Metabolomic responses triggered by arbuscular mycorrhiza enhance tolerance to water stress in wheat cultivars

Under global climate change forecasts, the pressure of environmental stressors (and in particular drought) on crop productivity is expected to rise and challenge further global food security. The application of beneficial microorganisms may represent an environment friendly tool to secure improved crop performance and yield stability. Accordingly, this current study aimed at elucidating the metabolomic responses triggered by mycorrhizal (Funneliformis mosseae) inoculation of durum (Triticum durum Desf.; cv. 'Mongibello') and bread wheat cultivars (Triticum aestivum L.; cv. 'Chinese Spring') under full irrigation and water deficit regimes. Metabolomics indicated a similar regulation of secondary metabolism in both bread and durum wheat cultivars following water limiting conditions. Nonetheless, a mycorrhizal fungi (AMF) x cultivar interaction could be observed, with the bread wheat cultivar being more affected by arbuscular colonization under water limiting conditions. Discriminant compounds could be mostly related to sugars and lipids, both being positively modulated by AMF colonization under water stress. Moreover, a regulation of metabolites related to oxidative stress and a tuning of crosstalk between phytohormones were also evidenced. Among the latter, the stimulation of the brassinosteroids biosynthetic pathway was particularly evident in inoculated wheat roots, supporting the hypothesis of their involvement in enhancing plant response to water stress and modulation of oxidative stress conditions. This study proposes new insights on the modulation of the tripartite interaction plant-AMF-environmental stress.

Proteomic insight into the mitigation of wheat root drought stress by arbuscular mycorrhizae

Arbuscular mycorrhizal fungi (AMF) are plant growth promoters that ameliorate plant-water relations and the nutrient uptake of wheat. In this work, two cultivars of Triticum spp., a bread and a durum wheat, grown under drought stress and inoculated or not by AMF, are evaluated through a shotgun proteomic approach. The AMF association had beneficial effects as compared to non-mycorrhizal roots, in both bread and durum wheat. The beneficial symbiosis was confirmed by measuring morphological and physiological traits. In our work, we identified 50 statistically differential proteins in the bread wheat cultivar and 66 differential proteins in the durum wheat cultivar. The findings highlighted a modulation of proteins related to sugar metabolism, cell wall rearrangement, cytoskeletal organization and sulphur-containing proteins, as well as proteins related to plant stress responses. Among differentially expressed proteins both cultivars evidenced a decrease in sucrose:fructan 6-fructosyltransferas. In durum wheat oxylipin signalling pathway was involved with two proteins: increased 12-oxo-phytodienoic acid reductase and decreased jasmonate-induced protein, both related to the biosynthesis of jasmonic acid. Interactome analysis highlighted the possible involvement of ubiquitin although not evidenced among differentially expressed proteins. The AMF association helps wheat roots reducing the osmotic stress and maintaining cellular integrity.

BIOLOGICAL SIGNIFICANCE

Drought is one of the major constraints that plants must face in some areas of the world, associated to climate change, negatively affecting the worldwide plant productivity. The adoption of innovative agronomic protocols may represent a winning strategy in facing this challenge. The arbuscular mycorrhizal fungi (AMF) inoculation may represent a natural and sustainable way to mitigate the negative effects due to drought in several crop, ameliorating plant growth and development. Studies on the proteomic responses specific to AMF in drought-stressed plants will help clarify how mycorrhization elicits plant growth, nutrient uptake, and stress-tolerance responses. Such studies also offer the potential to find biological markers and genetic targets to be used during breeding for new drought-resistant varieties.

Intraspecific variability of carbon isotope discrimination and its correlation with grain yield in safflower: prospects for selection in a Mediterranean climate

The goals of the present study were to obtain a first estimate of intraspecific variability of carbon isotope discrimination (Δ) in safflower, a thistle-like herbaceous plant, and to determine the statistical relationship between Δ and grain yield as well as its components in a collection of 45 accessions of different origins. Grain yield and aboveground biomass, harvest index, average grain weight, and Δ (measured on the bulk leaf organic matter) were investigated in experimental field conditions. A large variability was noted for all traits but a principal component analysis (PCA) allowed to identify several homogeneous groups of accessions. Average grain yield per plant varied between 1 and 39 g. Δ varied between 21.3 and 25.2 ‰, i.e. a large variation of 3.9 ‰. In our experiment, the variation of Δ was not significantly related to that of grain yield in the whole accession sample. However, we found contrasting trends for this relation within accession groups. These initial results motivate further experiments to assess more in depth correlation between Δ and yield in safflower and are encouraging regarding the possibility of using Δ as an effective selection index in safflower to obtain genotypes that efficiently consume water. This study also highlighted one accession that combines the two characters required in the Mediterranean regions, i.e. high yield performance and high water-use efficiency.

Population structure and genome-wide association analysis for frost tolerance in oat using continuous SNP array signal intensity ratios

Infinium SNP data analysed as continuous intensity ratios enabled associating genotypic and phenotypic data from heterogeneous oat samples, showing that association mapping for frost tolerance is a feasible option. Oat is sensitive to freezing temperatures, which restricts the cultivation of fall-sown or winter oats to regions with milder winters. Fall-sown oats have a longer growth cycle, mature earlier, and have a higher productivity than spring-sown oats, therefore improving frost tolerance is an important goal in oat breeding. Our aim was to test the effectiveness of a Genome-Wide Association Study (GWAS) for mapping QTLs related to frost tolerance, using an approach that tolerates continuously distributed signals from SNPs in bulked samples from heterogeneous accessions. A collection of 138 European oat accessions, including landraces, old and modern varieties from 27 countries was genotyped using the Infinium 6K SNP array. The SNP data were analyzed as continuous intensity ratios, rather than converting them into discrete values by genotype calling. PCA and Ward's clustering of genetic similarities revealed the presence of two main groups of accessions, which roughly corresponded to Continental Europe and Mediterranean/Atlantic Europe, although a total of eight subgroups can be distinguished. The accessions were phenotyped for frost tolerance under controlled conditions by measuring fluorescence quantum yield of photosystem II after a freezing stress. GWAS were performed by a linear mixed model approach, comparing different corrections for population structure. All models detected three robust QTLs, two of which co-mapped with QTLs identified earlier in bi-parental mapping populations. The approach used in the present work shows that SNP array data of heterogeneous hexaploid oat samples can be successfully used to determine genetic similarities and to map associations to quantitative phenotypic traits.

Occurrence of Fusarium langsethiae and T-2 and HT-2 Toxins in Italian Malting Barley

T-2 and HT-2 toxins are two of the most toxic members of type-A trichothecenes, produced by a number of Fusarium species. The occurrence of these mycotoxins was studied in barley samples during a survey carried out in the 2011-2014 growing seasons in climatically different regions in Italy. The percentage of samples found positive ranges from 22% to 53%, with values included between 26 and 787 μg/kg. The percentage of samples with a T-2 and HT-2 content above the EU indicative levels for barley of 200 μg/kg ranges from 2% to 19.6% in the 2011-2014 period. The fungal species responsible for the production of these toxins in 100% of positive samples has been identified as Fusarium langsethiae, a well-known producer of T-2 and HT-2 toxins. A positive correlation between the amount of F. langsethiae DNA and of the sum of T-2 and HT-2 toxins was found. This is the first report on the occurrence of F. langsethiae-and of its toxic metabolites T-2 and HT-2-in malting barley grown in Italy.

Changes and their possible causes in δ13C of dark-respired CO2 and its putative bulk and soluble sources during maize ontogeny

The issues of whether, where, and to what extent carbon isotopic fractionations occur during respiration affect interpretations of plant functions that are important to many disciplines across the natural sciences. Studies of carbon isotopic fractionation during dark respiration in C3 plants have repeatedly shown respired CO2 to be (13)C enriched relative to its bulk leaf sources and (13)C depleted relative to its bulk root sources. Furthermore, two studies showed respired CO2 to become progressively (13)C enriched during leaf ontogeny and (13)C depleted during root ontogeny in C3 legumes. As such data on C4 plants are scarce and contradictory, we investigated apparent respiratory fractionations of carbon and their possible causes in different organs of maize plants during early ontogeny. As in the C3 plants, leaf-respired CO2 was (13)C enriched whereas root-respired CO2 was (13)C depleted relative to their putative sources. In contrast to the findings for C3 plants, however, not only root- but also leaf-respired CO2 became more (13)C depleted during ontogeny. Leaf-respired CO2 was highly (13)C enriched just after light-dark transition but the enrichment rapidly decreased over time in darkness. We conclude that (i) although carbon isotopic fractionations in C4 maize and leguminous C3 crop roots are similar, increasing phosphoenolpyruvate-carboxylase activity during maize ontogeny could have produced the contrast between the progressive (13)C depletion of maize leaf-respired CO2 and (13)C enrichment of C3 leaf-respired CO2 over time, and (ii) in both maize and C3 leaves, highly (13)C enriched leaf-respired CO2 at light-to-dark transition and its rapid decrease during darkness, together with the observed decrease in leaf malate content, may be the result of a transient effect of light-enhanced dark respiration.

Changes in δ(13)C of dark respired CO2 and organic matter of different organs during early ontogeny in peanut plants

Carbon isotope composition in respired CO2 and organic matter of individual organs were measured on peanut seedlings during early ontogeny in order to compare fractionation during heterotrophic growth and transition to autotrophy in a species with lipid seed reserves with earlier results obtained on beans. Despite a high lipid content in peanut seeds (48%) compared with bean seeds (1.5%), the isotope composition of leaf- and root-respired CO2 as well as its changes during ontogeny were similar to already published data on bean seedlings: leaf-respired CO2 became (13)C-enriched reaching -21.5‰, while root-respired CO2 became (13)C-depleted reaching around -31‰ at the four-leaf stage. The opposite respiratory fractionation in leaves vs. roots already reported for C3 herbs was thus confirmed for peanuts. However, contrarily to beans, the peanut cotyledon-respired CO2 was markedly (13)C-enriched, and its (13)C-depletion was noted from the two-leaf stage onwards only. Carbohydrate amounts being very low in peanut seeds, this cannot be attributed solely to their use as respiratory substrate. The potential role of isotope fractionation during glyoxylate cycle and/or gluconeogenesis on the (13)C-enriched cotyledon-respired CO2 is discussed.

Opposite carbon isotope discrimination during dark respiration in leaves versus roots - a review

In general, leaves are (13) C-depleted compared with all other organs (e.g. roots, stem/trunk and fruits). Different hypotheses are formulated in the literature to explain this difference. One of these states that CO2 respired by leaves in the dark is (13) C-enriched compared with leaf organic matter, while it is (13) C-depleted in the case of root respiration. The opposite respiratory fractionation between leaves and roots was invoked as an explanation for the widespread between-organ isotopic differences. After summarizing the basics of photosynthetic and post-photosynthetic discrimination, we mainly review the recent findings on the isotopic composition of CO2 respired by leaves (autotrophic organs) and roots (heterotrophic organs) compared with respective plant material (i.e. apparent respiratory fractionation) as well as its metabolic origin. The potential impact of such fractionation on the isotopic signal of organic matter (OM) is discussed. Some perspectives for future studies are also proposed .

Harden the chloroplast to protect the plant

The chloroplast is the central switch of the plant's response to cold and light stress. The ability of many plant species to develop a cold tolerant phenotype is dependent on the presence of light and photosynthetic activity during low-temperature growth. Light exposure at low temperature stimulates an over-reduction of the plastoquinone pool as well as the accumulation of reactive oxygen species, and both metabolic conditions generate a retrograde signal controlling nuclear gene expression. At the same time the chloroplast is the target of many cold acclimation processes which are the results of the chloroplast-nucleus cross-talk. Often, the extent of cold acclimation of the chloroplast is tightly correlated with the overall plant tolerance to chilling and freezing temperatures, a finding suggesting that the chloroplast cold acclimation could be the rate limiting factor in the adaptation to low temperature.

On the 13C/12C isotopic signal of day and night respiration at the mesocosm level

While there is currently intense effort to examine the (13)C signal of CO(2) evolved in the dark, less is known on the isotope composition of day-respired CO(2). This lack of knowledge stems from technical difficulties to measure the pure respiratory isotopic signal: day respiration is mixed up with photorespiration, and there is no obvious way to separate photosynthetic fractionation (pure c(i)/c(a) effect) from respiratory effect (production of CO(2) with a different delta(13)C value from that of net-fixed CO(2)) at the ecosystem level. Here, we took advantage of new simple equations, and applied them to sunflower canopies grown under low and high [CO(2)]. We show that whole mesocosm-respired CO(2) is slightly (13)C depleted in the light at the mesocosm level (by 0.2-0.8 per thousand), while it is slightly (13)C enriched in darkness (by 1.5-3.2 per thousand). The turnover of the respiratory carbon pool after labelling appears similar in the light and in the dark, and accordingly, a hierarchical clustering analysis shows a close correlation between the (13)C abundance in day- and night-evolved CO(2). We conclude that the carbon source for respiration is similar in the dark and in the light, but the metabolic pathways associated with CO(2) production may change, thereby explaining the different (12)C/(13)C respiratory fractionations in the light and in the dark.