Acetylation is a ubiquitous modification on cell wall polymers, which alters physicochemical properties of cell wall and plays a structural role in plant growth and stress defenses. Recent progress indicates that at least three groups of proteins are involved in the polymer O-acetylation. Trichome Birefringence-Like (TBL) proteins belong to a super family, which have been found specifically responsible for acetylation of several polysaccharides, including a major hemicellulose xylan that possesses acetyl substituents at the backbone and sidechain. However, the mechanisms for how crop plants accomplish cell-wall polymer O-acetylation are largely unknown.
The group of Prof. ZHOU Yihua from the Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, recently isolated and characterized two trichome birefringence-like (tbl) mutants in rice, which are affected in xylan O-acetylation.
They found that ostbl1 and ostbl2 single mutant and the tbl1 tbl2 double mutant displayed a stunted growth phenotype with varied degree of dwarfism. Cell wall composition and nuclear magnetic resonance (NMR) spectroscopy analyses showed that the mutants have decreased acetylation level on xylan, suggesting that both genes are required for xylan acetylation modification.
They further demonstrated that
OsTBL1 is a functionally dominant gene because the expression level of
OsTBL1 is much higher than
OsTBL2. OsTBL1 was found localized in the Golgi apparatus, a place where polysaccharides are synthesized. They then expressed OsTBL1 in
Pichia incorporates. The purified recombinant OsTBL1 proteins were subjected to a series of biochemical assays. It was found that OsTBL1 can catalyze addition of acetyl groups onto xylan or
xylopentaose (X5). 2D-NMR spectroscopy specified that OsTBL1 transfers acetate to 2-
O or 3-
O site of xylosyl residues. In addition,
ostbl1 and
tbl1 tbl2 displayed susceptibility to rice blight disease, indicating that this xylan modification is required for pathogen resistance.
This is the first rice TBL protein being biochemically and genetically characterized, which offers us the insights into the mechanisms and functions of
xylan acetylation in crop plants.
This research was supported by the National Natural Science Foundation of China, the Ministry of Sciences and Technology of China, the Ministry of Agriculture of China for Transgenic Research, and Youth Innovation Promotion Association Chinese Academy of Sciences.
Figure . OsTBL1 is involved in xylan acetylation in rice. OsTBL1 represents xylan acetyltransferase, which catalyzes addition of acetyl groups onto the 2-O and 3-O position of xylosyl residues and is essential for pathogen resistance in rice. (Image by IGDB)
