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 differin g in growth under Mn or Zn deficiency. Soil microorganisms were extrac ted from three soils by two-phase aqueous partitioning and used to ino culate seedlings of four wheat (Triticum aestivum L.) genotypes mounte d in growth pouches and supplied with various nutrient solutions. The total count of colony-forming units (cfu) decreased after extraction c ompared 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 sho ot growth due to Zn or Mn deficiency. The most Mn-efficient wheat geno type C8MM (which grows better and yields more than Mn-inefficient geno types under Mn-deficiency conditions) had roots colonized with more ps eudomonads than other genotypes. Similarly, Zn-efficient Aroona sustai ned colonization of a greater number of non-pseudomonads under Zn defi ciency than under control conditions. Other genotypes showed the same tendency. Four out of 16 different Tn5-B20 insertion mutants of Pseudo monas fluorescens, which are routinely used in studying the role of ro ot exudates in influencing gene expression in root-colonizing bacteria , showed better colonization of wheat roots under Zn-sufficient condit ions than under Zn deficiency. In conclusion, Zn and Mn nutrition modi fies the quantitative make-up of the wheat rhizoflora, the effect that is strongly dependent on the genotype. The approaches used here for i nvestigating soil bacteria/plant/micronutrient interactions should hav e wider applications.