NITROGEN LIMITS XANTHOMONAS-CAMPESTRIS PV. CAMPESTRIS INVASION OF THEHOST XYLEM

Hydathodes are the primary portal of entry for Xanthomonas campestris pv. campestris in cruciferous plants. Once entry is gained, this vascu lar pathogen quickly multiplies and ramifies progressively in the xyle m. It was shown previously that X. campestris loses its motility when it interacts with the plant host and its movement in the xylem is rela tively passive; nonetheless, the bacterial cells are able to continue to spread in the xylem. The mechanism of bacterial invasion in the xyl em has remained largely unidentified. In the present study, X. campest ris pv. campestris chromosomally tagged with a constitutive lux report er cassette was used for real-rime analysis of the invasive characteri stic of this xylem-invading pathogen as a function of host nitrogen nu trition. High levels of nitrogen were found to slow down and even impe de the invasive activity of X. campestris pv. campestris. The type of nitrogen did not appear to affect the growth rate of X. campestris pv. campestris. Each nitrogen type had an appreciable effect on curbing b acterial invasion of the xylem in cabbage plants. These studies sugges t that host defence factors other than the conventional hypersensitive response are enhanced by nitrogen in the xylem. (C) 1998 Academic Pre ss Limited.