The Significance of Diversity in the Plant Actin Gene Family

Abstract

Recent progress in understanding the plant actin gene family is reviewed, focusing on the Arabidopsis actins. Taking an evolutionary perspective, we have focused on the functional significance of the conserved but ancient vegetative and reproductive actin classes, which date back to the origin of vascular plants. We propose that the conservation of ancient family members is due to differential gene regulation and/or to functional differences among isovariants. The eight functional actin genes are widely dispersed on four of the five Arabidopsis chromosomes. Each of the five actin gene subclasses are strongly expressed at some time and place during plant development, and they are highly differentially regulated. A handful of surface epitope differences among plant and vertebrate actins enabled the isolation of general and subclass-specific anti-plant actin monoclonal antisera. These reagents give an excellent resolution to the switch from vegetative to reproductive actin protein expression during floral development. Combined with refined fixation protocols, these reagents resolve the intimate relationship between the chloroplasts and the actin cytoskeleton in leaf cells. Sequence-based screening procedures were developed for the isolation of the first mutant alleles of plant actins. These mutants have strong deleterious effects on the survival of plants and are effectively lethal mutations over several generations. Sequence differences among the co-expressed plant actin isovariants should produce complex dynamics within actin filaments and with actin-binding proteins. Future work on the significance of this ancient family will focus on the cell biology, genetics, and biochemistry of the isovariants.