Phytohormones and induction of plant-stress tolerance and defense genes by seed and foliar inoculation with Azospirillum brasilense cells and metabolites promote maize growth

Josiane Fukami,Francisco Javier Ollero,Mariangela Hungria,Manuel Megías

doi:10.1186/s13568-017-0453-7

Josiane Fukami, Francisco Javier Ollero + Show 2 more

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https://doi.org/10.1186/s13568-017-0453-7

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Abstract

Azospirillum spp. are plant-growth-promoting bacteria used worldwide as inoculants for a variety of crops. Among the beneficial mechanisms associated with Azospirillum inoculation, emphasis has been given to the biological nitrogen fixation process and to the synthesis of phytohormones. In Brazil, the application of inoculants containing A. brasilense strains Ab-V5 and Ab-V6 to cereals is exponentially growing and in this study we investigated the effects of maize inoculation with these two strains applied on seeds or by leaf spray at the V2.5 stage growth—a strategy to relieve incompatibility with pesticides used for seed treatment. We also investigate the effects of spraying the metabolites of these two strains at V2.5. Maize growth was promoted by the inoculation of bacteria and their metabolites. When applied via foliar spray, although A. brasilense survival on leaves was confirmed by confocal microscopy and cell recovery, few cells were detected after 24 h, indicating that the effects of bacterial leaf spray might also be related to their metabolites. The major molecules detected in the supernatants of both strains were indole-3-acetic acid, indole-3-ethanol, indole-3-lactic acid and salicylic acid. RT-PCR of genes related to oxidative stress (APX1, APX2, CAT1, SOD2, SOD4) and plant defense (pathogenesis-related PR1, prp2 and prp4) was evaluated on maize leaves and roots. Differences were observed according to the gene, plant tissue, strain and method of application, but, in general, inoculation with Azospirillum resulted in up-regulation of oxidative stress genes in leaves and down-regulation in roots; contrarily, in general, PR genes were down-regulated in leaves and up-regulated in roots. Emphasis should be given to the application of metabolites, especially of Ab-V5 + Ab-V6 that in general resulted in the highest up-regulation of oxidative-stress and PR genes both in leaves and in roots. We hypothesize that the benefits of inoculation of Azospirillum on seeds or by leaf spray, as well as of leaf spraying of Azospirillum metabolites, are strongly correlated with the synthesis of phytohormones and by eliciting genes related to plant-stress tolerance and defense against pathogens.

Highlights

Inoculation with Azospirillum spp. has been the subject of several studies (Bashan and Holguin 1998) due to their remarkable capacity of promoting growth of importantFukami et al AMB Expr (2017) 7:153 secretion of phytohormones and other signaling molecules such as auxins (Spaepen and Vanderleyden 2015), cytokinins (Tien et al 1979), gibberellins (Bottini et al 1989) and salicylic acid (Sahoo et al 2014); biological nitrogen fixation (Marques et al 2017); and phosphate solubilization (Rodriguez et al 2004)
Effects of inoculation of Azospirillum brasilense and their metabolites on plant‐growth parameters In the greenhouse experiment performed to evaluate effects of inoculation on plant growth, it is worth mentioning that all treatments received the same amount of N-fertilizer, corresponding to 50% (60 kg of N ha−1) of the dose recommended for the maize crop in Brazil
When different methods of inoculation of A. brasilense strains Ab-V5 and Ab-V6, in single or combined mixtures, or foliar-spray application of their metabolites (MLS), at the V2.5 stage, were evaluated, statistically significant increases in chlorophyll content (CC) in relation to the non-inoculated control were observed in all treatments except for the seed inoculation (SI) with Ab-V5; the highest increases were observed in the treatments with MLS of Ab-V6 and MLS of Ab-V5 + Ab-V6, of 109 and 143%, respectively (Table 2)

Summary

Introduction

Inoculation with Azospirillum spp. has been the subject of several studies (Bashan and Holguin 1998) due to their remarkable capacity of promoting growth of importantFukami et al AMB Expr (2017) 7:153 secretion of phytohormones and other signaling molecules such as auxins (Spaepen and Vanderleyden 2015), cytokinins (Tien et al 1979), gibberellins (Bottini et al 1989) and salicylic acid (Sahoo et al 2014); biological nitrogen fixation (Marques et al 2017); and phosphate solubilization (Rodriguez et al 2004). Inoculation with Azospirillum spp. has been the subject of several studies (Bashan and Holguin 1998) due to their remarkable capacity of promoting growth of important. Azospirillum spp. seem remarkable in their apparent lack of specificity in promoting growth of practically every plant genus and species investigated so far (Pereg et al 2016), there are indications that species and strains may vary in determinants of niche-specific adaptation to the rhizosphere that affect plant–microbe interactions (Wisniewski-Dyé et al 2012). Examples of determinants of adaptation include reactive oxygen species (ROS) as shown with A. lipoferum strain 4B in the rice rhizosphere (Drogue et al 2014). ROS detoxification systems vary with plant species, cultivar, and age, and with the type and duration of abiotic and biotic stress (Hodges et al 1996)

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