Plasticity of xyloglucan composition in bean (Phaseolus vulgaris)-cultured cells during habituation and dehabituation to lethal concentrations of dichlobenil.

Bean cells that have been habituated to grow in a lethal concentration (12 µM) of 2,6-dichlorobenzonitrile (dichlobenil or DCB, a cellulose biosynthesis inhibitor) are known to have decreased cellulose content in their cell walls. Xyloglucan, which is bound to cellulose and together with it forms the main loading network of plant cell walls, has also been described to decrease in habituated cells, but whether the change on cellulose affects the xyloglucan structure besides its abundance has not been analyzed. Fragmentation analysis with xyloglucan-specific endoglucanase (XEG) and endocellulase revealed that habituation to DCB caused a change in the fine structure of xyloglucan, namely a decrease in fucosyl residues attached to the galactosyl-xylosyl residues along the glucan backbone. After the removal of herbicide from the medium (dehabituated cells), xyloglucan recovered its fucosyl residues. In addition, some cello-oligosaccharides could be detected only in habituated cells' xyloglucan digested by XEG and endocellulase, corresponding to a glucan covalently bound or co-precipitated with the hemicelluloses. These results show that structural flexibility of cell walls relies in part on the plasticity of xyloglucan composition and opens up new perspectives to further research in this field.

Increase in XET activity in bean (Phaseolus vulgaris L.) cells habituated to dichlobenil.

Bean (Phaseolus vulgaris L.) cells have been habituated to grow in lethal concentrations of dichlobenil (DCB), a specific inhibitor of cellulose biosynthesis. Bean callus cells were successively cultured in increasing DCB concentrations up to 2 microM. The 2-microM DCB habituated cells were impoverished in cellulose and xyloglucan, had an increased xyloglucan endotransglucosylase (XET; EC 2.4.1.207) activity, together with an increased growth rate and a decreased molecular size of xyloglucan. However, the application of lethal concentrations of two different cellulose-biosynthesis inhibitors (DCB and isoxaben) for a short period of time produced little effect on XET activity and xyloglucan molecular size. We propose that the weakening of plant cell wall provoked by decrease in cellulose content might promote the xyloglucan tethers and increase the ability of xyloglucan to bind to cellulose in order to give rigidity to the wall.

Characterization of cell walls in bean (Phaseolus vulgaris L.) callus cultures tolerant to dichlobenil.

The increase in dry weight during the culture of bean callus cultures was inhibited by the herbicide dichlobenil (2,6-dicholorobenzonitrile) with an I(50) of 0.5 µM. However bean calli became tolerant to a concentration of 12 µM by a stepwise increase in the concentration of the inhibitor in each subculture. Tolerant calli growing in 2,6-dicholorobenzonitrile developed with hollow protuberances. Groups of cells in these protuberances had irregular cell walls surrounded by a thicker cell wall with a lamellate structure and without a differentiated middle lamella. FTIR spectra of tolerant cell walls revealed an increase in both esterified and non-esterified pectins. Cell wall fractionation showed that in tolerant cell walls the xyloglucan-cellulose network of non-tolerant cell walls was partly replaced by a pectin-rich network mainly formed of cross-linked polyuronides with a large proportion of homogalacturonan. These modifications are comparable to those described for bean calli tolerant to isoxaben, pointing to a related mechanism of tolerance for both herbicides.