Multivariate associations of flavonoid and biomass accumulation in white clover (Trifolium repens) under drought

Phenylpropanoid Metabolism in Phaseolus vulgaris during Growth under Severe Drought

Drought limits the growth and development of Phaseolus vulgaris L. (known as common bean). Common bean plants contain various phenylpropanoids, but it is not known whether the levels of these metabolites are altered by drought. Here, BT6 and BT44, two white bean recombinant inbred lines (RILs), were cultivated under severe drought. Their respective growth and phenylpropanoid profiles were compared to those of well-irrigated plants. Both RILs accumulated much less biomass in their vegetative parts with severe drought, which was associated with more phaseollin and phaseollinisoflavan in their roots relative to well-irrigated plants. A sustained accumulation of coumestrol was evident in BT44 roots with drought. Transient alterations in the leaf profiles of various phenolic acids occurred in drought-stressed BT6 and BT44 plants, including the respective accumulation of two separate caftaric acid isomers and coutaric acid (isomer 1) relative to well-irrigated plants. A sustained rise in fertaric acid was observed in BT44 with drought stress, whereas the greater amount relative to well-watered plants was transient in BT6. Apart from kaempferol diglucoside (isomer 2), the concentrations of most leaf flavonol glycosides were not altered with drought. Overall, fine tuning of leaf and root phenylpropanoid profiles occurs in white bean plants subjected to severe drought.

Dissecting the role of salicylic acid in mediating stress response in mungbean cultivars concurrently exposed to Macrophomina phaseolina infection and drought stress

The combined stress studies provide fundamental knowledge that could assist in producing multiple stress resilient crops. The fungal phytopathogen, Macrophomina phaseolina is a major limiting factor in the productivity of the crop, Vigna radiata (mungbean). This fungal species tends to flourish under hot and dry conditions. Therefore, in this study the salicylic acid (SA) mediated stress responses in contrasting mungbean cultivars (Shikha and RMG-975) exposed to combined M. phaseolina infection (F) and drought stress (D) have been elucidated. The combined stress was applied to ten days seedlings in three orders i.e. drought followed by fungal infection (DF), drought followed by fungal infection with extended water deficit (DFD) and fungal infection followed by drought stress (FD). The severity of infection was analyzed using ImageJ analysis. Besides, the concentration of SA has been correlated with the phenylpropanoid pathway products, expression of pathogenesis-related proteins (β-1,3-glucanase and chitinase) and the specific activity of certain related enzymes (phenylalanine ammonia lyase, lipoxygenase and glutathione-S-transferase). The data revealed that the cultivar RMG-975 was relatively more tolerant than Shikha under individual stresses. However, the former became more susceptible to the infection under DFD treatment while the latter showed tolerance. Otherwise, the crown rot severity was reduced in both the cultivars under other combined treatments. The stress response analysis suggested that enhanced chitinase expression is vital for tolerance against both, the pathogen and drought stress. Also, it was noted that plants treat each stress combination differently and the role of SA was more prominently visible under individual stress conditions.

Drought stress-responsive abscisic acid and salicylic acid crosstalk with the phenylpropanoid pathway in soybean seeds

Crosstalk between hormones and secondary metabolites regulates the interactions between plants and stress. However, little is known about the effects of hormone crosstalk on the concentration of flavonoids in seeds. In this study, we identified abscisic acid (ABA) as a negative regulator of flavonoid accumulation in soybean seeds under drought-stress conditions. Alterations in flavonoid accumulation at several intensities of water stress, followed by a recovery period, were measured during the soybean seed-filling stage. Low soil moisture (SM 10%) significantly decreased the total flavonoid content in seeds. The decline in flavonoid content was proportional to the severity of drought stress and was dependent on the activities of phenylalanine ammonia-lyase (PAL) and chalcone synthase (CHS), two key phenylpropanoid pathway enzymes. The expression of phenylalanine ammonia-lyase 1 (GmPAL1), chalcone isomerase 1A (GmCHI1A), and chalcone synthase 8 (GmCHS8) was associated with phenolic and flavonoid accumulation in soybean seeds of plants subjected to drought stress. Interestingly, the expression levels of GmCHS8 were highly correlated with flavonoid levels under drought stress and water recovery conditions. Cinnamic acid, which is a biosynthesis precursor shared by both phenylpropanoid metabolism and salicylic acid (SA) biosynthesis, decreased under drought stress conditions. Notably, exogenous ABA suppressed the expression of GmPAL1, which encodes the first rate-limiting enzyme in the phenylpropanoid biosynthesis pathway and affects downstream products such as SA and flavonoids. In conclusion, drought stress altered the phenylpropanoid-derived compounds, at least with regard to flavonoid and SA accumulation in seeds, which was regulated by antagonistic interactions with ABA.

Responses to water stress extremes in diverse red clover germplasm accessions

Red clover (Trifolium pratense L.), a key perennial pastoral species used globally, can strengthen pastural mixes to withstand increasingly disruptive weather patterns from climate change. Breeding selections can be refined for this purpose by obtaining an in-depth understanding of key functional traits. A replicated randomized complete block glasshouse pot trial was used to observe trait responses critical to plant performance under control (15% VMC), water deficit (5% VMC) and waterlogged conditions (50% VMC) in seven red clover populations and compared against white clover. Twelve morphological and physiological traits were identified as key contributors to the different plant coping mechanisms displayed. Under water deficit, the levels of all aboveground morphological traits decreased, highlighted by a 41% decrease in total dry matter and 50% decreases in both leaf number and leaf thickness compared to the control treatment. An increase in root to shoot ratio indicated a shift to prioritizing root maintenance by sacrificing shoot growth, a trait attributed to plant water deficit tolerance. Under waterlogging, a reduction in photosynthetic activity among red clover populations reduced several morphological traits including a 30% decrease in root dry mass and total dry matter, and a 34% decrease in leaf number. The importance of root morphology for waterlogging was highlighted with low performance of red clover: there was an 83% decrease in root dry mass compared to white clover which was able to maintain root dry mass and therefore plant performance. This study highlights the importance of germplasm evaluation across water stress extremes to identify traits for future breeding programs.

Protective and defensive role of anthocyanins under plant abiotic and biotic stresses: An emerging application in sustainable agriculture

Global warming is the major cause of abiotic and biotic stresses that reduce plant growth and productivity. Various stresses such as drought, low temperature, pathogen attack, high temperature and salinity all negatively influence plant growth and development. Due to sessile beings, they cannot escape from these adverse conditions. However, plants develop a variety of systems that can help them to tolerate, resist, and escape challenges imposed by the environment. Among them, anthocyanins are a good example of stress mitigators. They aid plant growth and development by increasing anthocyanin accumulation, which leads to increased resistance to various biotic and abiotic stresses. In the primary metabolism of plants, anthocyanin improves the photosynthesis rate, membrane permeability, up-regulates many enzyme transcripts related to anthocyanin biosynthesis, and optimizes nutrient uptake. Generally, the most important genes of the anthocyanin biosynthesis pathways were up-regulated under various abiotic and biotic stresses. The present review will highlight anthocyanin mediated stress tolerance in plants under various abiotic and biotic stresses. We have also compiled literature related to genetically engineer stress-tolerant crops generated using over-expression of genes belonging to anthocyanin biosynthetic pathway or its regulation. To sum up, the present review provides an up-to-date description of various signal transduction mechanisms that modulate or enhance anthocyanin accumulation under stress conditions.

Interplay of silymarin and clove fruit extract effectively enhances cadmium stress tolerance in wheat (Triticum aestivum)

Introduction

Osmoprotectant supplementation can be used as a useful approach to enhance plant stress tolerance. However, the effect of silymarin and clove fruit extract (CFE) on wheat plants grown under cadmium (Cd) stress has not been studied.

Methods

Wheat seeds were planted in plastic pots filled with ions-free sand. A ½-strength Hoagland's nutrient solution was used for irrigation. Pots were treated with eight treatments thirteen days after sowing: 1) Control, 2) 0.5 mM silymarin foliar application [silymarin], 3) 2% CFE foliar application [CFE], 4) CFE enriched with silymarin (0.24 g silymarin L-1 of CFE) [CFE-silymarin], 5) Watering wheat seedlings with a nutritious solution of 2 mM Cd [Cd]. 6) Cadmium + silymarin, 7) Cadmium + CFE, and 8) Cadmium + CFE-silymarin. The experimental design was a completely randomized design with nine replicates.

Results and discussion

The Cd stress decreased grain yield, shoot dry weight, leaf area, carotenoids, chlorophylls, stomatal conductance, net photosynthetic rate, transpiration rate, membrane stability index, nitrogen, phosphorus, and potassium content by 66.9, 60.6, 56.7, 23.8, 33.5, 48.1, 41.2, 48.7, 42.5, 24.1, 39.9, and 24.1%, respectively. On the other hand, Cd has an Application of CFE, silymarin, or CEF-silymarin for wheat plants grown under Cd stress, significantly improved all investigated biochemical, morphological, and physiological variables and enhanced the antioxidant enzyme activities. Applying CFE and/or silymarin enhanced plant tolerance to Cd stress more efficiently. Our findings suggest using CFE-silymarin as a meaningful biostimulator for wheat plants to increase wheat plants' tolerance to Cd stress via enhancing various metabolic and physiological processes.

Clover Root Uptake of Cereal Benzoxazinoids (BXs) Caused Accumulation of BXs and BX Transformation Products Concurrently with Substantial Increments in Clover Flavonoids and Abscisic Acid

Metabolomic studies on root uptake and transformation of bioactive compounds, like cereal benzoxazinoids (BXs) in non-BX producing plants, are very limited. Therefore, a targeted mass-spectrometry-based metabolomics study was performed to elucidate the root uptake of BXs in white clover (Trifolium repens L.) and the impact of absorbed BXs on intrinsic clover secondary metabolites. Clover plants grew in a medium containing 100 μM of individual BXs (five aglycone and one glycoside BXs) for 3 weeks. Subsequently, plant tissues were analyzed by liquid chromatography-tandem mass spectrometry to quantify the BXs and clover secondary metabolite concentrations. All BXs were taken up by clover roots and translocated to the shoots. Upon uptake of 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA), 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), 2-hydroxy-1,4-benzoxazin-3-one (HBOA), and 2-β-d-glucopyranosyloxy-1,4-benzoxazin-3-one (HBOA-glc), the parent compounds and a range of transformation products were seen in the roots and shoots. The individual BX concentrations ranged from not detected (nd) to 469 μg/g of dry weight (dw) and from nd to 170 μg/g of dw in the roots and shoots, respectively. The root uptake of BXs altered the composition of intrinsic clover secondary metabolites. In particular, the concentration of flavonoids and the hormone abscisic acid increased substantially in comparison to control plants.

Abscisic acid induces the expression of AsKIN during the recovery period of garlic cryopreservation

Abscisic acid induced the expression of AsKIN during the recovery period of garlic cryopreservation. AsKIN was identified as a gene involved in cold and osmotic stress resistance. Cryopreservation has been proven to be effective in removing viruses from garlic. However, oxidative damage in cryopreservation has a significant impact on the survival after preservation. Abscisic acid (ABA) has been shown to reduce oxidative stress and promote the survival after cryopreservation. However, it is not clear which genes play important roles in this process. In this study, we added ABA to the dehydration step and analyzed the transcriptomic divergences between the ABA-treated group and the control group in three cryogenic steps (dehydration, unloading and recovery). By short time-series expression miner (STEM) analysis and weighted gene co-expression network analysis (WGCNA), the recovery step was identified as the period of significant changes in gene expression levels in cryopreservation. The addition of ABA promoted the upregulated expression of microtubule-related genes in the recovery step. We further identified AsKIN as a hub gene in the recovery step and verified its function. The results showed that overexpression of AsKIN enhanced the tolerance of Arabidopsis to oxidative stress in cryopreservation, influenced the expression of genes in response to cold and osmotic stress and promoted plant growth after stress. The AsKIN gene is likely to be involved in the plant response to cold stress and osmotic stress. These results reveal the molecular mechanisms of ABA in cryopreservation and elucidate the potential biological functions of the kinesin-14 subfamily.

Exogenous Application of Green Titanium Dioxide Nanoparticles (TiO2 NPs) to Improve the Germination, Physiochemical, and Yield Parameters of Wheat Plants under Salinity Stress

Agriculture is the backbone of every developing country. Among various crops, wheat (Triticum aestivum L.) belongs to the family Poaceae and is the most important staple food crop of various countries. Different biotic (viruses, bacteria and fungi) and abiotic stresses (water logging, drought and salinity) adversely affect the qualitative and quantitative attributes of wheat. Among these stresses, salinity stress is a very important limiting factor affecting the morphological, physiological, biochemical attributes and grain yield of wheat. This research work was carried out to evaluate the influence of phytosynthesized TiO₂ NPs on the germination, physiochemical, and yield attributes of wheat varieties in response to salinity. TiO₂ NPs were synthesized using TiO₂ salt and a Buddleja asiatica plant extract as a reducing and capping agent. Various concentrations of TiO₂ nanoparticles (20, 40, 60 and 80 mg/L) and salt solutions (NaCl) (100 and 150 mM) were used. A total of 20 mg/L and 40 mg/L improve germination attributes, osmotic and water potential, carotenoid, total phenolic, and flavonoid content, soluble sugar and proteins, proline and amino acid content, superoxide dismutase activity, and reduce malondialdhehyde (MDA) content at both levels of salinity. These two concentrations also improved the yield attributes of wheat varieties at both salinity levels. The best results were observed at 40 mg/L of TiO₂ NPs at both salinity levels. However, the highest concentrations (60 and 80 mg/L) of TiO₂ NPs showed negative effects on germination, physiochemical and yield characteristics and causes stress in both wheat varieties under control irrigation conditions and salinity stress. Therefore, in conclusion, the findings of this research are that the foliar application of TiO₂ NPs can help to improve tolerance against salinity stress in plants.

Transcriptomics analysis of field-droughted pear (Pyrus spp.) reveals potential drought stress genes and metabolic pathways

Drought acts as a major abiotic stress that hinders plant growth and crop productivity. It is critical, as such, to discern the molecular response of plants to drought in order to enhance agricultural yields under droughts as they occur with increasing frequency. Pear trees are among the most crucial deciduous fruit trees worldwide, and yet the molecular mechanisms of drought tolerance in field-grown pear remain unclear. In this study, we analyzed the differences in transcriptome profiles of pear leaves, branches, and young fruits in irrigation vs field-drought conditions over the growing seasons. In total, 819 differentially expressed genes (DEGs) controlling drought response were identified, among which 427 DEGs were upregulated and 392 DEGs were downregulated. Drought responsive genes were enriched significantly in monoterpenoid biosynthesis, flavonoid biosynthesis, and diterpenoid biosynthesis. Fourteen phenylpropanoid, five flavonoid, and four monoterpenoid structural genes were modulated by field drought stress, thereby indicating the transcriptional regulation of these metabolic pathways in fruit exposed to drought. A total of 4,438 transcription factors (TFs) belonging to 30 TF families were differentially expressed between drought and irrigation, and such findings signal valuable information on transcriptome changes in response to drought. Our study revealed that pear trees react to drought by modulating several secondary metabolic pathways, particularly by stimulating the production of phenylpropanoids as well as volatile organic compounds like monoterpenes. Our findings are of practical importance for agricultural breeding programs, while the resulting data is a resource for improving drought tolerance through genetic engineering of non-model, but economically important, perennial plants.

Specialized metabolites: Physiological and biochemical role in stress resistance, strategies to improve their accumulation, and new applications in crop breeding and management

Specialized plant metabolites (SPMs), traditionally referred to as 'secondary metabolites', are chemical compounds involved in a broad range of biological functions, including plant responses to abiotic and biotic stresses. Moreover, some of them have a role in end-product quality with potential health benefits in humans. For this reason, they became an important target of studies focusing on their mechanisms of action and use in crop breeding and management. In this review we summarize the specific role of SPMs in physiological processes and in plant resistance to abiotic and biotic stresses, and the different strategies to enhance their production/accumulation in plant tissues under stress, including genetic approaches (marker-assisted selection and biotechnological tools) and agronomic management (fertilizer applications, cultivation method and beneficial microorganisms). New crop management strategies based on the direct application of the most promising compounds in form of plant residuals or liquid formulations are also described.

Foliar Supplementation of Clove Fruit Extract and Salicylic Acid Maintains the Performance and Antioxidant Defense System of Solanum tuberosum L. under Deficient Irrigation Regimes

A field trial was conducted twice (in 2020 and 2021) to evaluate the effect of clove fruit extract (CFE) and/or salicylic acid (SA), which were used as a foliar nourishment, on growth and yield traits, as well as physiological and biochemical indices utilizing potato (Solanum tuberosum L.) plants irrigated with deficient regimes in an arid environment. Three drip irrigation regimes [e.g., well watering (7400 m3 ha−1), moderate drought (6200 m3 ha−1), and severe drought (5000 m3 ha−1)] were designed for this study. The tested growth, yield, and photosynthetic traits, along with the relative water content, were negatively affected, whereas markers of oxidative stress (hydrogen peroxide and superoxide), electrolyte leakage, and peroxidation of membrane lipids (assessed as malondialdehyde level) were augmented along with increased antioxidative defense activities under drought stress. These effects were gradually increased with the gradual reduction in the irrigation regime. However, under drought stress, CFE and/or SA significantly enhanced growth characteristics (fresh and dry weight of plant shoot and plant leaf area) and yield components (average tuber weight, number of plant tubers, and total tuber yield). In addition, photosynthetic attributes (chlorophylls and carotenoids contents, net photosynthetic and transpiration rates, and stomatal conductance) were also improved, and defensive antioxidant components (glutathione, free proline, ascorbate, soluble sugars, and α-tocopherol levels, and activities of glutathione reductase, peroxidase, superoxide dismutase, catalase, and ascorbate peroxidase) were further enhanced. The study findings advocate the idea of using a CFE+SA combined treatment, which was largely efficient in ameliorating potato plant growth and productivity by attenuating the limiting influences of drought stress in dry environments.

The Effect of Foliar Application of Melatonin on Changes in Secondary Metabolite Contents in Two Citrus Species Under Drought Stress Conditions

Plant secondary metabolites are compounds that play an important role in plant interactions and defense. Persian lime and Mexican lime as the two most important sour lime varieties with high levels of secondary metabolites, are widely cultivated in tropical and subtropical areas. Melatonin is a pleiotropic molecule that plays a key role in protecting plants against drought stress through regulating the secondary metabolite biosynthesis pathway. This study was performed as a factorial experiment consisting of three factors in a completely randomized design (CRD), including four concentrations of melatonin (0, 50, 100, and 150 μM), three levels of drought stress [100% (control), 75% (moderate stress), and 40% (severe stress) field capacity (FC)], and two Citrus cultivars. The experiment was conducted for 60 days in a greenhouse condition. Based on the results of this study under severe drought stress, melatonin-treated crops had higher total flavonoid and total phenolic contents than the untreated crops. The highest level of essential oils components was observed on 100 μM foliar application of melatonin under severe drought stress in both varieties. The main component of the essential oil was limonene in both Citrus species. Moreover, based on the analysis of the results, hesperidin was the main polyphenol in both varieties. Since the use of melatonin often increases the production of secondary metabolites, this study can be considered as a very effective method for controlling the adverse effects of drought stress in citrus for both industrial and horticultural aims.

Titanium dioxide nanoparticle is involved in mitigating NaCl-induced Calendula officinalis L. by activation of antioxidant defense system and accumulation of osmolytes

This study is aimed at evaluating the effects of TiO₂NPs on biochemical and physiological parameters of marigold (Calendula officinalis L.) under NaCl stress. Treatments included TiO₂NPs applied as foliar spraying in three levels (50, 100, and 200 mgL^-1), no foliar application as control, and different NaCl levels (0, 30, 60, and 90 mM) by adding NaCl to irrigation water. According to the results, the application of different concentrations of TiO₂ had various effects on the studied parameters. The use of 100 mgL^-1 TiO₂NPs increased flavonoid content of the leaves (at 30 mM NaCl), cell membrane injury (with no salinity), CAT and SOD activities (at 90 mM NaCl), and PPO activity (at 60 mM NaCl). On the other hand, using 100 mgL^-1 TiO₂NPs under salinity stress decreased MDA and H₂O₂ contents. Moreover, using 200 mgL^-1 TiO₂NPs increased photosynthetic pigments content, total flavonoid content of flowers, total soluble sugar, carotenoid content of flowers, and RWC (at the treatment with no salinity), aboveground and underground biomasses (at 30 mM and 60 mM NaCl, respectively), phenolic content and antioxidant activity (at 30 mM NaCl), and APX activity (at 90 mM NaCl). In conclusion, the findings of the present study indicated that TiO₂NPs could be a useful material to mitigate the harmful effects of salinity stress. Furthermore, the TiO₂NPs spraying could have beneficial effects on osmotic adjustment, biochemical compound, non-enzymatic and enzymatic antioxidant activities, and growth of marigold under NaCl stress conditions.

Shielded environments reduce stress in alien Asteraceae species during hot and dry summers along urban-to-rural gradients

Urban environments often host a greater abundance and diversity of alien plant species than rural areas. This is frequently linked to higher disturbance and propagule pressure, but could also be related to the additional establishment of species from warmer native ranges in cities, facilitated by the latter's higher air temperatures and drier soils. A hitherto unresolved question is how stressful the urban environments become during climate extremes such as heatwaves and droughts. Do such episodes still favor alien plant species, or set them back? We used in situ measured phenotypic leaf and development trait responses of the six most widespread alien Asteraceae species from various native climates along Belgian urban-to-rural gradients, measured during two unusually warm and dry summers. Urbanization was characterized by three factors: the percentage of artificially sealed surfaces (urbanity, measured at three spatial scales from in situ to satellite-based), the vegetation cover and the sky view factor (SVF, fraction of the hemisphere not blocked by buildings or vegetation). Across species, either from colder or warmer native climates, we found a predominant protective effect of shielded environments that block solar radiation (low SVF) along the entire urban-to-rural gradient. Such environments induced lower leaf anthocyanins and flavonols indices, indicating heat stress mitigation. Shielded environments also increased specific leaf area (SLA), a typical shade response. We found that vegetated areas had a secondary importance, increasing the chlorophyll content and decreasing the flavonols index, but these effects were not consistent across species. Finally, urbanity at the organism spatial scale decreased plant height, while broader-scale urbanity had no significant influence. Our results suggest that sealed surfaces constrain alien Asteraceae during unusually warm and dry summers, while shielded environments protect them, possibly canceling out the lack of light. These findings shed new light on alien plant species success along urban-to-rural gradients in a changing climate.

Abiotic stress-induced anthocyanins in plants: Their role in tolerance to abiotic stresses

Abiotic stresses, such as heat, drought, salinity, low temperature, and heavy metals, inhibit plant growth and reduce crop productivity. Abiotic stresses are becoming increasingly extreme worldwide due to the ongoing deterioration of the global climate and the increase in agrochemical utilization and industrialization. Plants grown in fields are affected by one or more abiotic stresses. The consequent stress response of plants induces reactive oxygen species (ROS), which are then used as signaling molecules to activate stress-tolerance mechanism. However, under extreme stress conditions, ROS are overproduced and cause oxidative damage to plants. In such conditions, plants produce anthocyanins after ROS signaling via the transcription of anthocyanin biosynthesis genes. These anthocyanins are then utilized in antioxidant activities by scavenging excess ROS for their sustainability. In this review, we discuss the physiological, biochemical, and molecular mechanisms underlying abiotic stress-induced anthocyanins in plants and their role in abiotic stress tolerance. In addition, we highlight the current progress in the development of anthocyanin-enriched transgenic plants and their ability to increase abiotic stress tolerance. Overall, this review provides valuable information that increases our understanding of the mechanisms by which anthocyanins respond to abiotic stress and protect plants against it. This review also provides practical guidance for plant biologists who are engineering stress-tolerant crops using anthocyanin biosynthesis or regulatory genes.

Medicago sativa and Medicago truncatula Show Contrasting Root Metabolic Responses to Drought

Drought is an environmental stressor that affects crop yield worldwide. Understanding plant physiological responses to stress conditions is needed to secure food in future climate conditions. In this study, we applied a combination of plant physiology and metabolomic techniques to understand plant responses to progressive water deficit focusing on the root system. We chose two legume plants with contrasting tolerance to drought, the widely cultivated alfalfa Medicago sativa (Ms) and the model legume Medicago truncatula (Mt) for comparative analysis. Ms taproot (tapR) and Mt fibrous root (fibR) biomass increased during drought, while a progressive decline in water content was observed in both species. Metabolomic analysis allowed the identification of key metabolites in the different tissues tested. Under drought, carbohydrates, abscisic acid, and proline predominantly accumulated in leaves and tapRs, whereas flavonoids increased in fibRs in both species. Raffinose-family related metabolites accumulated during drought. Along with an accumulation of root sucrose in plants subjected to drought, both species showed a decrease in sucrose synthase (SUS) activity related to a reduction in the transcript level of SUS1, the main SUS gene. This study highlights the relevance of root carbon metabolism during drought conditions and provides evidence on the specific accumulation of metabolites throughout the root system.

Phenolic Metabolites from Barley in Contribution to Phenome in soil Moisture Deficit

Eight barley varieties from Europe and Asia were subjected to moisture deficit at various development stages. At the seedling stage and the flag leaf stage combined stress was applied. The experiment was designed for visualization of the correlation between the dynamics of changes in phenolic compound profiles and the external phenome. The most significant increase of compound content in water deficiency was observed for chrysoeriol and apigenin glycoconjugates acylated with methoxylated hydroxycinnamic acids that enhanced the UV-protection effectiveness. Moreover, other good antioxidants such as derivatives of luteolin and hordatines were also induced by moisture deficit. The structural diversity of metabolites of the contents changed in response to water deficiency in barley indicates their multipath activities under stress. Plants exposed to moisture deficit at the seedling stage mobilized twice as many metabolites as plants exposed to this stress at the flag leaf stage. Specific metabolites such as methoxyhydroxycinnamic acids participated in the long-term acclimation. In addition, differences in phenolome mobilization in response to moisture deficit applied at the vegetative and generative phases were correlated with the phenotypical consequences. Observations of plant yield and biomass gave us the possibility to discuss the developmentally related consequences of moisture deficit for plants' fitness.

Melatonin regulates the functional components of photosynthesis, antioxidant system, gene expression, and metabolic pathways to induce drought resistance in grafted Carya cathayensis plants

The Chinese hickory (Carya cathayensis) is an economically important tree species popular for its nuts. However, the tree requires a long time to reach the nut-producing phase. To overcome this problem, grafting is widely used to reduce the time from the vegetative to the reproductive phase. This tree species also faces many environmental challenges due to climate change; drought is an important factor affecting growth and development. Here, we designed an experiment to assess the protective efficiency of melatonin in grafted Chinese hickory plants under drought stress. The results revealed that exogenously applied melatonin successfully recovered the growth of grafted Chinese hickory plants and improved photosynthetic efficiency. Exogenously applied melatonin also boosted the antioxidative defense system of the plants under drought stress, resulting in enhanced reactive oxygen species (ROS) scavenging. The accumulation of compatible solutes such as total soluble sugars and proline was also triggered by melatonin. Moreover, the analyses using metabolomics revealed that drought-stressed plants treated with melatonin regulated key metabolic pathways such as phenylpropanoid, chlorophyll and carotenoid biosynthesis, carbon fixation, and sugar metabolism. To further validate the physiological, biochemical, and metabolomic factors, we studied the molecular mechanisms by analyzing the expression of key genes involved in chlorophyll metabolism (chlorophyllase, CHLASE), antioxidative defense (superoxide dismutase, SOD; catalase, CAT; ascorbate peroxidase, APX; peroxidase, POD), and phenylalanine ammonia-lyase (PAL). Exogenously applied melatonin significantly regulated the transcript levels of key genes involved in the biological processes mentioned above. Melatonin also showed crosstalk with other hormones (zeatin, gibberellin A14, 24-epibrassinolide, jasmonic acid, and abscisic acid) to regulate the physiological processes. The results of this study show that melatonin regulates biological processes at the metabolic and molecular levels to resist drought stress.

Response of Phenylpropanoid Pathway and the Role of Polyphenols in Plants under Abiotic Stress

Phenolic compounds are an important class of plant secondary metabolites which play crucial physiological roles throughout the plant life cycle. Phenolics are produced under optimal and suboptimal conditions in plants and play key roles in developmental processes like cell division, hormonal regulation, photosynthetic activity, nutrient mineralization, and reproduction. Plants exhibit increased synthesis of polyphenols such as phenolic acids and flavonoids under abiotic stress conditions, which help the plant to cope with environmental constraints. Phenylpropanoid biosynthetic pathway is activated under abiotic stress conditions (drought, heavy metal, salinity, high/low temperature, and ultraviolet radiations) resulting in accumulation of various phenolic compounds which, among other roles, have the potential to scavenge harmful reactive oxygen species. Deepening the research focuses on the phenolic responses to abiotic stress is of great interest for the scientific community. In the present article, we discuss the biochemical and molecular mechanisms related to the activation of phenylpropanoid metabolism and we describe phenolic-mediated stress tolerance in plants. An attempt has been made to provide updated and brand-new information about the response of phenolics under a challenging environment.

Intraspecific differences in long-term drought tolerance in perennial ryegrass

Lolium perenne L. (perennial ryegrass) is the most important pasture grass species in temperate regions of the world. However, its growth is restricted in summer dry environments. Germplasm screening can be used to identify accessions or individual plants for incorporation into breeding programs for drought tolerance. We selected nine perennial ryegrass accessions from different global origins and from a range of climatic and environmental conditions. In addition, the perennial ryegrass cultivar ‘Grasslands Impact’ was chosen as a reference. The accessions were grown for 360 days in a controlled environment through six consecutive drought stress and recovery cycles. We observed intraspecific differences in drought stress responsiveness for shoot biomass and survival from the third stress cycle. An accession from Norway had 50% more shoot dry matter than the next best-performing accession after six drought cycles. Compared with the reference cultivar ‘Grasslands Impact’, shoot dry matter of the accession from Norway was more than seven times higher after six drought cycles, indicating superior performance of this ecotype under drought stress. Drought tolerance was characterized by osmotic adjustment and higher relative leaf water content at low soil moisture levels. Furthermore, the findings of this study identify solute potential as an early predictor of drought stress tolerance. These intraspecific differences can be used in breeding programs for the development of drought-tolerant perennial ryegrass cultivars.

Deep Sequencing of Suppression Subtractive Hybridisation Drought and Recovery Libraries of the Non-model Crop Trifolium repens L

White clover is a short-lived perennial whose persistence is greatly affected by abiotic stresses, particularly drought. The aim of this work was to characterize its molecular response to water deficit and recovery following re-hydration to identify targets for the breeding of tolerant varieties. We created a white clover reference transcriptome of 16,193 contigs by deep sequencing (mean base coverage 387x) four Suppression Subtractive Hybridization (SSH) libraries (a forward and a reverse library for each treatment) constructed from young leaf tissue of white clover at the onset of the response to drought and recovery. Reads from individual libraries were then mapped to the reference transcriptome and processed comparing expression level data. The pipeline generated four robust sets of transcripts induced and repressed in the leaves of plants subjected to water deficit stress (6,937 and 3,142, respectively) and following re-hydration (6,695 and 4,897, respectively). Semi-quantitative polymerase chain reaction was used to verify the expression pattern of 16 genes. The differentially expressed transcripts were functionally annotated and mapped to biological processes and pathways. In agreement with similar studies in other crops, the majority of transcripts up-regulated in response to drought belonged to metabolic processes, such as amino acid, carbohydrate, and lipid metabolism, while transcripts involved in photosynthesis, such as components of the photosystem and the biosynthesis of photosynthetic pigments, were up-regulated during recovery. The data also highlighted the role of raffinose family oligosaccharides (RFOs) and the possible delayed response of the flavonoid pathways in the initial response of white clover to water withdrawal. The work presented in this paper is to our knowledge the first large scale molecular analysis of the white clover response to drought stress and re-hydration. The data generated provide a valuable genomic resource for marker discovery and ultimately for the improvement of white clover.

Breaking through the feed barrier: options for improving forage genetics

Pasture based on perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) is the foundation for production and profit in the Australasian pastoral sectors. The improvement of these species offers direct opportunities to enhance sector performance, provided there is good alignment with industry priorities as quantified by means such as the forage value index. However, the rate of forage genetic improvement must increase to sustain industry competitiveness. New forage technologies and breeding strategies that can complement and enhance traditional approaches are required to achieve this. We highlight current and future research in plant breeding, including genomic and gene technology approaches to improve rate of genetic gain. Genomic diversity is the basis of breeding and improvement. Recent advances in the range and focus of introgression from wild Trifolium species have created additional specific options to improve production and resource-use-efficiency traits. Symbiont genetic resources, especially advances in grass fungal endophytes, make a critical contribution to forage, supporting pastoral productivity, with benefits to both pastures and animals in some dairy regions. Genomic selection, now widely used in animal breeding, offers an opportunity to lift the rate of genetic gain in forages as well. Accuracy and relevance of trait data are paramount, it is essential that genomic breeding approaches be linked with robust field evaluation strategies including advanced phenotyping technologies. This requires excellent data management and integration with decision-support systems to deliver improved effectiveness from forage breeding. Novel traits being developed through genetic modification include increased energy content and potential increased biomass in ryegrass, and expression of condensed tannins in forage legumes. These examples from the wider set of research emphasise forage adaptation, yield and energy content, while covering the spectrum from exotic germplasm and symbionts through to advanced breeding strategies and gene technologies. To ensure that these opportunities are realised on farm, continuity of industry-relevant delivery of forage-improvement research is essential, as is sustained research input from the supporting pasture and plant sciences.

Ragweed (Ambrosia artemisiifolia) pollen allergenicity: SuperSAGE transcriptomic analysis upon elevated CO2 and drought stress

BACKGROUND

Pollen of common ragweed (Ambrosia artemisiifolia) is a main cause of allergic diseases in Northern America. The weed has recently become spreading as a neophyte in Europe, while climate change may also affect the growth of the plant and additionally may also influence pollen allergenicity. To gain better insight in the molecular mechanisms in the development of ragweed pollen and its allergenic proteins under global change scenarios, we generated SuperSAGE libraries to identify differentially expressed transcripts.

RESULTS

Ragweed plants were grown in a greenhouse under 380 ppm CO2 and under elevated level of CO2 (700 ppm). In addition, drought experiments under both CO2 concentrations were performed. The pollen viability was not altered under elevated CO2, whereas drought stress decreased its viability. Increased levels of individual flavonoid metabolites were found under elevated CO2 and/or drought. Total RNA was isolated from ragweed pollen, exposed to the four mentioned scenarios and four SuperSAGE libraries were constructed. The library dataset included 236,942 unique sequences, showing overlapping as well as clear differently expressed sequence tags (ESTs). The analysis targeted ESTs known in Ambrosia, as well as in pollen of other plants. Among the identified ESTs, those encoding allergenic ragweed proteins (Amb a) increased under elevated CO2 and drought stress. In addition, ESTs encoding allergenic proteins in other plants were also identified.

CONCLUSIONS

The analysis of changes in the transcriptome of ragweed pollen upon CO2 and drought stress using SuperSAGE indicates that under global change scenarios the pollen transcriptome was altered, and impacts the allergenic potential of ragweed pollen.

Identification of key drought stress-related genes in the hyacinth bean

Hyacinth bean (Lablab purpureus [Linn.] Sweet) possesses excellent characteristics for field production, but the response of this plant to drought stress has not been described at the molecular level. Suppression subtraction hybridization (SSH) is an effective way to exploit key factors for plant responses to drought stress that are involved in transcriptional and metabolic activities. In this study, forward and reverse SSH libraries were generated from root tissues of the drought-tolerant hyacinth bean genotype MEIDOU 2012 under water-stress conditions. A total of 1,287 unigenes (94 contigs and 1,193 singletons) were derived from sequence alignment and cluster assembly of 1400 ESTs, and 80.6% of those hit against NCBI non-redundant (nr) database with E value <1E-06. BLASTX analysis revealed that the majority top matches were proteins form Glycine max (L.) Merrill. (61.5%). According to a gene ontology (GO) functional classification, 816 functionally annotated unigenes were assigned to the biological process category (74.1%), and 83.9% of them classified into molecular function and 69.2% involved in cellular component. A total of 168 sequences were further annotated with 207 Enzyme Commission (EC) codes and mapped to 83 different KEGG pathways. Seventeen functionally relevant genes were found to be overrepresented under drought stress using enrichment analysis. Differential expression of unigenes were confirmed by quantitative real-time PCR assays, and their transcript profiles generally divided into three patterns, depending on the expression peaked levels after 6, 8 or 10 days dehydration, which indicated that these genes are functionally associated in the drought-stress response.