Bacterial Inoculants Mitigating Water Scarcity in Tomato: The Importance of Long-Term in vivo Experiments

Abstract

Global population growth and climate change raise a challenge to agriculture, which, combined with the issues concerning the use of chemical fertilizers, have generated increasing attention in the use of plant-associated bacteria as a sustainable strategy in agri-food systems. The objective of this study is to evaluate the ability of five bacterial strains, previously isolated from the rhizosphere or endosphere of plants adapted to harsh environmental conditions, to act as potential plant biofertilizers in different conditions of water availability. The strain biosafety for a deliberate environmental release was investigated through a literature survey and antibiotic resistance testing. The selected strains were first characterized for their plant growth–promoting (PGP) and rhizocompetence-related traits through in vitro assays and then on short-term in vivo experiments on tomato plants. A long-term greenhouse experiment was further conducted to monitor the PGP effect of the bacteria during the entire life cycle of tomato plants subjected to full irrigation or to severe water deficit conditions, aiming to assess their actual effect on plant productivity, which is the ultimate target of the agricultural sector. Some of the strains showed a potential in improving water use efficiency and mitigating plant water stress. Under severe irrigation deficit, four of the tested strains, Micrococcus yunnanensis M1, Bacillus simplex RP-26, Pseudomonas stutzeri SR7-77, and Paenarthrobacter nitroguajacolicus 2–50, significantly increased the number of productive plants in comparison to non-bacterized control ones. Two of them, Bacillus simplex RP-26 and Paenarthrobacter nitroguajacolicus 2–50, demonstrated also, under full irrigation, to significantly improve the water productivity in comparison with non-bacterized plants. Despite all the strains showed promising PGP potential in short-term assays, the positive effect of the bacterial inoculants on plant physiology and fruit yield was observed in some cases but never corroborated by statistical significance. These results highlight the importance of performing long-term in vivo experiments to define the real PGP ability of a bacterial inoculant to positively impact plant production.

A bacterial formula with native strains as alternative to chemical fertiliser for tomato crop

“…Moreover, previous research on different crops concluded that PGPB inoculation must be combined with chemical treatment to have the best results on plant growth (Bona et al 2018;Cordero et al 2018;Angulo et al 2020). The same assertion was reported for the tomato species: several authors wrote about the positive effect of microbial inoculation under environmental or biological stresses such as drought, salinity (Mannino et al 2020;Riva et al 2021), nutrient de ciency in soils, and pathogens; anyway, the few studies that compared bio-and chemical fertilisation, described a lower yield in fruit production when chemicals have been completely replaced by PGPB inoculations (Adesemoye et al 2009; Ye et al 2020). The results of these mentioned works do not encourage the tomato growers to apply the biofertilisation.…”

“…After the positive results obtained by in vitro screening for the selected potential PGP bacteria, in agreement with the bottom-up approach suggested by Riva et al (2021), we performed eld experiments to test the real effect of these bacterial inoculants on plant production of three different tomato varieties.…”

“…Anyway, the greenhouse experimentations were essential because it is known that lab screening and small-scale experiments may provide only limited information and success in the open eld is often variable. In other studies, some consortia with multiple PGP-activities showed low e ciency when applied in eld experiments (Cardinale et al 2015;Compant et al 2019;Riva et al 2021), while other bacteria, that in typical assays did not display a promising set of PGP activities, proved to be the best growth promoters when tested directly on plants (Cardinale et al 2015). Therefore, if eld experiments are fundamental to select the most e cient bioinoculant, it is equally important to perform long-term experiments of biofertilisation and to evaluate the PGP effect exerted throughout the plant life cycle and especially in fruit production (Riva et al 2021).…”

Water Stress Alleviation Effects of Biostimulants on Greenhouse-Grown Tomato Fruit

“…Similar results were reported by Fernandes et al [39], who tested the same biostimulants on common bean and suggested water-stress-mitigating effects of the tested biostimulants on nutritional value parameters such as fat, proteins and ash content. These results could be associated with the improved nutrient status of plants, since previous studies have established the positive role of arbuscular mycorrhizal fungi (AMF), bacteria and seaweed extracts on nutrient uptake [31,35,[46][47][48]. Moreover, the application of biostimulants containing chelated metals improved specific nutritional aspects of tomato fruit under full irrigation, which could be considered as a sustainable tool to improve fruit quality.…”

“…In addition to their culinary role in the human diet, tomatoes represent a food source with low energetic value and with unique constituents that may positively affect health, since they are rich in vitamins, minerals, lycopene, β-carotene and anticancer agents [7,27]. The use of biostimulants in tomato crops is becoming popular, since positive results have been observed in terms of yield and fruit quality, especially under abiotic stress conditions, where plants showed increased resilience [28][29][30][31][32]. For example, the foliar application of a plant-based biostimulant containing flavonoids and organic acids on tomato plants grown under stress conditions showed protective effects on the functions of the photosynthetic machinery [33].…”