ACTIN7 Is Required for Perinuclear Clustering of Chloroplasts during Arabidopsis Protoplast Culture.

In Arabidopsis, the actin gene family comprises eight expressed and two non-expressed () genes. Of the eight expressed isoforms, , and are differentially expressed in vegetative tissues and may perform specific roles in development. Using tobacco mesophyll protoplasts, we previously demonstrated that actin-dependent clustering of chloroplasts around the nucleus prior to cell division ensures unbiased chloroplast inheritance. Here, we report that actin-dependent chloroplast clustering in Arabidopsis mesophyll protoplasts is defective in mutants, but not or . expression was upregulated during protoplast culture whereas and expression did not substantially change. In , expression increased in response to loss of , whereas in , neither nor expression changed appreciably in response to the absence of ACT7. Semi-quantitative immunoblotting revealed increased actin concentrations during culture, although total actin in was only two-thirds that of wild-type or after 96 h culture Over-expression of and under control of regulatory sequences restored normal levels of chloroplast clustering. These results are consistent with a requirement for ACT7 in actin-dependent chloroplast clustering due to reduced levels of actin protein and gene induction in mutants, rather than strong functional specialization of the ACT7 isoform.