In the monocot plant Commelina communis (L.), it was investigated whether or not the induction of de novo rhizogenesis had common steps with the auxin-dependent osmoregulation of the guard cell. Among differentiated tissues, these that were excised from the etiolated hypocotyl solely formed either calli, rooting calli or direct roots. Each of these morphogenic patterns was controlled in 90% of the cases, depending on the concentration of the plasma membrane non-permeating auxin, 2,4-dichlorophenoxyacetic acid (2,4-D). The cyclic GMP (cGMP) membrane-permeable derivative 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP) increased the mean number of roots per explant but, in the absence of 2,4-D, did not induce rhizogenesis. The rhizogenic response to 2,4-D was suppressed by treating the explants with the inhibitor of cGMP-producing guanylyl cyclase, 6-anilino-5,8-quinolinedione (LY 83583), the Ca 2 + buffer, 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), the intracellular Ca 2 + -release inhibitor, ruthenium red and the antagonist of cyclic ADP-ribose (cADPR) synthesis, nicotinamide. In the presence of both 2,4-D and 8-Br-cGMP, rhizogenesis was completely inhibited by the same pharmacological treatments, except that LY 83583 allowed the explants to form as many roots as did 2,4-D when applied alone. Together, these results suggested that cGMP was required to Ca 2 + -dependent auxin-induced rhizogenesis as it could be the case for auxin-induced stomatal opening.