Xylem-residing bacteria in alfalfa roots

Abstract

Bacteria were consistently isolated from the root and crown xylem of symptomless field-grown alfalfa plants. Most of the plants tested contained more than one bacterial genus. Pseudomonas spp. accounted for 52% of the 387 isolates identified and the fluorescent pseudomonads were the most frequent bacteria isolated. About 23% of the isolates were Erwinia-like bacteria. Bacterial population ranged from 6.0 × 103 to 4.3 × 104 CFU/g of fresh xylem, and was not affected by plant age or cultivar or by the sampling locations. The surface-sterilized seeds of the cultivars Iroquois and Titan were bacteria free and only 3 and 5% of the seeds of the cultivars Apica and Saranac, respectively, contained bacteria. In a greenhouse experiment, double antibotic resistant bacteria were inoculated into the soil of artificially wounded and intact roots of alfalfa plants, and on the stubble. The highest incidence of bacteria in the root xylem occurred when the roots were wounded. The highest numbers of bacteria (CFU/g fresh weight) were found when plants were wounded (stubble or roots) as compared with intact plants. The results suggest that bacteria are normal residents of the root xylem and that their main avenues of entrance to the xylem are natural root wounds and plant stubbles remaining after harvest.