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BACKGROUND: The plant growth-promoting bacteria (PGPB) Azospirillum brasilense is widely used as an inoculant for important grass crops, providing numerous benefits to the plants. However, one limitation to develop viable commercial inoculants is the control of PGPB survival, requiring strategies that guarantee their survival during handling and field application. The application of sublethal stress appears to be a promising strategy to increase bacterial cells tolerance to adverse environmental conditions since previous stress induces the activation of physiological protection in bacterial cell. In this work, we evaluated the effects of thermal and salt stresses on the survival of inoculant containing A. brasilense Ab-V5 and Ab-V6 strains and we monitored A. brasilense viability in inoculated maize roots after stress treatment of inoculant. RESULTS: Thermal stress application (> 35 °C) in isolated cultures for both strains, as well as salt stress [sodium chloride (NaCl) concentrations > 0.3 mol L-1], resulted in growth rate decline. The A. brasilense enumeration in maize roots obtained by propidium monoazide quantitative polymerase chain reaction (PMA-qPCR), for inoculated maize seedlings grown in vitro for 7 days, showed that there is an increased number of viable cells after the salt stress treatment, indicating that A. brasilense Ab-V5 and Ab-V6 strains are able to adapt to salt stress (0.3 mol L-1 NaCl) growth conditions. CONCLUSION: Azospirillum brasilense Ab-V5 and Ab-V6 strains had potential for osmoadaptation and salt stress, resulting in increased cell survival after inoculation in maize plants. © 2024 Society of Chemical Industry.

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The use of commercial bacterial inoculants formulated with plant-growth promoting bacteria (PGPB) in agriculture has shown significant prominence in recent years due to growth-promotion benefits provided to plants through different mechanisms. However, the survival and viability of bacterial cells in inoculants are affected during use and may decrease their effectiveness. Physiological adaptation strategies have attracted attention to solve the viability problem. This review aims to provide an overview of research on selecting sublethal stress strategies to increase the effectiveness of bacterial inoculants. The searches were performed in November 2021 using Web of Science, Scopus, PubMed, and Proquest databases. The keywords "nitrogen-fixing bacteria", "plant growth-promoting rhizobacteria", "azospirillum", "pseudomonas", "rhizobium", "stress pre-conditioning", "adaptation", "metabolic physiological adaptation", "cellular adaptation", "increasing survival", "protective agent" and "protective strategy" were used in the searches. A total of 2573 publications were found, and 34 studies were selected for a deeper study of the subject. Based on the studies analysis, gaps and potential applications related to sublethal stress were identified. The most used strategies included osmotic, thermal, oxidative, and nutritional stress, and the primary cell response mechanism to stress was the accumulation of osmolytes, phytohormones, and exopolysaccharides (EPS). Under sublethal stress, the inoculant survival showed positive increments after lyophilization, desiccation, and long-term storage processes. The effectiveness of inoculant-plants interaction also had positive increments after sublethal stress, improving plant development, disease control, and tolerance to environmental stresses compared to unappealed inoculants.

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The genus Herbaspirillum gained the spotlight due to the several reports of diazotrophic strains and promising results in plant-growth field assays. However, as diversity exploration of Herbaspirillum species gained momentum, it became clearer that the plant beneficial lifestyle was not the only form of ecological interaction in this genus, due to reports of phytopathogenesis and nosocomial infections. Here we performed a deep search across all publicly available Herbaspirillum genomes. Using a robust core genome phylogeny, we have found that all described species are well delineated, being the only exception H. aquaticum and H. huttiense clade. We also uncovered that the nif genes are only highly prevalent in H. rubrisubalbicans; however, irrespective to the species, all nif genes share the same gene arrangement with high protein identity, and are present in only two main types, in inverted strands. By means of a NifHDKENB phylogenetic tree, we have further revealed that the Herbaspirillum nif sequences may have been acquired from the same last common ancestor belonging to the Nitrosomonadales order.

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Herbaspirillum seropedicae is a plant growth-promoting bacteria isolated from diverse plant species. In this work, the main objective was to investigate the efficiency of H. seropedicae strain SmR1 in colonizing and increasing maize growth (DKB 390 variety) in the early stages of development under greenhouse conditions. Inoculation with H. seropedicae resulted in 19.43 % (regarding High and Low N controls) and 10.51% (regarding Low N control) in mean of increase of root biomass, for 1st and 2nd greenhouse experiments, respectively, mainly in the initial stages of plant development, at 21 days after emergence (DAE). Quantification of H. seropedicae in roots and leaves was performed by quantitative PCR. H. seropedicae was detected only in maize inoculated roots by qPCR, and a slight decrease in DNA copy number g-1 of fresh root weight was observed from 7 to 21 DAE, suggesting that there was initial effective colonization on maize plants. H. seropedicae strain SmR1 efficiently increased maize root biomass exhibiting its potential to be used as inoculant in agricultures systems.

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Azospirillum brasilense is a plant growth promoting bacteria used as an inoculant in diverse crops. Accurate analytical methods are required to enumerate viable cells in inoculant formulations or in planta. We developed a quantitative polymerase chain reaction (qPCR) assay associated to propidium monoazide (PMA) to evaluate the cell viability of A. brasilense in inoculant and in maize roots. A. brasilense was grown in culture medium and was exposed to 50 â„ƒ. Maize roots were grown in vitro and harvested 7 days after inoculation. Quantification was performed by qPCR, PMA-qPCR, and plate counting. Standard curves efficiency values ranged from 85 to 99%. The limit of detection was 104 CFU per gram of fresh root. Enumeration obtained in maize roots by qPCR where higher than enumeration by PMA-qPCR and by plate counting. PMA-qPCR assay was efficient in quantifying inoculant viable cells and provides reliable results in a quickly and accurately way compared to culture-dependent methods.

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Canola (Brassica napus L. var. oleifera) is cultivated in the Southern Brazil as an alternative winter crop in the rotation system. No-tillage and hydromorphic soils favor the formation of aliphatic organic acids with short chain and low molecular weight, as acetic, butyric, and propionic acids. These acids may cause negative effects on the canola production. Thus, this study aimed to evaluate the response of five canola hybrid genotypes under the effect of acetic, butyric and propionic acids on the germination and initial phase of seedling growth. The results showed the characters evaluated respond differently to treatments applied within the same genotype. Therefore, the selection of a genotype with a resilient performance for the characters evaluated under stress is complex. Nevertheless, a sensitive profile response was observed in Hyola 433 genotype under different organic acids. Furthermore, acetic acid showed no negative impact on germination and initial growth of the genotypes.

A Canola (Brassica napus L. var. oleifera) é cultivada no Sul do Brasil como uma alternativa para a estação fria no sistema de rotação de culturas. O sistema de plantio direto e a ocorrência de solos hidromórficos favorece a formação de ácidos orgânicos alifáticos com cadeia curta e baixo peso molecular, como os ácidos acético, butírico e propiônico. Estes ácidos podem causar efeitos negativos na produção de canola. Assim, este estudo teve por objetivo avaliar a resposta de cinco genótipos híbridos de canola sob o efeito dos ácidos acético, butírico e propiônico na germinação e na fase inicial do crescimento da plântula. Os resultados demonstraram que os caracteres avaliados respondem de forma diferente em relação aos tratamentos aplicados dentro do mesmo genótipo. Dessa forma, a seleção de um genótipo que manteve um comportamento padrão nos caracteres avaliados sob estresse é dificultado. Entretanto, em uma visão geral, o genótipo Hyola 433 manteve respostas características de sensibilidade nos caracteres avaliados em relação aos ácidos utilizados. Ainda, foi detectado que o ácido acético teve menor efeito na germinação e na fase inicial de crescimento dos genótipos estudados.

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Canola (Brassica napus L. var. oleifera) is cultivated in the Southern Brazil as an alternative winter crop in the rotation system. No-tillage and hydromorphic soils favor the formation of aliphatic organic acids with short chain and low molecular weight, as acetic, butyric, and propionic acids. These acids may cause negative effects on the canola production. Thus, this study aimed to evaluate the response of five canola hybrid genotypes under the effect of acetic, butyric and propionic acids on the germination and initial phase of seedling growth. The results showed the characters evaluated respond differently to treatments applied within the same genotype. Therefore, the selection of a genotype with a resilient performance for the characters evaluated under stress is complex. Nevertheless, a sensitive profile response was observed in Hyola 433 genotype under different organic acids. Furthermore, acetic acid showed no negative impact on germination and initial growth of the genotypes.(AU)

A Canola (Brassica napus L. var. oleifera) é cultivada no Sul do Brasil como uma alternativa para a estação fria no sistema de rotação de culturas. O sistema de plantio direto e a ocorrência de solos hidromórficos favorece a formação de ácidos orgânicos alifáticos com cadeia curta e baixo peso molecular, como os ácidos acético, butírico e propiônico. Estes ácidos podem causar efeitos negativos na produção de canola. Assim, este estudo teve por objetivo avaliar a resposta de cinco genótipos híbridos de canola sob o efeito dos ácidos acético, butírico e propiônico na germinação e na fase inicial do crescimento da plântula. Os resultados demonstraram que os caracteres avaliados respondem de forma diferente em relação aos tratamentos aplicados dentro do mesmo genótipo. Dessa forma, a seleção de um genótipo que manteve um comportamento padrão nos caracteres avaliados sob estresse é dificultado. Entretanto, em uma visão geral, o genótipo Hyola 433 manteve respostas características de sensibilidade nos caracteres avaliados em relação aos ácidos utilizados. Ainda, foi detectado que o ácido acético teve menor efeito na germinação e na fase inicial de crescimento dos genótipos estudados.(AU)

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