Peerakarn Banjerdkij creator Skorn Mongkolsuk Chairperson Preeda Pakpian Examination Committee Suvit Loprasert Examination committee Paiboon Vattanaviboon Examination committee Chongrak Polprasert Examination Committee Saovanee C. Dharmsthiti Examination committee Post-Graduate Education Scholarship donor Training and Research Program in Environmental Science, Technology and Management under Higher Education Development Project of the Ministry of University Affairs Scholarship donor text series technical report th Pathum Thani, Thailand Asian Institute of Technology 2003 continuing eng 127 p. Cadmium (Cd) and zinc (Zn) are important environmental pollutants contaminated in crop fields where phosphate fertilizers have been abundantly used and from industrial sources. The responses of Xanthomonas campestris, a soil and plant bacterial pathogen, against the exposure of Cd and Zn were investigated. Pretreatment of X campestris with sub-lethal concentrations of Cd induced adaptive protection against subsequent exposure to lethal doses of Cd. Moreover, Cd-induced cells also showed cross-resistance to lethal concentrations of Zn. These induced protections required newly synthesized proteins. Unexpectedly, Zn-induced cells did not exhibit adaptive protection against lethal concentrations of Zn or Cd. These data suggested that the increased resistance to Cd and Zn killing probably involved other protective mechanisms in addition to ion efflux. The effects of Cd and Zn on oxidative stress response in X campestris were investigated. Pretreatment of X campestris with inducing concentration of Cd stimulates onset of cross- protection against subsequent killing treatments with H20 2 and tert-Butyl hydroperoxide (tBOOH) but not with menadione (MD). Cross-resistance to H20 2 was fully dependent on OxyR, a peroxide sensor and transcription regulator, as shown by the completely loss of induced protection in the oxyR mutant pretreated cells. Induced resistance against tBOOH killing was only partially inhibited both in the oxyR and ohrR mutant suggests that these genes had crucial roles in induced protection to tBOOH. However, Cd induced cells of the oxyR and ohrR double mutant still retain partial resistance to tBOOH suggests the existence of other mechanisms involving in organic hydroperoxide protection. Exposure of exponential bacterial cultures to inducing concentration of Cd elevated the expression of various oxidative stress scavenging enzymes both in OxyR and OhrR regulons including alkyl hydroperoxide reductase (AhpC), monofunctional catalase (KatA) and organic hydroperoxide resistance (Ohr). The data elucidate that accumulated Cd in the environment could alter the responses against oxidative stress in soil and pathogenic bacteria. As a consequence, this alteration may affect plant/ microbe interaction leading to change in bacterial pathogenicity. A dissertation submitted in partial fulfillment of the requirements:· for the degree of Doctor of Technical Science Inter-University Program on Environmental Toxicology, Technology and Management Thesis (Ph.D.) - Asian Institute of Technology, 2003 Cadmium Xanthomonas campestris Asian Institute of Technology. http://203.159.5.9/ait-thesis/detail.php?q=B07636 http://203.159.5.9/ait-thesis/detail.php?q=B07636 040621 20260817172042.0