Cell- and development-specific degradation controls the levels of mixed-linkage glucan in sorghum leaves

Mixed-linkage glucan (MLG) is a component of the cell wall (CW) of grasses and is composed of glucose monomers linked by beta-1,3 and beta-1,4 bonds. MLG is believed to have several biological functions, such as the mobilizable storage of carbohydrates and structural support of the CW. The extracellular levels of MLG are largely controlled by rates of synthesis mediated by cellulose synthase-like (CSL) enzymes, and turnover by lichenases. Economically important crops like sorghum accumulate MLG to variable levels during development. While in sorghum, like other grasses, there is one major MLG synthase (CSLF6), the identity of lichenases is yet unknown. To fill this gap, we identified three sorghum lichenases (SbLCH1-3) and characterized them in leaves in relation to the expression of SbCSLF6, and the abundance of MLG and starch. We established that SbLCH1-3 are secreted to the apoplast, consistent with a role of degrading MLG extracellularly. Furthermore, while SbCSLF6 expression was associated with cell development, the SbLCH genes exhibited distinct development-, cell-type-specific and diel-regulated expression. Therefore, our study identifies three functional sorghum MLG lichenases and highlights that MLG accumulation in sorghum leaves is likely controlled by the activity of lichenases that tune MLG levels, possibly to suit distinct cell and developmental needs in planta. These findings have important implications for improving the growth, yield, and composition of sorghum as a feedstock.

Automated summary

Mixed-linkage glucan (MLG) is a component of the cell wall of grasses and plays important roles in carbohydrate storage and structural support. MLG levels are regulated by cellulose synthase-like (CSL) enzymes and lichenases. In this study, three functional sorghum lichenases were identified and characterized, showing that they are involved in MLG degradation in the extracellular space. The expression of the lichenase genes exhibited specific patterns related to development, cell type, and diurnal regulation. These findings have implications for improving sorghum as a feedstock.