eton plays an active role in the elaboration of these responses by directin g specific changes in cell morphology and cytoarchitecture. Here we demonst rate that the promoter and the protein product of one of the Arabidopsis ve getative actin genes, ACT7, are rapidly and strongly induced in response to exogenous auxin in the cultured tissues of Arabidopsis. Homozygous act7-1 mutant plants were slow to produce callus tissue in response to hormones, a nd the mutant callus contained at least two to three times lower levels of ACT7 protein than did the wild-type callus, On the other hand, a null mutat ion in ACT2, another vegetative actin gene, did not significantly affect ca llus formation from leaf or root tissue. Complementation of the act7-1 muta nts with the ACT7 genomic sequence restored their ability to produce callus at rates similar to those of wild-type plants, confirming that the ACT7 ge ne is required for callus formation. Immunolabeling of callus tissue with a ctinsubclass-specific antibodies revealed that the predominant ACT7 is coex pressed with the other actin proteins. We suggest that the coexpression, an d probably the copolymerization, of the abundant ACT7 with the other actin isovariants in cultured cells may facilitate isovariant dynamics well suite d for cellular responses to external stimuli such as hormones.