Both Vegetative and Reproductive Actin Isovariants Complement the Stunted Root Hair Phenotype of the Arabidopsisact2-1 Mutation

Abstract

Abstract The ACT2 gene, encoding one of eight actin isovariants in Arabidopsis, is the most strongly expressed actin gene in vegetative tissues. A search was conducted for physical defects inact2-1 mutant plants to account for their reduced fitness compared with wild type in population studies. Theact2-1 insertion fully disrupted expression ofACT2 RNA and significantly lowered the level of total actin protein in vegetative organs. The root hairs of theact2-1 mutants were 10% to 70% the length of wild-type root hairs, and they bulged severely at the base. The length of the mutant root hairs and degree of bulging at the base were affected by adjusting the osmolarity and gelling agent of the growth medium. Theact2-1 mutant phenotypes were fully rescued by anACT2 genomic transgene. When the act2-1mutation was combined with another vegetative actin mutation,act7-1, the resulting double mutant exhibited extensive synergistic phenotypes ranging from developmental lethality to severe dwarfism. Transgenic overexpression of the ACT7 vegetative isovariant and ectopic expression of the ACT1 reproductive actin isovariant also rescued the root hair elongation defects of the act2-1mutant. These results suggest normal ACT2 gene regulation is essential to proper root hair elongation and that even minor differences may cause root defects. However, differences in the actin protein isovariant are not significant to root hair elongation, in sharp contrast to recent reports on the functional nonequivalency of plant actin isovariants. Impairment of root hair functions such as nutrient mining, water uptake, and physical anchoring are the likely cause of the reduced fitness seen for act2-1 mutants in multigenerational studies.