All of the legume mutants described so far are common sym mutants impaired in root endosymbioses, mycorrhiza and nodulation. These mutants indicate the existence in legumes of partially shared genetic programs required for successful interaction with fungal and bacterial symbionts. A large number of legume mutants, including dmi1, dmi2 and dmi3 mutants (Catoira et al., 2000), and one tomato mutant (Barker et al., 1998), have been identified, which support appressoria formation but are compromised in intraradical invasion (reviewed in Marsh & Schultze, 2001). While in M. truncatula, three classes of mutants (dmi1, dmi2 and dmi3) have been reported, seven genes (LjSYMRK, LjCASTOR, LjPOLLUX, LjSYM3, LjSYM6, LjSYM15, LjSYM24) were found to be required for fungal penetration in L. japonicus (reviewed in Harrison, 2005; Kistner et al., 2005). The use of model legumes paved the way for isolating corresponding genes from mutant backgrounds and defining a signal transduction pathway essential for the two root symbioses (Fig. 2). As several authors have discussed this topic in detail (Parniske, 2004; Harrison, 2005; Hause & Fester, 2005; Oldroyd et al., 2005; Oldroyd & Downie, 2006), this article will only briefly summarize the current knowledge and describe recent progress taking a ‘mycocentric’ view. The genes revealed by these mutations all control fungal passage through the outer cell layers. The corresponding proteins, namely the Leu-rich repeat receptor-like kinase LjSYMRK/MsNORK/MtDMI2 (Endre et al., 2002; Stracke et al., 2002) as well as the plastid ion channels LjCASTOR/LjPOLLUX/MtDMI1 (Ane et al., 2004; Imaizumi-Anraku et al., 2004) and the nuclear localized calcium- and calmodulin-dependent protein kinase MtCCamK/MtDMI3 (Levy et al., 2004; Mitra et al., 2004), are needed for the early steps in Nod factor signaling and are probably also involved in signaling during the early stages of mycorrhizal colonization. The first genes isolated encoded the LjSYMRK/MsNORK/MtDMI2 receptor-like kinases (Endre et al., 2002; Stracke et al., 2002). The structure of the protein with an extensive extracellular domain suggests it is involved in the extracellular binding of a ligand and the intracellular transduction of the signaling event via its protein kinase domain. At present, however, it is not clear if these proteins in fact operate as primary receptors for a compound released by AM fungi. It is equally plausible that additional receptors are involved in the perception of a ‘Myc factor’ as was shown for the Nod factor receptors LjNFR1/MtLYK3 and LjNFR5 (Limpens et al., 2003; Madsen et al., 2003; Radutoiu et al., 2003). One of the early responses of legume root hair cells to the perception of Rhizobia or purified Nod factors is intracellular calcium oscillation (for a recent review, see Oldroyd & Downie, 2006). During Nod factor signaling, DMI1 (LjCASTOR and LjPOLLUX) and MtDMI2 (LjSYMRK, MsNORK) type proteins are necessary for the induction of calcium spiking, while MtDMI3 (MtCCamK) type proteins are assumed to be involved in deciphering the calcium signal (Fig. 2) (Levy et al., 2004; Mitra et al., 2004). Although calcium oscillation has not been shown for mycorrhizal signaling, the participation of proteins associated with calcium signaling suggests calcium also functions as a second messenger within the fungus-related signaling pathway. Recently, another member of the SYM pathway required for calcium spiking during nodulation has been discovered: the gene affected in the L. japonicus sym3 mutant encodes a homologue of the mammalian nucleoporin NUP133 and was therefore termed Lotus NUP133 (Kanamori et al., 2006). Consistent with a function as a nucleoporin, an eYFP-NUP133 fusion protein localized to the nuclear rim assembly with the nuclear pore complex. The absence of pleiotropic phenotypes of Lotus nup133 mutant plants indicates the specific involvement of the LjNUP133 protein in root symbioses. Although its function during symbiosis is not clear, it is intriguing that the early symbiotic signal transduction pathway includes ion fluxes, plastid ion channels and nuclear pore complexes. Cloning of the mutated genes from the remaining Lotus sym mutants is currently in progress in different laboratories. The establishment of a comprehensive picture of how the individual components of the SYM pathway tie up should be possible in the near future. Furthermore, because of the ancient nature of the AM symbiosis, a high degree of functional conservation of these SYM factors among members of plant families other than legumes can be anticipated. Noticeably, a first supporting example showed that the nodulation phenotype of the M. truncatula dmi3 mutant can be rescued by introducing a rice gene putatively orthologous to the leguminous CCamK (Godfroy et al., 2006). It will be important to verify complementation of the mycorrhizal mutant phenotype of the transformed dmi3 line and also to address the function of this gene for the interaction of rice with AM fungi. Similarly, for other putative SYM factor orthologs from non-leguminous dicotyledonous or from monocotyledonous plants (Zhu et al., 2006), the relevance for the AM symbiosis can be addressed via reverse genetics.
The identification of additional components within the symbiotic nitrogen fixation signaling cascade indicates that proteins with related functions might be involved in mycorrhizal signaling. For example, a functional equivalent of the putative Nod factor receptors LjNFR1 and LjNFR5 (reviewed in Oldroyd et al., 2005; Oldroyd & Downie, 2006) could exist for ‘Myc factor’ perception (Fig. 2). It is also likely that components corresponding to the GRAS transcription factors MtNSP1 and MtNSP2 (Kalo et al., 2005; Smit et al., 2005) are present to modulate gene expression in response to calcium oscillation during mycorrhizal signaling (Fig. 2). In summary, genetically dissecting the common SYM signal transduction pathway required for bacterial and fungal root endosymbiosis not only unraveled the players involved but also provided a first glimpse at conservation and specialization of signaling cascades essential for nodulation and mycorrhiza development.