초록
<P>The <I>brittle culm</I> (<I>bc</I>) mutants of Gramineae plants having brittle skeletal structures are valuable materials for studying secondary cell walls. In contrast to other recessive <I>bc</I> mutants, rice <I>Bc6</I> is a semi-dominant <I>bc</I> mutant with easily breakable plant bodies. In this study, the <I>Bc6</I> gene was cloned by positional cloning. <I>Bc6</I> encodes a cellulose synthase catalytic subunit, OsCesA9, and has a missense mutation in its highly conserved region. In culms of the <I>Bc6</I> mutant, the proportion of cellulose was reduced by 38%, while that of hemicellulose was increased by 34%. Introduction of the semi-dominant <I>Bc6</I> mutant gene into wild-type rice significantly reduced the percentage of cellulose, causing brittle phenotypes. Transmission electron microscopy analysis revealed that <I>Bc6</I> mutation reduced the cell wall thickness of sclerenchymal cells in culms. In rice expressing a reporter construct, <I>BC6</I> promoter activity was detected in the culms, nodes, and flowers, and was localized primarily in xylem tissues. This expression pattern was highly similar to that of <I>BC1</I>, which encodes a COBRA-like protein involved in cellulose synthesis in secondary cell walls in rice. These results indicate that BC6 is a secondary cell wall-specific CesA that plays an important role in proper deposition of cellulose in the secondary cell walls.</P>