A Root-Colonizing Pseudomonad Lessens Stress Responses in Wheat Imposed by CuO Nanoparticles

Nanoparticle (NPs) containing essential metals are being considered in formulations of fertilizers to boost plant nutrition in soils with low metal bioavailability. This paper addresses whether colonization of wheat roots by the bacterium, Pseudomonas chlororaphis O6 (PcO6), protected roots from the reduced elongation caused by CuO NPs. There was a trend for slightly elongated roots when seedlings with roots colonized by PcO6 were grown with CuO NPs; the density of bacterial cells on the root surface was not altered by the NPs. Accumulations of reactive oxygen species in the plant root cells caused by CuO NPs were little affected by root colonization. However, bacterial colonization did reduce the extent of expression of an array of genes associated with plant responses to stress induced by root exposure to CuO NPs. PcO6 colonization also reduced the levels of two important chelators of Cu ions, citric and malic acids, in the rhizosphere solution; presumably because these acids were used as nutrients for bacterial growth. There was a trend for lower levels of soluble Cu in the rhizosphere solution and reduced Cu loads in the true leaves with PcO6 colonization. These studies indicate that root colonization by bacterial cells modulates plant responses to contact with CuO NPs.含必需金属的纳米颗粒（Nanoparticle, NPs）目前被纳入肥料配方，用于提升金属生物利用度较低的土壤中作物的营养供给水平。本研究旨在探讨绿针假单胞菌O6（Pseudomonas chlororaphis O6, PcO6）定殖小麦根系，是否能够缓解氧化铜纳米颗粒（CuO NPs）诱导的根系伸长抑制效应。当根系被PcO6定殖的小麦幼苗与CuO NPs共培养时，根系呈现出轻微伸长的趋势；且纳米颗粒并未改变根表细菌的定植密度。CuO NPs诱导植物根系细胞产生的活性氧（reactive oxygen species, ROS）积累量，几乎未受根系定殖的影响。但细菌定殖确实下调了一系列与植物根系暴露于CuO NPs诱导的胁迫响应相关的基因的表达幅度。PcO6定殖还降低了根际溶液中两种重要的铜离子螯合剂——柠檬酸与苹果酸的含量；推测这是由于这些有机酸被细菌用作生长所需的营养基质。经PcO6定殖后，根际溶液中的可溶性铜含量以及真叶中的铜负载量均呈现下降趋势。本研究表明，细菌根系定殖可调控植物对CuO NPs接触的响应过程。