Functional Characterization of Arabidopsis Formin Homologues Afh1, Afh5, Afh6, Afh7 and Afh8

Dynamic actin remodeling is at the core of a number of fundamental cellular processes in a variety of organisms ranging from animal neuronal outgrowth to pollen tube growth during plant reproduction and asymmetrical cell division in budding yeast. Such dynamism results from a concerted effort of a number of temporally and spatially regulated actin binding proteins. The polymerization and depolymerization of elaborate F-actin networks takes place by the addition and removal of actin subunits at the two filament ends with each end possessing distinct properties, making the filaments themselves polar structures with a fast growing “barbed end” and a slow growing “pointed end”. Although actin polymerization at the barbed end is an efficient and favorable process, the initial filament nucleation step is a much more inefficient and thermodynamically unfavorable process requiring the need for a variety of actin nucleating proteins. These nucleating proteins not only determine the precise location of actin assembly at the cell membrane during polarized cell growth but also directly control the number of force producing filaments. Here we have concentrated our efforts in characterizing a number of Arabidopsis thaliana , Group I membrane bound, formin homologues AFH1, AFH5, AFH6, AFH7, AFH8 and AFH11 using a range of functional genomics tools. Consequently we have shown that AFH5 is a pollen expressed actin nucleating protein localized at the tip of the polarized tube. Genetic alterations of the AFH5 gene using T-DNA gene knockout and gene overexpression both show distinct deformities at the tip of the rapidly growing pollen tube leading to inefficient fertilization during plant reproduction and reduced silique length. In the tip growing root hair cells of Arabidopsis the afh5 mutants show a wavy and bulgy phenotype at the hypocotyl region while the afh1 mutants project branched, split root hairs along the primary root. Although our results do indicate that AFH1 and AFH5 are expressed in the polarized pollen tubes and root hairs their, expression and hence activity is spatially controlled and restricted to different parts of the cell