N-fixation in cereals by root-associated bacteria is a promising solution to reducing chemical nitrogen fertilizer in agriculture. However, plant-bacterial responses are unpredictable across environments. We hypothesized that cereal responses to N-fixing bacteria are dynamic, depending on N supply and time. To quantify dynamics, the gnotobiotic, fabricated ecosystem (EcoFAB) was adapted to analyze N mass-balance, image shoot and root growth and measure gene expression of Brachypodium distachyon (Brachypodium) inoculated with N-fixing bacterium, Herbaspirillum seropedicae (Herbaspirillum). Phenotyping throughput of EcoFAB-N was 25-30 plants/h with open software and imaging systems. Herbaspirillum inoculation of Brachypodium shifted root and shoot growth, nitrate versus ammonium uptake, and gene expression with time; directions and magnitude depending on N availability. Primary roots were longer and root hairs shorter regardless of N, with stronger changes at low N. At higher N, Herbaspirillum provided 11% total plant N from sources other than seed or nutrient solution. The time-resolved phenotypic and molecular data, points to distinct modes of action: At 5 mM the benefit appears through N fixation; while at 0.5 mM the mechanism appears to be plant physiological, with Herbaspirillum promoting uptake of N from the root medium.Future work could fine-tune plant and root-associated microorganisms to growth and nutrient dynamics.
Pseudomonas spp. make up 1.6% of the bacteria in the soil and are found throughout the world. More than 140 species of this genus have been identified, some beneficial to the plant. Several species in the family Pseudomonadaceae, including Azotobacter vinelandii AvOP, Pseudomonas stutzeri A1501, Pseudomonas stutzeri DSM4166, Pseudomonas szotifigens 6HT33bT and Pseudomonas sp. K1 can fix nitrogen from the air. The genes required for these reactions are organized in a nitrogen fixation island, obtained via horizontal gene transfer from Klebsiella pneumoniae, Pseudomonas stutzeri and Azotobacter vinelandii. Today, this island is conserved in Pseudomonas spp. from different geographical locations, which in turn have evolved to deal with different geo-climatic conditions. Here, we summarize the molecular mechanisms behind Pseudomonas driven plant growth promotion, with particular focus on improving plant performance at limiting nitrogen (N), and improving plant N content. We describe Pseudomonas-plant interaction strategies in the soil, noting that the mechanisms of denitrification, ammonification, and secondary metabolite signalling are only marginally explored. Plant growth promotion is dependent on the abiotic conditions, and differs at sufficient and deficient N. The molecular controls behind different plant response are not fully elucidated. We suggest that superposition of transcriptome, proteome, and metabolome data and their integration with plant phenotype development through time will help fill these gaps. The aim of this review is to summarize the knowledge behind Pseudomonas driven nitrogen fixation and to point to possible agricultural solutions
<p>The use of microorganisms for improving plant performance under limiting conditions can be traced throughout history. Interestingly the first commercial biological plant growth promotor was patented in 1896. However, the understanding how the organisms interact on molecular level really took off after the advent of the genomic era which produced the tools needed for understanding how plants and microorganisms modulate each-other&#8217;s gene expression and metabolism. Today more than ever, the holistic understanding of plant nutrient uptake and novel strategies to improve nutrient uptake are of utmost importance. Our work focuses on nitrogen (N) &#8211; the second most abundant nutrient in plants and phosphorus (P) &#8211; a finite global resource. We present studies where use of plant growth promoting rhizobacteria (PGPR) resulted in improved plant performance under limited N or P in Brachypodium - a model plant for cereals. Plant roots were analyzed with the non-invasive root phenotyping platform GrowScreen Page [1], or with the 3D printed EcoFab microcosms [2]. The latter was adapted and used in combination with Plant Screen Mobile [3], for non-invasive shoot area estimation, in conjunction with root scanning, over time. On the other hand, the performance of barley plants under the influence of 2 fungal interaction partners were investigated in soil system, using magnetic resonance imaging [4].</p><p>The plant response to a micro-organism is largely dependent on the surrounding conditions. Examples of plants treated with plant growth promoting rhizobacteria (PGPR) and grown under high and low N show that: the plant phenotype, N content within the plant and molecular response vary depending on the N availability in the surrounding medium.</p><p>Furthermore, we were able to dissect the plant phenotype of plants grown under limiting P in soil-less medium, and found that plant biomass was higher in plants inoculated with PGPR. A time series image-analysis of root phenotype showed the changes in root architecture, pin-pointing the time-window when growth promotion took effect after inoculation. A sand experiment confirmed these results.</p><p>Finally, the interaction between Barley roots and two fungi (a pathogen and a putative beneficial partner) was investigated to find dynamic response in root growth in soil that varied in soil depth, and had a different progression through time based on treatment.</p><p>We argue that for successful use of PGPR in context of nutrient uptake we need to account for: time in context of plant developmental stage [5] and moment of application, the organisms in question and the surrounding condition. Efforts are needed to elucidate the proper interaction partners and application points to result in a sustainable solution for agriculture.</p><ol><li>Funct Plant Biol, 2017. 44(1)</li> <li>New Phytol. 2019; 222(2): 1149&#8211;1160</li> <li>Plant Methods 2019 <strong>15:</strong>2</li> <li>Plant Physiol 170(3): 1176-1188.</li> <li>New Phytol. 2019 doi: 10.1111/nph.15955</li> </ol>
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