Brassinosteroids (BRs) play important roles throughout plant growth and development. Despite the importance of clarifying the mechanism of BR-related growth regulation in cereal crops, BR-related cereal mutants have been identified only in rice (Oryza sativa). We previously found that semidwarf barley (Hordeum vulgare) accessions carrying the "uzu" gene, called "uzu" barley in Japan, are non-responding for brassinolide (BL). We then performed chemical and molecular analyses to clarify the mechanisms of uzu dwarfism using isogenic line pairs of uzu gene. The response of the uzu line to BL was significantly lower than that of its corresponding normal line. Measurement of BRs showed that the uzu line accumulates BRs, similar to known BR-insensitive mutants. The marker synteny of rice and barley chromosomes suggests that the uzu gene may be homologous to rice D61, a rice homolog of Arabidopsis BR-insensitive 1 (BRI1), encoding a BR-receptor protein.A barley homolog of BRI1, HvBRI1, was isolated by using degenerate primers. A comparison of HvBRI1 sequences in uzu and normal barley varieties showed that the uzu phenotype is correlated with a single nucleotide substitution. This substitution results in an amino acid change at a highly conserved residue in the kinase domain of the BR-receptor protein.These results may indicate that uzu dwarfism is caused by the missense mutation in HvBRI1. The uzu gene is being introduced into all hull-less barley cultivars in Japan as an effective dwarf gene for practical use, and this is the first report about an agronomically important mutation related to BRs.Brassinolide (BL) is a firstly identified plant steroid hormone, isolated from rape (Brassica napus) pollen (Grove et al., 1979). Diverse plant species have been found to contain BL and a variety of structural analogs, called brassinosteroids (BRs). With their characteristic physiological effect on plant growth and development, BRs should be included as essential plant hormones, along with GAs, auxins, cytokinins (CKs), abscisic acid (ABA), and ethylene. The effect of BRs on germination, elongation growth, flowering, and sex expressions of plants have been reported, and various application techniques have been tested in the greenhouse and in the field (Yokota, 1999). BR applications have often increased grain and vegetable yields. Plants treated with BRs also acquired resistance to or tolerance against such stresses as cold, drought, salt, disease, and herbicide. In field tests, however, BR effects were unstable and not replicable. The biological activity of these BRs disappeared rapidly due to deactivation and was influenced by environmental conditions (Kamuro and Takatsuto, 1999).In addition to studies on agricultural applications of BRs, BR physiology has also been studied (Yokota, 1997; Altmann, 1999; Bishop and Yokota, 2001). After many BR-deficient and -insensitive mutants were identified in Arabidopsis, BR biosynthesis and signaling have been rapidly clarified. BR biosynthesis mutants such as deetiolated 2 (det2; Chory et...

A sensitive method to examine the brassinolide (BL) response of barley (Hordeum vulgare L.) using dark-grown leaf segments was established based on the known method for wheat. BL responses of 53 dwarf isogenic lines of barley were examined, and two lines were found having a uzu gene that doesn't respond significantly. These results indicate that uzu dwarfism may be caused by the non-responding character to BL.