ROOT COLONIZATION OF MAIZE AND LETTUCE BY BIOLUMINESCENT RHIZOBIUM-LEGUMINOSARUM BIOVAR PHASEOLI

Two strains of Rhizobium leguminosarum bv. phaseoli and three other pl ant growth-promoting rhizobacteria (PGPR) were examined for the potent ial of maize and lettuce root colonization. All of these strains were selected in vitro for their phosphate-solubilizing abilities. Maize an d lettuce seeds were treated with derivatives of all strains marked wi th lux genes for bioluminescence and resistance to kanamycin and rifam pin prior to planting in nonsterile Premix and natural soil, The intro duced bacterial strains were quantified on roots by dilution plating o n antibiotic media together with observation of bioluminescence. Rhizo bia were superior colonizers compared with other tested bacteria; rhiz obial root populations averaged log 4.1 CFU/g (fresh weight) on maize roots 4 weeks after seeding and log 3.7 CFU/g (fresh weight) on lettuc e roots 5 weeks after seeding. The average populations of the recovere d PGPR strains were log 3.5 and log 3.0 CFU/g (fresh weight) on maize and lettuce roots, respectively. One of the three PGPR was not recover ed later than the first week after seeding in Premix. Bioluminescence also permitted visualization of in situ root colonization in rhizoboxe s and demonstrated the efficiency of rhizobial strains to colonize and survive on maize and lettuce roots.