Heat stress increases the use of cytosolic pyruvate for isoprene biosynthesis

Drivers of biogenic volatile organic compound emissions in hygrophytic bryophytes

Bryophytes can both emit and take up biogenic volatile organic compounds (BVOCs) to and from the environment. Despite the scarce study of these exchanges, BVOCs have been shown to be important for a wide range of ecological roles. Bryophytes are the most ancient clade of land plants and preserve very similar traits to those first land colonisers. Therefore, the study of these plants can help understand the early processes of BVOC emissions as an adaptation to terrestrial life. Here, we determine the emission rates of BVOCs from different bryophyte species to understand what drives such emissions. We studied 26 bryophyte species from temperate regions that can be found in mountain springs located in NE Spain. Bryophyte BVOC emission presented no significant phylogenetic signal for any of the compounds analysed. Hence, we used mixed linear models to investigate the species-specific differences and eco-physiological and environmental drivers of bryophyte BVOC emission. In general, species-specific variability was the main factor explaining bryophyte BVOC emissions; but additionally, photosynthetic rates and light intensity increased BVOC emissions. Despite emission measurements reported here were conducted at 30°, and may not directly correspond to emission rates in natural conditions, most of the screened species have never been measured before for BVOC emissions and therefore this information can help understand the drivers of the emissions of BVOCs in bryophytes.

Optimal seasonal schedule for the production of isoprene, a highly volatile biogenic VOC

The leaves of many trees emit volatile organic compounds (abbreviated as BVOCs), which protect them from various damages, such as herbivory, pathogens, and heat stress. For example, isoprene is highly volatile and is known to enhance the resistance to heat stress. In this study, we analyze the optimal seasonal schedule for producing isoprene in leaves to mitigate damage. We assume that photosynthetic rate, heat stress, and the stress-suppressing effect of isoprene may vary throughout the season. We seek the seasonal schedule of isoprene production that maximizes the total net photosynthesis using Pontryagin's maximum principle. The isoprene production rate is determined by the changing balance between the cost and benefit of enhanced leaf protection over time. If heat stress peaks in midsummer, isoprene production can reach its highest levels during the summer. However, if a large portion of leaves is lost due to heat stress in a short period, the optimal schedule involves peaking isoprene production after the peak of heat stress. Both high photosynthetic rate and high isoprene volatility in midsummer make the peak of isoprene production in spring. These results can be clearly understood by distinguishing immediate impacts and the impacts of future expectations.

Leaf-level metabolic changes in response to drought affect daytime CO2 emission and isoprenoid synthesis pathways

In the near future, climate change will cause enhanced frequency and/or severity of droughts in terrestrial ecosystems, including tropical forests. Drought responses by tropical trees may affect their carbon use, including production of volatile organic compounds (VOCs), with implications for carbon cycling and atmospheric chemistry that are challenging to predict. It remains unclear how metabolic adjustments by mature tropical trees in response to drought will affect their carbon fluxes associated with daytime CO2 production and VOC emission. To address this gap, we used position-specific 13C-pyruvate labeling to investigate leaf CO2 and VOC fluxes from four tropical species before and during a controlled drought in the enclosed rainforest of Biosphere 2 (B2). Overall, plants that were more drought-sensitive had greater reductions in daytime CO2 production. Although daytime CO2 production was always dominated by non-mitochondrial processes, the relative contribution of CO2 from the tricarboxylic acid cycle tended to increase under drought. A notable exception was the legume tree Clitoria fairchildiana R.A. Howard, which had less anabolic CO2 production than the other species even under pre-drought conditions, perhaps due to more efficient refixation of CO2 and anaplerotic use for amino acid synthesis. The C. fairchildiana was also the only species to allocate detectable amounts of 13C label to VOCs and was a major source of VOCs in B2. In C. fairchildiana leaves, our data indicate that intermediates from the mevalonic acid (MVA) pathway are used to produce the volatile monoterpene trans-β-ocimene, but not isoprene. This apparent crosstalk between the MVA and methylerythritol phosphate pathways for monoterpene synthesis declined with drought. Finally, although trans-β-ocimene emissions increased under drought, it was increasingly sourced from stored intermediates and not de novo synthesis. Unique metabolic responses of legumes may play a disproportionate role in the overall changes in daytime CO2 and VOC fluxes in tropical forests experiencing drought.

Pachira aquatica (Malvaceae): An unconventional food plant with food, technological, and nutritional potential to be explored

Pachira aquatica (Malvaceae) is an unconventional food plant (UFP) native to Mexico and found all over Brazil, where it is commonly known as monguba. It has an arboreal shape, exotic flowers, and a fruit similar to cocoa with several seeds. Although its main application is in urban ornamentation and folk medicine, monguba's fruit has a great potential for use in the food, pharmacology, cosmetic, and bioenergy industry, mainly due to its oil's characteristics. This review aims to compile the nutritional composition, bioactive and antioxidant activities, and technological and nutritional potential of monguba's seed, leaf, and fruit pericarp. It reviews studies of different databases between January 2018 and October 2021. Monguba seeds are rich in lipids, proteins, and minerals; the bark is rich in fiber; and all parts of the fruit have bioactive compounds. Discussing the use of UFP is a way of finding new alternative food sources, usually discarded, offering products with high nutritional value allied to technological and consumption potential, such as the monguba fruit.

Isoprene in urban Atlantic forests: Variability, origin, and implications on the air quality of a subtropical megacity

Biosphere-atmosphere interactions play a key role in urban chemistry because of biogenic volatile organic compound (BVOC) emissions. Of the BVOC, isoprene is the most emitted compound; however, it also has anthropogenic origins in urban areas. In this study, we aimed to investigate the spatio-temporal variability and atmospheric impacts of biogenic and anthropogenic isoprene in the subtropical megacity of São Paulo (MASP), Brazil. Several measurement campaigns were conducted in three different urban Atlantic forests (Matão, PEFI, and RMG), and an urban background site (IAG); this equated to a total of 268 samples for the 2018-2019 period. For all sampling points, daytime average concentrations of isoprene were two to three times higher during the rainy season (IAG: 1.75 ± 0.93 ppb; Matão: 0.87 ± 0.35 ppb; PEFI: 0.50 ± 0.30 ppb; RMG: 0.37 ± 0.18 ppb), than those observed during the dry season (IAG: 0.46 ± 0.24 ppb; Matão: 0.31 ± 0.17 ppb; PEFI: 0.17 ± 0.11 ppb; RMG: 0.11 ± 0.07 ppb). Average isoprene concentrations were similar to those observed in other places worldwide, with the exception of the Amazon forest. Our results indicate differences in isoprene concentrations between sites, suggesting that environmental conditions such as the urban heat island and vegetation types, may play a role in spatial variability. Estimates of the isoprene fraction indicated that the biogenic fraction (85%) surpassed the anthropogenic fraction during the rainy season. By contrast, the anthropogenic fraction (52%) exceeded the biogenic fraction during dry periods. These fractions have an impact on potentially forming secondary pollutants gaseous (ozone formation potential: 7.19-33.32 μg m^-3), and aerosols (secondary organic aerosols formation potential: 0.41-1.88 μg m^-3). These results highlight the role of biogenic isoprene and its potential impact on urban air quality in subtropical megacities; this requires further investigation under future climate change scenarios.

Heat Stress Resistance Mechanisms of Two Cucumber Varieties from Different Regions

High temperatures affect the yield and quality of vegetable crops. Unlike thermosensitive plants, thermotolerant plants have excellent systems for withstanding heat stress. This study evaluated various heat resistance indexes of the thermotolerant cucumber (TT) and thermosensitive cucumber (TS) plants at the seedling stage. The similarities and differences between the regulatory genes were assessed through transcriptome analysis to understand the mechanisms for heat stress resistance in cucumber. The TT plants exhibited enhanced leaf status, photosystem, root viability, and ROS scavenging under high temperature compared to the TS plants. Additionally, transcriptome analysis showed that the genes involved in photosynthesis, the chlorophyll metabolism, and defense responses were upregulated in TT plants but downregulated in TS plants. Zeatin riboside (ZR), brassinosteroid (BR), and jasmonic acid (JA) levels were higher in TT plants than in TS. The heat stress increased gibberellic acid (GA) and indoleacetic acid (IAA) levels in both plant lines; however, the level of GA was higher in TT. Correlation and interaction analyses revealed that heat cucumber heat resistance is regulated by a few transcription factor family genes and metabolic pathways. Our study revealed different phenotypic and physiological mechanisms of the heat response by the thermotolerant and thermosensitive cucumber plants. The plants were also shown to exhibit different expression profiles and metabolic pathways. The heat resistant pathways and genes of two cucumber varieties were also identified. These results enhance our understanding of the molecular mechanisms of cucumber response to high-temperature stress.

Capturing the microbial volatilome: an oft overlooked 'ome'

Among the diverse metabolites produced by microbial communities, some are volatile. Volatile organic compounds (VOCs) are vigorously cycled by microbes as metabolic substrates and products and as signaling molecules. Yet, current microbial metabolomic studies predominantly focus on nonvolatile metabolites and overlook VOCs, which therefore represent a missing component of the metabolome. Advances in VOC detection now allow simultaneous observation of the numerous VOCs constituting the 'volatilome' of microbial systems. We present a roadmap for integrating and advancing VOC and other 'omics approaches and highlight the potential for realtime VOC measurements to help overcome limitations in discrete 'omics sampling. Including volatile metabolites in metabolomics, both conceptually and in practice, will build a more comprehensive understanding of microbial processes across ecological communities.

Drought affects carbon partitioning into volatile organic compound biosynthesis in Scots pine needles

The effect of drought on the interplay of processes controlling carbon partitioning into plant primary and secondary metabolisms, such as respiratory CO₂ release and volatile organic compound (VOC) biosynthesis, is not fully understood. To elucidate the effect of drought on the fate of cellular C sources into VOCs vs CO₂ , we conducted tracer experiments with ¹³ CO₂ and position-specific ¹³ C-labelled pyruvate, a key metabolite between primary and secondary metabolisms, in Scots pine seedlings. We determined the stable carbon isotope composition of leaf exchanged CO₂ and VOC. Drought reduced the emission of the sesquiterpenes α-farnesene and β-farnesene but did not affect ¹³ C-incorporation from ¹³ C-pyruvate. The labelling patterns suggest that farnesene biosynthesis partially depends on isopentenyl diphosphate crosstalk between chloroplasts and cytosol, and that drought inhibits this process. Contrary to sesquiterpenes, drought did not affect emission of isoprene, monoterpenes and some oxygenated compounds. During the day, pyruvate was used in the TCA cycle to a minor degree but was mainly consumed in pathways of secondary metabolism. Drought partly inhibited such pathways, while allocation into the TCA cycle increased. Drought caused a re-direction of pyruvate consuming pathways, which contributed to maintenance of isoprene and monoterpene production despite strongly inhibited photosynthesis. This underlines the importance of these volatiles for stress tolerance.

Metabolic exchange between pathways for isoprenoid synthesis and implications for biosynthetic hydrogen isotope fractionation

Hydrogen isotope ratios of plant lipids are used for paleoclimate reconstruction, but are influenced by both source water and biosynthetic processes. Measuring ² H : ¹ H ratios of multiple compounds produced by different pathways could allow these effects to be separated, but hydrogen isotope fractionations during isoprenoid biosynthesis remain poorly constrained. To investigate how hydrogen isotope fractionation during isoprenoid biosynthesis is influenced by molecular exchange between the cytosolic and plastidial production pathways, we paired position-specific ¹³ C-pyruvate labeling with hydrogen isotope measurements of lipids in Pachira aquatica saplings. We find that acetogenic compounds primarily incorporated carbon from ¹³ C2-pyruvate, whereas isoprenoids incorporated ¹³ C1- and ¹³ C2-pyruvate equally. This indicates that cytosolic pyruvate is primarily introduced into plastidial isoprenoids via glyceraldehyde 3-phosphate and that plastidial isoprenoid intermediates are incorporated into cytosolic isoprenoids. Probably as a result of the large differences in hydrogen isotope fractionation between plastidial and cytosolic isoprenoid pathways, sterols from P. aquatica are at least 50‰ less ² H-enriched relative to phytol than sterols in other plants. These results provide the first experimental evidence that incorporation of plastidial intermediates reduces ² H : ¹ H ratios of sterols. This suggests that relative offsets between the ² H : ¹ H ratios of sterols and phytol can trace exchange between the two isoprenoid synthesis pathways.

Heatwave frequency and seedling death alter stress-specific emissions of volatile organic compounds in Aleppo pine

Biogenic volatile organic compounds (BVOC) play important roles in plant stress responses and can serve as stress indicators. While the impacts of gradual environmental changes on BVOCs have been studied extensively, insights in emission responses to repeated stress and recovery are widely absent. Therefore, we studied the dynamics of shoot gas exchange and BVOC emissions in Pinus halepensis seedlings during an induced moderate drought, two four-day-long heatwaves, and the combination of drought and heatwaves. We found clear stress-specific responses of BVOC emissions. Reductions in acetone emissions with declining soil water content and transpiration stood out as a clear drought indicator. All other measured BVOC emissions responded exponentially to rising temperatures during heat stress (maximum of 43 °C), but monoterpenes and methyl salicylate showed a reduced temperature sensitivity during the second heatwave. We found that these decreases in monoterpene emissions between heatwaves were not reflected by similar declines in their internal storage pools. Because stress intensity was extremely severe, most of the seedlings in the heat-drought treatment died at the end of the second heatwave (dark respiration ceased). Interestingly, BVOC emissions (methanol, monoterpenes, methyl salicylate, and acetaldehyde) differed between dying and surviving seedlings, already well before indications of a reduced vitality became visible in gas exchange dynamics. In summary, we could clearly show that the dynamics of BVOC emissions are sensitive to stress type, stress frequency, and stress severity. Moreover, we found indications that stress-induced seedling mortality was preceded by altered methanol, monoterpene, and acetaldehyde emission dynamics.

Physiological responses of date palm (Phoenix dactylifera) seedlings to seawater and flooding

In their natural environment along coast lines, date palms are exposed to seawater inundation and, hence, combined stress by salinity and flooding. To elucidate the consequences of this combined stress on foliar gas exchange and metabolite abundances in leaves and roots, date palm seedlings were exposed to flooding with seawater and its major constituents under controlled conditions. Seawater flooding significantly reduced CO₂ assimilation, transpiration and stomatal conductance, but did not affect isoprene emission. A similar effect was observed upon NaCl exposure. By contrast, flooding with distilled water or MgSO₄ did not affect CO₂ /H₂ O gas exchange or stomatal conductance significantly, indicating that neither flooding itself, nor seawater sulfate, contributed greatly to stomatal closure. Seawater exposure increased Na and Cl contents in leaves and roots, but did not affect sulfate contents significantly. Metabolite analyses revealed reduced abundances of foliar compatible solutes, such as sugars and sugar alcohols, whereas nitrogen compounds accumulated in roots. Reduced transpiration upon seawater exposure may contribute to controlling the movement of toxic ions to leaves and, therefore, can be seen as a mechanism to cope with salinity. The present results indicate that date palm seedlings are tolerant towards seawater exposure to some extent, and highly tolerant to flooding.

Source of 12C in Calvin-Benson cycle intermediates and isoprene emitted from plant leaves fed with 13CO2

Feeding 14CO2 was crucial to uncovering the path of carbon in photosynthesis. Feeding 13CO2 to photosynthesizing leaves emitting isoprene has been used to develop hypotheses about the sources of carbon for the methylerythritol 4-phosphate pathway, which makes the precursors for terpene synthesis in chloroplasts and bacteria. Both photosynthesis and isoprene studies found that products label very quickly (<10 min) up to 80-90% but the last 10-20% of labeling requires hours indicating a source of 12C during photosynthesis and isoprene emission. Furthermore, studies with isoprene showed that the proportion of slow label could vary significantly. This was interpreted as a variable contribution of carbon from sources other than the Calvin-Benson cycle (CBC) feeding the methylerythritol 4-phosphate pathway. Here, we measured the degree of label in isoprene and photosynthetic metabolites 20 min after beginning to feed 13CO2. Isoprene labeling was the same as labeling of photosynthesis intermediates. High temperature reduced the label in isoprene and photosynthesis intermediates by the same amount indicating no role for alternative carbon sources for isoprene. A model assuming glucose, fructose, and/or sucrose reenters the CBC as ribulose 5-phosphate through a cytosolic shunt involving glucose 6-phosphate dehydrogenase was consistent with the observations.

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.

Heat Waves Change Plant Carbon Allocation Among Primary and Secondary Metabolism Altering CO2 Assimilation, Respiration, and VOC Emissions

Processes controlling plant carbon allocation among primary and secondary metabolism, i.e., carbon assimilation, respiration, and VOC synthesis are still poorly constrained, particularly regarding their response to stress. To investigate these processes, we simulated a 10-day 38°C heat wave, analysing real-time carbon allocation into primary and secondary metabolism in the Mediterranean shrub Halimium halimifolium L. We traced position-specific ¹³C-labeled pyruvate into daytime VOC and CO₂ emissions and during light-dark transition. Net CO₂ assimilation strongly declined under heat, due to three-fold higher respiration rates. Interestingly, day respiration also increased two-fold. Decarboxylation of the C1-atom of pyruvate was the main process driving daytime CO₂ release, whereas the C2-moiety was not decarboxylated in the TCA cycle. Heat induced high emissions of methanol, methyl acetate, acetaldehyde as well as mono- and sesquiterpenes, particularly during the first two days. After 10-days of heat a substantial proportion of ¹³C-labeled pyruvate was allocated into de novo synthesis of VOCs. Thus, during extreme heat waves high respiratory losses and reduced assimilation can shift plants into a negative carbon balance. Still, plants enhanced their investment into de novo VOC synthesis despite associated metabolic CO₂ losses. We conclude that heat stress re-directed the proportional flux of key metabolites into pathways of VOC biosynthesis most likely at the expense of reactions of plant primary metabolism, which might highlight their importance for stress protection.