J.A. Downie, S. Walker, B. Hogg, A.E. Davies, V. Viprey
John Innes Centre, Colney Lane, Norwich NR4 7UH, UK 1. Introduction
Many plant-host and bacterial mutants have been identified that block, or substantially reduce, the establishment of nitrogen-fixing symbioses between Rhizobium leguminosarum bv. viciae and pea. Here we focus on some bacterial and plant mutants that are affected in the initiation of nodules, particularly focusing on early stages of the interaction and plant-bacterial genotype interactions.
R. 1. viciae strain A3 4 makes a mixture of four Nod factors which are oligomers of four or five //-acetyl glucosamine residues carrying either aC18:l oraC18:4 acyl chain on the amino group of the terminal non-reducing residue, which also carries an O-linked acetyl group (Spaink et al. 1991; Firmin et al. 1993). Mutation of nodE results in the inability to produce the Nod factors carrying the CI8:4 group, whereas the CI8:1 Nod factors are still made (Spaink et al. 1991). nodE mutants are reduced for nodulation of pea, forming nodules which are delayed in emergence and are at around 30-40% of wild-type numbers. Nodulation is almost completely abolished if nodO is mutated in addition to the nodE (Economou et al. 1994). The nodO gene encodes a secreted calcium-binding protein that forms cation-selective pores in membranes (Sutton et al. 1994); it is thought that this pore-forming property in some way enhances the ability of the bacteria to infect peas and vetch.
Some strains of R. I. viciae contain an additional nodulation gene nodX (Davis et al. 1988) that mediates an acetylation of the terminal reducing glucosamine (Firmin et al. 1993). nodX is required for nodulation of cv. Afghanistan and cv. Iran peas, which carry an allele (sym2A) that confers resistance to nodulation by strains of R. I. viciae lacking nodX (Kozik et al. 1995). The pea sym2A allele causes growth of infection threads to be arrested (Geurts et al. 1997), and this can be overcome if the bacteria carry nodX. nodZ, which causes a fucosylation of the Nod factor, can also overcome the sym2A-mediated nodulation resistance (Ovtsyna et al. 1998) demonstrating that it is not the precise structure of the Nod factor that overcomes the nodulation resistance.
Another characteristic of cv. Afghanistan peas is that some strains of R. I. viciae, that are Nod" because they lack nodX, can competitively inhibit nodulation by strains carrying nodX (Dowling et al. 1987). This phenomenon has been called competitive nodulation blocking (Cnb) and requires the fully decorated Nod factor structure, because mutations in nodE or nodL abolish the Cnb phenotype (Dowling et al. 1989; Firmin et al. 1993). It was predicted that the Cnb effect occurred prior to infection, because an exopolysaccharide-defective mutant that does not induce infections is Cnb+ (Firmin et al. 1993).
In addition to sym2A several pea mutants that are completely nodulation deficient have been identified (Sagan et al. 1994; Engvild et al. 1987; Kozik et al. 1996; Kneen el al. 1994; Tsyganov et al. 1998 and references therein). The study of these mutants has given an insight into aspects of the pea-./?. I. viciae symbiosis (e.g. Albrecht et al. 1998; Schneider et al. 1999). Here we further that analysis and examine establishment of infections in mutant and wild-type plants inoculated with wild-type and mutant bacteria, that are compromised for nodulation due to mutations in nod genes, or blocked in competitive nodulation assays on cv. Afghanistan peas.
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