Impact of Soil Warming on the Plant Metabolome of Icelandic Grasslands

Long-term soil warming changes the profile of primary metabolites in fine roots of Norway spruce in a temperate montane forest

Climate warming poses major threats to temperate forests, but the response of tree root metabolism has largely remained unclear. We examined the impact of long-term soil warming (>14 years, +4°C) on the fine root metabolome across three seasons for 2 years in an old spruce forest, using a liquid chromatography-mass spectrometry platform for primary metabolite analysis. A total of 44 primary metabolites were identified in roots (19 amino acids, 12 organic acids and 13 sugars). Warming increased the concentration of total amino acids and of total sugars by 15% and 21%, respectively, but not organic acids. We found that soil warming and sampling date, along with their interaction, directly influenced the primary metabolite profiles. Specifically, in warming plots, concentrations of arginine, glycine, lysine, threonine, tryptophan, mannose, ribose, fructose, glucose and oxaloacetic acid increased by 51.4%, 19.9%, 21.5%, 19.3%, 22.1%, 23.0%, 38.0%, 40.7%, 19.8% and 16.7%, respectively. Rather than being driven by single compounds, changes in metabolite profiles reflected a general up- or downregulation of most metabolic pathway network. This emphasises the importance of metabolomics approaches in investigating root metabolic pathways and understanding the effects of climate change on tree root metabolism.

Soil warming duration and magnitude affect the dynamics of fine roots and rhizomes and associated C and N pools in subarctic grasslands

BACKGROUND AND AIMS

The response of subarctic grassland's below-ground to soil warming is key to understanding this ecosystem's adaptation to future climate. Functionally different below-ground plant organs can respond differently to changes in soil temperature (Ts). We aimed to understand the below-ground adaptation mechanisms by analysing the dynamics and chemistry of fine roots and rhizomes in relation to plant community composition and soil chemistry, along with the duration and magnitude of soil warming.

METHODS

We investigated the effects of the duration [medium-term warming (MTW; 11 years) and long-term warming (LTW; > 60 years)] and magnitude (0-8.4 °C) of soil warming on below-ground plant biomass (BPB), fine root biomass (FRB) and rhizome biomass (RHB) in geothermally warmed subarctic grasslands. We evaluated the changes in BPB, FRB and RHB and the corresponding carbon (C) and nitrogen (N) pools in the context of ambient, Ts < +2 °C and Ts > +2 °C scenarios.

KEY RESULTS

BPB decreased exponentially in response to an increase in Ts under MTW, whereas FRB declined under both MTW and LTW. The proportion of rhizomes increased and the C-N ratio in rhizomes decreased under LTW. The C and N pools in BPB in highly warmed plots under MTW were 50 % less than in the ambient plots, whereas under LTW, C and N pools in warmed plots were similar to those in non-warmed plots. Approximately 78 % of the variation in FRB, RHB, and C and N concentration and pools in fine roots and rhizomes was explained by the duration and magnitude of soil warming, soil chemistry, plant community functional composition, and above-ground biomass. Plant's below-ground biomass, chemistry and pools were related to a shift in the grassland's plant community composition - the abundance of ferns increased and BPB decreased towards higher Ts under MTW, while the recovery of below-ground C and N pools under LTW was related to a higher plant diversity.

CONCLUSION

Our results indicate that plant community-level adaptation of below ground to soil warming occurs over long periods. We provide insight into the potential adaptation phases of subarctic grasslands.

Chromolaena odorata Linn leaf extract - Geothermal versus nongeothermal: Phytochemical, antioxidant, and cytotoxicity screenings

Chromolaena odorata Linn, a popular yet underutilized ethnomedicinal plant, is hypothesized to possess higher bioactive phytoconstituents when it grows in geothermal areas. In this study, the comparison of ethanolic extract from geothermal and nongeothermal C. odorata leaves was carried out based on the phytochemical profile, antioxidant activity, and cytotoxicity. The leaf extracts were produced from a maceration using ethanol 96%, where the products were identified using reagents and gas chromatography-mass spectrometry (GC-MS). Antioxidant activities of both samples were measured based on their 2,2-diphenyl-1-picrylhydrazyl (DPPH) scavenging activities. Cytotoxicity was determined by brine shrimp lethality test using Artemia salina. Phenols were found to be more abundant in geothermal sample based on the qualitative screening and GC-MS analysis (i.e. higher relative abundance of phytol - 3.97%). DPPH antioxidant was higher in geothermal sample than in nongeothermal sample (median inhibitory concentration =13.04 ± 3.35 mg/L vs. 41.09 ± 4.13 mg/L, respectively). Geothermal sample was noncytotoxic (median lethal concentration [LC50] =2139.30 mg/L), whereas the nongeothermal sample had low cytotoxicity (LC50 = 491.48 mg/L). Taken altogether, geothermal C. odorata leaves contain higher bioactive compounds with potent antioxidant activities.

Implications of mistletoe parasitism for the host metabolome: A new plant identity in the forest canopy

Mistletoe-host systems exemplify an intimate and chronic relationship where mistletoes represent protracted stress for hosts, causing long-lasting impact. Although host changes in morphological and reproductive traits due to parasitism are well known, shifts in their physiological system, altering metabolite concentrations, are less known due to the difficulty of quantification. Here, we use ecometabolomic techniques in the plant-plant interaction, comparing the complete metabolome of the leaves from mistletoe (Viscum album) and needles from their host (Pinus nigra), both parasitized and unparasitized, to elucidate host responses to plant parasitism. Our results show that mistletoe acquires metabolites basically from the primary metabolism of its host and synthesizes its own defence compounds. In response to mistletoe parasitism, pines modify a quarter of their metabolome over the year, making the pine canopy metabolome more homogeneous by reducing the seasonal shifts in top-down stratification. Overall, host pines increase antioxidant metabolites, suggesting oxidative stress, and also increase part of the metabolites required by mistletoe, which act as a permanent sink of host resources. In conclusion, by exerting biotic stress and thereby causing permanent systemic change, mistletoe parasitism generates a new host-plant metabolic identity available in forest canopy, which could have notable ecological consequences in the forest ecosystem.

Metabolomics of Dry Versus Reanimated Antarctic Lichen-Dominated Endolithic Communities

Cryptoendolithic communities are almost the sole life form in the ice-free areas of the Antarctic desert, encompassing among the most extreme-tolerant organisms known on Earth that still assure ecosystems functioning, regulating nutrient and biogeochemical cycles under conditions accounted as incompatible with active life. If high-throughput sequencing based studies are unravelling prokaryotic and eukaryotic diversity, they are not yet characterized in terms of stress adaptations and responses, despite their paramount ecological importance. In this study, we compared the responses of Antarctic endolithic communities, with special focus on fungi, both under dry conditions (i.e., when dormant), and after reanimation by wetting, light, and optimal temperature (15 °C). We found that several metabolites were differently expressed in reanimated opposite sun exposed communities, suggesting a critical role in their success. In particular, the saccharopine pathway was up-regulated in the north surface, while the spermine/spermidine pathway was significantly down-regulated in the shaded exposed communities. The carnitine-dependent pathway is up-regulated in south-exposed reanimated samples, indicating the preferential involvement of the B-oxidation for the functioning of TCA cycle. The role of these metabolites in the performance of the communities is discussed herein.

Global warming shifts the composition of the abundant bacterial phyllosphere microbiota as indicated by a cultivation-dependent and -independent study of the grassland phyllosphere of a long-term warming field experiment

The leaf-colonizing bacterial microbiota was studied in a long-term warming experiment on a permanent grassland, which had been continuously exposed to increased surface temperature (+2°C) for more than six years. Two abundant plant species, Arrhenatherum elatius and Galium album, were studied. Surface warming reduced stomata opening and changed leaf metabolite profiles. Leaf surface colonization and the concentration of leaf-associated bacterial cells were not affected. However, bacterial 16S ribosomal RNA (rRNA) gene amplicon Illumina sequencing showed significant temperature effects on the plant species-specific phyllosphere microbiota. Warming partially affected the concentrations of cultured bacteria and had a significant effect on the composition of most abundant cultured plant species-specific bacteria. The abundance of Sphingomonas was significantly reduced. Sphingomonas isolates from warmed plots represented different phylotypes, had different physiological traits and were better adapted to higher temperatures. Among Methylobacterium isolates, a novel phylotype with a specific mxaFtype was cultured from plants of warmed plots while the most abundant phylotype cultured from control plots was strongly reduced. This study clearly showed a correlation of long-term surface warming with changes in the plant physiology and the development of a physiologically and genetically adapted phyllosphere microbiota.

Different "metabolomic niches" of the highly diverse tree species of the French Guiana rainforests

Tropical rainforests harbor a particularly high plant diversity. We hypothesize that potential causes underlying this high diversity should be linked to distinct overall functionality (defense and growth allocation, anti-stress mechanisms, reproduction) among the different sympatric taxa. In this study we tested the hypothesis of the existence of a metabolomic niche related to a species-specific differential use and allocation of metabolites. We tested this hypothesis by comparing leaf metabolomic profiles of 54 species in two rainforests of French Guiana. Species identity explained most of the variation in the metabolome, with a species-specific metabolomic profile across dry and wet seasons. In addition to this “homeostatic” species-specific metabolomic profile significantly linked to phylogenetic distances, also part of the variance (flexibility) of the metabolomic profile was explained by season within a single species. Our results support the hypothesis of the high diversity in tropical forest being related to a species-specific metabolomic niche and highlight ecometabolomics as a tool to identify this species functional diversity related and consistent with the ecological niche theory.

A systemic overreaction to years versus decades of warming in a subarctic grassland ecosystem

Temperature governs most biotic processes, yet we know little about how warming affects whole ecosystems. Here we examined the responses of 128 components of a subarctic grassland to 5-8 or >50 years of soil warming. Warming of >50 years drove the ecosystem to a new steady state possessing a distinct biotic composition and reduced species richness, biomass and soil organic matter. However, the warmed state was preceded by an overreaction to warming, which was related to organisms’ physiologies and was evident after 5-8 years. Ignoring this overreaction yielded errors of more than 100% for 83 variables when predicting their responses to a realistic warming scenario of 1 ºC over 50 years, although some, including soil carbon content, remained stable after 5-8 years. This study challenges long-term ecosystem predictions made from short-term observations, and provides a framework for characterising ecosystem responses to sustained climate change.

Crop metabolomics: from diagnostics to assisted breeding

BACKGROUND

Until recently, plant metabolomics have provided a deep understanding on the metabolic regulation in individual plants as experimental units. The application of these techniques to agricultural systems subjected to more complex interactions is a step towards the implementation of translational metabolomics in crop breeding.

AIM OF REVIEW

We present here a review paper discussing advances in the knowledge reached in the last years derived from the application of metabolomic techniques that evolved from biomarker discovery to improve crop yield and quality.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Translational metabolomics applied to crop breeding programs.

Root exudate metabolomes change under drought and show limited capacity for recovery

Root exudates comprise a large variety of compounds released by plants into the rhizosphere, including low-molecular-weight primary metabolites (particularly saccharides, amino acids and organic acids) and secondary metabolites (phenolics, flavonoids and terpenoids). Changes in exudate composition could have impacts on the plant itself, on other plants, on soil properties (e.g. amount of soil organic matter), and on soil organisms. The effects of drought on the composition of root exudates, however, have been rarely studied. We used an ecometabolomics approach to identify the compounds in the exudates of Quercus ilex (holm oak) under an experimental drought gradient and subsequent recovery. Increasing drought stress strongly affected the composition of the exudate metabolome. Plant exudates under drought consisted mainly of secondary metabolites (71% of total metabolites) associated with plant responses to drought stress, whereas the metabolite composition under recovery shifted towards a dominance of primary metabolites (81% of total metabolites). These results strongly suggested that roots exude the most abundant root metabolites. The exudates were changed irreversibly by the lack of water under extreme drought conditions, and the plants could not recover.