Disruption of Brachypodium Lichenase Alters Metabolism of Mixed-linkage Glucan and Starch

Mixed-linkage glucan (MLG), which is widely distributed in grasses, is a polysaccharide highly abundant in cell walls of grass endosperm and young vegetative tissues. Lichenases are enzymes that hydrolyze MLG first identified in MLG-rich lichens. In this study, we identify a gene encoding a lichenase we name Brachypodium distachyon LICHENASE 1 (BdLCH1), which is highly expressed in the endosperm of germinating seeds and coleoptiles and at lower amounts in mature shoots. RNA in situ hybridization showed that BdLCH1 is primarily expressed in chlorenchyma cells of mature leaves and internodes. Disruption of BdLCH1 resulted in an eight-fold increase in MLG content in senesced leaves. Consistent with the in situ hybridization data, immunolocalization results showed that MLG was not removed in chlorenchyma cells of lch1 mutants as it was in wild type and implicate the BdLCH1 enzyme in removing MLG in chlorenchyma cells in mature vegetative tissues. We also show that MLG accumulation in lch1 mutants was resistant to dark induced degradation, and eight-week-old lch1 plants showed a faster rate of starch breakdown than wild type in darkness. Our results suggest a role for BdLCH1 in modifying the cell wall to support highly metabolically active cells.