ULTRACURVATA1, a SHAGGY-like Arabidopsis gene required for cell elongation

To better understand the genetic mechanisms underlying plant leaf developme nt, we have performed a large-scale screening for Arabidopsis thaliana muta nts to identify those displaying abnormally shaped or sized leaves. One of the stronger mutant phenotypes found was that of the ultracurvata1 (ucu1) m utants, whose vegetative and cauline leaves are spirally rolled downwards a nd show a reduced expansion along the longitudinal axis, in contrast to wil d type leaves, which are flattened organs. We have identified one recessive and two semidominant ucu1 alleles, the most extreme of which cause severe dwarfism and a constitutive photomorphogenic response. Following a map-base d strategy, we have cloned the UCU1 gene, which was found to encode an intr acellular kinase closely related to SHAGGY, one of the components of the Wi ngless/Wnt animal signalling pathway. The responses of ucu1 mutants to exog enous plant hormones and the genetic analyses of double mutants involving u cu1 alleles indicate that UCU1 is a key component in several signalling pat hways controlling cell expansion and overall plant growth, including those of auxins and brassinosteroids.