Tracing of Two Pseudomonas Strains in the Root and Rhizoplane of Maize, as Related to Their Plant Growth-Promoting Effect in Contrasting Soils

Abstract

TaqMan-based quantitative PCR (qPCR) assays were developed to study the persistence of two well-characterized strains of plant growth-promoting rhizobacteria (PGPR), Pseudomonas fluorescens Pf153 and Pseudomonas sp. DSMZ 13134, in the root and rhizoplane of inoculated maize plants. This was performed in pot experiments with three contrasting field soils (Buus, Le Caron and DOK-M). Potential cross-reactivity of the qPCR assays was assessed with indigenous Pseudomonas and related bacterial species, which had been isolated from the rhizoplane of maize roots grown in the three soils and then characterized by Matrix-Assisted Laser Desorption Ionization (MALDI) Time-of-Flight (TOF) mass spectrometry (MS). Sensitivity of the qPCR expressed as detection limit of bacterial cells spiked into a rhizoplane matrix was 1.4 × 102 CFU and 1.3 × 104 CFU per gram root fresh weight for strain Pf153 and DSMZ 13134, respectively. Four weeks after planting and inoculation, both strains could readily be detected in root and rhizoplane, whereas only Pf153 could be detected after 8 weeks. The colonization rate of maize roots by strain Pf153 was significantly influenced by the soil type, with a higher colonization rate in the well fertile and organic soil of Buus. Inoculation with strain DSMZ 13134, which colonized roots and rhizoplane to the same degree, independently of the soil type, increased yield of maize, in terms of biomass accumulation, only in the acidic soil of Le Caron, whereas inoculation with strain Pf153 reduced yield in the soil Buus, despite of its high colonization rate and persistence. These results indicate that the colonization rate and persistence of inoculated Pseudomonas strains can be quantitatively assessed by the TaqMan-based qPCR technique, but that it cannot be taken for granted that inoculation with a well-colonizing and persistent Pseudomonas strain has a positive effect on yield of maize.

Potential of three microbial bio-effectors to promote maize growth and nutrient acquisition from alternative phosphorous fertilizers in contrasting soils

“…Consequently, depending on the properties of the soil and the structure of the naturally occurring rhizosphere community, their plant growth-promoting effect has often shown limited reproducibility. As reported in Mosimann [25], soil properties such as pH can have impacts on the BEs' ability to exert plant growth-promoting effects. Moreover, the amount of total and plant available P seems to be a decisive factor for the performance of bacterial BEs [26,27].…”

“…The strain Pseudomonas DSMZ 13134 is an efficient root colonizer [38] and has been shown to solubilize inorganic phosphate under in vitro conditions [39]. Moreover, it has recently shown the potential to improve plant growth and phosphorus acquisition when inoculated to maize [10,25]. The gram-positive strain Bacillus amyloliquefaciens FZB42 is mainly studied for its biocontrol potential but other studies also indicate its ability to act as a biofertilizer [40,41].…”

“…Various studies have shown that soil pH, soil organic carbon content, soil microbial activity, and soil nutrient status are all influencing the success of BE application [16,25,[55][56][57]. Our findings indicate that the most responsive soils to BE application (in combination with P fertilizers) were either alkaline (Castel-IT) or depleted in soil nutrients (Taastrup-DK and Vörden-DE for P acquisition), confirming the influence of soil pH and soil nutrient statues on the effectiveness of BEs.…”

Densely rooted rhizosphere hotspots induced around subsurface NH4 +-fertilizer depots: a home for soil PGPMs?

“…This study also showed that soil zones around NH 4 + -depots with high root densities may be sites for effective root colonization by inoculated PGPMs. Nevertheless, in addition to cultures on selective media, molecular tracing techniques like qPCR [34] as already shown for Pseudomonas sp. DSMZ 13134 would provide more precision in tracing.…”

Ammonium excretion, auxin production and effects of maize inoculation with ethylenediamine-resistant mutants of Pseudomonas sp.

“…Plant inoculation with beneficial bacteria, as the Pseudomonas strain used in this study, are known to induce changes in plant architecture due to modification of the plant endogenous hormonal balance (Mosimann et al 2016;Calvo et al 2017). The potential of Pseudomonas AZM-01 and derivative EDA-resistant mutants to produce auxins was demonstrated in the presented study, and this trait should be related to the modifications on roots of plants inoculated with the wild and mutant strains.…”