Plant growth-promoting bacteria (PGPB) can mitigate the effect of abiotic stresses on plant growth and development; however, the degree of plant response is host-specific. The present study aimed to assess the growth-promoting effect of Herbaspirillum (AP21, AP02), Azospirillum (D7), and Pseudomonas (N7) strains (single and co-inoculated) in perennial ryegrass plants subjected to drought. The plants were grown under controlled conditions and subjected to water deficit for 10 days. A significant increase of approximately 30% in dry biomass production was observed using three co-inoculation combinations (p < 0.01). Genomic analysis enabled the detection of representative genes associated with plant colonization and growth promotion. In vitro tests revealed that all the strains could produce indolic compounds and exopolysaccharides and suggested that they could promote plant growth via volatile organic compounds. Co-inoculations mostly decreased the in vitro-tested growth-promoting traits; however, the co-inoculation of Herbaspirillum sp. AP21 and Azospirillum brasilense D7 resulted in the highest indolic compound production (p < 0.05). Although the Azospirillum strain showed the highest potential in the in vitro and in silico tests, the plants responded better when PGPB were co-inoculated, demonstrating the importance of integrating in silico, in vitro, and in vivo assessment results when selecting PGPB to mitigate drought stress.

Plant growth promoting bacteria (PGPB) enhances the growth of their hosts and can protect them from biotic and abiotic stresses. Bacterial inoculants contain one or more of these beneficial strains in a carrier material, which must be able to maintain the viability of the cells during the time of storage, and also guarantee the biological activity of the strains once applied in the soil. These inoculants can be solid, liquid, gel or oil-based, depending on the characteristics of the strains and the shelf life expected by the producers. In this study, we used a method of accelerated degradation to select a polymer and a concentration to maintain cell stability of a liquid inoculant based on the strain C16 Azospirillum brasilense. A screening at 45°C was made to compare the protectant effect of five polymers on the viability of the strain (p/v): carrageenan (1.5%), sodium alginate (1%), trehalose (10 mM), polyvinylpyrrolidone (2%), glycerol (10 mM) and phosphate saline buffer as control. Carrageenan and sodium alginate showed significant differences in cell viability over the use of other polymers (P < 0.05). We evaluated cell viability with these two polymers at three concentrations and three different temperatures (4, 28 and 45°C) for 60 days and determined the bacterial degradation rates. Based on the Arrhenius thermodynamic model, we calculated the time required to reduce cell concentration in three log units, and observed that the protectant activity of each polymer and each concentration depends on the temperature of storage. Cell viability was best preserved in all treatments at 4°C. In general, alginate prolonged cell viability at 28°C, and carrageenan at 45°C. Alginate at 1% and carrageenan at 0.75% showed a stable behavior (superior to the control) in the three evaluated temperatures, so we conclude that they can be used for a formulation of a liquid inoculant based on the strain C16 of A. brasilense. Key words: Energy of activation, degradation, cell death, kinetics, formulation.

Perennial ryegrass is a forage commonly used in temperate regions for livestock feeding; however, its yield is affected by reduced biomass production under water deficit. In a previous study, three co-inoculations of beneficial bacteria were selected based on their ability to promote plant growth under reduced water availability. The aim of this work was to elucidate some mechanisms by which the selected bacteria can help improve the response of perennial ryegrass to water deficit. Ryegrass plants were inoculated with each of the co-inoculations (Herbaspirillum sp. AP02–Herbaspirillum sp. AP21; Herbaspirillum sp. AP02–Pseudomonas sp. N7; Herbaspirillum sp. AP21–Azospirillum brasilense D7) and subjected to water deficit for 10 days. Physiological and biochemical measurements were taken 10 days after stress and shortly after rehydration. The results showed that bacteria had a positive effect on shoot biomass production, dissipation of excess energy, and proline and chlorophyll pigments during the days of water deficit (p < 0.05). The leaf water status of the inoculated plants was 12% higher than that of the uninoculated control after rehydration. Two Herbaspirillum strains showed greater potential for use as biofertilizers that help ameliorate the effects of water deficit.

Una alternativa a los fertilizantes de síntesis química para lograr una producción sostenible es la utilización de productos amigables con el ambiente, con base en microorganismos promotores de crecimiento vegetal, en forma de biofertilizantes. Estos productos tienen la capacidad de movilizar los nutrientes en el suelo de forma natural sin afectar el ambiente y, sobre todo, sin perjudicar el rendimiento de las plantas. Esta cartilla enseña qué es un biofertilizante y cómo usarlo en un sistema productivo, con miras a reducir los costos de producción sin afectar los rendimientos de las praderas.