STRUCTURAL REQUIREMENTS OF RHIZOBIUM CHITOLIPOOLIGOSACCHARIDES FOR UPTAKE AND BIOACTIVITY IN LEGUME ROOTS AS REVEALED BY SYNTHETIC ANALOGS AND FLUORESCENT-PROBES

Rhizobium chitolipooligosaccharides (CLOSs) are heterogeneous fatty ac ylated N-acetyl glucosamine oligomers with variations in both the pola r (hydrophilic) oligosaccharide head group and the non-polar (hydropho bic) fatty acyl chain. They trigger root hair deformation and cortical cell divisions in legume roots during development of the nitrogen-fix ing root-nodule symbiosis. It has been proposed that only certain uniq ue molecular species of CLOSs made by a particular rhizobia can elicit these responses on the corresponding legume host, suggesting that rec eptor-mediated perception of CLOSs serves as a basis of symbiotic spec ificity. We evaluated the relative symbiotic importance of the hydroph ilic and hydrophobic structural domains of CLOSs by comparing the biol ogical activities of CLOSs from wild type R. leguminosarum by. trifoli i ANU843 with that of various synthetic analogs. These tests were perf ormed in axenic bioassays on the compatible symbiotic host, white clov er (Trifolium repens) and the incompatible non-host legume, alfalfa (M edicago sativa). Fluorochrome-tagged derivatives of the native CLOSs a nd the analogs were also prepared in order to evaluate the uptake and localization patterns of these molecules within host root cells. The r esults indicate a direct link between uptake and biological activities of Rhizobium CLOSs on legume roots. The smallest CLOS analog taken up and biologically active on white clover and alfalfa was a N-fatty acy lglucosamine, without an essential requirement of oligomerization, fat ty N-acyl unsaturation, or acetate/sulfate functionalization.(jlr) Thi s suggests that N-fattyacylglucosamine is the common minimum structure required and sufficient for uptake and biological activity of CLOS gl ycolipids in these legumes, and that the various specific modification s of its polar head group and hy- drophobic tail modulate its inherent ability to further express these activities, thus influencing which l egumes are capable of responding to CLOSs rather than dictating their biological activities per se.