Rhizosphere colonization of hexaploid wheat by Pseudomonas fluorescens strains Q8r1-96 and Q2-87 is cultivar-variable and associated with changes in gross root morphology

In the Pacific Northwest, natural suppression of take-all disease of wheat (take-all decline) has been attributed to certain strains of the gram-negative bacterium Pseudomonas fluorescens. To examine naturally occurring variation in the ability of Triticum aestivum L. (hexaploid wheat) to support these bacteria, we have surveyed 27 Pacific Northwest (PNW) cultivars for the ability to undergo root colonization with the aggressive colonizer P. fluorescens strain Q8r1-96, and P. fluorescens strain Q2-87, a less effective colonizer. In seed inoculation experiments, Q8r1-86 colonized roots of all of the cultivars equally or more effectively than did Q2-87 in a non-pasteurized, non-agricultural soil. Seven cultivars supported significantly (P<0.05) higher rhizosphere populations of Q8r1-96 than Q2-87 within 14 days post-inoculation (dpi), two cultivars supported relatively high population densities of each bacterial strain, and three cultivars supported low population densities of the strains. Population densities normalized to root weight reached maximum steady-state levels within 4 dpi, and differential colonization was seen as early as 7 dpi. In pairwise comparisons, the bacterial treatments differentially affected the root morphology of some of the cultivars at 14 dpi. However, principal components (factor) and correlation analysis showed that preferential colonization by Q8r1-96 was independent of root fresh weight, total length, surface area, volume, and average diameter, and that differential colonization was not correlated with changes in any specific root morphometric variable. Variation in root colonization of specific cultivars suggests useful genetic stocks for mapping and identifying host genes involved in wheat–rhizosphere interactions.