Annual Report 2003

Isolation and characterization of a ricedwarf mutant with a defectin brassinosteroid biosynthesisMasaki MoriMolecular Genetics DepartmentBrassinosteroids (BRs) have various physiologicaland morphological effects on plants,and are involved in the elongation of stems,unrolling of leaves, responses to environmentalstress, and differentiation of trachearyelements. In contrast to the many studiesperformed in dicots, only a few have beenperformed on the physiological effects of BRson the growth and development of monocots.We have isolated a new recessive dwarfmutant of rice (Oryza sativa L. cv ‘Nipponbare’).Under normal growth conditions, the mutanthas very short leaf sheaths; has short,curled, and frizzled leaf blades; has fewtillers (Fig. 1); and is sterile. Longitudinalsections of the leaf sheaths revealed that thecell length along the longitudinal axis isreduced, which explains the short leafsheaths. Transverse sections of the leafblades revealed enlargement of the motorcells along the dorsal–ventral axis, whichexplains the curled and frizzled leaf blades.Fig. 1Morphology of brd1 plant. (A) 80-day-old wild-type and brd1 plants. (B)Close-up of brd1.Fig. 2Phenotypic restoration of brd1 plant by BL.Fig. 3Brassinolide biosynthetic pathway and brassinosteroidcontent. Brassinosteroid amounts, in ng/g fresh weight,of wild-type and brd1 plants are shown in white and redboxes, respectively. ND, not detected. BR6ox,Brassinosteroid-6-oxidase.36 Annual Report 2003