LIPO-CHITOOLIGOSACCHARIDE NODULATION SIGNALS FROM RHIZOBIUM-MELILOTI INDUCE THEIR RAPID DEGRADATION BY THE HOST-PLANT ALFALFA

Extracellutar enzymes from alfalfa (Medicago sativa L.) involved in th e degradation of nodulation (Nod) factors could be distinguished by th eir different cleavage specificities and were separated by lectin affi nity chromatography. A particular glycoprotein was able to release an acylated lipo-disaccharide from all tested Nod factors having an oligo saccharide chain length of four or five residues. Structural modificat ions of the basic lipo-chitooligosaccharide did not affect the cleavag e site and had only weak influence on the cleavage efficiency of Nod f actors tested. The acylated lipo-trisaccharide was resistant to degrad ation. When alfalfa roots were preincubated with Nod factors at nanomo lar concentrations, the activity of the dimer-forming enzyme was stimu lated up to 6-fold within a few hours. The inducing activity of Nod fa ctors decreased in the order NodRm-IV(C16:2,Ac,S) > NodRm-IV(C16:2,S) and NodRm-V(C16:2,Ac,S) > NodRm-V(C16:2,S) > NodRm-IV(C16: O,S) > NodR m-IV(C16:2). Pretreatment with NodRm-III(C16:P) as well as unmodified chitooligosaccharides did not stimulate the dimer-forming enzyme. Root s preincubated with Rhizobium meliloti showed similar stimulation of t he dimer-forming activity. Mutant strains unable to produce Nod factor s did not enhance the hydrolytic activity. These results indicate a ra pid feedback inactivation of Nod signals after their perception by the host plant alfalfa.