Isoprene production in transgenic tobacco alters isoprenoid, non-structural carbohydrate and phenylpropanoid metabolism, and protects photosynthesis from drought stress

Photosynthesis: Genetic Strategies Adopted to Gain Higher Efficiency

The global challenge of feeding an ever-increasing population to maintain food security requires novel approaches to increase crop yields. Photosynthesis, the fundamental energy and material basis for plant life on Earth, is highly responsive to environmental conditions. Evaluating the operational status of the photosynthetic mechanism provides insights into plants' capacity to adapt to their surroundings. Despite immense effort, photosynthesis still falls short of its theoretical maximum efficiency, indicating significant potential for improvement. In this review, we provide background information on the various genetic aspects of photosynthesis, explain its complexity, and survey relevant genetic engineering approaches employed to improve the efficiency of photosynthesis. We discuss the latest success stories of gene-editing tools like CRISPR-Cas9 and synthetic biology in achieving precise refinements in targeted photosynthesis pathways, such as the Calvin-Benson cycle, electron transport chain, and photorespiration. We also discuss the genetic markers crucial for mitigating the impact of rapidly changing environmental conditions, such as extreme temperatures or drought, on photosynthesis and growth. This review aims to pinpoint optimization opportunities for photosynthesis, discuss recent advancements, and address the challenges in improving this critical process, fostering a globally food-secure future through sustainable food crop production.

The effect of constitutive root isoprene emission on root phenotype and physiology under control and salt stress conditions

Isoprene, a volatile hydrocarbon, is typically emitted from the leaves of many plant species. Given its well-known function in plant growth and defense aboveground, we examined its effects on root physiology. We used isoprene-emitting (IE) lines and a non-emitting (NE) line of Arabidopsis and investigated their performance by analyzing root phenotype, hormone levels, transcriptome, and metabolite profiles under both normal and salt stress conditions. We show that IE lines emitted tiny amounts of isoprene from roots and showed an increased root/shoot ratio compared with NE line. Isoprene emission exerted a noteworthy influence on hormone profiles related to plant growth and stress response, promoting root development and salt-stress resistance. Methyl erythritol 4-phosphate pathway metabolites, precursors of isoprene and hormones, were higher in the roots of IE lines than in the NE line. Transcriptome data indicated that the presence of isoprene increased the expression of key genes involved in hormone metabolism/signaling. Our findings reveal that constitutive root isoprene emission sustains root growth under saline conditions by regulating and/or priming hormone biosynthesis and signaling mechanisms and expression of key genes relevant to salt stress defense.

Deoxyxylulose 5-Phosphate Synthase Does Not Play a Major Role in Regulating the Methylerythritol 4-Phosphate Pathway in Poplar

The plastidic 2-C-methylerythritol 4-phosphate (MEP) pathway supplies the precursors of a large variety of essential plant isoprenoids, but its regulation is still not well understood. Using metabolic control analysis (MCA), we examined the first enzyme of this pathway, 1-deoxyxylulose 5-phosphate synthase (DXS), in multiple grey poplar (Populus × canescens) lines modified in their DXS activity. Single leaves were dynamically labeled with 13CO2 in an illuminated, climate-controlled gas exchange cuvette coupled to a proton transfer reaction mass spectrometer, and the carbon flux through the MEP pathway was calculated. Carbon was rapidly assimilated into MEP pathway intermediates and labeled both the isoprene released and the IDP+DMADP pool by up to 90%. DXS activity was increased by 25% in lines overexpressing the DXS gene and reduced by 50% in RNA interference lines, while the carbon flux in the MEP pathway was 25-35% greater in overexpressing lines and unchanged in RNA interference lines. Isoprene emission was also not altered in these different genetic backgrounds. By correlating absolute flux to DXS activity under different conditions of light and temperature, the flux control coefficient was found to be low. Among isoprenoid end products, isoprene itself was unchanged in DXS transgenic lines, but the levels of the chlorophylls and most carotenoids measured were 20-30% less in RNA interference lines than in overexpression lines. Our data thus demonstrate that DXS in the isoprene-emitting grey poplar plays only a minor part in controlling flux through the MEP pathway.

The isoprene-responsive phosphoproteome provides new insights into the putative signalling pathways and novel roles of isoprene

Many plants, especially trees, emit isoprene in a highly light- and temperature-dependent manner. The advantages for plants that emit, if any, have been difficult to determine. Direct effects on membranes have been disproven. New insights have been obtained by RNA sequencing, proteomic and metabolomic studies. We determined the responses of the phosphoproteome to exposure of Arabidopsis leaves to isoprene in the gas phase for either 1 or 5 h. Isoprene effects that were not apparent from RNA sequencing and other methods but were apparent in the phosphoproteome include effects on chloroplast movement proteins and membrane remodelling proteins. Several receptor kinases were found to have altered phosphorylation levels. To test whether potential isoprene receptors could be identified, we used molecular dynamics simulations to test for proteins that might have strong binding to isoprene and, therefore might act as receptors. Although many Arabidopsis proteins were found to have slightly higher binding affinities than a reference set of Homo sapiens proteins, no specific receptor kinase was found to have a very high binding affinity. The changes in chloroplast movement, photosynthesis capacity and so forth, found in this work, are consistent with isoprene responses being especially useful in the upper canopy of trees.

A Novel Isoprene Synthase from the Monocot Tree Copernicia prunifera (Arecaceae) Confers Enhanced Drought Tolerance in Transgenic Arabidopsis

The capacity to emit isoprene, among other stresses, protects plants from drought, but the molecular mechanisms underlying this trait are only partly understood. The Arecaceae (palms) constitute a very interesting model system to test the involvement of isoprene in enhancing drought tolerance, as their high isoprene emissions may have contributed to make them hyperdominant in neotropical dry forests, characterized by recurrent and extended periods of drought stress. In this study we isolated and functionally characterized a novel isoprene synthase, the gene responsible for isoprene biosynthesis, from Copernicia prunifera, a palm from seasonally dry tropical forests. When overexpressed in the non-emitter Arabidopsis thaliana, CprISPS conferred significant levels of isoprene emission, together with enhanced tolerance to water limitation throughout plant growth and development, from germination to maturity. CprISPS overexpressors displayed higher germination, cotyledon/leaf greening, water usage efficiency, and survival than WT Arabidopsis under various types of water limitation. This increased drought tolerance was accompanied by a marked transcriptional up-regulation of both ABA-dependent and ABA-independent key drought response genes. Taken together, these results demonstrate the capacity of CprISPS to enhance drought tolerance in Arabidopsis and suggest that isoprene emission could have evolved in Arecaceae as an adaptive mechanism against drought.

Automatization of metabolite extraction for high-throughput metabolomics: case study on transgenic isoprene-emitting birch

Metabolomics studies are becoming increasingly common for understanding how plant metabolism responds to changes in environmental conditions, genetic manipulations and treatments. Despite the recent advances in metabolomics workflow, the sample preparation process still limits the high-throughput analysis in large-scale studies. Here, we present a highly flexible robotic system that integrates liquid handling, sonication, centrifugation, solvent evaporation and sample transfer processed in 96-well plates to automatize the metabolite extraction from leaf samples. We transferred an established manual extraction protocol performed to a robotic system, and with this, we show the optimization steps required to improve reproducibility and obtain comparable results in terms of extraction efficiency and accuracy. We then tested the robotic system to analyze the metabolomes of wild-type and four transgenic silver birch (Betula pendula Roth) lines under unstressed conditions. Birch trees were engineered to overexpress the poplar (Populus × canescens) isoprene synthase and to emit various amounts of isoprene. By fitting the different isoprene emission capacities of the transgenic trees with their leaf metabolomes, we observed an isoprene-dependent upregulation of some flavonoids and other secondary metabolites as well as carbohydrates, amino acid and lipid metabolites. By contrast, the disaccharide sucrose was found to be strongly negatively correlated to isoprene emission. The presented study illustrates the power of integrating robotics to increase the sample throughput, reduce human errors and labor time, and to ensure a fully controlled, monitored and standardized sample preparation procedure. Due to its modular and flexible structure, the robotic system can be easily adapted to other extraction protocols for the analysis of various tissues or plant species to achieve high-throughput metabolomics in plant research.

Heterologous expression of Zygophyllum xanthoxylon zinc finger protein gene (ZxZF) enhances the tolerance of poplar photosynthetic function to drought stress

The ZxZF transcription factor (TF) of Zygophyllum xanthoxylon (Bunge) Maxim, an extremely drought-resistant woody plant, is a C₂H₂ zinc finger protein. Studies have shown that C₂H₂ zinc finger proteins play important roles in activating stress-related genes and enhancing plant resistance. However, their function in regulating plant photosynthesis under drought stress is not well understood. Since poplar is an important greening and afforestation tree species, it is particularly important to cultivate excellent drought-tolerant varieties. The ZxZF transcription factor (TF) was heterogeneously expressed in Euroamerican poplar (Populus × euroameracana cl.'Bofengl') by genetic transformation. Based on the mechanism and potential function of poplar photosynthesis regulated by ZxZF under drought stress, transcriptomic and physiological determinations were used to reveal the important role of this gene in improving the drought resistance of poplar. The results showed that the overexpression of ZxZF TF in transgenic poplars could improve the inhibition of Calvin cycle by regulating stomatal opening and increasing the concentration of intercellular CO₂. The chlorophyll content, photosynthetic performance index, and photochemical efficiency of transgenic lines under drought stress were significantly higher than those of the wild type (WT). The overexpression of ZxZF TFs could alleviate the degree of photoinhibition of photosystems II and I under drought stress and maintain the efficiency of light energy capture and the photosynthetic electron transport chain. The transcriptomic data also showed that differentially expressed genes between the transgenic poplar and WT under drought stress were primarily enriched in metabolic pathways related to photosynthesis, such as photosynthesis, photosynthesis-antenna protein, porphyrin and chlorophyll metabolism, and photosynthetic carbon fixation, and the downregulation of genes related to chlorophyll synthesis, photosynthetic electron transport and Calvin cycle were alleviated. In addition, the overexpression of ZxZF TF can alleviate the inhibition of NADH dehydrogenase-like (NDH) cyclic electron flow of the poplar NDH pathway under drought stress, which plays an important role in reducing the excess pressure of electrons on the photosynthetic electron transport chain and maintaining the normal photosynthetic electron transport. In summary, the overexpression of ZxZF TFs can effectively alleviate the inhibition of drought on the assimilation of carbon in poplar and have a positive impact on light energy capture, the orderly transport of photosynthetic electron transport chain and the integrity of the photosystem, which is highly significant to acheivean in-depth understanding of the function of ZxZF TFs. This also provides an important basis for the breeding of new transgenic poplar varieties.

Characterization of promoter elements of isoprene-responsive genes and the ability of isoprene to bind START domain transcription factors

Isoprene has recently been proposed to be a signaling molecule that can enhance tolerance of both biotic and abiotic stress. Not all plants make isoprene, but all plants tested to date respond to isoprene. We hypothesized that isoprene interacts with existing signaling pathways rather than requiring novel mechanisms for its effect on plants. We analyzed the cis-regulatory elements (CREs) in promoters of isoprene-responsive genes and the corresponding transcription factors binding these promoter elements to obtain clues about the transcription factors and other proteins involved in isoprene signaling. Promoter regions of isoprene-responsive genes were characterized using the Arabidopsis cis-regulatory element database. CREs bind ARR1, Dof, DPBF, bHLH112, GATA factors, GT-1, MYB, and WRKY transcription factors, and light-responsive elements were overrepresented in promoters of isoprene-responsive genes; CBF-, HSF-, WUS-binding motifs were underrepresented. Transcription factors corresponding to CREs overrepresented in promoters of isoprene-responsive genes were mainly those important for stress responses: drought-, salt/osmotic-, oxidative-, herbivory/wounding and pathogen-stress. More than half of the isoprene-responsive genes contained at least one binding site for TFs of the class IV (homeodomain leucine zipper) HD-ZIP family, such as GL2, ATML1, PDF2, HDG11, ATHB17. While the HD-zipper-loop-zipper (ZLZ) domain binds to the L1 box of the promoter region, a special domain called the steroidogenic acute regulatory protein-related lipid transfer, or START domain, can bind ligands such as fatty acids (e.g., linolenic and linoleic acid). We tested whether isoprene might bind in such a START domain. Molecular simulations and modeling to test interactions between isoprene and a class IV HD-ZIP family START-domain-containing protein were carried out. Without membrane penetration by the HDG11 START domain, isoprene within the lipid bilayer was inaccessible to this domain, preventing protein interactions with membrane bound isoprene. The cross-talk between isoprene-mediated signaling and other growth regulator and stress signaling pathways, in terms of common CREs and transcription factors could enhance the stability of the isoprene emission trait when it evolves in a plant but so far it has not been possible to say what how isoprene is sensed to initiate signaling responses.

Isoprene-Emitting Tobacco Plants Are Less Affected by Moderate Water Deficit under Future Climate Change Scenario and Show Adjustments of Stress-Related Proteins in Actual Climate

Isoprene-emitting plants are better protected against thermal and oxidative stresses, which is a desirable trait in a climate-changing (drier and warmer) world. Here we compared the ecophysiological performances of transgenic isoprene-emitting and wild-type non-emitting tobacco plants during water stress and after re-watering in actual environmental conditions (400 ppm of CO₂ and 28 °C of average daily temperature) and in a future climate scenario (600 ppm of CO₂ and 32 °C of average daily temperature). Furthermore, we intended to complement the present knowledge on the mechanisms involved in isoprene-induced resistance to water deficit stress by examining the proteome of transgenic isoprene-emitting and wild-type non-emitting tobacco plants during water stress and after re-watering in actual climate. Isoprene emitters maintained higher photosynthesis and electron transport rates under moderate stress in future climate conditions. However, physiological resistance to water stress in the isoprene-emitting plants was not as marked as expected in actual climate conditions, perhaps because the stress developed rapidly. In actual climate, isoprene emission capacity affected the tobacco proteomic profile, in particular by upregulating proteins associated with stress protection. Our results strengthen the hypothesis that isoprene biosynthesis is related to metabolic changes at the gene and protein levels involved in the activation of general stress defensive mechanisms of plants.

Effects of soil drought and nitrogen deposition on BVOC emissions and their O3 and SOA formation for Pinus thunbergii

Soil drought and nitrogen (N) deposition can influence the biogenic volatile organic compound (BVOC) emissions and thereby their ozone (O₃) and secondary organic aerosol (SOA) formation. This study addressed their single and combined effects on BVOC emissions of Pinus thunbergii by laboratory simulation experiments. The results showed that light drought (LD, 50% soil volumetric water content (VWC)) stimulated isoprene, monoterpene, sesquiterpene, and total BVOC emissions, while moderate drought (MD, 30% and 40% VWC) and severe drought (SD, 10% and 20% VWC) inhibited their emissions (except for sesquiterpene in 20% VWC). N deposition decreased other VOC emissions and increased isoprene and sesquiterpene emissions. Total BVOCs and monoterpene were stimulated in low N deposition (LN, 2 g N/(m²·yr)) and inhibited in moderate (MN, 5 g N/(m²·yr)) and high N deposition (HN, 10 g N/(m²·yr)). Under combined treatment of soil drought and N deposition, total BVOC, monoterpene, and other VOC emissions were inhibited, sesquiterpene had no significant change, and isoprene emission was inhibited in MD combined treatment but promoted in SD. The O₃ formation potential (OFP) and SOA formation potential (SOAP) from the changed BVOC emissions were calculated, OFP and SOAP of BVOC emissions and their compositions varied significantly among the treatments. Our study provided theoretical basis for assessing the impact of climate change and atmospheric pollution on BVOC emissions and their contribution to the formation of secondary atmospheric pollution.

Plant volatiles as regulators of hormone homeostasis

Some canonical plant hormones such as auxins and gibberellins have precursors that are biogenic volatiles (indole, indole acetonitrile, phenylacetaldoxime and ent-kaurene). Cytokinins, abscisic acid and strigolactones are hormones comprising chemical moieties that have distinct volatile analogues, and are synthesised alongside constitutively emitted volatiles (isoprene, sesquiterpenes, lactones, benzenoids and apocarotenoid volatiles). Nonvolatile hormone analogues and biogenic volatile organic compounds (BVOCs) evolved in tandem as growth and behavioural regulators in unicellular organisms. In plants, however, nonvolatile hormones evolved as regulators of growth, development and differentiation, while endogenous BVOCs (often synthesised lifelong) became subtle regulators of hormone synthesis, availability, activity and turnover, all supported by functionally redundant components of hormone metabolism. Reciprocal changes in the abundance and activity of hormones, nitric oxide, and constitutive plant volatiles constantly bridge retrograde and anterograde signalling to maintain hormone equilibria even in unstressed plants. This is distinct from transient interference in hormone signalling by stress-induced and exogenously received volatiles.

Transcriptome Meta-Analysis Associated Targeting Hub Genes and Pathways of Drought and Salt Stress Responses in Cotton (Gossypium hirsutum): A Network Biology Approach

Abiotic stress tolerance is an intricate feature controlled through several genes and networks in the plant system. In abiotic stress, salt, and drought are well known to limit cotton productivity. Transcriptomics meta-analysis has arisen as a robust method to unravel the stress-responsive molecular network in crops. In order to understand drought and salt stress tolerance mechanisms, a meta-analysis of transcriptome studies is crucial. To confront these issues, here, we have given details of genes and networks associated with significant differential expression in response to salt and drought stress. The key regulatory hub genes of drought and salt stress conditions have notable associations with functional drought and salt stress-responsive (DSSR) genes. In the network study, nodulation signaling pathways 2 (NSP2), Dehydration-responsive element1 D (DRE1D), ethylene response factor (ERF61), cycling DOF factor 1 (CDF1), and tubby like protein 3 (TLP3) genes in drought and tubby like protein 1 (TLP1), thaumatin-like proteins (TLP), ethylene-responsive transcription factor ERF109 (EF109), ETS-Related transcription Factor (ELF4), and Arabidopsis thaliana homeodomain leucine-zipper gene (ATHB7) genes in salt showed the significant putative functions and pathways related to providing tolerance against drought and salt stress conditions along with the significant expression values. These outcomes provide potential candidate genes for further in-depth functional studies in cotton, which could be useful for the selection of an improved genotype of Gossypium hirsutum against drought and salt stress conditions.

Isoprene Emission Influences the Proteomic Profile of Arabidopsis Plants under Well-Watered and Drought-Stress Conditions

Isoprene is a small lipophilic molecule synthesized in plastids and abundantly released into the atmosphere. Isoprene-emitting plants are better protected against abiotic stresses, but the mechanism of action of isoprene is still under debate. In this study, we compared the physiological responses and proteomic profiles of Arabidopsis which express the isoprene synthase (ISPS) gene and emit isoprene with those of non-emitting plants under both drought-stress (DS) and well-watered (WW) conditions. We aimed to investigate whether isoprene-emitting plants displayed a different proteomic profile that is consistent with the metabolic changes already reported. Only ISPS DS plants were able to maintain the same photosynthesis and fresh weight of WW plants. LC-MS/MS-based proteomic analysis revealed changes in protein abundance that were dependent on the capacity for emitting isoprene in addition to those caused by the DS. The majority of the proteins changed in response to the interaction between DS and isoprene emission. These include proteins that are associated with the activation of secondary metabolisms leading to ABA, trehalose, and proline accumulations. Overall, our proteomic data suggest that isoprene exerts its protective mechanism at different levels: under drought stress, isoprene affects the abundance of chloroplast proteins, confirming a strong direct or indirect antioxidant action and also modulates signaling and hormone pathways, especially those controlling ABA synthesis. Unexpectedly, isoprene also alters membrane trafficking.

Exposure to different light intensities affects emission of volatiles and accumulations of both pigments and phenolics in Azolla filiculoides

Many agronomic trials demonstrated the nitrogen-fixing ability of the ferns Azolla spp. and its obligate cyanobiont Trichormus azollae. In this study, we have screened the emission of volatile organic compounds (VOCs) and analyzed pigments (chlorophylls, carotenoids) as well as phenolic compounds in Azolla filiculoides-T. azollae symbionts exposed to different light intensities. Our results revealed VOC emission mainly comprising isoprene and methanol (~82% and ~13% of the overall blend, respectively). In particular, by dissecting VOC emission from A. filiculoides and T. azollae, we found that the cyanobacterium does not emit isoprene, whereas it relevantly contributes to the methanol flux. Enhanced isoprene emission capacity (15.95 ± 2.95 nmol m^-2  s^-1 ), along with increased content of both phenolic compounds and carotenoids, was measured in A. filiculoides grown for long-term under high (700 μmol m^-2  s^-1 ) rather than medium (400 μmol m^-2  s^-1 ) and low (100 μmol m^-2  s^-1 ) light intensity. Moreover, light-responses of chlorophyll fluorescence demonstrated that A. filiculoides was able to acclimate to high growth light. However, exposure of A. filiculoides from low (100 μmol m^-2  s^-1 ) to very high light (1000 μmol m^-2  s^-1 ) did not affect, in the short term, photosynthesis, but slightly decreased isoprene emission and leaf pigment content whereas, at the same time, dramatically raised the accumulation of phenolic compounds (i.e. deoxyanthocyanidins and phlobaphenes). Our results highlight a coordinated photoprotection mechanism consisting of isoprene emission and phenolic compounds accumulation employed by A. filiculoides to cope with increasing light intensities.

Sandbur Drought Tolerance Reflects Phenotypic Plasticity Based on the Accumulation of Sugars, Lipids, and Flavonoid Intermediates and the Scavenging of Reactive Oxygen Species in the Root

The perennial grass Cenchrus spinifex (common sandbur) is an invasive species that grows in arid and semi-arid regions due to its remarkable phenotypic plasticity, which confers the ability to withstand drought and other forms of abiotic stress. Exploring the molecular mechanisms of drought tolerance in common sandbur could lead to the development of new strategies for the protection of natural and agricultural environments from this weed. To determine the molecular basis of drought tolerance in C. spinifex, we used isobaric tags for relative and absolute quantitation (iTRAQ) to identify proteins differing in abundance between roots growing in normal soil and roots subjected to moderate or severe drought stress. The analysis of these proteins revealed that drought tolerance in C. spinifex primarily reflects the modulation of core physiological activities such as protein synthesis, transport and energy utilization as well as the accumulation of flavonoid intermediates and the scavenging of reactive oxygen species. Accordingly, plants subjected to drought stress accumulated sucrose, fatty acids, and ascorbate, shifted their redox potential (as determined by the NADH/NAD ratio), accumulated flavonoid intermediates at the expense of anthocyanins and lignin, and produced less actin, indicating fundamental reorganization of the cytoskeleton. Our results show that C. spinifex responds to drought stress by coordinating multiple metabolic pathways along with other adaptations. It is likely that the underlying metabolic plasticity of this species plays a key role in its invasive success, particularly in semi-arid and arid environments.

OsLSD1.1 is involved in the photosystem II reaction and affects nitrogen allocation in rice

Nitrogen (N) is an essential nutrient element for plants; however, high N accumulation often leads to a decrease in photosynthetic nitrogen use efficiency (PNUE). In rice (Oryza sativa L.), well-developed aerenchyma is formed to promote oxygen transport from the shoot to the root tips as an adaptation to submerged and oxygen-deficient environment. Total N concentrations were increased in the rice root by changes in O₂ levels in the rhizosphere. However, few reports have focused on how aerenchyma formation-related genes participate in photosynthesis and affect nitrogen allocation in rice. In this study, we found that OsLSD1.1, located in the chloroplast, cell membrane, and nucleus, may be involved in the photosystem II reaction and affect chloroplast development. OsLSD1.1 knockout was found to significantly reduce the quantum efficiency of the PSII reaction center (ΦPSII). Furthermore, we observed that the nitrogen accumulation decreased in the grain of OsLSD1.1 mutants. RNA-Seq transcriptome analysis revealed that OsPEPC3, OsPsbR1, OsNRG2, OsNRT1.5A, OsNRT1.7, and OsAMT3;2 were downregulated in m12 compared with N-WT (wild-type Nipponbare), which may be a reason that photosynthesis and nitrogen transport were inhibited. Taken together, our findings demonstrated that OsLSD1.1 may be key in plant growth, photosynthesis, and nitrogen allocation in rice. Our results may provide theoretical support for the discovery of key genes for nitrogen physiological use efficiency.

Isoprene: An Antioxidant Itself or a Molecule with Multiple Regulatory Functions in Plants?

Isoprene (C₅H₈) is a small lipophilic, volatile organic compound (VOC), synthesized in chloroplasts of plants through the photosynthesis-dependent 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway. Isoprene-emitting plants are better protected against thermal and oxidative stresses but only about 20% of the terrestrial plants are able to synthesize isoprene. Many studies have been performed to understand the still elusive isoprene protective mechanism. Isoprene reacts with, and quenches, many harmful reactive oxygen species (ROS) like singlet oxygen (¹O₂). A role for isoprene as antioxidant, made possible by its reduced state and conjugated double bonds, has been often suggested, and sometimes demonstrated. However, as isoprene is present at very low concentrations compared to other molecules, its antioxidant role is still controversial. Here we review updated evidences on the function(s) of isoprene, and outline contrasting indications on whether isoprene is an antioxidant directly scavenging ROS, or a membrane strengthener, or a modulator of genomic, proteomic and metabolomic profiles (perhaps as a secondary effect of ROS removal) eventually leading to priming of antioxidant plant defenses, or a signal of stress for neighbor plants alike other VOCs, or a hormone-like molecule, controlling the metabolic flux of other hormones made by the MEP pathway, or acting itself as a growth and development hormone.

Molecular Programming of Drought-Challenged Trichoderma harzianum-Bioprimed Rice (Oryza sativa L.)

Trichoderma biopriming enhances rice growth in drought-stressed soils by triggering various plant metabolic pathways related to antioxidative defense, secondary metabolites, and hormonal upregulation. In the present study, transcriptomic analysis of rice cultivar IR64 bioprimed with Trichoderma harzianum under drought stress was carried out in comparison with drought-stressed samples using next-generation sequencing techniques. Out of the 2,506 significant (p < 0.05) differentially expressed genes (DEGs), 337 (15%) were exclusively expressed in drought-stressed plants, 382 (15%) were expressed in T. harzianum-treated drought-stressed plants, and 1,787 (70%) were commonly expressed. Furthermore, comparative analysis of upregulated and downregulated genes under stressed conditions showed that 1,053 genes (42%) were upregulated and 733 genes (29%) were downregulated in T. harzianum-treated drought-stressed rice plants. The genes exclusively expressed in T. harzianum-treated drought-stressed plants were mostly photosynthetic and antioxidative such as plastocyanin, small chain of Rubisco, PSI subunit Q, PSII subunit PSBY, osmoproteins, proline-rich protein, aquaporins, stress-enhanced proteins, and chaperonins. The Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis states that the most enriched pathways were metabolic (38%) followed by pathways involved in the synthesis of secondary metabolites (25%), carbon metabolism (6%), phenyl propanoid (7%), and glutathione metabolism (3%). Some of the genes were selected for validation using real-time PCR which showed consistent expression as RNA-Seq data. Furthermore, to establish host-T. harzianum interaction, transcriptome analysis of Trichoderma was also carried out. The Gene Ontology (GO) analysis of T. harzianum transcriptome suggested that the annotated genes are functionally related to carbohydrate binding module, glycoside hydrolase, GMC oxidoreductase, and trehalase and were mainly upregulated, playing an important role in establishing the mycelia colonization of rice roots and its growth. Overall, it can be concluded that T. harzianum biopriming delays drought stress in rice cultivars by a multitude of molecular programming.

Mechanisms Regulating the Dynamics of Photosynthesis Under Abiotic Stresses

Photosynthesis sustains plant life on earth and is indispensable for plant growth and development. Factors such as unfavorable environmental conditions, stress regulatory networks, and plant biochemical processes limits the photosynthetic efficiency of plants and thereby threaten food security worldwide. Although numerous physiological approaches have been used to assess the performance of key photosynthetic components and their stress responses, though, these approaches are not extensive enough and do not favor strategic improvement of photosynthesis under abiotic stresses. The decline in photosynthetic capacity of plants due to these stresses is directly associated with reduction in yield. Therefore, a detailed information of the plant responses and better understanding of the photosynthetic machinery could help in developing new crop plants with higher yield even under stressed environments. Interestingly, cracking of signaling and metabolic pathways, identification of some key regulatory elements, characterization of potential genes, and phytohormone responses to abiotic factors have advanced our knowledge related to photosynthesis. However, our understanding of dynamic modulation of photosynthesis under dramatically fluctuating natural environments remains limited. Here, we provide a detailed overview of the research conducted on photosynthesis to date, and highlight the abiotic stress factors (heat, salinity, drought, high light, and heavy metal) that limit the performance of the photosynthetic machinery. Further, we reviewed the role of transcription factor genes and various enzymes involved in the process of photosynthesis under abiotic stresses. Finally, we discussed the recent progress in the field of biodegradable compounds, such as chitosan and humic acid, and the effect of melatonin (bio-stimulant) on photosynthetic activity. Based on our gathered researched data set, the logical concept of photosynthetic regulation under abiotic stresses along with improvement strategies will expand and surely accelerate the development of stress tolerance mechanisms, wider adaptability, higher survival rate, and yield potential of plant species.

Leaf isoprene emission as a trait that mediates the growth-defense tradeoff in the face of climate stress

Plant isoprene emissions are known to contribute to abiotic stress tolerance, especially during episodes of high temperature and drought, and during cellular oxidative stress. Recent studies have shown that genetic transformations to add or remove isoprene emissions cause a cascade of cellular modifications that include known signaling pathways, and interact to remodel adaptive growth-defense tradeoffs. The most compelling evidence for isoprene signaling is found in the shikimate and phenylpropanoid pathways, which produce salicylic acid, alkaloids, tannins, anthocyanins, flavonols and other flavonoids; all of which have roles in stress tolerance and plant defense. Isoprene also influences key gene expression patterns in the terpenoid biosynthetic pathways, and the jasmonic acid, gibberellic acid and cytokinin signaling networks that have important roles in controlling inducible defense responses and influencing plant growth and development, particularly following defoliation. In this synthesis paper, using past studies of transgenic poplar, tobacco and Arabidopsis, we present the evidence for isoprene acting as a metabolite that coordinates aspects of cellular signaling, resulting in enhanced chemical defense during periods of climate stress, while minimizing costs to growth. This perspective represents a major shift in our thinking away from direct effects of isoprene, for example, by changing membrane properties or quenching ROS, to indirect effects, through changes in gene expression and protein abundances. Recognition of isoprene's role in the growth-defense tradeoff provides new perspectives on evolution of the trait, its contribution to plant adaptation and resilience, and the ecological niches in which it is most effective.

Nucleoredoxin Gene TaNRX1 Positively Regulates Drought Tolerance in Transgenic Wheat (Triticum aestivum L.)

Drought is the main abiotic stress factor limiting the growth and yield of wheat (Triticum aestivum L.). Therefore, improving wheat tolerance to drought stress is essential for maintaining yield. Previous studies have reported on the important role of TaNRX1 in conferring drought stress tolerance. Therefore, to elucidate the regulation mechanism by which TaNRX1 confers drought resistance in wheat, we generated TaNRX1 overexpression (OE) and RNA interference (RNAi) wheat lines. The results showed that the tolerance of the OE lines to drought stress were significantly enhanced. The survival rate, leaf chlorophyll, proline, soluble sugar content, and activities of the antioxidant enzymes (catalase, superoxide dismutase, and peroxidase) of the OE lines were higher than those of the wild type (WT); however, the relative electrical conductivity and malondialdehyde, hydrogen peroxide, and superoxide anion levels of the OE lines were lower than those of the WT; the RNAi lines showed the opposite results. RNA-seq results showed that the common differentially expressed genes of TaNRX1 OE and RNAi lines, before and after drought stress, were mainly distributed in the plant-pathogen interaction, plant hormone signal transduction, phenylpropane biosynthesis, starch and sucrose metabolism, and carbon metabolism pathways and were related to the transcription factors, including WRKY, MYB, and bHLH families. This study suggests that TaNRX1 positively regulates drought stress tolerance in wheat.

Responses of isoprene emission and photochemical efficiency to severe drought combined with prolonged hot weather in hybrid Populus

Isoprene emissions have been considered as a protective response of plants to heat stress, but there is limited information of how prolonged heat spells affect isoprene emission capacity, particularly under the drought conditions that often accompany hot weather. Under combined long-term stresses, presence of isoprene emission could contribute to the maintenance of the precursor pool for rapid synthesis of essential isoprenoids to repair damaged components of leaf photosynthetic apparatus. We studied changes in leaf isoprene emission rate, photosynthetic characteristics, and antioxidant enzyme activities in two hybrid Populus clones, Nanlin 1388 (relatively high drought tolerance) and Nanlin 895 (relatively high thermotolerance) that were subjected to long-term (30 d) soil water stress (25% versus 90% soil field capacity) combined with a natural heat spell (day-time temperatures of 35-40 °C) that affected both control and water-stressed plants. Unexpectedly, isoprene emissions from both the clones were similar and the overall effects of drought on the emission characteristics were initially minor; however, treatment effects and clonal differences increased with time. In particular, the isoprene emission rate only increased slightly in the Nanlin 895 control plants after 15 d of treatment, whereas it decreased by more than 5-fold in all treatment × clone combinations after 30 d. The reduction in isoprene emission rate was associated with a decrease in the pool size of the isoprene precursor dimethylallyl diphosphate in all cases at 30 d after the start of treatment. Net assimilation rate, stomatal conductance, the openness of PSII centers, and the effective quantum yield all decreased, and non-photochemical quenching and catalase activity increased in both control and water-stressed plants. Contrary to the hypothesis of protection of leaf photosynthetic apparatus by isoprene, the data collectively indicated that prolonged stress affected isoprene emissions more strongly than leaf photosynthetic characteristics. This primarily reflected the depletion of isoprene precursor pools under long-term severe stress.

Genome-Wide Identification of Key Candidate microRNAs and Target Genes Associated with Peanut Drought Tolerance

Peanut is an important crash crop worldwide, and it is often threatened by drought stress due to unexpected extreme weather events. In this work, NH5 and FH18 were selected as drought-tolerant and drought-sensitive varieties, respectively. Comparison of their physiological responses revealed that NH5 showed less wilting, higher relative water content and lower water loss rate of detached leaves, lower electrolyte leakage, and stronger antioxidant ability under drought stress than did FH18. Based on comparative transcriptomic analysis, 5376 differentially expressed mRNAs were commonly identified in the two varieties, and 2993 genes specifically changed in the drought-tolerant variety and were mainly enriched in photosynthesis-antenna proteins and photosynthetic pathways. Furthermore, 73 microRNAs (miRNAs) were differentially expressed in the drought tolerance variety specifically under drought stress; of these, two key candidate miRNAs, novel miR_416 and novel miR_73, were identified, and the majority of their target genes were enriched in phenylpropanoid biosynthesis, linoleic acid metabolism, and cutin, suberine, and wax biosynthesis. This study lays the foundation for the analysis of the molecular mechanism of drought tolerance and promotes the genetic improvement of peanut drought tolerance.

Are Flavonoids Effective Antioxidants in Plants? Twenty Years of Our Investigation

Whether flavonoids play significant antioxidant roles in plants challenged by photooxidative stress of different origin has been largely debated over the last few decades. A critical review of the pertinent literature and our experimentation as well, based on a free-of-scale approach, support an important antioxidant function served by flavonoids in plants exposed to a wide range of environmental stressors, the significance of which increases with the severity of stress. On the other side, some questions need conclusive answers when the putative antioxidant functions of plant flavonoids are examined at the level of both the whole-cell and cellular organelles. This partly depends upon a conclusive, robust, and unbiased definition of "a plant antioxidant", which is still missing, and the need of considering the subcellular re-organization that occurs in plant cells in response to severe stress conditions. This likely makes our deterministic-based approach unsuitable to unveil the relevance of flavonoids as antioxidants in extremely complex biological systems, such as a plant cell exposed to an ever-changing stressful environment. This still poses open questions about how to measure the occurred antioxidant action of flavonoids. Our reasoning also evidences the need of contemporarily evaluating the changes in key primary and secondary components of the antioxidant defense network imposed by stress events of increasing severity to properly estimate the relevance of the antioxidant functions of flavonoids in an in planta situation. In turn, this calls for an in-depth analysis of the sub-cellular distribution of primary and secondary antioxidants to solve this still intricate matter.

Seed Priming: A Feasible Strategy to Enhance Drought Tolerance in Crop Plants

Drought is a serious threat to the farming community, biasing the crop productivity in arid and semi-arid regions of the world. Drought adversely affects seed germination, plant growth, and development via non-normal physiological processes. Plants generally acclimatize to drought stress through various tolerance mechanisms, but the changes in global climate and modern agricultural systems have further worsened the crop productivity. In order to increase the production and productivity, several strategies such as the breeding of tolerant varieties and exogenous application of growth regulators, osmoprotectants, and plant mineral nutrients are followed to mitigate the effects of drought stress. Nevertheless, the complex nature of drought stress makes these strategies ineffective in benefiting the farming community. Seed priming is an alternative, low-cost, and feasible technique, which can improve drought stress tolerance through enhanced and advanced seed germination. Primed seeds can retain the memory of previous stress and enable protection against oxidative stress through earlier activation of the cellular defense mechanism, reduced imbibition time, upsurge of germination promoters, and osmotic regulation. However, a better understanding of the metabolic events during the priming treatment is needed to use this technology in a more efficient way. Interestingly, the review highlights the morphological, physiological, biochemical, and molecular responses of seed priming for enhancing the drought tolerance in crop plants. Furthermore, the challenges and opportunities associated with various priming methods are also addressed side-by-side to enable the use of this simple and cost-efficient technique in a more efficient manner.

Changes in abscisic acid content during and after drought are related to carbohydrate mobilization and hydraulic recovery in poplar stems

Drought compromises plant's ability to replace transpired water vapor with water absorbed from the soil, leading to extensive xylem dysfunction and causing plant desiccation and death. Short-term plant responses to drought rely on stomatal closure, and on the plant's ability to recover hydraulic functioning after drought relief. We hypothesize a key role for abscisic acid (ABA) not only in the control of stomatal aperture, but also in hydraulic recovery. Young plants of Populus nigra L. were used to investigate possible relationships among ABA, non-structural carbohydrates (NSC) and xylem hydraulic function under drought and after re-watering. In Populus nigra L. plants subjected to drought, water transport efficiency and hydraulic recovery after re-watering were monitored by measuring the percentage loss of hydraulic conductivity (PLC) and stem specific hydraulic conductivity (Kstem). In the same plants ABA and NSC were quantified in wood and bark. Drought severely reduced stomatal conductance (gL) and markedly increased the PLC. Leaf and stem water potential, and stem hydraulic efficiency fully recovered within 24 h after re-watering, but gL values remained low. After re-watering, we found significant correlations between changes in ABA content and hexoses concentration both in wood and bark. Our findings suggest a role for ABA in the regulation of stem carbohydrate metabolism and starch mobilization upon drought relief, possibly promoting the restoration of xylem transport capacity.

Overexpression of Isoprene Synthase Affects ABA- and Drought-Related Gene Expression and Enhances Tolerance to Abiotic Stress

Isoprene is the most abundant single biogenic volatile compound emitted by plants. Despite the relevance of this molecule to plant abiotic resistance and its impact on global atmospheric chemistry, little is known about the details of its mechanism of action. Here, we characterized through both physiological and molecular methods the mechanisms of action of isoprene using model transgenic arabidopsis lines overexpressing a monocot isoprene synthase gene. Our results demonstrated the effect that isoprene had on ABA signaling at different tissue-specific, spatial, and temporal scales. In particular, we found that isoprene enhanced stomatal sensitivity to ABA through upregulation of RD29B signaling gene. By contrast, isoprene decreased sensitivity to ABA in germinating seeds and roots, suggesting tissue-specific mechanisms of action. In leaves, isoprene caused the downregulation of COR15A and P5CS genes, suggesting that the enhanced tolerance to water-deprivation stress observed in isoprene-emitting plants may be mediated chiefly by an enhanced membrane integrity and tolerance to osmotic stress.

High productivity in hybrid-poplar plantations without isoprene emission to the atmosphere

Hybrid-poplar tree plantations provide a source for biofuel and biomass, but they also increase forest isoprene emissions. The consequences of increased isoprene emissions include higher rates of tropospheric ozone production, increases in the lifetime of methane, and increases in atmospheric aerosol production, all of which affect the global energy budget and/or lead to the degradation of air quality. Using RNA interference (RNAi) to suppress isoprene emission, we show that this trait, which is thought to be required for the tolerance of abiotic stress, is not required for high rates of photosynthesis and woody biomass production in the agroforest plantation environment, even in areas with high levels of climatic stress. Biomass production over 4 y in plantations in Arizona and Oregon was similar among genetic lines that emitted or did not emit significant amounts of isoprene. Lines that had substantially reduced isoprene emission rates also showed decreases in flavonol pigments, which reduce oxidative damage during extremes of abiotic stress, a pattern that would be expected to amplify metabolic dysfunction in the absence of isoprene production in stress-prone climate regimes. However, compensatory increases in the expression of other proteomic components, especially those associated with the production of protective compounds, such as carotenoids and terpenoids, and the fact that most biomass is produced prior to the hottest and driest part of the growing season explain the observed pattern of high biomass production with low isoprene emission. Our results show that it is possible to reduce the deleterious influences of isoprene on the atmosphere, while sustaining woody biomass production in temperate agroforest plantations.

Morphological, physiochemical and antioxidant responses of Maclura pomifera to drought stress

Drought is one of the most important environmental factor limiting the growth of woody and non woody plants. In the present paper, we aimed to explore the performance of Maclura pomifera under a prolonged drought period followed by re-watering. M. pomifera plants were exposed to four different watering regimes (100%, 75%, 50% and 30% of the field capacity (FC)) for three weeks and then rewatered. The exposure to drought affected physiological, morphological and biochemical traits of M. pomifera. Leaf area, relative water content and water potential of leaf decreased in parallel with increased water deficit. Malondialdehyde content increased along with the drought stress experiment. Soluble carbohydrates (sucrose, glucose and fructose) accumulated during drought stress, but decreased after 22 days of water deficit in severe stressed plants (30% FC). Proline and mannitol, two compatible osmolytes, were higher in drought stresses plants than in control plants. Additionally the activity of antioxidant enzymes (SOD, APX, DHAR and GR) resulted affected by drought stress. In the recovery period, the physiological parameters as well as the proline content recovered at control levels, whereas soluble sugars, mannitol and total activity of antioxidant enzymes remained slight higher than in control plants, presumably to allow plants a complete recovery after stress. Our results suggest that M. pomifera has a good adaptive response to drought stress, probably corresponded to decreasing oxidative injury by induction of the antioxidant system and accumulation of stable and protective osmolytes such as proline and mannitol at higher rates.

Impact of Drought and Salinity on Sweetgum Tree (Liquidambar styraciflua L.): Understanding Tree Ecophysiological Responses in the Urban Context

Understanding urban tree responses to drought, salt stress, and co-occurring stresses, as well as the capability to recover afterward, is important to prevent the cited stresses’ negative effects on tree performance and ecological functionality. We investigated the impact of drought and salinity, alone and in combination, on leaf water potential, gas exchange, chlorophyll a fluorescence, xanthophyll cycle pigments, and isoprene emission of the urban tree species Liquidambar styraciflua L. Generally, drought had a rapid negative impact, while the effect of salt stress was more long lasting. Both stressors significantly decreased photosynthesis, transpiration, and stomatal conductance, as well as the maximum quantum efficiency of photosystem II (Fv/Fm) and the photochemical efficiency of PSII (ΦPSII), but increased nonphotochemical quenching (NPQ). Under stress conditions, a strong negative correlation between the PSII efficiency and the xanthophyll cycle pigment composition indicated a nocturnal retention of zeaxanthin and antheraxanthin in a state primed for energy dissipation. Drought and salt stress inhibited isoprene emission from leaves, although its emission was less responsive to stresses than stomatal conductance and photosynthesis. Full recovery of photosynthetic parameters took place after rewatering and washing off of excess salt, indicating that no permanent damage occurred, and suggesting downregulation rather than permanent impairment of the photosynthetic apparatus. Sweetgum trees were capable of withstanding and surviving moderate drought and salt events by activating defense mechanisms conferring tolerance to environmental stresses, without increasing the emission in the atmosphere of the highly reactive isoprene.

Dynamic changes in ABA content in water-stressed Populus nigra: effects on carbon fixation and soluble carbohydrates

BACKGROUND AND AIMS

Hydraulic and chemical signals operate in tandem to regulate systemic plant responses to drought. Transport of abscisic acid (ABA) through the xylem and phloem from the root to shoot has been suggested to serve as the main signal of water deficit. There is evidence that ABA and its ABA-glycosyl-ester (ABA-GE) are also formed in leaves and stems through the chloroplastic 2-C-methylerythritol-5-phosphate (MEP) pathway. This study aimed to evaluate how hormonal and hydraulic signals contribute to optimize stomatal (gs), mesophyll (gm) and leaf hydraulic (Kleaf) conductance under well-watered and water-stressed conditions in Populus nigra (black poplar) plants. In addition, we assessed possible relationships between ABA and soluble carbohydrates within the leaf and stem.

METHODS

Plants were subjected to three water treatments: well-watered (WW), moderate stress (WS1) and severe stress (WS2). This experimental set-up enabled a time-course analysis of the response to water deficit at the physiological [leaf gas exchange, plant water relations, (Kleaf)], biochemical (ABA and its metabolite/catabolite quantification in xylem sap, leaves, wood, bark and roots) and molecular (gene expression of ABA biosynthesis) levels.

KEY RESULTS

Our results showed strong coordination between gs, gm and Kleaf under water stress, which reduced transpiration and increased intrinsic water use efficiency (WUEint). Analysis of gene expression of 9-cis-epoxycarotenoid dioxygenase (NCED) and ABA content in different tissues showed a general up-regulation of the biosynthesis of this hormone and its finely-tuned catabolism in response to water stress. Significant linear relationships were found between soluble carbohydrates and ABA contents in both leaves and stems, suggesting a putative function for this hormone in carbohydrate mobilization under severe water stress.

CONCLUSIONS

This study demonstrates the tight regulation of the photosynthetic machinery by levels of ABA in different plants organs on a daily basis in both well-watered and water stress conditions to optimize WUEint and coordinate whole plant acclimation responses to drought.

Isoprene: New insights into the control of emission and mediation of stress tolerance by gene expression

Isoprene is a volatile compound produced in large amounts by some, but not all, plants by the enzyme isoprene synthase. Plants emit vast quantities of isoprene, with a net global output of 600 Tg per year, and typical emission rates from individual plants around 2% of net carbon assimilation. There is significant debate about whether global climate change resulting from increasing CO2 in the atmosphere will increase or decrease global isoprene emission in the future. We show evidence supporting predictions of increased isoprene emission in the future, but the effects could vary depending on the environment under consideration. For many years, isoprene was believed to have immediate, physical effects on plants such as changing membrane properties or quenching reactive oxygen species. Although observations sometimes supported these hypotheses, the effects were not always observed, and the reasons for the variability were not apparent. Although there may be some physical effects, recent studies show that isoprene has significant effects on gene expression, the proteome, and the metabolome of both emitting and nonemitting species. Consistent results are seen across species and specific treatment protocols. This review summarizes recent findings on the role and control of isoprene emission from plants.

Dissecting Adaptation Mechanisms to Contrasting Solar Irradiance in the Mediterranean Shrub Cistus incanus

Molecular mechanisms that are the base of the strategies adopted by Mediterranean plants to cope with the challenges imposed by limited or excessive solar radiation during the summer season have received limited attention. In our study, conducted on C. incanus plants growing in the shade or in full sunlight, we performed measurements of relevant physiological traits, such as leaf water potential, gas exchange and PSII photochemistry, RNA-Seq with de-novo assembly, and the analysis of differentially expressed genes. We also identified and quantified photosynthetic pigments, abscisic acid, and flavonoids. Here, we show major mechanisms regulating light perception and signaling which, in turn, sustain the shade avoidance syndrome displayed by the 'sun loving' C. incanus. We offer clear evidence of the detrimental effects of excessive light on both the assembly and the stability of PSII, and the activation of a suite of both repair and effective antioxidant mechanisms in sun-adapted leaves. For instance, our study supports the view of major antioxidant functions of zeaxanthin in sunny plants concomitantly challenged by severe drought stress. Finally, our study confirms the multiple functions served by flavonoids, both flavonols and flavanols, in the adaptive mechanisms of plants to the environmental pressures associated to Mediterranean climate.

Molecular regulatory mechanism of isoprene emission under short-term drought stress in the tropical tree Ficus septica

Isoprene is emitted by many plants and is thought to function as an antioxidant under stressful conditions. However, the detailed regulatory mechanism of isoprene emission in relation to the antioxidant system remains unclear. Therefore, in this study, we explored the molecular regulatory mechanism of isoprene emission under short-term drought stress in the tropical tree Ficus septica Burm.f. We found that the soil moisture content gradually decreased from 55% on Day 1 (D1) to 23% (wilting point) on D5 after withholding water for 4 days and then returning to the initial level following re-watering on D6. On D5, drought-stressed plants had more than twofold higher isoprene emission and 90.6% lower photosynthesis rates, 99.5% lower stomatal conductance and 82.3% lower transpiration rates than well-watered control plants. It was also estimated that the isoprene concentration inside the leaf greatly increased on D5 due to the increased isoprene emission rate and reduced stomatal conductance. Among the traits related to the 2-C-methyl-d-erythritol-4-phosphate (MEP) pathway, which is responsible for isoprene biosynthesis, the isoprene synthase (IspS) protein level was positively correlated with the isoprene emission rate in stressed plants. The transcripts of the antioxidant genes peroxidase 2 (POD2), POD4, copper-zinc superoxide dismutase 2 (Cu-ZnSOD2) and manganese superoxide dismutase 1 (Mn-SOD1) also increased during the drying period, while those of ascorbate peroxidase 1 (APX1) decreased. However, there was only a weak correlation between isoprene emission and antioxidant enzyme gene expression, indicating that the regulation of isoprene biosynthesis is not directly linked to the antioxidant defense network in drought-stressed F. septica. These findings suggest that the post-transcriptional regulation of IspS led to the observed change in isoprene emission rate, which enhanced the quenching of reactive oxygen species (ROS) and, in combination with the increased antioxidant enzyme activity, conferred tolerance to drought stress in this species.

Physiological and structural adjustments of two ecotypes of Platanus orientalis L. from different habitats in response to drought and re-watering

Platanus orientalis covers a very fragmented area in Europe and, at the edge of its natural distribution, is considered a relic endangered species near extinction. In our study, it was hypothesized that individuals from the edge of the habitat, with stronger climate constrains (drier and warmer environment, Italy, IT ecotype), developed different mechanisms of adaptation than those growing under optimal conditions at the center of the habitat (more humid and colder environment, Bulgaria, BG ecotype). Indeed, the two P. orientalis ecotypes displayed physiological, structural and functional differences already under control (unstressed) conditions. Adaptation to a dry environment stimulated constitutive isoprene emission, determined active stomatal behavior, and modified chloroplast ultrastructure, ultimately allowing more effective use of absorbed light energy for photochemistry. When exposed to short-term acute drought stress, IT plants showed active stomatal control that enhanced instantaneous water use efficiency, and stimulation of isoprene emission that sustained photochemistry and reduced oxidative damages to membranes, as compared to BG plants. None of the P. orientalis ecotypes recovered completely from drought stress after re-watering, confirming the sensitivity of this mesophyte to drought. Nevertheless, the IT ecotype showed less damage and better stability at the level of chloroplast membrane parameters when compared to the BG ecotype, which we interpret as possible adaptation to hostile environments and improved capacity to cope with future, likely more recurrent, drought stress.

Metabolic plasticity in the hygrophyte Moringa oleifera exposed to water stress

Over the past decades, introduction of many fast-growing hygrophilic, and economically valuable plants into xeric environments has occurred. However, production and even survival of these species may be threatened by harsh climatic conditions unless an effective physiological and metabolic plasticity is available. Moringa oleifera Lam., a multipurpose tree originating from humid sub-tropical regions of India, is widely cultivated in many arid countries because of its multiple uses. We tested whether M. oleifera can adjust primary and secondary metabolism to efficiently cope with increasing water stress. It is shown that M. oleifera possesses an effective isohydric behavior. Water stress induced a quick and strong stomatal closure, driven by abscisic acid (ABA) accumulation, and leading to photosynthesis inhibition with consequent negative effects on biomass production. However, photochemistry was not impaired and maximal fluorescence and saturating photosynthesis remained unaffected in stressed leaves. We report for the first time that M. oleifera produces isoprene, and show that isoprene emission increased three-fold during stress progression. It is proposed that higher isoprene biosynthesis helps leaves cope with water stress through its antioxidant or membrane stabilizing action, and also indicates a general MEP (methylerythritol 4-phosphate) pathway activation that further helps protect photosynthesis under water stress. Increased concentrations of antioxidant flavonoids were also observed in water stressed leaves, and probably cooperate in limiting irreversible effects of the stress in M. oleifera leaves. The observed metabolic and phenotypic plasticity may facilitate the establishment of M. oleifera in xeric environments, sustaining the economic and environmental value of this plant.

Biochemical characterization of isoprene synthase from Ipomoea batatas

The bio-production process of isoprene, an essential chemical used in industry, is strongly limited by isoprene synthase. In our previous work, relatively high isoprene production was observed with isoprene synthase from Ipomoea batatas (IspS_ib). In this work the biochemical properties of IspS_ib were analyzed and compared with those of isoprene synthase from Populus alba (IspS_pa) and other species. Firstly, IspS_ib and IspS_pa were expressed, purified, and identified by SDS-PAGE and western blot analysis. Secondly, pH and temperature dependence of IspS_ib were performed and an optimum pH of 8.6 and an optimum temperature of 42 °C were resulted. Mg²⁺ with optimum concentration of 56 mM was proved to be needed for enzyme activation. In addition, in vivo and in vitro study of the thermostabilities of IspS_ib and IspS_pa were performed. The enzyme activity of IspS_ib and IspS_pa dropped very rapidly after incubation at 30 °C; almost 80% enzyme activity of IspS_ib was lost after 20 min of incubation. Moreover, the Michaelis-Menten constant was measured. IspS_ib showed a lower K_m, 0.2 mM, and a higher k_cat, 0.37 s^-1, as compared with IspS_pa. The high catalytic efficiency, which was reflected by the high k_cat/K_m ratio, indicates that IspS_ib is a good candidate for the bio-isoprene production, while its thermal instability remains as a challenge. Enzyme engineering efforts, such as direction evolution or semi-rational evolution, are planned for further research.

Drought response of Mucuna pruriens (L.) DC. inoculated with ACC deaminase and IAA producing rhizobacteria

Drought is one of the major constraints limiting agricultural production worldwide and is expected to increase in the future. Limited water availability causes significant effects to plant growth and physiology. Plants have evolved different traits to mitigate the stress imposed by drought. The presence of plant growth-promoting rhizobacteria (PGPR) could play an important role in improving plant performances and productivity under drought. These beneficial microorganisms colonize the rhizosphere of plants and increase drought tolerance by lowering ethylene formation. In the present study, we demonstrate the potential to improve the growth of velvet bean under water deficit conditions of two different strains of PGPR with ACCd (1-Aminocyclopropane-1-Carboxylate deaminase) activity isolated from rainfed farming system. We compared uninoculated and inoculated plants with PGPR to assess: a) photosynthetic performance and biomass; b) ACC content and ethylene emission from leaves and roots; c) leaf isoprene emission. Our results provided evidence that under drought conditions inoculation with PGPR containing the ACCd enzyme could improve plant growth compared to untreated plants. Ethylene emission from roots and leaves of inoculated velvet bean plants was significantly lower than uninoculated plants. Moreover, isoprene emission increased with drought stress progression and was higher in inoculated plants compared to uninoculated counterparts. These findings clearly illustrate that selected PGPR strains isolated from rainfed areas could be highly effective in promoting plant growth under drought conditions by decreasing ACC and ethylene levels in plants.

Dissecting the role of isoprene and stress-related hormones (ABA and ethylene) in Populus nigra exposed to unequal root zone water stress

Isoprene is synthesized through the 2-C-methylerythritol-5-phosphate (MEP) pathway that also produces abscisic acid (ABA). Increases in foliar free ABA concentration during drought induce stomatal closure and may also alter ethylene biosynthesis. We hypothesized a role of isoprene biosynthesis in protecting plants challenged by increasing water deficit, by influencing ABA production and ethylene evolution. We performed a split-root experiment on Populus nigra L. subjected to three water treatments: well-watered (WW) plants with both root sectors kept at pot capacity, plants with both root compartments allowed to dry for 5 days (DD) and plants with one-half of the roots irrigated to pot capacity, while the other half did not receive water (WD). WD and WW plants were similar in photosynthesis, water relations, foliar ABA concentration and isoprene emission, whereas these parameters were significantly affected in DD plants: leaf isoprene emission increased despite the fact that photosynthesis declined by 85% and the ABA-glucoside/free ABA ratio decreased significantly. Enhanced isoprene biosynthesis in water-stressed poplars may have contributed to sustaining leaf ABA biosynthesis by keeping the MEP pathway active. However, this enhancement in ABA was accompanied by no change in ethylene biosynthesis, likely confirming the antagonistic role between ABA and ethylene. These results may indicate a potential cross-talk among isoprene, ABA and ethylene under drought.

In Planta Recapitulation of Isoprene Synthase Evolution from Ocimene Synthases

Isoprene is the most abundant biogenic volatile hydrocarbon compound naturally emitted by plants and plays a major role in atmospheric chemistry. It has been proposed that isoprene synthases (IspS) may readily evolve from other terpene synthases, but this hypothesis has not been experimentally investigated. We isolated and functionally validated in Arabidopsis the first isoprene synthase gene, AdoIspS, from a monocotyledonous species (Arundo donax L., Poaceae). Phylogenetic reconstruction indicates that AdoIspS and dicots isoprene synthases most likely originated by parallel evolution from TPS-b monoterpene synthases. Site-directed mutagenesis demonstrated invivo the functional and evolutionary relevance of the residues considered diagnostic for IspS function. One of these positions was identified by saturating mutagenesis as a major determinant of substrate specificity in AdoIspS able to cause invivo a dramatic change in total volatile emission from hemi- to monoterpenes and supporting evolution of isoprene synthases from ocimene synthases. The mechanism responsible for IspS neofunctionalization by active site size modulation by a single amino acid mutation demonstrated in this study might be general, as the very same amino acidic position is implicated in the parallel evolution of different short-chain terpene synthases from both angiosperms and gymnosperms. Based on these results, we present a model reconciling in a unified conceptual framework the apparently contrasting patterns previously observed for isoprene synthase evolution in plants. These results indicate that parallel evolution may be driven by relatively simple biophysical constraints, and illustrate the intimate molecular evolutionary links between the structural and functional bases of traits with global relevance.

Isoprene Responses and Functions in Plants Challenged by Environmental Pressures Associated to Climate Change

The functional reasons for isoprene emission are still a matter of hot debate. It was hypothesized that isoprene biosynthesis evolved as an ancestral mechanism in plants adapted to high water availability, to cope with transient and recurrent oxidative stresses during their water-to-land transition. There is a tight association between isoprene emission and species hygrophily, suggesting that isoprene emission may be a favorable trait to cope with occasional exposure to stresses in mesic environments. The suite of morpho-anatomical traits does not allow a conservative water use in hygrophilic mesophytes challenged by the environmental pressures imposed or exacerbated by drought and heat stress. There is evidence that in stressed plants the biosynthesis of isoprene is uncoupled from photosynthesis. Because the biosynthesis of isoprene is costly, the great investment of carbon and energy into isoprene must have relevant functional reasons. Isoprene is effective in preserving the integrity of thylakoid membranes, not only through direct interaction with their lipid acyl chains, but also by up-regulating proteins associated with photosynthetic complexes and enhancing the biosynthesis of relevant membrane components, such as mono- and di-galactosyl-diacyl glycerols and unsaturated fatty acids. Isoprene may additionally protect photosynthetic membranes by scavenging reactive oxygen species. Here we explore the mode of actions and the potential significance of isoprene in the response of hygrophilic plants when challenged by severe stress conditions associated to rapid climate change in temperate climates, with special emphasis to the concomitant effect of drought and heat. We suggest that isoprene emission may be not a good estimate for its biosynthesis and concentration in severely droughted leaves, being the internal concentration of isoprene the important trait for stress protection.

Modulating role of ROS in re-establishing desiccation tolerance in germinating seeds of Caragana korshinskii Kom

In close agreement with visible germination, orthodox seeds lose desiccation tolerance (DT). This trait can be regained under osmotic stress, but the mechanisms are poorly understood. In this study, germinating seeds of Caragana korshinskii Kom. were investigated, focusing on the potential modulating roles of reactive oxygen species (ROS) in the re-establishment of DT. Germinating seeds with 2 mm long radicles can be rendered tolerant to desiccation by incubation in a polyethylene glycol (PEG) solution (-1.7 MPa). Upon PEG incubation, ROS accumulation was detected in the radicles tip by nitroblue tetrazolium chloride staining and further confirmed by confocal microscopy. The PEG-induced re-establishment of DT was repressed when ROS scavengers were added to the PEG solution. Moreover, ROS act downstream of abscisic acid (ABA) to modulate PEG-mediated re-establishment of DT and serve as a new inducer to re-establish DT. Transcriptomic analysis revealed that re-establishment of DT by ROS involves the up-regulation of key genes in the phenylpropanoid-flavonoid pathway, and total flavonoid content and key enzyme activity increased after ROS treatment. Furthermore, DT was repressed by an inhibitor of phenylalanine ammonia lyase. Our data suggest that ROS play a key role in the re-establishment of DT by regulating stress-related genes and the phenylpropanoid-flavonoid pathway.

Phenotypic differences determine drought stress responses in ecotypes of Arundo donax adapted to different environments

Arundo donax has been identified as an important biomass and biofuel crop. Yet, there has been little research on photosynthetic and metabolic traits, which sustain the high productivity of A. donax under drought conditions. This study determined phenotypic differences between two A. donax ecotypes coming from stands with contrasting adaptation to dry climate. We hypothesized that the Bulgarian (BG) ecotype, adapted to drier conditions, exhibits greater drought tolerance than the Italian (IT) ecotype, adapted to a more mesic environment. Under well-watered conditions the BG ecotype was characterized by higher photosynthesis, mesophyll conductance, intrinsic water use efficiency, PSII efficiency, isoprene emission rate and carotenoids, whereas the IT ecotype showed higher levels of hydroxycinnamates. Photosynthesis of water-stressed plants was mainly limited by diffusional resistance to CO2 in BG, and by biochemistry in IT. Recovery of photosynthesis was more rapid and complete in BG than in IT, which may indicate better stability of the photosynthetic apparatus associated to enhanced induction of volatile and non-volatile isoprenoids and phenylpropanoid biosynthesis. This study shows that a large phenotypic plasticity among A. donax ecotypes exists, and may be exploited to compensate for the low genetic variability of this species when selecting plant productivity in constrained environments.

Physiological significance of isoprenoids and phenylpropanoids in drought response of Arundinoideae species with contrasting habitats and metabolism

Physiological, biochemical and morpho-anatomical traits that determine the phenotypic plasticity of plants under drought were tested in two Arundinoideae with contrasting habitats, growth traits and metabolism: the fast-growing Arundo donax, which also is a strong isoprene emitter, and the slow-growing Hakonechloa macra that does not invest on isoprene biosynthesis. In control conditions, A. donax displayed not only higher photosynthesis but also higher concentration of carotenoids and lower phenylpropanoid content than H. macra. In drought-stressed plants, photosynthesis was similarly inhibited in both species, but substantially recovered only in A. donax after rewatering. Decline of photochemical and biochemical parameters, increased concentration of CO2 inside leaves, and impairment of chloroplast ultrastructure were only observed in H. macra indicating damage of photosynthetic machinery under drought. It is suggested that volatile and non-volatile isoprenoids produced by A. donax efficiently preserve the chloroplasts from transient drought damage, while H. macra invests on phenylpropanoids that are less efficient in preserving photosynthesis but likely offer better antioxidant protection under prolonged stress.

De Novo Assembly and Comparative Transcriptome Analyses of Red and Green Morphs of Sweet Basil Grown in Full Sunlight

Sweet basil (Ocimum basilicum), one of the most popular cultivated herbs worldwide, displays a number of varieties differing in several characteristics, such as the color of the leaves. The development of a reference transcriptome for sweet basil, and the analysis of differentially expressed genes in acyanic and cyanic cultivars exposed to natural sunlight irradiance, has interest from horticultural and biological point of views. There is still great uncertainty about the significance of anthocyanins in photoprotection, and how green and red morphs may perform when exposed to photo-inhibitory light, a condition plants face on daily and seasonal basis. We sequenced the leaf transcriptome of the green-leaved Tigullio (TIG) and the purple-leaved Red Rubin (RR) exposed to full sunlight over a four-week experimental period. We assembled and annotated 111,007 transcripts. A total of 5,468 and 5,969 potential SSRs were identified in TIG and RR, respectively, out of which 66 were polymorphic in silico. Comparative analysis of the two transcriptomes showed 2,372 differentially expressed genes (DEGs) clustered in 222 enriched Gene ontology terms. Green and red basil mostly differed for transcripts abundance of genes involved in secondary metabolism. While the biosynthesis of waxes was up-regulated in red basil, the biosynthesis of flavonols and carotenoids was up-regulated in green basil. Data from our study provides a comprehensive transcriptome survey, gene sequence resources and microsatellites that can be used for further investigations in sweet basil. The analysis of DEGs and their functional classification also offers new insights on the functional role of anthocyanins in photoprotection.

Drought and flooding have distinct effects on herbivore-induced responses and resistance in Solanum dulcamara

In the field, biotic and abiotic stresses frequently co-occur. As a consequence, common molecular signalling pathways governing adaptive responses to individual stresses can interact, resulting in compromised phenotypes. How plant signalling pathways interact under combined stresses is poorly understood. To assess this, we studied the consequence of drought and soil flooding on resistance of Solanum dulcamara to Spodoptera exigua and their effects on hormonal and transcriptomic profiles. The results showed that S. exigua larvae performed less well on drought-stressed plants than on well-watered and flooded plants. Both drought and insect feeding increased abscisic acid and jasmonic acid (JA) levels, whereas flooding did not induce JA accumulation. RNA sequencing analyses corroborated this pattern: drought and herbivory induced many biological processes that were repressed by flooding. When applied in combination, drought and herbivory had an additive effect on specific processes involved in secondary metabolism and defence responses, including protease inhibitor activity. In conclusion, drought and flooding have distinct effects on herbivore-induced responses and resistance. Especially, the interaction between abscisic acid and JA signalling may be important to optimize plant responses to combined drought and insect herbivory, making drought-stressed plants more resistant to insects than well-watered and flooded plants.

Biochemical characterization of an isoprene synthase from Campylopus introflexus (heath star moss)

Each year, plants emit terragram quantities of the reactive hydrocarbon isoprene (2-methyl-1,3-butadiene) into the earth's atmosphere. In isoprene-emitting plants, the enzyme isoprene synthase (ISPS) catalyzes the production of isoprene from the isoprenoid intermediate dimethylallyl diphosphate (DMADP). While isoprene is emitted from all major classes of land plants, to date ISPSs from angiosperms only have been characterized. Here, we report the identification and initial biochemical characterization of a DMADP-dependent ISPS from the isoprene-emitting bryophyte Campylopus introflexus (heath star moss). The partially-purified C. introflexus ISPS (CiISPS) exhibited a Km for DMADP of 0.37 ± 0.28 mM, a pH optimum of 8.6 ± 0.5, and a temperature optimum of 40 ± 3 °C in vitro. Like ISPSs from angiosperms, the CiISPS required the presence of a divalent cation. However, unlike angiosperm ISPSs, the CiISPS utilized Mn(2+) preferentially over Mg(2+). Efforts are currently underway in our laboratory to further purify the CiISPS and clone the cDNA sequence encoding this novel enzyme. Our discovery of the first bryophyte ISPS paves the way for future studies concerning the evolutionary origins of isoprene emission in land plants and may help generate new bryophyte model systems for physiological and biochemical research on plant isoprene function.

Isoprenoids and phenylpropanoids are part of the antioxidant defense orchestrated daily by drought-stressed Platanus × acerifolia plants during Mediterranean summers

The hypothesis was tested that isoprenoids and phenylpropanoids play a prominent role in countering photooxidative stress, following the depletion of antioxidant enzyme activity in plants exposed to severe drought stress under high solar irradiance and high temperatures. Platanus × acerifolia, a high isoprene-emitting species, was drought-stressed during summer (WS) and compared with unstressed controls (WW). Water relations and photosynthetic parameters were measured under mild, moderate, and severe drought stress conditions. Volatile and nonvolatile isoprenoids, antioxidant enzymes, and phenylpropanoids were measured with the same time course, but in four different periods of the day. Drought severely inhibited photosynthesis, whereas it did not markedly affect the photochemical machinery. Isoprene emission and zeaxanthin concentration were higher in WS than in WW leaves, particularly at mild and moderate stresses, and during the hottest hours of the day. The activities of catalase and ascorbate peroxidase steeply declined during the day, while the activity of guaiacol peroxidase and the concentration of quercetin increased during the day, peaking in the hottest hours in both WW and WS plants. Our experiment reveals a sequence of antioxidants that were used daily by plants to orchestrate defense against oxidative stress induced by drought and associated high light and high temperature. Secondary metabolites seem valuable complements of antioxidant enzymes to counter oxidative stress during the hottest daily hours.