Abstract
The beneficial effects of plant growth–promoting Rhizobacteria (PGPR) entail several interaction mechanisms with the plant or with other root-associated microorganisms. These microbial functions are carried out by multiple taxa within functional groups and contribute to rhizosphere functioning. It is likely that the inoculation of additional PGPR cells will modify the ecology of these functional groups. We also hypothesized that the inoculation effects on functional groups are site specific, similarly as the PGPR phytostimulation effects themselves. To test this, we assessed in the rhizosphere of field-grown maize the effect of seed inoculation with the phytostimulatory PGPR Azospirillum lipoferum CRT1 on the size and/or diversity of selected microbial functional groups important for plant growth, using quantitative polymerase chain reaction and/or Illumina MiSeq metabarcoding. The functional groups included bacteria able to fix nitrogen (a key nutrient for plant growth), producers of 1-aminocyclopropane-1-carboxylate (ACC) deaminase (which modulate ethylene metabolism in plant and stimulate root growth), and producers of 2,4-diacetylphloroglucinol (an auxinic signal enhancing root branching). To test the hypothesis that such ecological effects were site-specific, the functional groups were monitored at three different field sites, with four sampling times over two consecutive years. Despite poor inoculant survival, inoculation enhanced maize growth. It also increased the size of functional groups in the three field sites, at the maize six-leaf and flowering stages for diazotrophs and only at flowering stage for ACC deaminase and 2,4-diacetylphloroglucinol producers. Sequencing done in the second year revealed that inoculation modified the composition of diazotrophs (and of the total bacterial community) and to a lesser extent of ACC deaminase producers. This study revealed an ecological impact that was field specific (even though a few taxa were impacted in all fields) and of unexpected magnitude with the phytostimulatory Azospirillum inoculant, when considering microbial functional groups. Further methodological developments are needed to monitor additional functional groups important for soil functioning and plant growth under optimal or stress conditions.
Highlights
IntroductionPlant growth–promoting Rhizobacteria (PGPR) benefit plants mainly by (i) stimulating root system development, thereby allowing seedlings to explore larger soil volumes to gain access to water and mineral nutrients (Vacheron et al, 2013); (ii) enhancing nutrient availability, e.g., via N2 reduction or P solubilization (Dobbelaere et al, 2003; Basu et al, 2021); (iii) improving root system functioning in terms of ion uptake, by stimulating nutrient transporter expressions and/or activities in roots (Bertrand et al, 2000; Vacheron et al, 2013; Pii et al, 2015); and/or (iv) controlling root parasites via competition or antagonism, which leads to parasite inhibition (Raaijmakers et al, 2009; Basu et al, 2021)
At site L, inoculation resulted in statistically higher number of nifH bacteria in nonfertilized plots but lower number in fertilized plots (Table 2)
The seed inoculant A. lipoferum CRT1 did not manage to establish itself in the maize rhizosphere
Summary
Plant growth–promoting Rhizobacteria (PGPR) benefit plants mainly by (i) stimulating root system development, thereby allowing seedlings to explore larger soil volumes to gain access to water and mineral nutrients (Vacheron et al, 2013); (ii) enhancing nutrient availability, e.g., via N2 reduction or P solubilization (Dobbelaere et al, 2003; Basu et al, 2021); (iii) improving root system functioning in terms of ion uptake, by stimulating nutrient transporter expressions and/or activities in roots (Bertrand et al, 2000; Vacheron et al, 2013; Pii et al, 2015); and/or (iv) controlling root parasites via competition or antagonism, which leads to parasite inhibition (Raaijmakers et al, 2009; Basu et al, 2021). Effective PGPR strains have increased crop yields in many (but not all) field trials (Okon and LabanderaGonzalez, 1994; El Zemrany et al, 2006; García de Salamone et al, 2010) Their use as crop inoculants is promising to reduce chemical inputs and improve farming sustainability, despite plant growth-promotion effects that can fluctuate according to the field, the year, or other farming/environmental parameters (Okon and Labandera-Gonzalez, 1994; Dobbelaere et al, 2003; CastroSowinski et al, 2007; Raaijmakers et al, 2009; Rozier et al, 2017; Basu et al, 2021). Associative nitrogen fixation occurs in Azospirillum, its contribution is considered minor (Steenhoudt and Vanderleyden, 2000)
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