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.