Transfer of Rhizobium meliloti pSym genes into Agrobacterium tumefaciens: host-specific nodulation by atypical infection

Abstract

The pSym megaplasmid of Rhizobium meliloti 2011 mobilized by plasmid RP4, or plasmid pGMI42, an RP4-prime derivative which carries a 290-kilobase pSym fragment including nitrogenase and nod genes, was introduced into Agrobacterium tumefaciens. The resulting transconjugants induced root deformations specifically on the homologous hosts Medicago sativa and Melilotus alba and not on the heterologous hosts Trifolium pratense and Trifolium repens. The root deformations were shown to be genuine nodules by physiological and cytological studies. Thus, host specificity nodulation genes are located on the pSym megaplasmid. Host nodulation specificity did not seem to require recognition at the root hair level since no infection threads could be detected in the root hairs. Cytological observations indicated that bacteria penetrated only the superficial layers of the host root tissue by an atypical infection process. The submeristematic zone and the central tissue of the nodules were bacteria free. Thus, nodule organogenesis was probably triggered from a distance by the bacteria. Agrobacterium transconjugants carrying pSym induced the formation of more numerous and larger nodules than those carrying the RP4-prime plasmid pGMI42, suggesting that some genes influencing nodule organogenesis are located in a pSym region(s) outside that which has been cloned into pGMI42.

Cooperative Action of Rhizobium meliloti Nodulation and Infection Mutants during the Process of Forming Mixed Infected Alfalfa Nodules

“…That cortical cell divisions could be induced from a distance was concluded earlier from microscopy studies of the first observable plant responses following inoculation (Calvert et al, 1984;Dudley, Jacobs, and Long, 1987) as well as from studies of nodules induced by noninvasive mutants or heterologous, Sym-plasmid carrying strains (Truchet et al, 1980;Hirsch et al, 1984;Truchet et al, 1984;Finan et al, 1985). Bauer et al (1985) showed, by seria1 sectioning, that cell divisions could be induced in the host root cortex by the homologous Rhizobium even though physical contact was prevented by an interposed Millipore membrane, but they did not mention whether empty nodules were formed.…”

Nodulin Gene Expression and ENOD2 Localization in Effective, Nitrogen-Fixing and Ineffective, Bacteria-Free Nodules of Alfalfa

“…However, alfalfa roots are more susceptible than many other legumes to agents that elicit the formation of ineffective, bacteria-free nodules. R. meliloti mutants defective in ex- (Finan et al, 1985;Leigh et al, 1987) and Agrobacterium tumefaciens transconjugants carrying R. meliloti nodulation genes (Wong et al, 1983;Truchet et al, 1984;Hirsch et al, 1984Hirsch et al, ,1985 induce nodules that superficially resemble normal, nitrogen-fixing nodules. They have a central tissue surrounded by nodule parenchyma and peripheral vascular bundles, but cells of the central tissue are devoid of bacteria.…”

Characterization of the Rhizobium leguminosarum genes nodLMN involved in efficient host‐specific nodulation

“…Transfer of these plasmids (or subcloned regions of them) to appropriate strains o( Rhizobium or Agrobacterium can result in concomitant transfer of host-specific nodulation (Johnston etal., 1978;Hooykaas ef at., 1981;Kondorosi ef al., 1982;Wong ef Received 10 August, 1987;revised 23 October, 1987, tFor correspondence. al., 1983Downie ef al., 1983;Schofield ef ai, 1984;Truchet et a/., 1984;Fisher ef a/., 1985).…”