Procedure for Obtaining Efficient Root Nodulation of a Pea Cultivar by a Desired Rhizobium Strain and Preempting Nodulation by Other Strains

Identification and cloning of the bacterial nodulation specificity gene in the Sinorhizobium meliloti--Medicago laciniata symbiosis

Medicago laciniata (cut-leaf medic) is an annual medic that is highly nodulation specific, nodulating only with a restricted range of Sinorhizobium meliloti. e.g., strain 102L4, but not with most strains that nodulate Medicago sativa (alfalfa), e.g., strains RCR2011 and Rm41. Our aim was to identify and clone the S. meliloti 102L4 gene implicated in the specific nodulation of M. laciniata and to characterize the adjacent nodulation (nod) region. An 11-kb EcoRI DNA fragment from S. meliloti 102L4 was shown to complement strain RCR2011 for nodulation of M. laciniata. Nucleotide sequencing revealed that this fragment contained nodABCIJ genes whose overall arrangement was similar to those found in strains RCR2011 and Rm41, which do not nodulate M. laciniata. Data for Tn5 mutagenesis of the nodABCIJ region of strain 102L4 suggested that the nodC gene was involved in the specific nodulation of M. laciniata. Tn5 insertions in the nodIJ genes gave mutants with nodulation delay phenotypes on both M. laciniata and M. sativa. Only subclones of the 11-kb DNA fragment containing a functional nodC gene from strain 102L4 were able to complement strain RCR2011 for nodulation of M. laciniata. The practical implications of these findings are discussed in the context of the development of a specific M. sativa - S. meliloti combination that excludes competition for nodulation by bacterial competitors resident in soil.

HOST VARIATION FOR INTERACTIONS WITH BENEFICIAL PLANT-ASSOCIATED MICROBES

Beneficial plant-associated microbes can profoundly influence plant health by suppressing disease, enhancing nutrient uptake, fixing atmospheric nitrogen, and promoting plant growth. Host variation, among cultivars or plant genotypes, for response to beneficial microorganisms suggests that plant genes play a role in supporting these interactions. Such host variation can be found among diverse groups of microorganisms including rhizobia, mycorrhizal fungi, and microbial biocontrol agents. Discrete variation among plant genotypes for interaction with beneficial microbes has led to the discovery of single genes that specify compatible interactions. Continuous variation for interaction phenotypes such as disease suppression, plant growth, or nutrient uptake have led to hypotheses, and in some cases genetic descriptions, of multigenic control of these interactions. Future research into the role of plant genes involved in hosting beneficial plant-associated microbes will provide greater insight into this relatively unexplored area of biology and should provide new tools to improve plant health in agriculture.

Preference in the nodulation ofPhaseolus vulgariscv. RAB39. II. Effect of delayed inoculation or low cell representation in the inoculant on nodule occupancy byRhizobium tropiciUMR1899

Common bean (Phaseolus vulgaris L.) is a traditional crop in much of Latin America, where it is often planted into soils containing numerous, sometimes ineffective, indigenous rhizobia. The presence of these indigenous organisms can limit response to inoculation. Because of this, we have sought bean cultivars that will nodulate preferentially with the inoculant strain, and have previously reported on the preference between the bean cultivar RAB39 and strains of Rhizobium tropici. We have detailed this interaction using the inoculant-quality strain UMR1899. In the present study the root tip marking (RTM) technique was used to demonstrate that this preference in nodulation was evident, even when inoculation with UMR1899 was delayed up to 8 relative to that with Rhizobium etli UMR1632. In contrast to studies with other legumes, roots of RAB39 were not predisposed to nodulate with UMR1632, even though preexposed to this strain for considerable periods of time. The presence of UMR1899 actually reduced nodulation by UMR1632 substantially, even when inoculation with UMR1899 was significantly delayed. When UMR1899 and UMR1632 were applied to separate halves of a split-root system, the number of nodules on the side receiving UMR1632 was less than for the half root inoculated with UMR1899, but the differences were not significant. This suggests that the preference response is not systemic but requires proximity between the strains involved. UMR1899 produced more than 50% of the nodules even when the ratio of UMR1632:UMR1899 in the inoculant was 10:1. The results are further evidence of a stable and marked preference of RAB39 for UMR1899, which warrants a more detailed study at the field level.Key words: Phaseolus vulgaris L., common bean, delayed inoculation, strain preference, cell proportions.