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

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.

Assessing Forms of Application of Azospirillum brasilense Associated with Silicon Use on Wheat

“…Although the exact mechanisms underlying the PGPB effect on wheat development were not evaluated in the present study, it is very likely that the improvement in LCI and N uptake (N foliar concentration and N root and shoot accumulation), reflected in the improved root and shoot dry matter, plant height, spike length, and grain yield by wheat inoculated with A. brasilense, is associated with its well-known ability to promote plant growth [4,13,19,48,49]. Specifically, it was previously demonstrated in draft genome sequences that the strains Ab-V5 and Ab-V6 of A. brasilense, which were tested in the present study, carry similar nif and fix genes that confer their ability to fix atmospheric N [50].…”

The impact of microbes in the orchestration of plants’ resistance to biotic stress: a disease management approach

“…Application of the metabolites and live bacteria on the leaves of maize equally enhanced plant growth as a result of the phytohormone produced and priming of plant defense genes (Fukami et al 2017; Vacheron et al 2015). …”

Technical and Economic Viability of Corn with Azospirillum brasilense Associated with Acidity Correctives and Nitrogen

“…Hungria et al (2010) also obtained increases in corn yield, and depending on the strain of A. brasilense evaluated, the increase in productivity was of the order of 24 to 30%, corresponding to 662 to 823 kg ha -1 . The analysis of results from a large number of field trials with various non-legume crops, conducted worldwide over 20 years, under different soil and weather conditions, has demonstrated that yield increases of up to 30% could be obtained 70% of the time (Fukami et al, 2016;Fukami et al, 2017) in response to inoculation with Azospirillum.…”

“…Moreover, it is inferred that the application of Azospirillum is also responsible for higher rates of absorption of water and minerals by the plant (Dardanelli et al, 2008) and higher tolerance to abiotic stresses, such as drought and salinity (Zawonski et al, 2011). The relationship between different soil microorganisms and the role of metabolites secreted by them on growth of other surrounding microbial species and plants has been the subject of numerous studies (Marks et al, 2015;Fukami et al, 2016;Fukami et al, 2017), and according to Bashan and de-Bashan (2010), due to the wide array of mechanisms proposed for stimulation of plant growth by Azospirillum spp., probably this bacteria promoves multiple mechanisms that might act either in a cumulative or sequential pattern.…”

“…is also being increasingly adopted in several countries, especially for crops such as corn and wheat (Díaz-Zorita & Fernandez-Canigia, 2009;Hartmann & Bashan, 2009;Marks et al, 2015). The analysis of results from a large number of field trials with various non-legume crops, conducted worldwide over 20 years, under different soil and weather conditions, has demonstrated that yield increases of up to 30% could be obtained 70% of the time (Fukami et al, 2016;Fukami et al, 2017) in response to inoculation with Azospirillum.…”