Yaping Gao, Congwu He, Dongmei Zhang, Xiangling Liu, Zuopeng Xu,, Baocai Zhang, and Yihua Zhou*

　　State Key Laboratory of Plant Genomics, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing 100101, China

　　* Email: yhzhou@genetics.ac.cn

　　The most common modification on cell wall polymers is O-acetylation, which affects physiochemical and rheological properties of polymers. The degree of O-acetylation is tightly modulated across different plant species, tissues and development stages, indicating that such modification is vital for plant growth and development. Xylan, the secondly most abundant polymer in cell wall, is heavily acetylated on both the backbone and the side chain of xylan. Trichome birefringence-like (TBL) proteins have been demonstrated to be involved in the polymer O-acetylation in Arabidopsis. However, none of polysaccharide acetyltransferase has been identified in rice.  Based on the bioinformatic and expression analyses, 8 TBLs were predicted to participate in xylan acetylation. Through characterization of ostb1 and ostbl2 mutants and generation of the double mutants and transgenic plants, we found both TBLs are xylan acetyltransferases. The mutants show dwarf phenotypes. Chemical analyses revealed that the content of acetyl esters is decreased in the mutants in contrast to the invariant contents of polysaccharides. 2D-NMR analyses assigned the reduction in acetyl esters to xylan backbone. OsTBL1 was found to be Golgi-localized. The recombinant OsTBL1 can transfer up to four acetyl residues onto xylopentaose and this activity showed saturable kinetics. 2D-NMR spectroscopy specified that OsTBL1 transfers acetate to both 2-O and 3-O site of xylosyl residues. We also found that mutation in TBL1 disturbs the resistance of rice plants to leaf blight pathogen. Moreover, additional mutants of several TBL candidates have been isolation. These findings further our understanding on biochemical and biological functions of TBLs in rice.