ACTIVATION OF THE CELL-CYCLE MACHINERY AND THE ISOFLAVONOID BIOSYNTHESIS PATHWAY BY ACTIVE RHIZOBIUM-MELILOTI NOD SIGNAL MOLECULES IN MEDICAGO-MICROCALLUS SUSPENSIONS

We have shown that treatment of Medicago microcallus suspensions with the cognate Rhizobium meliloti Nod signal molecule NodRm-IV(C16:2,S) c an modify gene expression both qualitatively and quantitatively. At co ncentrations of 10(-6)-10(-9) M, this host specific plant morphogen bu t not the inactive non-sulfated molecule stimulated cell cycle progres sion as indicated by the significantly enhanced thymidine incorporatio n, elevated number of S phase cells, increase in kinase activity of th e p34(cdc2)-related complexes and enhancement of the level of expressi on of several cell cycle marker genes, the histone H3-1, the cdc2Ms an d the cyclin cycMs2. The presented data suggest that at least part of the physiological role of the Nod factor may be linked to molecular ev ents involved in the control of the plant cell division cycle. In situ hybridization experiments with antisense H3-1 RNA probe indicated tha t only certain cells of the calli were able to respond to the Nod fact or. High (10(-6) M) but not low (10(-9) M) concentrations of the activ e Nod factors induced the expression of the isoflavone reductase gene (IFR), a marker gene of the isoflavonoid biosynthesis pathway in most callus cells. Our results indicate that Medicago cell responses to the Nod signal molecules can be investigated in suspension cultures.