Loss-of-function of a rice brassinosteroid biosynthetic enzyme,C-6 oxidase,prevents the organized arrangement and polar elongation of cells in the leaves and stem

Molecular genetic and physiological studies on brassinosteroid (BR)-related mutants of dicot plants have revealed that BRs play important roles in normal plant growth and development. However, little is known about the function of BR in monocots (grasses), except for the phenotypic analysis of a rice mutant partially insensitive to BR signaling. To investigate the function of BR in monocots, we identified and characterized BR-deficient mutants of rice, BR-deficient dwarf1 (brd1). The brd1 mutants showed a range of abnormalities in organ development and growth, the most striking of which were defects in the elongation of the stem and leaves. Light microscopic observations revealed that this abnormality was primarily owing to a failure in the organization and polar elongation of the leaf and stem cells. The accumulation profile of BR compounds in the brd1 mutants suggested that these plants may be deficient in the activity of BR C-6 oxidase. Therefore, we cloned a rice gene, OsDWARF, which has a high sequence similarity to the tomato C-6 oxidase gene, DWARF. Introduction of the wild-type OsDWARF gene into brd1 rescued the abnormal phenotype of the mutants. The OsDWARF gene was expressed at a low level in all of the examined tissues, with preferential expression in the leaf sheath, and the expression was negatively regulated by brassinolide treatment. On the basis of these findings, we discuss the biological function of BRs in rice plants.     

The gibberellin signaling pathway is regulated by the appearance and disappearance of SLENDER RICE1 in nuclei

The slender rice1 mutant (slr1) shows a constitutive gibberellin (GA) response phenotype. To investigate the mode of action of SLR1, we generated transgenic rice expressing a fusion protein consisting of SLR1 and green fluorescent protein (SLR1-GFP) and analyzed the phenotype of the transformants and the subcellular localization of GFP in vivo. SLR1-GFP worked in nuclei to repress the GA signaling pathway; its overproduction caused a dwarf phenotype. Application of GA(3) to SLR1-GFP overproducers induced GA actions such as shoot elongation, downregulation of GA 20-oxidase expression, and upregulation of SLR1 expression linked with the disappearance of the nuclear SLR1-GFP protein. We also performed domain analyses of SLR1 using transgenic plants overproducing different kinds of truncated SLR1 proteins. The analyses revealed that the SLR1 protein can be divided into four parts: a GA signal perception domain located at the N terminus, a regulatory domain for its repression activity, a dimer formation domain essential for signal perception and repression activity, and a repression domain at the C terminus. We conclude that GA signal transduction is regulated by the appearance or disappearance of the nuclear SLR1 protein, which is controlled by the upstream GA signal.     

Where do gibberellin biosynthesis and gibberellin signaling occur in rice plants?

To identify where gibberellin (GA) biosynthesis and signaling occur, we analyzed the expression of four genes involved in GA biosynthesis, GA 20-oxidase1 and GA 20-oxidase2 (OsGA20ox1 and OsGA20ox2), and GA 3-oxidase1 and GA 3-oxidase2 (OsGA3ox1 and OsGA3ox2), and two genes involved in GA signaling, namely, the gene encoding the alpha-subunit of the heterotrimeric GTP-binding protein (Galpha), and SLENDER RICE1 (SLR1), which encodes a repressor of GA signaling. At the vegetative stage, the expression of OsGA20ox2, OsGA3ox2, Galpha, and SLR1 was observed in rapidly elongating or dividing organs and tissues, whereas the expression of OsGA20ox1 or OsGA3ox1 could not be detected. At the inflorescence or floral stage, the expression of OsGA20ox2, OsGA3ox2, Galpha, and SLR1 was also observed in the shoot meristems and stamen primordia. The overlapping expression of genes for GA biosynthesis and signaling indicates that in these tissues and organs, active GA biosynthesis occurs at the same site as does GA signaling. In contrast, no GA-biosynthesis genes were expressed in the aleurone cells of the endosperm; however, the two GA-signaling genes were actively expressed, indicating that the aleurone does not produce bioactive GAs, but can perceive GAs. The expression of OsGA20ox1 and OsGA3ox1 was observed only in the epithelium of the embryo and the tapetum of the anther. Based on the specific expression pattern of OsGA20ox1 and OsGA3ox1 in these tissues, we discuss the unique nature of the epithelium and the tapetum in terms of GA biosynthesis. The epithelium and the tapetum are considered to be an important source of bioactive GAs for aleurone and other organs of the flower, respectively.     

Identification, isolation and pyramiding of quantitative trait loci for rice breeding

Many agronomically important traits are governed by several genes known as quantitative trait loci (QTLs). The identification of important, QTL-controlled agricultural traits has been difficult because of their complex inheritance; however, completion of the rice genomic sequence has facilitated the cloning of QTLs and their pyramiding for breeding. Because QTLs are derived from natural variation, the use of a wider range of variations such as that found in wild species is important. In addition, Introgression Lines (ILs) developed from wild species in combination with Marker Assisted Selection should facilitate efficient gene identification. This review describes recent developments in rice QTL analysis including mapping, cloning and pyramiding QTLs.     

Identification of Delta12-fatty acid desaturase from arachidonic acid-producing mortierella fungus by heterologous expression in the yeast Saccharomyces cerevisiae and the fungus Aspergillus oryzae.

Based on the sequence information for the omega3-desaturase genes (from Brassica napus and Caenorhabditis elegans), which are involved in the desaturation of linoleic acid (Delta9, Delta12-18 : 2) to alpha-linolenic acid (Delta9, Delta12, Delta15-18 : 3), a cDNA was cloned from the filamentous fungal strain, Mortierella alpina 1S-4, which is used industrially to produce arachidonic acid. Homology analysis with protein databases revealed that the amino acid sequence showed 43.7% identity as the highest match with the microsomal omega6-desaturase (from Glycine max, soybean), whereas it exhibited 38.9% identity with the microsomal omega3-desaturase (from soybean). The evolutionary implications of these enzymes will be discussed. The cloned cDNA was confirmed to encode a Delta12-desaturase, which was involved in the desaturation of oleic acid (Delta9-18 : 1) to linoleic acid, by its expression in both the yeast Saccharomyces cerevisiae and the fungus Aspergillus oryzae. Analysis of the fatty acid composition of yeast and fungus transformants demonstrated that linoleic acid (which was not contained in the control strain of S. cerevisiae) was accumulated in the yeast transformant and that the fungal transformant contained a large amount of linoleic acid (71.9%). Genomic Southern blot analysis of the transformants with the Mortierella Delta12-desaturase gene as a probe confirmed integration of this gene into the genome of A. oryzae. The M. alpina 1S-4 Delta12-desaturase is the first example of a cloned nonplant Delta12-desaturase.     

Rice growth adapting to deepwater

Flooding is one of the most hazardous natural disasters, and there are several levels of flooding. Recently, research on flood-tolerant rice plants revealed that some rice varieties have evolved to overcome two different flood types, 'flash flood' and 'deepwater flood', using two different mechanisms, and their molecular mechanisms were determined. During flash flooding, the tolerant plants that are fully submerged for a few weeks stop elongating and thus avoid energy consumption that will be needed to restart growth when the water recedes. On the contrary, during deepwater flooding, with water depth up to several meters for several months, the deepwater-flood-tolerant rice plants promote elongation of internodes to keep the foliage above the water surface and thus allow respiration and photosynthesis.