Small Changes in the Regulation of One Arabidopsis Profilin Isovariant, PRF1, Alter Seedling Development

Abstract

Profilin (PRF) is a low-molecular-weight actin binding protein encoded by a diverse gene family in plants. Arabidopsis PRF1 transcripts are moderately well expressed in all vegetative organs. A regulatory mutant in PRF1 , prf1-1 , was isolated from a library of T-DNA insertions. The insertion disrupted the promoter region of PRF1 100 bp upstream from the transcriptional start site. Although steady state levels of PRF1 transcripts appeared normal in mature prf1-1 plants, the levels in young seedlings were only one-half those observed in wild type. Reactions with a PRF1 isovariant-specific monoclonal antiserum and general anti-profilin antisera demonstrated that PRF1 protein levels also were one-half those found in wild-type seedlings, although total profilin levels were unaffected. Mutant seedlings no longer could downregulate PRF1 levels in the light, as did wild type. Consistent with their molecular phenotypes, young mutant seedlings displayed several morphological phenotypes but developed into apparently normal adult plants. Their initial germination rate and development were slow, and they produced excessive numbers of root hairs. Mutant seedlings had abnormally raised cotyledons, elongated hypocotyls, and elongated cells in the hypocotyl, typical of phenotypes associated with some defects in light and circadian responses. A wild-type PRF1 transgene fully complements the hypocotyl phenotypes in the prf1-1 mutant. The ability of profilin to regulate actin polymerization and participate directly in signal transduction pathways is discussed in light of the prf1-1 phenotypes. INTRODUCTIONThe actin cytoskeleton contributes to many of the dynamic processes directing plant development: cell polarity, division plane determination, cell elongation, and cell wall deposition (Meagher et al., 1999b). As seedlings emerge from the soil and respond to light, significant changes occur in plant morphology that must be directed at the cellular level by alterations in the cytoskeleton (Meagher et al., 2000). Because the actin binding protein profilin is particularly important to the dynamics of actin polymerization and sequestration, the regulation of plant profilin expression in cells and organs responding to changes in light levels should affect plant development. This article provides initial data demonstrating the connection between profilin regulation and plant morphology.Profilin binds monomeric G-actin and participates in several cytoskeletal functions (Gibbon and Staiger, 2000) with the potential to alter cellular and organismal development. Profilin accelerates assembly at the barbed ends of actin filaments and sequesters actin monomers, which decreases the pool of polymerizable actin once barbed ends are capped (Carlier et al., 1993;Pantaloni and Carlier, 1993;Perelroizen et al., 1996). Profilin activates actin for assembly by catalyzing ADP-to-ATP exchange on actin monomers. Profilin binds tightly to poly-L -proline, a motif present in some proteins involved in regulating the actin cytoskeleton (Ahern-Djamali et al....

Profilin Is Essential for Tip Growth in the MossPhyscomitrella patens

“…Phenotypes observed included reduced cell expansion and shorter swollen roots and root hairs (Ramachandran et al, 2000). However, another study analyzed a T-DNA insertion in the most highly expressed profilin gene during seedling formation in Arabidopsis, which also reduced profilin levels by approximately twofold during germination, and this study observed an increase in cell expansion (McKinney et al, 2001).…”

New Views on the Plant Cytoskeleton

“…It is likely that profilins have a similar role in formin-mediated actin polymerization in plant cells. Suppression of profilin gene expression affects cell growth, morphogenesis, and seedling growth, highlighting the important physiological function of profilin-mediated actin dynamics (Ramachandran et al, 2000;McKinney et al, 2001).…”

ArabidopsisCAP1 – a key regulator of actin organisation and development

“…With the exception of AIP1 (Ketelaar et al, 2004a), most plant ABP mutants and suppression constructs affect the morphogenesis of only a variable subset of cell types. The tissue-specific nature of formin phenotypes (Deeks et al, 2005;Ingouff et al, 2005;Yi et al, 2005) and profilin (McKinney et al, 2001) can be considered to be a symptom of large gene families with the potential for genetic redundancy, but the relatively mild phenotypes of components of the Arp2/3 complex together with the unexpectedly severe Arabidopsis AIP1 phenotype suggests that plants place functional emphasis upon individual classes of ABPs in a pattern that differs from animals and fungi. Here, we show that the Arabidopsis homologue of CAP (CAP1) is essential for the development of multiple cell types and that null mutant phenotypes of these tissues correlate with actin organisational defects.…”