Selection of plant growth promoting rhizobacteria sharing suitable features to be commercially developed as biostimulant products

Maria Vasseur-Coronado, Herv´e Dupr´e du Boulois, Ilaria Pertot, Gerardo Puopolo

INFORMATIVE ABSTRACT - Chemical fertilisers have a negative impact on the environment. One of the best alternatives is represented by the Plant Biostimulants, which are microorganisms or natural substances applied to plants with the aim of improving nutritional efficiency, tolerance to abiotic stressess (due to the lack or excess of an environmental factor, like water or light) and quality crop traits. Plant biostimulants ensure optimal nutrient uptake and increase crop yield with nearly zero impact on environment and human health. However, the procedure to select and develop them as commercial products is not well defined and optimized. The study wanted to propose and validate a new procedure to select plant growth-promoting rhizobacteria (PGPR) for the development as plant biostimulants. PGPR colonize plant roots and promote the decomposition and mineralisation of organic substances; facilitate nutrient uptake; promote the growth of the root system; fix nitrogen making it available to the plant.

However, PGPR are not all the same! This work aims to select the best PGPR in six phases:
(i) determination of the target crop and commercial strategy,
(ii) selection of growth rate for the isolation of microbial candidates,
(iii) screening for traits giving major agronomical advantages,
(iv) screening for traits related to product development,
(v) characterisation of the mode of action of PGPR,
(vi) assessment of plant growth efficacy.

The operative strategy was validated using a case study, combining the PGPR and humic acids on tomato plants. Assessing the bacterial strains by their ability to tolerate drought, produce high cell mass, colonize effectively tomato plant roots, produce phytohormones and solubilise soil minerals, the screening led to the selection of two candidates belonging to Pantoea and Pseudomonas genera that have shown the ability to promote tomato plant growth in experiments carried out in greenhouse conditions. In doing so, the research has provided a flow diagram for the selection of PGPR to be successfully developed and commercialized as Plant Biostimulants.