The dose/response curve for auxin's action as a plant growth promoter is extremely wide. How does the cell vary its response to such a wide range of signal intensity? We suggest that cell wall-bound cellulase, and the products of its action on the wall polysaccharides xyloglucan and cellulose, provide a complex regulatory system, acting at three or possibly four different levels. Our recent observations will be presented in terms of the following speculations. (1) SPECULATION: Low levels of auxin activate cellulase (by inducing H(+)-secretion?), which cuts inter-microfibrillar xyloglucan chains, loosening the cell wall and tending to promote growth.
Observation: auxins induce xyloglucan turnover and decrease the mean Mr of wall xyloglucan. Re-incorporation of xyloglucan fragments suggests that transglycosylation predominates over hydrolysis. (2) SPECULATION: Moderate levels of auxin activate cellulase further; the enzyme generates small amounts (nM) of oligosaccharides, some of which are growth-inhibitory. This process tends to moderate growth.
Observation: Xyloglucan nonasaccharide (XG9-see Fig. 2) at 1-10 nM antagonises auxin-induced growth. The effect is seen with fucose-containing oligosaccharides (XG9, XG5, fucosyl-lactose), but not with XG7 or XG8. XG9 (1 nM) and XG10 (0.1 nM) and cellohexaose (10 nM) also block H(+)-induced growth. (3) SPECULATION: Higher levels of auxin induce cellulase synthesis leading to the generation of higher concentrations of oligosaccharides that can mimic auxin. This process thus tends to restore growth.
Observation: all xyloglucan-oligosaccharides tested (HPLC-pure XG7, XG8, XG9, XG9n - Fig. 2) promote growth at 1 microM, the order of effectiveness being XG9n greater than XG8 greater than XG7 conquent to XG9. The same oligosaccharides (at 1-100 microM) promote the mid-chain hydrolysis of xyloglucan by plant cellulase, while slightly inhibiting total hydrolysis. The order of effectiveness is XG9n congruent to XG8 greater than XG7 congruent to XG9. The promotion of mid-chain hydrolysis (and/or transglycosylation) might be expected to be particularly effective at loosening the cell wall: most of the wall's xyloglucan molecules might suffer a single cleavage rather than a few of them undergoing complete degradation. (4) SPECULATION: Very high levels of auxin induce excessive levels of cellulase, and therefore of oligosaccharides. This inhibits growth.
Observation: XG7, XG9 and XG9n lose their auxin-like activity at 10 microM.