Treatment of non-stressed mung bean cuttings with 24-epibrassinolide (BR) inhibited elongation of adventitious roots, but it stimulated root elongation in plants exposed to 70 mM NaCl. These findings confirm that brassinosteroids are involved in control of division and elongation of root cells. We found that both positive and negative effects are counteracted by the presence of geldanamycin in the root medium. Because of the high specificity of geldanamycin for the chaperone Hsp90, this suggests that, as in animal cells, Hsp90 is involved in expression or transduction of the brassinosteroid signals in plants. Moreover, a detailed analysis of root ranking showed an additional geldanamycin-insensitive mode of action of BR, especially observed under salt stress conditions. These two modes differently affected elongation of the adventitious roots, depended on the phases of differentiation and on the presence of an environmental constraint.