DISTINCT RESPONSE OF MEDICAGO SUSPENSION-CULTURES AND ROOTS TO NOD FACTORS AND CHITIN OLIGOMERS IN THE ELICITATION OF DEFENSE-RELATED RESPONSES

The induction of plant defense-related responses by chitin oligomers a nd the Rhizobium meliloti lipo-chito-oligosaccharide nodulation signal s (Nod factors) in Medicago cell cultures and roots was investigated b y following the expression of genes encoding enzymes of the isoflavono id biosynthetic pathway, such as chalcone synthase, chalcone reductase , isoflavone reductase, as well as genes encoding a pathogenesis-relat ed protein and a peroxidase. In suspension-cultured cells, all genes e xcept the peroxidase gene were induced by both the R. meliloti Nod fac tor NodRm-IV(C16:2,S) and chitin oligomers with a minimum of three sug ar residues. However, activation of these genes was not elicited by th e symbiotically inactive, desulfated NodRm-IV(C16:2). Moreover, the ce lls were more sensitive to the chitin oligosaccharides than to the Nod factor. Analysis of flavonoids in Medicago microcallus cultures revea led differences between cells treated with N-acetylchitotetraose and t hose treated with Nod factor and demonstrated increased production of the phytoalexin medicarpin in the presence of Nod factor. In Medicago roots, none of the tested genes was activated by the N-acetylchitotetr aose, whereas the Nod factor at micromolar concentration enhanced tran sient expression of the isoflavonoid biosynthetic genes. The different ial responses to Nod factors and chitin oligomers suggest that Medicag o cells possess distinct perception systems for these related molecule s.