Background and aims
Root-released carboxylates enhance the availability of manganese (Mn), which enters roots through transporters with low substrate specificity. Leaf Mn concentration (Mn) has been ...proposed as a signature for phosphorus (P)-mobilising carboxylates in the rhizosphere. Here we test whether leaf Mn provides a signature for root functional types related to P acquisition.
Methods
Across 727 species at 66 sites in Australia and New Zealand, we measured leaf Mn as related to root functional type, while also considering soil and climate variables. To further assess the specific situations under which leaf Mn is a suitable proxy for rhizosphere carboxylate concentration, we studied leaf Mn along a strong gradient in water availability on one representative site. In addition, we focused on two systems where a species produced unexpected results.
Results
Controlling for background site-specific variation in leaf Mn with soil pH and mean annual precipitation, we established that mycorrhizal species have significantly lower leaf Mn than non-mycorrhizal species with carboxylate-releasing root structures, e.g., cluster roots. In exception to the general tendency, leaf Mn did not provide information about root functional types under seasonally waterlogged conditions, which increase iron availability and thereby interfere with Mn-uptake capacity. Two further exceptions were scrutinised, leading to the conclusion that they were ‘anomalous’ in not functioning like typical species in their families, as expected according to the literature.
Conclusions
Leaf Mn variation provides considerable insights on differences in belowground functioning among co-occurring species. Using this approach, we concluded that, within typical mycorrhizal families, some species actually depend on a carboxylate-releasing P-mobilising strategy. Likewise, within families that are known to produce carboxylate-releasing cluster roots, some do not produce functional cluster roots when mature. An analysis of leaf Mn can alert us to such ‘anomalous’ species.
Phosphorus (P) availability severely limits plant growth due to its immobility and inaccessibility in soils. Yet, visualization and measurements of P uptake from different root types or regions in ...soil are methodologically challenging. Here, we explored the potential of phosphor imaging combined with local injection of radioactive 33 P to quantitatively visualize P uptake and translocation along roots of maize grown in soils. Rhizoboxes (20 × 40 × 1 cm) were filled with sandy field soil or quartz sand, with one maize plant per box. Soil compartments were created using a gravel layer to restrict P transfer. After 2 weeks, a compartment with the tip region of a seminal root was labeled with a NaH 2 33 PO4 solution containing 12 MBq of 33 P. Phosphor imaging captured root P distribution at 45 min, 90 min, 135 min, 180 min, and 24 h post-labeling. After harvest, 33 P levels in roots and shoots were quantified. 33 P uptake exhibited a 50% increase in quartz sand compared to sandy soil, likely attributed to higher P adsorption to the sandy soil matrix than to quartz sand. Notably, only 60% of the absorbed 33 P was translocated to the shoot, with the remaining 40% directed to growing root tips of lateral or seminal roots. Phosphor imaging unveiled a continuous rise in 33 P signal in the labeled seminal root from immediate post-labeling until 24 h after labeling. The highest 33 P activities were concentrated just above the labeled compartment, diminishing in locations farther away. Emerging laterals from the labeled root served as strong sinks for 33 P, while a portion was also transported to other seminal roots. Our study quantitatively visualized 33 P uptake and translocation dynamics, facilitating future investigations into diverse root regions/types and varying plant growth conditions. This improves our understanding of the significance of different P sources for plant nutrition and potentially enhances models of plant P uptake.
• Aluminum (Al) toxicity is a major limiting factor of crop production on acid soils, but the implication of oxidative stress in this process is controversial. A multidisciplinary approach was used ...here to address this question in the forage legume Lotus corniculatus. • Plants were treated with low Al concentrations in hydroponic culture, and physiological and biochemical parameters, together with semiquantitative metabolic and proteomic profiles, were determined. • The exposure of plants to 10 μM Al inhibited root and leaf growth, but had no effect on the production of reactive oxygen species or lipid peroxides. By contrast, exposure to 20 μM Al elicited the production of superoxide radicals, peroxide and malondialdehyde. In response to Al, there was a progressive replacement of the superoxide dismutase isoforms in the cytosol, a loss of ascorbate and consistent changes in amino acids, sugars and associated enzymes. • We conclude that oxidative stress is not a causative factor of Al toxicity. The increased contents in roots of two powerful Al chelators, malic and 2‐isopropylmalic acids, together with the induction of an Al‐activated malate transporter gene, strongly suggest that both organic acids are implicated in Al detoxification. The effects of Al on key proteins involved in cytoskeleton dynamics, protein turnover, transport, methylation reactions, redox control and stress responses underscore a metabolic dysfunction, which affects multiple cellular compartments, particularly in plants exposed to 20 μM Al.
C sink/source balance and N assimilation have been identified as target processes conditioning crop responsiveness to elevated CO₂. However, little is known about phenology‐driven modifications of C ...and N primary metabolism at elevated CO₂ in cereals such as wheat. Here, we examined the differential effect of elevated CO₂ at two development stages (onset of flowering, onset of grain filling) in durum wheat (Triticum durum, var. Sula) using physiological measurements (photosynthesis, isotopes), metabolomics, proteomics and ¹⁵N labelling. Our results show that growth at elevated CO₂ was accompanied by photosynthetic acclimation through a lower internal (mesophyll) conductance but no significant effect on Rubisco content, maximal carboxylation or electron transfer. Growth at elevated CO₂ altered photosynthate export and tended to accelerate leaf N remobilization, which was visible for several proteins and amino acids, as well as lysine degradation metabolism. However, grain biomass produced at elevated CO₂ was larger and less N rich, suggesting that nitrogen use efficiency rather than photosynthesis is an important target for improvement, even in good CO₂‐responsive cultivars.
Protein turnover is a well-controlled process in which polypeptides are constantly being degraded and subsequently replaced with newly synthesized copies. Extraction of composite spectral envelopes ...from complex LC/MS shotgun proteomics data can be a challenging task, due to the inherent complexity of biological samples. With partial metabolic labeling experiments this complexity increases as a result of the emergence of additional isotopic peaks. Automated spectral extraction and subsequent protein turnover calculations enable the analysis of gigabytes of data within minutes, a prerequisite for systems biology high throughput studies. Here we present a fully automated method for protein turnover calculations from shotgun proteomics data. The approach enables the analysis of complex shotgun LC/MS 15N partial metabolic labeling experiments. Spectral envelopes of 1419 peptides can be extracted within an hour. The method quantifies turnover by calculating the Relative Isotope Abundance (RIA), which is defined as the ratio between the intensity sum of all heavy (15N) to the intensity sum of all light (14N) and heavy peaks. To facilitate this process, we have developed a computer program based on our method, which is freely available to download at http://promex.pph.univie.ac.at/protover.
Increasing food demand coupled with climate change pose a great challenge to agricultural systems. In this review we summarize recent advances in our knowledge of how plants, together with their ...associated microbiota, shape rhizosphere processes. We address (molecular) mechanisms operating at the plant–microbe-soil interface and aim to link this knowledge with actual and potential avenues for intensifying agricultural systems, while at the same time reducing irrigation water, fertilizer inputs and pesticide use. Combining in-depth knowledge about above and belowground plant traits will not only significantly advance our mechanistic understanding of involved processes but also allow for more informed decisions regarding agricultural practices and plant breeding. Including belowground plant-soil-microbe interactions in our breeding efforts will help to select crops resilient to abiotic and biotic environmental stresses and ultimately enable us to produce sufficient food in a more sustainable agriculture in the upcoming decades.
Most legume species establish a symbiotic association with soil bacteria. The plant accommodates the differentiated rhizobia in specialized organs, the root nodules. In this environment, the ...microsymbiont reduces atmospheric nitrogen (N) making it available for plant metabolism. Symbiotic N-fixation is driven by the respiration of the host photosynthates and thus constitutes an additional carbon sink for the plant. Molecular phenotypes of symbiotic and non-symbiotic Medicago truncatula are identified. The implication of nodule symbiosis on plant abiotic stress response mechanisms is not well understood. In this study, we exposed nodulated and non-symbiotic N-fertilized plants to salt and drought conditions. We assessed the stress effects with proteomic and metabolomic methods and found a nutritionally regulated phenotypic plasticity pivotal for a differential stress adjustment strategy.
Purpose
Root exudates are key components driving belowground interaction between plant, microbes and soil. High-end analytical approaches provide advanced insights into exudate metabolite diversity, ...however, the amount of total carbon (C) released by roots should always be determined as the most basic parameter when characterizing root exudation as it (i) provides quantitative information of C exuded into the surrounding soil and (ii) allows to relate the abundance of individual exudate compounds to total C released. Here we propose a simple and quick, spectrophotometry-based method to quantify total dissolved organic carbon (DOC) concentration in exudation samples that is based on measuring the absorption of a pre-filtered but otherwise untreated exudate sample at 260 nm (DOC
260
).
Method
Exudate samples collected from different grass genotypes (
Zea mays, Oryza sativa, Hordeum vulgare)
grown in various experimental settings (soil, hydroponic) were analysed with the DOC
260
assay and results were compared with C concentrations obtained by liquid TOC-analyser.
Conclusion
We demonstrated that the DOC
260
method allowed for quick and inexpensive measurements of total dissolved organic carbon concentrations in exudate samples from grass species grown under nutrient sufficient as well as under P deficient conditions. Interestingly, DOC
260
failed to predict DOC concentrations in exudate samples from plants grown under Zn and Fe deficiency suggesting a strong shift in metabolite composition under micronutrient deficiency. Even though the applicability of the DOC
260
method remains to be tested on exudate samples originating from dicots and plants exposed to other environmental stresses (e.g. pathogen attack, heavy metal stress, etc), it will help to increase our understanding of root exudation and related rhizosphere processes in the future.
Main conclusion
Different lupin species exhibited varied biomass, P allocation, and physiological responses to P-deprivation. White and yellow lupins had higher carboxylate exudation rates, while ...blue lupin showed the highest phosphatase activity.
White lupin (
Lupinus albus
) can produce specialized root structures, called cluster roots, which are adapted to low-phosphorus (P) soil. Blue lupin (
L. angustifolius
) and yellow lupin (
L. luteus
), which are two close relatives of white lupin, do not produce cluster roots. This study characterized plant responses to nutrient limitation by analyzing biomass accumulation and P distribution, absorption kinetics and root exudation in white, blue, and yellow lupins. Plants were grown in hydroponic culture with (64 µM NaH
2
PO
4
) or without P for 31 days. Under P limitation, more biomass was allocated to roots to improve P absorption. Furthermore, the relative growth rate of blue lupin showed the strongest inhibition. Under + P conditions, the plant total-P contents of blue lupin and yellow lupin were higher than that of white lupin. To elucidate the responses of lupins via the perspective of absorption kinetics and secretion analysis, blue and yellow lupins were confirmed to have stronger affinity and absorption capacity for orthophosphate after P-deprivation cultivation, whereas white lupin and yellow lupin had greater ability to secrete organic acids. The exudation of blue lupin had higher acid phosphatase activity. This study elucidated that blue lupin was more sensitive to P-scarcity stress and yellow had the greater tolerance of P-deficient condition than either of the other two lupin species. The three lupin species have evolved different adaptation strategies to cope with P deficiency.
Root secreted acid phosphatases and organic anions are widely perceived as major players of plant phosphorus (P) mobilisation from the rhizosphere under P limiting growth conditions. Previous ...research indicated that other mechanisms play a role, especially in species with fine roots, such as wheat. In this study we characterised the plant-derived extracellular proteome of wheat roots by profiling root tip mucilage, soluble root secreted and root tip proteomes. Extracellular acid phosphatases and enzymes of the central carbon metabolism were targeted using selected reaction monitoring. More than 140 proteins with extracellular localisation prediction were identified in mucilage. P starvation induced proteins predicted to be localised to the apoplast which are related to cell wall modification and defence in both, root tip and soluble root-secreted proteomes. Glycolytic enzymes were strongly increased in abundance by P limitation in root tips, as were PEPC and plastidial MDH. Soluble acid phosphatases were not identified in extracellular protein samples. Our results indicate that root tip mucilage contains proteins with the functional potential to actively shape their immediate environment by modification of plant structural components and biotic interactions. Wheat acid phosphatases appear to play a minor role in P mobilisation beyond the immediate root surface.
Phosphorus (P) is a plant growth limiting nutrient in many agricultural situations and the development of phosphorus efficient crops is of paramount importance for future agricultural management practices. As P is relatively immobile in soils, processes occurring at the root-soil interface, the rhizosphere, are suspected to play a key role in plant-induced P mobilisation. According to the current view, the secretion of extracellular acid phosphatases and organic anions enhances P mobilisation within several millimetres beyond the root surface, either directly or indirectly through the selection and appropriate soil microbes. However, the mechanisms of P mobilisation in species with fine roots, such as wheat, and the role of other secreted root proteins are poorly understood. Here, we carried out the profiling of wheat root tip mucilage, soluble root secreted and root tip proteomes. We analysed proteome changes in response to P starvation. We found that proteins with a predicted localisation to the apoplast made up a major proportion of stress-responsive proteins. Acid phosphatases were not identified within extracellular protein samples, which were enriched in proteins with predicted extracellular localisation. The absence of extracellular APases was further validated by multiple reaction monitoring. Our data indicates that wheat acid phosphatases play a minor role in P mobilisation beyond the immediate root surface and provides a resource for breeding strategies and further investigations of the functional roles of root tip-released proteins in the rhizosphere under P limitation.
Display omitted
•Profiling of plant-derived extracellular proteome of wheat roots under P limitation•Protein set with extracellular localisation prediction identified in root mucilage•Increased abundance of proteins with predicted apoplastic localisation and glycolytic enzymes•Targeted proteomics of glycolytic enzymes and acid phosphatases under P limitation•Minor importance of acid phosphatases beyond the root surface in P mobilisation
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