Root Metabolic Profiles Research Articles

Salt stress alters root meristem definition, vascular differentiation and metabolome in Sorghum bicolor (L.) genotypes

Knowledge on salt tolerance requires further investigation, particularly in plants of agro-food interest. Sorghum is a potentially useful plant because it is a emerging food species that combines high levels of salt tolerance with interesting nutritional characteristics. In sorghum different genotypes respond differently to saline stress and the early events characterizing the salt stress tolerance are not yet fully understood. Moreover, the number of salt resistant genotypes needs to be extended. The genotypes Bianca and Tonkawa are two possible candidates for extending sorghum cultivation to soils characterized by high levels of salinity. The root is the first organ that responds to soil conditions, especially during the initial stages of plant developmental. The research aim was to analyse the root system responses to salt stress (NaCl) of Bianca and Tonkawa genotypes to identify the morpho-functional and metabolic changes that occur during the initial stages of the root system development and to use them as discriminating parameters for assessing the different plant’s susceptibility to the salt. The results showed that salt stress negatively affected many morphological and cyto-histological root parameters, from seed germination to root system establishment. The salt altered the root meristem organization and quiescent centre (QC) definition, but similarly in both genotypes. By contrast, it reduced primary root (PR) length and induced a more extended oxidative stress in the adventitious roots (ARs) and lateral root primordia (LRPs) of Tonkawa in comparison with Bianca. The stele area and the number of protoxylem and phloem elements in the ARs were more reduced in 150 mM NaCl-treated Tonkawa seedlings in comparison with those of Bianca. Moreover, the salt enhanced lignin deposition in protoxylem, early metaxylem and endodermis and changed the root metabolic profiles significantly increasing the levels of leucine, isoleucine, alanine, proline, trigonelline, allantoin and glutamine in Bianca compared to Tonkawa. Altogether, specific morpho-anatomical and metabolic differences between the genotypes were identified as discriminating markers of genotype salt susceptibility.

The Role of Medicago lupulina Interaction with Rhizophagus irregularis in the Determination of Root Metabolome at Early Stages of AM Symbiosis.

The nature of plant–fungi interaction at early stages of arbuscular mycorrhiza (AM) development is still a puzzling problem. To investigate the processes behind this interaction, we used the Medicago lupulina MlS-1 line that forms high-efficient AM symbiosis with Rhizophagus irregularis. AM fungus actively colonizes the root system of the host plant and contributes to the formation of effective AM as characterized by a high mycorrhizal growth response (MGR) in the host plant. The present study is aimed at distinguishing the alterations in the M. lupulina root metabolic profile as an indicative marker of effective symbiosis. We examined the root metabolome at the 14th and 24th day after sowing and inoculation (DAS) with low substrate phosphorus levels. A GS-MS analysis detected 316 metabolites. Results indicated that profiles of M. lupulina root metabolites differed from those in leaves previously detected. The roots contained fewer sugars and organic acids. Hence, compounds supporting the growth of mycorrhizal fungus (especially amino acids, specific lipids, and carbohydrates) accumulated, and their presence coincided with intensive development of AM structures. Mycorrhization determined the root metabolite profile to a greater extent than host plant development. The obtained data highlight the importance of active plant–fungi metabolic interaction at early stages of host plant development for the determination of symbiotic efficiency.

Elucidating Drought-Tolerance Mechanisms in Plant Roots through 1H NMR Metabolomics in Parallel with MALDI-MS, and NanoSIMS Imaging Techniques.

As direct mediators between plants and soil, roots play an important role in metabolic responses to environmental stresses such as drought, yet these responses are vastly uncharacterized on a plant-specific level, especially for co-occurring species. Here, we aim to examine the effects of drought on root metabolic profiles and carbon allocation pathways of three tropical rainforest species by combining cutting-edge metabolomic and imaging technologies in an in situ position-specific 13C-pyruvate root-labeling experiment. Further, washed (rhizosphere-depleted) and unwashed roots were examined to test the impact of microbial presence on root metabolic pathways. Drought had a species-specific impact on the metabolic profiles and spatial distribution in Piper sp. and Hibiscus rosa sinensis roots, signifying different defense mechanisms; Piper sp. enhanced root structural defense via recalcitrant compounds including lignin, while H. rosa sinensis enhanced biochemical defense via secretion of antioxidants and fatty acids. In contrast, Clitoria fairchildiana, a legume tree, was not influenced as much by drought but rather by rhizosphere presence where carbohydrate storage was enhanced, indicating a close association with symbiotic microbes. This study demonstrates how multiple techniques can be combined to identify how plants cope with drought through different drought-tolerance strategies and the consequences of such changes on below-ground organic matter composition.

Revealing interactions between root phenolic metabolomes and rhizosphere bacterial communities in Populus euphratica plantations

How the root metabolic profiles and rhizosphere bacterial communities of dioecious plants respond to soil properties and sex identity is largely unknown. In this study, we analyzed root phenolic metabolomes and rhizosphere bacterial microbiomes of Populus euphratica females and males in two P. euphratica plantations with different soil properties to reveal the relative importance of soil and sex effects, and to decipher associations of certain phenolic compounds with specific bacterial taxa. We found that the relative abundances of bacterial OTUs and phenolic metabolites were closely linked to soil properties and sex identity. Soil is the main filter influencing the root phenolic metabolic profiles and rhizosphere bacterial communities of P. euphratica, while sexes and their interactions with soil properties are secondary factors. Differences in the diversity and evenness of phenolic metabolites were affected by plant sex, but not by soil properties. Conversely, the diversity and evenness of bacterial communities were affected by soil properties independent of plant sex. A multiple regression model indicated the presence of associations between root phenolic metabolites and specific soil bacteria taxa. Furthermore, all bacterial phyla and families correlated with at least one phenolic metabolite. Especially, both Nitrosomonadaceae and Cytophagaceae positively correlated with salicylic acid. Thus, our study provides new insights into the ecological mechanism that maintains rhizosphere bacterial communities in P. euphratica plantations in the desert area.

Osmotic Stress Leads to Significant Changes in Rice Root Metabolic Profiles between Tolerant and Sensitive Genotypes.

To breed osmotic stress-tolerant rice, the mechanisms involved in maintaining root growth under osmotic stress is important to elucidate. In this study, two rice (Oryza sativa L.) cultivars, IR 58 (stress-tolerant cultivar) and Basilanon (stress-sensitive cultivar), were used. After 1, 3, and 7 days of −0.42 MPa osmotic stress treatment induced by polyethylene glycol (PEG) 6000, root metabolomes were analyzed, yielding 276 detected compounds. Among 276 metabolites, 102 metabolites increased with the duration of the stress treatment in IR 58 roots, and only nine metabolites decreased. In contrast, 51 metabolites increased, and 45 metabolites decreased in Basilanon roots. Principal component analysis (PCA) scores clearly indicated differences between the cultivars and the treatments. Pathway analysis showed that the metabolites exhibiting stress-induced increases in IR 58 were those involved in sugar metabolism (such as sucrose 6’-phosphate, glucose 1-phosphate), polyamine and phenylpropanoid metabolisms (such as spermine, spermidine, gamma-aminobutyric acid (GABA)), and glutathione metabolism (such as glutathione, cysteine, cadaverine). IR 58 roots showed an increase in the most proteinogenic amino acids such as proline, serine, glutamine and asparagine. It was also maintained or increased the tricarboxylic acid (TCA) cycle intermediates (citric acid, cis-Aconitic acid, isocitric acid, fumaric acid, malic acid) under osmotic stress compared with that under control. Therefore, IR 58 actively synthesized various metabolites, and the increase in these metabolites contributed to the maintenance of important biological functions such as energy production and antioxidant defense to promote root development under osmotic stress.

Exogenous strigolactones impact metabolic profiles and phosphate starvation signalling in roots.

Strigolactones (SLs) are important ex-planta signalling molecules in the rhizosphere, promoting the association with beneficial microorganisms, but also affecting plant interactions with harmful organisms. They are also plant hormones in-planta, acting as modulators of plant responses under nutrient-deficient conditions, mainly phosphate (Pi) starvation. In the present work, we investigate the potential role of SLs as regulators of early Pi starvation signalling in plants. A short-term pulse of the synthetic SL analogue 2'-epi-GR24 promoted SL accumulation and the expression of Pi starvation markers in tomato and wheat under Pi deprivation. 2'-epi-GR24 application also increased SL production and the expression of Pi starvation markers under normal Pi conditions, being its effect dependent on the endogenous SL levels. Remarkably, 2'-epi-GR24 also impacted the root metabolic profile under these conditions, promoting the levels of metabolites associated to plant responses to Pi limitation, thus partially mimicking the pattern observed under Pi deprivation. The results suggest an endogenous role for SLs as Pi starvation signals. In agreement with this idea, SL-deficient plants were less sensitive to this stress. Based on the results, we propose that SLs may act as early modulators of plant responses to P starvation.

Mycorrhizal fungi enhance nutrient uptake but disarm defences in plant roots, promoting plant-parasitic nematode populations

Arbuscular mycorrhizal (AM) fungi are ubiquitous components of the soil biota which live symbiotically with terrestrial plants. Plant-parasitic nematodes are an important group of soil-dwelling invertebrates that inflict considerable damage to crops, representing a serious threat to food security. The effects of the AM symbiosis on plant-parasitic nematodes can be variable, and the mechanisms driving such variability remain ambiguous.We tested the impacts of inoculation with AM fungi on the root metabolic profile and nutritional chemistry of two varieties of wheat (Triticum aestivum), and how this affected populations of the plant-parasitic nematode Pratylenchus neglectus.AM fungi reduced plant biomass by almost 24%, yet increased root concentrations of phosphorus, potassium and zinc by 50%, 15% and 16%, respectively. Contrary to our predictions, nematode populations were 47–117% higher on AM inoculated plants, depending on variety. Untargeted metabolomic profiling revealed significant effects of mycorrhizal colonisation on certain markers of biological interest, these compounds were the benzoxazinoid glucoside defence compounds DIBOA-Glc, HMBOA-Glc and HDMBOA-Glc. Overall, mycorrhizae reduced abundances of these defence metabolites, which were potentially driving AM fungi – nematode interactions; although for DIBOA-Glc this was dependent on wheat variety. Moreover, there was a negative correlation between total AM colonisation and DIBOA-Glc concentrations.Our results demonstrate AM fungi can reduce plant biomass and supress root defence compounds associated with plant resistance to invertebrate pests, while still providing nutritional benefit to the host plant. This highlights that mycorrhizal colonisation of wheat varieties can have simultaneous positive and negative effects on different plant traits which drive plant-herbivore interactions. In working towards effective exploitation of the AM symbiosis in sustainable plant production, the context dependent outcomes of mycorrhizal-plant-nematode interactions is a key challenge. Untargeted metabolomic profiling offers the ability to reveal some of the driving mechanisms underpinning such complex tripartite interactions in the soil.

A Vegetal Biopolymer-Based Biostimulant Promoted Root Growth in Melon While Triggering Brassinosteroids and Stress-Related Compounds.

Plant biostimulants are receiving great interest for boosting root growth during the first phenological stages of vegetable crops. The present study aimed at elucidating the morphological, physiological, and metabolomic changes occurring in greenhouse melon treated with the biopolymer-based biostimulant Quik-link, containing lateral root promoting peptides, and lignosulphonates. The vegetal-based biopolymer was applied at five rates (0, 0.06, 0.12, 0.24, or 0.48 mL plant-1) as substrate drench. The application of biopolymer-based biostimulant at 0.12 and 0.24 mL plant-1 enhanced dry weight of melon leaves and total biomass by 30.5 and 27.7%, respectively, compared to biopolymer applications at 0.06 mL plant-1 and untreated plants. The root dry biomass, total root length, and surface in biostimulant-treated plants were significantly higher at 0.24 mL plant-1 and to a lesser extent at 0.12 and 0.48 mL plant-1, in comparison to 0.06 mL plant-1 and untreated melon plants. A convoluted biochemical response to the biostimulant treatment was highlighted through UHPLC/QTOF-MS metabolomics, in which brassinosteroids and their interaction with other hormones appeared to play a pivotal role. Root metabolic profile was more markedly altered than leaves, following application of the biopolymer-based biostimulant. Brassinosteroids triggered in roots could have been involved in changes of root development observed after biostimulant application. These hormones, once transported to shoots, could have caused an hormonal imbalance. Indeed, the involvement of abscisic acid, cytokinins, and gibberellin related compounds was observed in leaves following root application of the biopolymer-based biostimulant. Nonetheless, the treatment triggered an accumulation of several metabolites involved in defense mechanisms against biotic and abiotic stresses, such as flavonoids, carotenoids, and glucosinolates, thus potentially improving resistance toward plant stresses.

Cell-Type-Specific H+-ATPase Activity in Root Tissues Enables K+ Retention and Mediates Acclimation of Barley (Hordeum vulgare) to Salinity Stress.

While the importance of cell type specificity in plant adaptive responses is widely accepted, only a limited number of studies have addressed this issue at the functional level. We have combined electrophysiological, imaging, and biochemical techniques to reveal the physiological mechanisms conferring higher sensitivity of apical root cells to salinity in barley (Hordeum vulgare). We show that salinity application to the root apex arrests root growth in a highly tissue- and treatment-specific manner. Although salinity-induced transient net Na+ uptake was about 4-fold higher in the root apex compared with the mature zone, mature root cells accumulated more cytosolic and vacuolar Na+, suggesting that the higher sensitivity of apical cells to salt is not related to either enhanced Na+ exclusion or sequestration inside the root. Rather, the above differential sensitivity between the two zones originates from a 10-fold difference in K+ efflux between the mature zone and the apical region (much poorer in the root apex) of the root. Major factors contributing to this poor K+ retention ability are (1) an intrinsically lower H+-ATPase activity in the root apex, (2) greater salt-induced membrane depolarization, and (3) a higher reactive oxygen species production under NaCl and a larger density of reactive oxygen species-activated cation currents in the apex. Salinity treatment increased (2- to 5-fold) the content of 10 (out of 25 detected) amino acids in the root apex but not in the mature zone and changed the organic acid and sugar contents. The causal link between the observed changes in the root metabolic profile and the regulation of transporter activity is discussed.

Metabolomic characterization of the possible involvement of a Cytochrome P450, CYP81F4, in the biosynthesis of indolic glucosinolate in Arabidopsis

We studied the biosynthetic role of a cytochrome P450, CYP81F4, in Arabidopsis. The CYP81F4 gene was found within a gene expression network containing the transcription factor gene MYB34/ATR1 involved in tryptophan metabolisms. Root metabolic profiles in null cyp81f4 mutant lines were analyzed using an in-house metabolomics scheme based on Fourier transform ion cyclotron resonance mass spectrometry. The cyp81f4 mutant plants exhibited a build-up of indole-3-yl-methyl glucosinolate with concomitant loss of 1-methoxy-indole-3-yl-methyl glucosinolate. A pathway prediction supported by a detailed glucosinolate analysis indicated that CYP81F4, together with an unidentified methyltransferase, is involved in the formation of the methoxy group in the indole ring to yield 1-methoxy-indole-3-yl-methyl glucosinolate in Arabidopsis roots. Current results suggest that 1-methoxy-indole-3-yl-methyl glucosinolate is produced by the activity of CYP81F4 in Arabidopsis roots.