Split-Root Assays Using Trifolium subterraneum Show that Rhizobium Infection Induces a Systemic Response That Can Inhibit Nodulation of Another Invasive Rhizobium Strain

Subterranean clover plants possessing two equally infectible and robust lateral root systems (“split roots”) were used in conjunction with several specific mutant strains (derived from Rhizobium trifolii ANU843) to investigate a systemic plant response induced by infective Rhizobium strains. This plant response controls and inhibits subsequent nodulation on the plant. When strain ANU843 was inoculated onto both root systems simultaneously or 24, 48, 72, or 96 h apart, an inhibitory response occurred which retarded nodulation on the root exposed to the delayed inoculum but only when the delay period between inocula was greater than 24 h. Equal numbers of nodules were generated on both roots when ANU843 was inoculated simultaneously or 24 h apart. The ability to infect subterranean clover plants was required to initiate the plant inhibitory response since preexposure of one root system to non-nodulating strains did not retard the ability of the wild-type strain to nodulate the opposing root system (even when the delay period was 96 h). Moreover, the use of specific Tn 5 -induced mutants subtly impaired in their ability to nodulate demonstrated that the plant could effectively and rapidly discriminate between infections initiated by either the parent or the mutant strains. When inoculated alone onto clover plants, these mutant strains were able to infect the most susceptible plant cells at the time of inoculation and induce nitrogen-fixing nodules. However, the separate but simultaneous inoculation on opposing root systems of the parent and the mutant strains resulted in the almost complete inhibition of the nodulation ability of the mutant strains. We concluded that the mutants were affected in their competitive ability, and this finding was reflected by poor nodule occupancy when the mutants were coinoculated with the parent strain onto a single root system. Thus the split-root system may form the basis of a simple screening method for the ranking of competitiveness of various rhizobia on small seeded legumes.

Cloning of Rhizobium leguminosarum genes for competitive nodulation blocking on peas

One type of competitive interaction among rhizobia is that between nonnodulating and nodulating strains of Rhizobium leguminosarum on primitive pea genotypes. Pisum sativum cv. Afghanistan nodulates effectively with R. leguminosarum TOM, and this can be blocked in mixed inoculations by R. leguminosarum PF2, which does not nodulate this cultivar. We termed this PF2 phenotype Cnb+, for competitive nodulation blocking. Strain PF2 contains three large plasmids including a 250-kilobase-pair symbiotic (Sym) plasmid. Transfer of this plasmid, pSymPF2, to nonblocking rhizobia conferred the Cnb+ phenotype on recipients in mixed inoculations on cultivar Afghanistan with TOM. A library of the PF2 genome constructed in the vector pMMB33 was used to isolate two cosmid clones which hybridize to pSymPF2. These cosmids, pDD50 and pDD58, overlapped to the extent of 23 kilobase pairs and conferred a Cnb+ phenotype on recipient Cnb- rhizobia, as did pSD1, a subclone from the common region.

Nodulation of specific legumes is controlled by several distinct loci in Rhizobium trifolii

Three distinct loci (designated regions III, IV and V) were identified in the 14 kb Nod region of Rhizobium trifolii strain ANU843 and were found to determine the host range characteristics of this strain. Deletion of region III or region V only from the 14 kb Nod region affected clover nodulation capacity. The introduction to R. Leguminosarum of DNA fragments on multicopy vectors carrying regions III, IV and V (but not smaller fragments) extended the host range of R. leguminosarum so that infection threads and nodules occurred on white clover plants. The same DNA fragments were introduced to the Sym plasmid-cured strain (ANU845) carrying the R. meliloti recombinant nodulation plasmid pRmSL26. Plasmid pRmSL26 alone does not confer root hair curling or nodulation on clover plants. However, the introduction to ANU845 (pRmSL26) of a 1.4 kb fragment carrying R. trifolii region IV only, resulted in the phenotypic activation of marked root hair curling ability to this strain on clovers but no infection events or nodules resulted. Only the transfer of regions III, IV and V to strain ANU845 (pRmSL26) conferred normal nodulation and host range ability of the original wild type R. trifolii strain. These results indicate that the host range genes determine the outcome of early plant-bacterial interactions primarily at the stage of root hair curling and infection.