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 K_m (392.82 μM) and V_max (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.