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

Abstract

Summary 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.

Grapes on Steroids. Brassinosteroids Are Involved in Grape Berry Ripening

“…This suggests that in grape berries the main bioactive BR is likely to be CS, and the conversion of 6-DeoxoCS to CS (catalyzed by the VvBR6OX1 gene product) may be an important BR activation step in this species. This scenario is similar to the reported situation in vegetative tissues of tomato (Yokota, 1997;Bishop et al, 1999) and rice (Hong et al, 2002), where CS is considered to be the main bioactive BR. However, in many circumstances BL is thought to be the major bioactive BR.…”

RAV-Like1Maintains Brassinosteroid Homeostasis via the Coordinated Activation ofBRI1and Biosynthetic Genes in Rice

“…Microscopy inspection of dark-green leaves revealed that the number of bulliform cells was increased in the mutants when compared with wild-type plants ( Figure 2B). An increase in the number and size of bulliform cells is also observed in BR-deficient dwarf1 (brd1) rice mutants (Hong et al, 2002). In addition, the other ravl1 phenotypes (i.e., late germination, retarded shoot growth, and dark-green leaves) are also often found in BR-related mutants (Yamamuro et al, 2000;Hong et al, 2003).…”

Rice leaf inclination2, a VIN3-like protein, regulates leaf angle through modulating cell division of the collar

“…To date, several rice mutants with altered leaf inclination or QTLs related to lamina joint angle have been genetically identified [3,[7][8][9][10], and the major detected QTL for leaf angle is tiller angle 1 (ta1) [7,8]. However, the underlying mechanism of the leaf angle alteration has not been clarified.…”

“…Rice lamina joint angle increases proportionally to BR levels [11,12], and deficiency of OsDWARF4, a key gene in BR biosynthesis, resulted in increased leaf erectness and enhanced grain yields in dense planting [3]. The erect leaves were also observed in other BR biosynthesis-defective mutants including ebisu dwarf (d2, deficiency of CYD90D2/D2 [10]) and brassinosteroid-deficient dwarf1 (brd1, deficiency of OsDWARF [9]). In addition, the BR signaling-defective mutant d61-7, the weakest mutant of rice BRASSINOSTEROID INSENSITIVE1 (OsBRI1, a protein kinase severing as rice BR receptor), and transgenic rice plants with suppressed expression of OsBZR1 (a transcription factor involved in BR signaling pathway), showed erect leaves [13][14][15].…”