Small changes in the regulation of one Arabidopsis profilin isovariant, PRF1, Alter seedling development

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 disrupte d the promoter region of PRF1 100 bp upstream from the transcriptional star t site. Although steady state levels of PRF1 transcripts appeared normal in mature prf1-1 plants, the levels in young seedlings were only one-half tho se observed in wild type. Reactions with a PRF1 isovariant-specific monoclo nal antiserum and general anti-profilin antisera demonstrated that PRF1 pro tein levels also were one-half those found in wild-type seedlings, although total profilin levels were unaffected. Mutant seedlings no longer could do wnregulate PRF1 levels in the light, as did wild type. Consistent with thei r molecular phenotypes, young mutant seedlings displayed several morphologi cal phenotypes but developed into apparently normal adult plants. Their ini tial germination rate and development were slow, and they produced excessiv e numbers of root hairs. Mutant seedlings had abnormally raised cotyledons, elongated hypocotyls, and elongated cells in the hypocotyl, typical of phe notypes associated with some defects in light and circadian responses. A wi ld-type PRF1 transgene fully complements the hypocotyl phenotypes in the pr f1-1 mutant. The ability of profilin to regulate actin polymerization and p articipate directly in signal transduction pathways is discussed in light o f the prf1-1 phenotypes.