Plant genotype and micronutrient status influence colonization of wheat roots by soil bacteria

Wheat genotypes show differential growth under manganese (Mn) or zinc (Zn) deficiency. A contribution of rhizoplane microorganisms to such a differential response is unclear. This study was conducted to assess changes in bacterial colonization of roots of wheat genotypes differing in growth under Mn or Zn deficiency. Soil microorganisms were extracted from three soils by two-phase aqueous partitioning and used to inoculate seedlings of four wheat (Triticum aestivum L.) genotypes mounted in growth pouches and supplied with various nutrient solutions. The total count of colony-forming units (cfu) decreased after extraction compared to soils-before extraction, while pseudomonad numbers remained largely unaffected by extraction. Axenic plants grown in pouches and supplied with solutions containing no Zn or Mn showed a decline in shoot growth due to Zn or Mn deficiency. The most Mn-efficient wheat genotype C8MM (which grows better and yields more than Mn-inefficient genotypes under Mn-deficiency conditions) had roots colonized with more pseudomonads than other genotypes. Similarly, Zn-efficient Aroona sustained colonization of a greater number of non-pseudomonads under Zn deficiency than under control conditions. Other genotypes showed the same tendency. Four out of 16 different Tn5-B20 insertion mutants of Pseudomonas fluorescens, which are routinely used in studying the role of root exudates in influencing gene expression in root-colonizing bacteria, showed better colonization of wheat roots under Zn-sufficient conditions than under Zn deficiency. In conclusion, Zn and Mn nutrition modifies the quantitative make-up of the wheat rhizoflora, the effect that is strongly dependent on the genotype. The approaches used here for investigating soil bacteria/plant/micronutrient interactions should have wider applications.