The growth of a plant-parasitic bacterium, Clavibacter xyli subsp cynodontis, is enhanced by xylem fluid components

Clavibacter xyli subspecies cynodontis is a gram-positive coryneform bacterium, originally isolated from Bermuda grass. The bacterium is xylem-limited, slow-growing and nutritionally fastidious. We found that extracts of Bermuda grass and maize, as well as xylem fluid collected from decapitated maize plants; enhanced the growth of the bacteria in vitro. The growth was enhanced even with 1 % (v/v) maize xylem sap in a rich culture medium, giving both an increased growth rate and a higher stationary phase cell density. The relative increase in growth was inversely correlated with the size of bacterial inoculum. When liquid medium was stored for more than 2 months, the bacteria did not grow at all unless the medium was conditioned with a plant exudate. We fractionated maize xylem fluid by ultrafiltration and chromatographic methods and tested all the fractions for bacterial growth induction. The xylem sap was found to contain two distinct growth-enhancing compounds: a small hydrophilic compound and a proteinaceous compound of more than 10 kDa. (C) 2000 Academic Press.