Probing expansin action using cellulose/hemicellulose composites

Cellulose-based composite materials containing xyloglucans or mannan-based polysaccharides have been shown to possess organisational features with man y characteristics similar to primary plant cell walls. We have tested the e ffects of a typical alpha-expansin (CsExp1) on these composites using two d ifferent mechanical assays. We show that CsExp1 induces very rapid extensio n in composites containing tamarind xyloglucan under constant load. In cont rast, expansin treatment had no effect in constant load extension assays us ing cellulose-only materials or in those carried out on composites containi ng glucomannan or galactomannan. We show that the effect of expansins is mu ch smaller on composites made with short chain length xyloglucans than on t hose containing longer chains. In uniaxial extension tests we found that ex pansin could double the total extension (before failure) in xyloglucan comp osites and that the effects were again lower in composites containing short er xyloglucans. We found no effect of expansin on uniaxial extensions with glucomannan or galactomannan. However, a significant effect of expansin on the uniaxial extension behaviour of cellulose-only material was observed. T hese experiments suggest that the target of CsExp1 in cell walls is probabl y the cellulose xyloglucan matrix, but that other (1-4) beta-glucan to (1-4 ) beta-glucan hydrogen bonded contacts can also serve as substrates.