EXPRESSION OF THE EARLY NODULIN, ENOD40, IN SOYBEAN ROOTS IN RESPONSETO VARIOUS LIPO-CHITIN SIGNAL MOLECULES

The lipo-chitin (LCO) nodulation signal (nod signal) purified from Bra dyrhizobium japonicum induced nodule primordia on soybean (i.e. Glycin e soja) roots. These primordia were characterized by a bifurcated vasc ular connection, cortical cell division, and the accumulation of mRNA of the early nodulin gene, ENOD40. A chemically synthesized LCO identi cal in structure to the Nod signal purified from B. japonicum cultures showed the same activity when inoculated on to soybean roots. Surpris ingly, synthetic LCO or chitin pentamer, inactive in inducing root hai r curling (HAD) or cortical cell division (NOI) in G. soja, induced th e transient accumulation of ENOD40 mRNA. In roots inoculated with such LCO, ENOD40 mRNA was abundant at 40 h after inoculation but decreased to the background levels 6 days after inoculation. In contrast, nod s ignals active in inducing HAD and NOI induced high levels of ENOD40 ac cumulation at 40 h and 6 days after inoculation. In situ hybridization analysis showed that ENOD40 mRNA accumulated in the pericycle of the vascular bundle at 24 h after root inoculation with nod signal. At 6 d ays post-inoculation with nod signal, ENOD40 expression was seen in di viding subepidermal cortical cells. These results provide morphologica l and molecular evidence that nodule induction in soybean in response to purified or synthetic nod signal is similar, if not identical, to n odule formation induced by bacterial inoculation. Surprisingly, ENOD40 mRNA accumulation occurs in response to non-specific chitin signals. This suggests that, in the case of ENOD4D, nodulation specificity is n ot determined at the level of initial gene expression.