Acid‑modulated phenotypes as desirable targets for biofertilizer formulations

Abstract

Acidity is a widespread stressful condition in nature. The characteristics of the edaphic material, or anthropic activities, have led to an increase in the occurrence of acidic soils worldwide. Soil microbiomes have evolved to overcome this form of stress, with pH being a fundamental characteristic in determining the diversity of soil microbiota. The delivery of external bacteria into acid soils, such as those carried by biofertilizers to inoculate commercial crops, confronts those microorganisms with a sudden change in hydrogen-ion concentration that could be deleterious for triggering beneficial traits. To overcome acid-shock conditions, bacteria have evolved an acid-tolerant response (ATR), a trait displayed by acid-habituated bacteria that enhances their survival when faced with sudden changes in pH values. This response was found to be active in severalphylogenetically distant bacterial species, such as Proteobacteria and Firmicutes, although strain-specific responses were also reported. The ATR is triggered by acidity, which induces a acid-dependent multigene activation that not only favors bacterial viability at low pH, but also modifies the interaction of bacterial cells with partner organisms. During acid habituation, several genes are differentially expressed to protect cells from acidity, although not all of those loci are associated with the observed increase in acid tolerance. This review is an overview of the ATR in bacteria and discusses the potential advantages of incorporating acid-adapted (ATR+) plant-growth-promoting bacteria in the formulations of commercial inoculants for their application in acidic soil environments