Interactions between arbuscular mycorrhizal fungi, rhizobacteria, soil phosphorus and plant cytokinin deficiency change the root morphology, yield and quality of tobacco

Arbuscular mycorrhizal fungi (AMF) and rhizobacteria (Pseudomonas fluorescens) have been targeted for plant growth promotion in order to reduce agrochemical inputs. However, the effects of their interaction on root morphology under different nutrient levels are unknown. Moreover, both soil microorganisms can alter the levels of plant hormonal regulators but no in vivo evidence is available for their interplay with cytokinin (CK) on root morphology. In a full-factorial greenhouse experiment we investigated the effects of interaction between Rhizophagus intraradices, P. fluorescens 8569r, phosphorus (P) amendment and plant CK deficiency on tobacco root morphology, shoot yield and quality. Our results suggest that reduced CK levels may be involved in plant signaling to stimulate AMF hyphal growth in the roots. In addition, we document that a bacteria isolated from the rhizosphere of a non-mycorrhizal plant can function as mycorrhizal helper bacteria, most likely via interplay with phytohormones. The two soil microorganisms, depending on the inoculum combination, P amendment and plant CK levels, modified tobacco root morphology. Our results suggest that the positive interactions between P. fluorescens and AMF depend on soil nutrient status and root hormonal balance. Both microorganism modified shoot yield and these effects seem to result from altered root morphology. Overall, our study support early conclusions that the classification of a soil microorganism as detrimental or beneficial should be based upon their net effects on the plant growth according to circumstances. We suggest this consideration to be extended to the effects of interaction between soil microorganisms on root morphology.