Tania Taurian

Arsenate [As(V)] competes with phosphate for uptake, creating a challenge for peanut production in P-limited, As-affected agricultural systems. We investigated whether phosphate-solubilizing plant growth-promoting bacteria (PS-PGPB) with contrasting As(V)-tolerance phenotypes remained functional and benefited peanut when coinoculated with Bradyrhizobium sp. SEMIA6144. We screened four PS strains: native Enterobacter sp. J49, Serratia sp. S119, and Serratia sp. J260; and commercial Pseudomonas pergaminensis PMT1. J49 and PMT1 tolerated millimolar As(V), whereas S119 and J260 were inhibited at micromolar concentrations. As(V) increased soluble P in J49 and PMT1, maintained it in S119, and decreased it in J260. In J49 and S119, As(V) increased arsenate reductase and superoxide dismutase activities, but only S119 showed lipid peroxidation and induced glutathione-related enzymes, revealing different redox responses under strain-specific exposure conditions. Despite this divergence, both strains converged in planta. At a regionally relevant 3 µM As(V), coinoculation with either strain increased shoot P concentration in tricalcium phosphate (PTC)-supplied plants relative to PTC alone and restricted shoot As to below the method detection limit without significantly reducing root As concentrations. Nevertheless, As accumulated in nodules with J49 but remained undetected with S119. These findings identify integrated strain–plant–rhizobium performance as a key criterion for selecting inoculants that couple P acquisition with restriction of detectable total As translocation to shoots.

Phosphorus (P) is an essential macronutrient that plays a critical role in plant growth, development, and productivity. However, limited soil phosphorus availability can reduce crop performance in maize (Zea mays L.) and peanut (Arachis hypogaea L.). Native phosphate-solubilizing bacteria (PSB) convert insoluble P forms into plant-accessible forms. The aim of this study was to select efficient plant growth promotion native PSB to be used as a biological input in the development of sustainable biotechnological biofertilizers. For this, the effects of the inoculation of PSB isolates on maize and peanut’s phosphorus uptake, growth, and yield were evaluated. The assays were developed both under controlled greenhouse conditions and in field trials. Inoculation with native PSB strains significantly enhanced the plant growth and increased the phosphorus content of maize and peanut by 38–58% and 49.6%, respectively. These effects became evident earlier in the peanut than in maize. In field trials, inoculation with Serratia sp. S119 without chemical fertilizer application significantly increased maize yield. In conclusion, native PSB strains significantly promote plant growth, enhance phosphorus acquisition, and improve crop yield. The use of Serratia sp. S119 as a phosphate biofertilizer represents a promising strategy to reduce chemical fertilizer inputs and to promote more sustainable agricultural systems.

Phosphorus (P) deficiency and soil-borne fungal diseases are major constraints to peanut (Arachis hypogaea L.) production. Phosphate-solubilizing bacteria (PSB) can improve P availability in the soil, thereby promoting plant growth. However, their potential to improve plant resistance against pathogens under P-limited conditions remains poorly understood. In this study, we first evaluated the ability of two PSB strains, Enterobacter sp. J49 and Serratia sp. S119, to induce systemic resistance (ISR) in peanut plants against the fungal pathogen Sclerotium rolfsii. Results showed that strain S119 reduced disease severity by 40%, whereas strain J49 reduced both incidence (30%) and severity (40%). The protective effect produced by strain J49 was mediated by ISR, as evidenced by the early increase in phenolic compounds accumulation (48 h) and total peroxidase activity (72 h) in inoculated plants. Under P-deficient conditions, the J49 strain was also able to protect peanut plants against S. rolfsii, as demonstrated by a significant reduction in disease severity (55%). These findings highlight the potential of multifunctional bacterium Enterobacter sp. J49 to enhance sustainable peanut production by simultaneously improving P acquisition and strengthening plant defense mechanisms.

Insoluble phosphorous compounds solubilization by soil bacteria is of great relevance since it puts available the phosphorus to be used by plants. The production of organic acids is the main microbiological mechanism by which insoluble inorganic phosphorus compounds are solubilized. In Gram negative bacteria, gluconic acid is synthesized by the activity of the holoenzyme glucose dehydrogenase-pyrroloquinoline quinine named GDH-PQQ. The use of marker genes is a very useful tool to evaluate the persistence of the introduced bacteria and allow to follow-up the effect of biotic and abiotic factors on these beneficial microorganisms in the soil. In previous studies we detected the presence of the pqqE gene in a great percentage of both non-culturable and culturable native soil bacteria. The objective of this study was to analyze the phylogeny of the sequence of pqqE gene and its potential for the study of phosphate solubilizing bacteria from pure and mixed bacterial cultures and rhizospheric soil samples. For this, the presence of the pqqE gene in the genome of phosphate solubilizing bacteria that belong to several bacteria was determined by PCR. Also, this gene was analyzed from mixed bacterial cultures and rhizospheric soil associated to peanut plants inoculated or not with phosphate solubilizing bacteria. For this, degenerate primers designed from several bacterial genera and specific primers for the genus Pseudomonas spp., designed in this study, were used. DNA template used from simple or mixed bacterial cultures and from rhizospheric soil samples was obtained using two different DNA extraction techniques. Results indicated that pqqE gene amplification product was found in the genome of all Gram negative phosphate solubilizing bacteria analyzed. It was possible to detect this gene in the DNA obtained from mixed cultures where these bacteria grew in interaction with other microorganisms and in that obtained from rhizospheric soil samples inoculated or not with these bacteria. The phylogenetic analysis indicated that pqqE gene is a conserved gene within related genera. In conclusion, pqqE gene could be a potential marker for the study of phosphate solubilizing bacterial populations.

In agricultural soils the productivity is determined by several factors and among them are the metabolic activities of the microorganisms that reside in it. The inoculation of plants with these bacteria is an alternative to the use of agrochemicals in crops. In particular, in those soils in which P levels are low, phosphate-solubilizing bacteria became an important group of soil microorganisms. In order to propose a potential P-biofertilizer to replace chemical fertilizers, the objective of this study was to evaluate the response of peanut and maize plants to the inoculation with the phosphate solubilizer Enterobacter sp. J49 individually or in combination with chemical fertilizers on growth, yield, and nutrient contents on peanut and maize plants in field trials. Two field assays in the peanut growing region of Córdoba Province (Argentina) were carried out. The inoculation of peanut with Enterobacter sp. J49 showed an increase in the yield with respect to the other treatments. Maize plants inoculated with this strain, alone or combined with half dose of chemical fertilizer, presented the highest yields. The results indicated that Enterobacter sp. J49 has a growth-promoting effect on the yield of peanut and maize mainly under drought stress. In conclusion, the inoculation with this strain would be a more sustainable agricultural practice for improving yield of peanut and maize crops in Argentinian agricultural area.

Phosphorus (P) is a limiting factor of plant development due to its low availability in the soil. The use of endophytic phosphate solubilizing bacteria as a more sustainable alternative to the use of chemical phosphorus fertilizers is proposed in this study. The objectives were to analyze the effect of simple inoculations of native peanut endophytic phosphate solubilizing bacteria on plant growth promotion and P content of soybean and maize and to evaluate their survival and endophytic colonization capacity on these plants. In addition, bacterial plant cell wall degrading enzymes activities in presence or absence of root exudates was determined. Soybean, maize and peanut plants were grown on a microcosm scale and inoculated with Enterobacter sp. J49 or Serratia sp. S119. It was observed that phosphate solubilizing strains promoted the growth of maize and soybean plants and contributed significantly P to their tissues. A significant increase in the phosphate solubilizing capacity of the plant rhizosphere after the end of the assay was observed. The strains showed to survive in plant’s growth substrate and in the case of Enterobacter sp. J49, it showed also to colonize endophytically maize and soybean. Root exudates of the three plants showed to produce changes in pectinase and cellulase activities of the strains. The bacterial strains analyzed in this study constitutes potential sources for the formulation of biofertilizers for their application for several crops in agricultural soils with low P content.