Autolysis-like release of homogalacturonan from bean ( Phaseolus vulgaris L.) callus cell walls

Abstract

After 12 h incubation at 35 °C, cell walls isolated from bean calluses released approximately 3% of the cell wall dry weight as pectic polysaccharides (2.7%), largely composed of a t-1,2-diaminecyclohexane- N, N, N′, N′-tetraacetic acid-soluble homogalacturonan, and monosaccharides (0.3%). The release of sugars occurred over a broad pH range and was affected by the type of buffer used in cell wall incubation. The effect of the addition of calcium and chelators on the amount of released sugars indicated that the homogalacturonan was released by the cleavage of calcium bridges rather than an enzymatic reaction. This result is likely to be a consequence of the high amount of loosely-bound pectins in the cell walls of calluses. The monosaccharide fraction was exclusively composed of neutral sugars, mainly glucose. It seems reasonable to postulate that most of these monosaccharides would have been released in a reaction mediated by enzymes since the highest enzymatic activity detected in these cell walls corresponded to a β- d-glucosidase. A very small peak of released sugars at pH 5.5 is also consistent with an autolysis mediated through enzymes associated with the cell walls. The results presented suggest that the mechanism of sugar release from incubated bean cell walls varies with the cell wall composition and structure.