Ethylene Inhibits the Nod Factor Signal Transduction Pathway of Medicago truncatula
The Plant Cell
Legumes form a mutualistic symbiosis with bacteria collectively referred to as rhizobia. The bacteria induce the formation of nodules on the roots of the appropriate host plant, and this process requires the bacterial signaling molecule Nod factor. Although the interaction is beneficial to the plant, the number of nodules is tightly regulated. The gaseous plant hormone ethylene has been shown to be involved in the regulation of nodule number. The mechanism of the ethylene inhibition on
... n is unclear, and the position at which ethylene acts in this complex developmental process is unknown. Here, we used direct and indirect ethylene application and inhibition of ethylene biosynthesis, together with comparison of wild-type plants and an ethylene-insensitive supernodulating mutant, to assess the effect of ethylene at multiple stages of this interaction in the model legume Medicago truncatula . We show that ethylene inhibited all of the early plant responses tested, including the initiation of calcium spiking. This finding suggests that ethylene acts upstream or at the point of calcium spiking in the Nod factor signal transduction pathway, either directly or through feedback from ethylene effects on downstream events. Furthermore, ethylene appears to regulate the frequency of calcium spiking, suggesting that it can modulate both the degree and the nature of Nod factor pathway activation. Walker, S.A., and Downie, J.A. (2000). Entry of Rhizobium leguminosarum bv. viciae into root hairs requires minimal nod factor specificity, but subsequent infection thread growth requires nodO and nodE. Mol. Plant-Microbe Interact. 13, 754-762. Walker, S.A., Viprey, V., and Downie, J.A. (2000). Dissection of nodulation signaling using pea mutants defective for calcium spiking induced by Nod factors and chitin oligomers. Proc. Natl. Acad. Sci. USA 97, 13413-13418. Woods, N.M., Cuthbertson, K.S.R., and Cobbold, P.H. (1986). Repetitive transient rises in cytoplasmic free calcium in hormone stimulated hepatocytes. Nature 319, 600-602. Yang, S.F., and Hoffman, N.E. (1984). Ethylene biosynthesis and its regulation in higher plants. Annu. Rev. Plant Physiol. 35, 155-189.