Feruloyl esterase activity and its role in regulating the feruloylation of maize cell walls.

Cell walls of grasses have ferulic acid (FA) ester-linked to the arabinosyl substitutions of arabinoxylan (AX). Feruloyl esterases (FAE) are carboxylic acid esterases that release FA from cell walls and synthetic substrates. Despite the importance of FA for cell wall recalcitrance and in response to biotic and abiotic stresses, the physiological function of plant FAEs remains unclear. Here, we developed a simple method for the determination of FAE activity (ZmFAE) in maize using the total protein extract and investigated its role in regulating the feruloylation of cell wall. The method includes a single protein extraction and enzymatic reaction with protein concentration as low as 65 µg at 35 °C for 30 min, using methyl ferulate as the substrate. The methodology allowed the determination of the apparent Km (392.82 µM) and Vmax (79.15 pkat mg-1 protein). We also found that ZmFAE activity was correlated (r = 0.829) with the levels of FA in seedling roots, plant roots and leaves of maize. Furthermore, the exposure to osmotic stress resulted in a 50% increase in ZmFAE activity in seedling roots. These data suggest that FAE-catalyzed reaction is important for cell wall feruloylation during plant development and in response to abiotic stress. We conclude proposing a model for the feruloylation and deferuloylation of AX, which explains the role of FAE in regulating the levels of ester-linked FA. Our model might orient further studies investigating the role of plant FAEs and assist strategies for genetic engineering of grasses to obtain plants with reduced biomass recalcitrance.

Cell wall remodeling under salt stress: Insights into changes in polysaccharides, feruloylation, lignification, and phenolic metabolism in maize.

Although cell wall polymers play important roles in the tolerance of plants to abiotic stress, the effects of salinity on cell wall composition and metabolism in grasses remain largely unexplored. Here, we conducted an in-depth study of changes in cell wall composition and phenolic metabolism induced upon salinity in maize seedlings and plants. Cell wall characterization revealed that salt stress modulated the deposition of cellulose, matrix polysaccharides and lignin in seedling roots, plant roots and stems. The extraction and analysis of arabinoxylans by size-exclusion chromatography, 2D-NMR spectroscopy and carbohydrate gel electrophoresis showed a reduction of arabinoxylan content in salt-stressed roots. Saponification and mild acid hydrolysis revealed that salinity also reduced the feruloylation of arabinoxylans in roots of seedlings and plants. Determination of lignin content and composition by nitrobenzene oxidation and 2D-NMR confirmed the increased incorporation of syringyl units in lignin of maize roots. Salt stress also induced the expression of genes and the activity of enzymes enrolled in phenylpropanoid biosynthesis. The UHPLC-MS-based metabolite profiling confirmed the modulation of phenolic profiling by salinity and the accumulation of ferulate and its derivatives 3- and 4-O-feruloyl quinate. In conclusion, we present a model for explaining cell wall remodeling in response to salinity.