ReviewPlant growth-promoting rhizobacteria act as biostimulants in horticulture

- PGPRs are a sustainable tool to overcome present and future constrains of horticulture.
- The mode of action of PGPRs are wide diverse and interconnected.
- Inoculation procedure and selection of the proper bacteria is crucial for the outcome in terms of final yield.

To overcome the challenge of increasing food production with a significant reduction of agrochemical use and environmental pollution, and an increase of natural resource productivity, the use of soil microorganisms in horticulture is essential. One group of microorganisms consists of plant growth-promoting rhizobacteria (PGPR), which have been studied from the beginning of the twentieth century and their mode of action at the physiological level is currently well understood. PGPR mechanisms include hormone release or hormonal changes within plants, the production of volatile organic compounds, the improvement in the availability of nutrients and the enhancement of tolerance to abiotic stresses. All these mechanisms are described in the present review. However, to maximize the effects of these mechanisms, the proper PGPR strain needs to be selected in each soil-plant-PGPR system and the mode of inoculation must be optimized in both greenhouse and open-field experiments. This review summarizes recent progress in our understanding of the PGPR-plant interaction and highlights future lines of research that should increase our knowledge on plant-bacterial communication and that can help to improve the effective use of PGPR in horticulture.