The Arabidopsis Class I formin AtFH5 contributes to seedling resistance to salt stress.

The family of formins, evolutionarily conserved multidomain proteins engaged in the control of actin and microtubule cytoskeleton organization, exhibits considerable diversity in plants. Angiosperms have two formin clades consisting of multiple paralogs, Class I and Class II, the former being often transmembrane proteins located at the plasmalemma or endomembranes. According to available transcriptome data, the Arabidopsis thaliana Class I transmembrane formin AtFH5 (At5g54650) exhibits a distinct pattern of transcript abundance in various seedling root tissues with massive increase of transcript level upon salinity stress. To examine a possible role of AtFH5 in NaCl stress response, we generated transgenic plants expressing green fluorescent protein (GFP)-tagged AtFH5 under its native promoter and characterized its tissue and intracellular localization under standard culture conditions and under NaCl stress. While we confirmed the induction of AtFH5 expression by salt treatment, the distribution of tagged protein, with maxima in the border-like cells of the root cap, in the phloem and at lateral root emergence sites, did not reflect previously reported transcript abundance, suggesting posttranscriptional regulation of gene expression. Subcellular localization studies employing also membrane trafficking inhibitors suggested that AtFH5 protein level may be modulated by endocytosis and autophagy. Notably, loss-of-function atfh5 mutants exhibited increased sensitivity to NaCl stress, indicating that AtFH5 contributes to the development of seedling salt tolerance. These findings highlight the functional importance of AtFH5 in abiotic stress responses.