Resistance of wheat aleurone cell walls to acid and xylanase action

Isolated wheal (Triticum aestivum var. Potam) aleurone layers have a high capacity to acidify their environment, and secrete hydrolytic enzymes (endoxylanase, glucanase, alpha -amylase, proteases, etc.) under the control of GA(3). Acidic pH and xylanases are found to be essential for cell wall relaxation in growing tissues, but aleurone is a non-growing, non-dividing tissue. In this tissue, we studied the effect of these loosening factors on aleurone cell walls. Exposure to pH 3.0 caused the release of carbohydrates and calcium ions from the pericarp, and a small amount of carbohydrates, mainly polysaccharides, from aleurone layers from which pericarp tissue had been removed. 50% of the total sugars released into the incubation medium by these isolated aleurone tissue was arabinose, but no xylose, calcium ions, or phenolic compounds were found. Acid preincubation decreased by 30% the susceptibility of aleurone cell walls to degradation by exogenously-applied endoxylanase, and also modified the architecture of cell wail as observed by autofluorescence of phenolic groups. These findings suggest that acid treatment and endoxylanase action, rather than having a loosening effect on aleurone cell wall, can have an opposite effect, increasing the resistance of aleurone cell walls to loosening.