COOPERATIVE ACTION OF LIPO-CHITIN NODULATION SIGNALS ON THE INDUCTIONOF THE EARLY NODULIN, ENOD2, IN SOYBEAN ROOTS

Various lipo-chitin molecules were tested for their ability to induce the expression of the early nodulin, ENOD2, in Glycine soja roots. Whe n inoculated separately onto G. soja roots, LCO-V (C18:1 Delta 11,Mefu c), LCO-V (C18:1 Delta 9,Mefuc), LCO-V (C16:0,Mefuc), and LCO-IV (C16: 0) were unable to induce ENOD2 expression, even though these compounds had previously been shown to induce root hair curling, the formation of nodule-like primordia, and induction of the early nodulin, ENOD40. ENOD2 expression, however, was induced when any two of these molecules were inoculated in combination. Thus, the lipo-chitin nodulation sign als appear to act cooperatively to induce ENOD2 expression, B. japonic um strains USDA110 and USDA135 and B. elkanii strain USDA61, all symbi onts of soybean, were found to produce at least two distinct nod signa ls ([i.e., NodBj-V[C18:1,Mefuc] and NodBj-V[C16:0,Mefuc]). These two c ompounds were mixed in various ratios and tested for their ability to induce ENOD2 expression, The results indicate that the former compound must be present in equivalent or excess amount in order to obtain max imum ENOD2 expression, Additional nonspecific LCOs (e.g., LCO-IV[C16:2 Delta 2,9; SO3]), incapable of inducing root hair curling or cortical fell division, were tested in combination with the four active LCOs l isted above, It was found that any combination of one active LCO with a nonspecific LCO was sufficient to induce ENOD2 mRNA expression. The ENOD2 mRNA. expression pattern detected by in situ hybridization close ly resembled that found in bacterial-induced nodules with expression d etected in cortical cells between primary and secondary meristems and around the vascular strands. These data demonstrate that the cooperati ve action of at least two LCO nodulation signals leads to a greater pr ogression of nodule ontogeny as demonstrated by the expression of ENOD 2, a marker gene for the differentiation of nodule parenchyma.