Arbuscular mycorrhiza (AM) is a root endosymbiosis between plants and glomeromycete fungi. It is the most widespread terrestrial plant symbiosis, improving plant uptake of water and mineral nutrients. Yet, despite its crucial role in land ecosystems, molecular mechanisms leading to its formation are just beginning to be unravelled. Recent evidence suggests that AM fungi produce diffusible symbiotic signals. Here we show that Glomus intraradices secretes symbiotic signals that are a mixture of sulphated and non-sulphated simple lipochitooligosaccharides (LCOs), which stimulate formation of AM in plant species of diverse families (Fabaceae, Asteraceae and Umbelliferae). In the legume Medicago truncatula these signals stimulate root growth and branching by the symbiotic DMI signalling pathway. These findings provide a better understanding of the evolution of signalling mechanisms involved in plant root endosymbioses and will greatly facilitate their molecular dissection. They also open the way to using these natural and very active molecules in agriculture.

Rhizobia are symbiotic bacteria that elicit the formation on leguminous plants of specialized organs, root nodules, in which they fix nitrogen. In various Rhizobium species, such as R. leguminosarum and R. meliloti, common and host-specific nodulation (nod) genes have been identified which determine infection and nodulation of specific hosts. Common nodABC genes as well as host-specific nodH and nodQ genes were shown recently, using bioassays, to be involved in the production of extracellular Nod signals. Using R. meliloti strains overproducing symbiotic Nod factors, we have purified the major alfalfa-specific signal, NodRm-1, by gel permeation, ion exchange and C18 reverse-phase high performance liquid chromatography. From mass spectrometry, nuclear magnetic resonance, (35)S-labelling and chemical modification studies, NodRm-1 was shown to be a sulphated beta-1,4-tetrasaccharide of D-glucosamine (Mr 1,102) in which three amino groups were acetylated and one was acylated with a C16 bis-unsaturated fatty acid. This purified Nod signal specifically elicited root hair deformation on the homologous host when added in nanomolar concentration.

Rhizobium nodulation (Nod) factors are lipo-chitooligosaccharides that act as symbiotic signals, eliciting several key developmental responses in the roots of legume hosts. Using nodulation-defective mutants of Medicago truncatula , we have started to dissect the genetic control of Nod factor transduction. Mutants in four genes ( DMI1 , DMI2 , DMI3 , and NSP ) were pleiotropically affected in Nod factor responses, indicating that these genes are required for a Nod factoractivated signal transduction pathway that leads to symbiotic responses such as root hair deformations, expressions of nodulin genes, and cortical cell divisions. Mutant analysis also provides evidence that Nod factors have a dual effect on the growth of root hair: inhibition of endogenous (plant) tip growth, and elicitation of a novel tip growth dependent on (bacterial) Nod factors. dmi1 , dmi2 , and dmi3 mutants are also unable to establish a symbiotic association with endomycorrhizal fungi, indicating that there are at least three common steps to nodulation and endomycorrhization in M. truncatula and providing further evidence for a common signaling pathway between nodulation and mycorrhization. INTRODUCTIONSymbiotic bacteria of the genera Rhizobium , Bradyrhizobium , Azorhizobium , and Sinorhizobium , collectively referred to as rhizobia, are able to elicit on their leguminous hosts the formation of specialized organs, called nodules, capable of fixation of atmospheric nitrogen. Among the earliest visible manifestations of symbiotic development are the infection of root hair cells by bacteria, contained in plant-derived infection structures called infection threads, and the induction of cortical divisions to form the nascent nodule primordium (Mylona et al., 1995;Schultze and Kondorosi, 1998).Genetic analysis of rhizobia has led to identification of nod genes, which are involved in the control of host specificity, infection, and nodulation. nod gene expression is under the control of plant signals, essentially flavonoids, excreted in the rhizosphere. Once activated, nod genes specify the synthesis of nodulation (Nod) factors, which are lipo-chitooligosaccharides (Dénarié et al., 1996). Purified Nod factors are capable of eliciting in the roots of the legume hosts many of the plant responses characteristic of the bacteria themselves (Dénarié and Cullimore, 1993): root hair deformations, activation of the plant genes that are specifically induced during early stages of nodulation (early nodulin genes), initiation of cortical cell division, and triggering of a plant organogenic program that leads to nodule formation. Nod factors thus act as symbiotic signaling molecules for initiating nodule development. They also play a key role in the control of specificity of infection and nodulation, the result of particular substitutions present on the basic backbone of the Nod factor molecule (reviewed in Long, 1996; Cohn et al., 1998;Schultze and Kondorosi, 1998).One of the most challenging areas of research in the study of the Rhizobium-legume symbiosis...