Xyloglucan is hypothesized to bind to cellulose in an extended conformation in plant primary cell walls and to tether microfibrils into a load-bearing network, implying a key role in wall mechanics and cell growth. However, this view is challenged by newer results based on mechanics and solid-state NMR. Here we combined FESEM with selective endoglucanase digestions and with nanogold affinity tags to assess the spatial location and conformation of xyloglucan in onion cell walls. Wall digestion with xyloglucanases did not remove distinctive structures such as tethers or amorphous coils. Cell wall labeling with xyloglucan-specific carbohydrate binding module 76 (CBM76) and LM15 antibody indicated xyloglucans were loosely associated with fibril surfaces. Cell wall labeling with CBM3, which binds the hydrophobic surface of crystalline cellulose, was infrequent until the wall was predigested with xyloglucanase, whereupon CBM3 labeling of microfibrils was extensive. When tamarind xyloglucan was allowed to bind to the surface of xyloglucan-depleted onion walls, CBM76 labeling gave positive evidence for xyloglucans in both extended and coiled configurations on the wall surface, but individual xyloglucan chains were not discernable by FESEM. In contrast, isolated xyloglucan chains assumed remarkably different, aggregated conformations. These results indicate that an appreciable, but still small, surface of cellulose microfibrils in the onion wall is tightly bound with extended xyloglucan chains. The form and organization of xyloglucan in the onion wall implies an assembly process in which individual xyloglucan chains bind to the hydrophobic surfaces of cellulose microfibrils during assembly of growing cell walls.

Xyloglucan-cellulose organization in plant primary cell walls: Reassessment by imaging approaches

255th National Meeting (2018)

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