Effects on Glomus mosseae Root Colonization by Paenibacillus polymyxa and Paenibacillus brasilensis Strains as Related to Soil P-Availability in Winter Wheat
Greenhouse experiments were conducted to assess the effects of inoculating winter wheat (Triticum aestivum) with plant growth promoting rhizobacteria (PGPR) of the genus Paenibacillus under phosphate P-limited soil conditions in the presence or absence of the arbuscular mycorrhizal fungus (AMF) Glomus mosseae. Four P. polymyxa strains and one P. brasilensis strain were compared at two cell concentrations (106 and 108？cells？g？1 seeds) of inoculation, and surface sterilized AMF spores were added to pots. Mycorrhizal root colonization, plant growth, and plant uptake of phosphorus were analyzed. Bacterial phosphate solubilization was examined separately in vitro. Most P. polymyxa strains, isolated from wheat, had dramatic effects per se on root growth and root P-content. No treatment gave significant effect on shoot growth. AMF root colonization levels and total plant uptake of P were much stimulated by the addition of most P. polymyxa strains. The AM fungus alone and the P. brasilensis, alone or in combination with the fungus, did not affect total plant P-levels. Our results indicate that practical application of inoculation with plant host-specific rhizobacteria (i.e., P. polymyxa) could positively influence uptake of phosphorus in P-deficient soils by wheat plants, provided that suitable AM fungi (e.g., G. mosseae) are present. 1. Introduction Increased environmental awareness is progressively leading to a shift from conventional intensive agriculture to low-input sustainable agricultural cropping systems relying on biological processes rather than agrochemicals to maintain crop health and productivity. This shift has resulted in greater interest in naturally occurring soil microorganisms that facilitate improvement of soil fertility and/or stimulate plant nutrition and health, either alone or via specific interactions. One example of beneficial microbial interactions is the association between arbuscular mycorrhizal (AM) fungi and bacteria [1–3]. AM fungi and bacteria interact synergistically to stimulate plant growth via a range of mechanisms. For instance, certain bacterial species directly affect AM fungal germination and growth rate [4–7], thereby stimulating plant root colonization [8]. Earlier studies by our group showed that Glomus mosseae colonization of clover and wheat roots in pot cultures containing sterilized soil was higher upon co-inoculation with Paenibacillus brasilensis PB177, compared to controls without bacterial inoculates [8, 9]. Consistently, similar results regarding beneficial impact of Paenibacillus spp. on AM fungi have been
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