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 bacter ium, originally isolated from Bermuda grass. The bacterium is xylem-limited , slow-growing and nutritionally fastidious. We found that extracts of Berm uda grass and maize, as well as xylem fluid collected from decapitated maiz e plants; enhanced the growth of the bacteria in vitro. The growth was enha nced even with 1 % (v/v) maize xylem sap in a rich culture medium, giving b oth an increased growth rate and a higher stationary phase cell density. Th e relative increase in growth was inversely correlated with the size of bac terial 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 chromato graphic methods and tested all the fractions for bacterial growth induction . The xylem sap was found to contain two distinct growth-enhancing compound s: a small hydrophilic compound and a proteinaceous compound of more than 1 0 kDa. (C) 2000 Academic Press.