Plants can develop an enhanced defensive capacity in response to infection by arbuscular mycorrhizal fungi (AMF). This 'mycorrhiza-induced resistance' (MIR) provides systemic protection against a wide range of attackers and shares characteristics with systemic acquired resistance (SAR) after pathogen infection and induced systemic resistance (ISR) following root colonisation by non-pathogenic rhizobacteria. It is commonly assumed that fungal stimulation of the plant immune system is solely responsible for MIR. In this opinion article, we present a novel model of MIR that integrates different aspects of the induced resistance phenomenon. We propose that MIR is a cumulative effect of direct plant responses to mycorrhizal infection and indirect immune responses to ISR-eliciting rhizobacteria in the mycorrhizosphere.

Benzoxazinoids, such as 2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one (DIMBOA), are secondary metabolites in grasses. In addition to their function in plant defence against pests and diseases above-ground, benzoxazinoids (BXs) have also been implicated in defence below-ground, where they can exert allelochemical or antimicrobial activities. We have studied the impact of BXs on the interaction between maize and Pseudomonas putida KT2440, a competitive coloniser of the maize rhizosphere with plant-beneficial traits. Chromatographic analyses revealed that DIMBOA is the main BX compound in root exudates of maize. In vitro analysis of DIMBOA stability indicated that KT2440 tolerance of DIMBOA is based on metabolism-dependent breakdown of this BX compound. Transcriptome analysis of DIMBOA-exposed P. putida identified increased transcription of genes controlling benzoate catabolism and chemotaxis. Chemotaxis assays confirmed motility of P. putida towards DIMBOA. Moreover, colonisation essays in soil with Green Fluorescent Protein (GFP)-expressing P. putida showed that DIMBOA-producing roots of wild-type maize attract significantly higher numbers of P. putida cells than roots of the DIMBOA-deficient bx1 mutant. Our results demonstrate a central role for DIMBOA as a below-ground semiochemical for recruitment of plant-beneficial rhizobacteria during the relatively young and vulnerable growth stages of maize.

The dynamics of phosphorus (P) in the environment is important for regulating nutrient cycles in natural and managed ecosystems and an integral part in assessing biological resilience against environmental change. Organic P (P_o) compounds play key roles in biological and ecosystems function in the terrestrial environment being critical to cell function, growth and reproduction.

Summary: Phosphorus cycling exerts significant influence upon soil fertility and productivity – processes largely controlled by microbial activity. We adopted phenotypic and metagenomic approaches to investigate phosphatase genes within soils. Microbial communities in bare fallowed soil showed a marked capacity to utilise phytate for growth compared with arable or grassland soil communities. Bare fallowed soil contained lowest concentrations of orthophosphate. Analysis of metagenomes indicated phoA , phoD and phoX , and histidine acid and cysteine phytase genes were most abundant in grassland soil which contained the greatest amount of NaOH‐EDTA extractable orthophosphate. Beta‐propeller phytase genes were most abundant in bare fallowed soil. Phylogenetic analysis of metagenome sequences indicated the phenotypic shift observed in the capacity to mineralise phytate in bare fallow soil was accompanied by an increase in phoD , phoX and beta‐propeller phytase genes coding for exoenzymes. However, there was a remarkable degree of genetic similarity across the soils despite the differences in land‐use. Predicted extracellular ecotypes were distributed across a greater range of soil structure than predicted intracellular ecotypes, suggesting that microbial communities subject to the dual stresses of low nutrient availability and reduced access to organic material in bare fallowed soils rely upon the action of exoenzymes.