Brassinosteroids (BRs) are plant growth-promoting natural products required for plant growth and development. Physiological studies have demonstrated that exogenous BR, alone or in combination with auxin, enhance bending of the lamina joint of rice. However, little is known about the function of endogenous BR in rice or other grass species. We report here the phenotypical and molecular characterization of a rice dwarf mutant, d61 , that is less sensitive to BR compared to the wild type. We cloned a rice gene, OsBRI1 , with extensive sequence similarity to that of the Arabidopsis BRI gene, which encodes a putative BR receptor kinase. Linkage analysis showed that the OsBRI1 gene is closely linked to the d61 locus . Single nucleotide substitutions found at different sites of the d61 alleles would give rise to amino acid changes in the corresponding polypeptides. Furthermore, introduction of the entire OsBRI1 coding region, including the 5 Ј and 3 Ј flanking sequences, into d61 plants complemented the mutation to display the wild-type phenotype. Transgenic plants carrying the antisense strand of the OsBRI1 transcript showed similar or even more severe phenotypes than those of the d61 mutants. Our results show that OsBRI1 functions in various growth and developmental processes in rice, including (1) internode elongation, by inducing the formation of the intercalary meristem and the longitudinal elongation of internode cells; (2) bending of the lamina joint; and (3) skotomorphogenesis.

Peroxisome proliferator-activated receptor gamma (PPARgamma) is induced in leptin-deficient (ob/ob) mouse liver and is critical for the development of hepatic steatosis. The present study shows that fat-specific protein 27 (Fsp27) in ob/ob liver is a direct target gene of PPARgamma and can elevate hepatic triglyceride levels. FSP27 belongs to the CIDE family, composed of CIDE A, CIDE B, and FSP27/CIDE C, all of which contain a conserved CIDE-N domain. FSP27 was recently reported to be a lipid droplet-binding protein and to promote lipid accumulation in adipocytes. The Fsp27 gene was expressed at high levels in ob/ob liver and at markedly lower levels in ob/ob livers lacking PPARgamma. Forced expression of FSP27 by adenovirus in hepatocytes in vitro or in vivo led to increased triglyceride levels. Knockdown by adenovirus expressing FSP27 shRNA resulted in lower accumulation of hepatic triglycerides compared to control adenovirus-infected liver. Taken together, these results indicate that FSP27 is a direct mediator of PPARgamma-dependent hepatic steatosis.

Seven dwarf mutants resembling brassinosteroid (BR)-biosynthetic dwarfs were isolated that did not respond significantly to the application of exogenous BRs. Genetic and molecular analyses revealed that these were novel alleles of BRI1 (BrassinosteroidInsensitive 1), which encodes a receptor kinase that may act as a receptor for BRs or be involved in downstream signaling. The results of morphological and molecular analyses indicated that these represent a range of alleles from weak to null. The endogenous BRs were examined from 5-week-old plants of a null allele (bri1-4) and two weak alleles (bri1-5 and bri1-6). Previous analysis of endogenous BRs in several BR-biosynthetic dwarf mutants revealed that active BRs are deficient in these mutants. However, bri1-4 plants accumulated very high levels of brassinolide, castasterone, and typhasterol (57-, 128-, and 33-fold higher, respectively, than those of wild-type plants). Weaker alleles (bri1-5 and bri1-6) also accumulated considerable levels of brassinolide, castasterone, and typhasterol, but less than the null allele (bri1-4). The levels of 6-deoxoBRs in bri1 mutants were comparable to that of wild type. The accumulation of biologically active BRs may result from the inability to utilize these active BRs, the inability to regulate BR biosynthesis in bri1 mutants, or both. Therefore, BRI1 is required for the homeostasis of endogenous BR levels.Based on their wide distribution in the plant kingdom, their diverse physiological effects at nanomolar levels, and the discovery of mutants deficient in their biosynthesis, it is now widely accepted that brassinosteroids (BRs) are important hormones that regulate growth and development

The Arabidopsis bas1-D mutation suppresses the long hypocotyl phenotype caused by mutations in the photoreceptor phytochrome B (phyB). The adult phenotype of bas1-D phyB-4 double mutants mimics that of brassinosteroid biosynthetic and response mutants. bas1-D phyB-4 has reduced levels of brassinosteroids and accumulates 26-hydroxybrassinolide in feeding experiments. The basis for the mutant phenotype is the enhanced expression of a cytochrome P450 (CYP72B1). bas1-D suppresses a phyB-null allele, but not a phyA-null mutation, and partially suppresses a cryptochrome-null mutation. Seedlings with reduced BAS1 expression are hyperresponsive to brassinosteroids in a light-dependent manner and display reduced sensitivity to light under a variety of conditions. Thus, BAS1 represents one of the control points between multiple photoreceptor systems and brassinosteroid signal transduction.