Abstract

Genome Sequence and Mutational Analysis of Plant-Growth-Promoting Bacterium Agrobacterium tumefaciens CCNWGS0286 Isolated from a Zinc-Lead Mine Tailing Xiuli Hao a , b , Pin Xie a , Laurel Johnstone c , Susan J. Miller d , Christopher Rensing b and Gehong Wei a a College of Life Sciences, State Key Laboratory of Crop Stress Biology in Arid Areas, Northwest A & F University, Yangling, Shaanxi, China b Center for Agricultural and Environmental Biotechnology, RTI International, Research Triangle Park, North Carolina, USA c University of Arizona Genetics Core, Tucson, Arizona, USA d Biotechnology Computing Facility, Arizona Research Laboratories, University of Arizona, Tucson, Arizona, USA ABSTRACT The plant-growth-promoting bacterium Agrobacterium tumefaciens CCNWGS0286, isolated from the nodules of Robinia pseudoacacia growing in zinc-lead mine tailings, both displayed high metal resistance and enhanced the growth of Robinia plants in a metal-contaminated environment. Our goal was to determine whether bacterial metal resistance or the capacity to produce phytohormones had a larger impact on the growth of host plants under zinc stress. Eight zinc-sensitive mutants and one zinc-sensitive mutant with reduced indole-3-acetic acid (IAA) production were obtained by transposon mutagenesis. Analysis of the genome sequence and of transcription via reverse transcriptase PCR (RT-PCR) combined with transposon gene disruptions revealed that ZntA-4200 and the transcriptional regulator ZntR1 played important roles in the zinc homeostasis of A. tumefaciens CCNWGS0286. In addition, interruption of a putative oligoketide cyclase/lipid transport protein reduced IAA synthesis and also showed reduced zinc and cadmium resistance but had no influence on copper resistance. In greenhouse studies, R. pseudoacacia inoculated with A. tumefaciens CCNWGS0286 displayed a significant increase in biomass production over that without inoculation, even in a zinc-contaminated environment. Interestingly, the differences in plant biomass improvement among A. tumefaciens CCNWGS0286, A. tumefaciens C58, and zinc-sensitive mutants 12-2 ( zntA ::Tn 5 ) and 15-6 (low IAA production) revealed that phytohormones, rather than genes encoding zinc resistance determinants, were the dominant factor in enhancing plant growth in contaminated soil.