Senior Lecturer, INMS
B.Sc., M.Sc., Ph.D.
physical: Building 14.02 // The Station Crescent // Oteha Rohe
tel: +64 9 4140800 ext 41191 // email: x.x.zhang1(at)massey.ac.nz
My research focuses on the ecological genetics of traits that affect the performance of bacteria in natural environments.
Like humans, the health and well being of plants is to a large extent determined by the microbes with which they co-exist. While some bacteria are harmful, others have the capacity to promote plant growth. Together with Prof Paul Rainey I am interested in the biology of Pseudomonas fluorescens SBW25 – a bacterium that can promote plant growth. One of the most significant challenges that we face is in understanding the function of P. fluorescens in the plant environment. If P. fluorescens was the size of a bird and possessed similar morphological complexity then progress could be made by observation alone. In the absence of readily observable phenotypes we have taken to detecting changes in patterns of gene expression. Genes expressed in one environment, but not in another are likely to encode traits relevant to the former environment, but not the latter. Understanding the biological significance of these traits and their contribution to ecological performance is our primary aim. Accordingly, much of our current research concerns the regulation, biological function and ecological significance of plant-inducible genes.
The role of a unique two component regulatory system (CbrAB) in global carbon and nitrogen metabolism.
Bacteria need to monitor their environment in terms of the availability of nutrients. The two-component CbrAB regulatory system plays an important role in controlling the overall activity of genes involved in extracting carbon, nitrogen and energy from a range of organic compounds. Current efforts are focused at understanding how CbrA senses its environment and how CbrB co-ordinates its transcriptional activities with other regulators that are also responsive to the carbon:nitrogen ratio. We are also interesting in the molecular mechanisms of carbon catabolite repression in Pseudomonas and the role of both CbrAB and NtrBC.
Molecular mechanisms of copper homeostasis inPseudomonas fluorescens SBW25.
Copper is an essential metal, but it is also toxic: how the cell maintains a correct balance of copper is of much interest. We recently characterized a copper transporter system (CueA) that is expressed in the rhizosphere (in the absence of exogenously applied copper) and plays a role in competitive colonization in planta (see Zhang & Rainey (2007) MPMI 20, 581).
CueA, is regulated by the MerR-type regulator, CueR, which is also required for the expresison of a copper chaperone (CopZ). Together these genes function to export copper from the cell and they are activite at high copper concentrations. Recently I have analysed another set of copper-related genes: copCD(which are regulated by CopRS). These genes are similar to the well-studied plasmid-born copper resistance genes, but in SBW25 they are chromosomally located and copAB, which is typically part of the Cop system, is absent from the SBW25 genome. My work indicates that the Cop system is activate at low copper concentrations and serves to bring copper into the cell. Together it seems that the two systems (Cue and Cop) -- one of which is active at low copper concentrations and serves to bring copper into the cell and the other that is active at high copper concentraitons and functions to export excess copper -- function to maintain a correct balance of copper. A paper describing this work is currently under revision at Env. Microbiol
Ecological significance of histidine and urocanate utilization in plant-colonizing Pseudomonas populations
Making a living in any environment is dependent upon obtaining an adequate supply of nutrients. Amino acids, such as histidine, are an important source of carbon, nitrogen, and energy for many bacteria. Having spent much time studying the genetics of histidine (and urocanate) uptake we have become interested in the differences among strains ofPseudomonas in terms of their abilitiy to uptake histidine and urocanate. Along with Hao Chang we are studying the variation among strains from different populations and have recently begun to obtain insight into the ecological signfiicance of the differences and also the evolutionary causes of these differences.
Xue-Xian Zhang and Paul B Rainey (2008) Regulation of copper homeostasis in Pseudomonas fluorescens SBW25. Environmental Microbiology (under revision).
Xue-Xian Zhang and Paul B Rainey (2008) Dual involvement of CbrAB and NtrBC in the regulation of histidine utilization inPseudomonas fluorescens SBW25. Genetics 178: 185-195.
Christina D. Moon, Xue-Xian Zhang, Sandra Matthijs and Paul. B. Rainey (2008) Genomic and genetic analysis of pyoverdine-mediated iron acquisition in Pseudomonas fluorescens SBW25. BMC Microbiology 8: 7.
Xue-Xian Zhang and Paul B. Rainey (2007) Genetic analysis of the histidine utilization (hut) genes in Pseudomonas fluorescens SBW25. Genetics 176: 2165-2176.
Xue-Xian Zhang, Ken Scott, Rebecca Meffin and Paul B. Rainey (2007) Genetic characterization of psp encoding the DING protein in Pseudomonas fluorescens SBW25. BMC Microbiology 7:114.
Stephen R. Giddens, Robert W. Jackson, Christina D. Moon, Michael A. Jacobs, Xue-Xian Zhang, Stefanie M. Gehrig and Paul B. Rainey (2007) Mutational activation of niche-specific genes provides insight into regulatory networks and bacterial function in a complex environment. Proc Natl Acad Sci USA104: 18247-18252.
Xue-Xian Zhang and Paul B. Rainey (2007) Construction and validation of a neutrally-marked strain of Pseudomonas fluorescens SBW25. Journal of Microbiological Methods 71:78-81.
Adrian Tett, Andrew J Spiers, Lisa C Crossman, Duane Ager, Lena Ciric, J Maxwell Dow, John C Fry, David Harris, Andrew Lilley, Anna Oliver, Julian Parkhill, Michael A Quail, Paul B Rainey, Nigel J Saunders, Kathy Seeger, Lori A S Snyder, Rob Squares, Christopher M Thomas, Sarah L Turner, Xue-Xian Zhang, Dawn Field and Mark J Bailey (2007) Sequence-based analysis of pQBR103; a representative of a unique, transfer-proficient mega plasmid resident in the microbial community of sugar beet. The ISME Journal 1: 331-340. (doi:10.1038/ismej.2007.47)
Xue-Xian Zhang and Paul B. Rainey (2007) The role of a P1-type ATPase from Pseudomonas fluorescens SBW25 in copper homeostasis and plant colonization. Molecular Plant-Microbe Interactions 20: 581-588.
Tadashi Fukami, Hubertus J. E. Beaumont, Xue-Xian Zhangand Paul B. Rainey (2007) Immigration history controls diversification in experimental adaptive radiation. Nature 446:436-439.
Xue-Xian Zhang, Andrew George, Mark J. Bailey and Paul B. Rainey (2006) The histidine utilization (hut) genes ofPseudomonas fluorescens SBW25 are active on the plant surfaces, but are not required for competitive colonization of sugar beet seedlings. Microbiology 152: 1867-1875.
Xue-Xian Zhang, Andrew K. Lilley, Mark J. Bailey and Paul B. Rainey (2004) The indigenous Pseudomonas plasmid pQBR103 encodes plant-inducible genes, including three putative helicases. FEMS Microbiology Ecology 51: 9-17.
Xue-Xian Zhang, Andrew K. Lilley, Mark J. Bailey and Paul B. Rainey (2004) Functional and phylogenetic analysis of a plant-inducible oligoribonuclease (orn) gene from an indigenous Pseudomonas plasmid. Microbiology 150: 2889-2898.
Sarah L. Turner, Xue-Xian Zhang, Fu-Di Li and J. Peter W. Young (2002) What does a bacterial genome sequence represent? Misassignment of MAFF303099 to the genospecies Mesorhizobium loti. Microbiology 148: 3330-3331.
Xue-Xian Zhang, Bob Kosier and Ursula B. Priefer (2001) Symbiotic Plasmid Rearrangement in Rhizobium leguminosarum bv. viciae VF39SM. Journal of Bacteriology183: 2141-2144.
Xue-Xian Zhang, Bob Kosier and Ursula B. Priefer (2001) Genetic diversity of indigenous Rhizobium leguminosarum bv. viciae isolates nodulating two different host plants during soil restoration fields with alfalfa. Molecular Ecology 10: 2297-2305.
Xue-Xian Zhang, Turner SL, Guo XW, Yang HJ, Debelle F, Denarie J, Young JPW & Li FD (2000) The common nodulation genes of Astragalus sinicus rhizobia are conserved despite chromosomal diversity. Applied and Environmental Microbiology66: 2988-2995.