Isabel Nogues

Increased Thermostability of Thylakoid Membranes in Isoprene-Emitting Leaves Probed with Three Biophysical Techniques

Three biophysical approaches were used to get insight into increased thermostability of thylakoid membranes in isoprene-emittingplants.Arabidopsis (Arabidopsis thaliana) plants genetically modified to make isoprene and Platanus orientalis leaves, in which isoprene emission was chemically inhibited, were used. First, in the circular dichroism spectrum the transition temperature of the main band at 694 nm was higher in the presence of isoprene, indicating that the heat stability of chiral macrodomains of chloroplast membranes, and specifically the stability of ordered arrays of light-harvesting complex II-photosystem II in the stacked region of the thylakoid grana, was improved in the presence of isoprene. Second, the decay of electrochromic absorbance changes resulting from the electric field component of the proton motive force (ΔA515) was evaluated following single-turnover saturating flashes. The decay of ΔA515 was faster in the absence of isoprene when leaves of Arabidopsis and Platanus were exposed to high temperature, indicating that isoprene protects the thylakoid membranes against leakiness at elevated temperature. Finally, thermoluminescence measurements revealed that S2QB− charge recombination was shifted to higher temperature in Arabidopsis and Platanus plants in the presence of isoprene, indicating higher activation energy for S2QB− redox pair, which enables isoprene-emitting plants to perform efficient primary photochemistry of photosystem II even at higher temperatures. The data provide biophysical evidence that isoprene improves the integrity and functionality of the thylakoid membranes at high temperature. These results contribute to our understanding of isoprene mechanism of action in plant protection against environmental stresses.

Dimethylallyl Diphosphate and Geranyl Diphosphate Pools of Plant Species Characterized by Different Isoprenoid Emissions

Dimethylallyl diphosphate (DMADP) and geranyl diphosphate (GDP) are the last precursors of isoprene and monoterpenes emitted by leaves, respectively. DMADP and GDP pools were measured in leaves of plants emitting isoprene (Populus alba), monoterpenes (Quercus ilex and Mentha piperita), or nonemitting isoprenoids (Prunus persica). Detectable pools were found in all plant species, but P. persica showed the lowest pool size, which indicates a limitation of the whole pathway leading to isoprenoid biosynthesis in nonemitting species. The pools of DMADP and GDP of nonemitting, isoprene-emitting, and monoterpene-emitting species were partially labeled (generally 40%–60% of total carbon-incorporated 13C) within the same time by which volatile isoprenoids are fully labeled (15 min). This indicates the coexistence of two pools for both precursors, the rapidly labeled pool presumably occurring in chloroplasts and thereby synthesized by the methylerythritol phosphate pathway and the nonlabeled pool presumably located in the cytosol and synthesized by the mevalonic pathway. In M. piperita storing monoterpenes in specialized leaf structures, the GDP pool remained totally unlabeled, indicating either that monoterpenes are totally formed by the mevalonic pathway or that labeling occurs slowly in comparison to the large pool of stored monoterpenes in this plant. The pools of DMADP and GDP increased during the season (from May to July) but decreased when the leaf was darkened or exposed to very high temperature. In the dark, the pool of DMADP of the isoprene-emitting species decreased faster than the pool of GDP. However, after 6 h of darkness, both pools were depleted to about 10% of the pool size in illuminated leaves. This indicates that both the chloroplastic and the cytosolic pools of precursors are depleted in the dark. When comparing measurements over the season and at different temperatures, an inverse correlation was observed between isoprene emission by P. alba and the DMADP pool size and between monoterpene emission by Q. ilex and the GDP pool size. This suggests that the pool size does not limit the emission of isoprenoids. Rather, it indicates that the flux of volatile isoprenoids effectively controls the size of their pools of precursors.

Physiological and antioxidant responses of Quercus ilex to drought in two different seasons

Climate change projections forecast a warming and an associated change in the distribution and intensity of rainfalls. In the case of the Mediterranean area, this will be reflected in more frequent and severe drought periods in the future. Within a long-term (9 years) manipulation experiment, we aimed to study the effect of the soil drought projected for the coming decades (an average of 10% soil moisture reduction) onto photosynthetic rates and water relations, and onto the antioxidant and anti-stress defense capacity of Quercus ilex, a dominant species in Mediterranean forests, in two different seasons, spring and summer. Results showed that photosynthesis was limited by stomatal closure in summer. However, a decrease in photosynthesis as a consequence of drought was observed only during spring, possibly due to a low pigment concentration and to an insufficient antioxidant protection. In summer, the increased resistance to CO2 entry reduced photosynthesis in control and drought-treated leaves, though the higher pigment content and antioxidant levels in summer leaves prevented a further decrease in photosynthesis as a consequence of drought. Also total monoterpene emission rates were higher in summer than in spring, though they did not change with drought, as happened with photosynthetic pigments. On the other hand, the antioxidant defense system was induced by drought in both studied seasons, indicating an efficient activation of defense responses aiming at scavenging reactive oxygen species produced in Q. ilex leaves under drought.

The Impact of Root Temperature on Photosynthesis and Isoprene Emission in Three Different Plant Species

Most of the perennial plant species, particularly trees, emit volatile organic compounds (BVOCs) such as isoprene and monoterpenes, which in several cases have been demonstrated to protect against thermal shock and more generally against oxidative stress. In this paper, we show the response of three strong isoprene emitter species, namely, Phragmites australis, Populus x euramericana, and Salix phylicifolia exposed to artificial or natural warming of the root system in different conditions. This aspect has not been investigated so far while it is well known that warming the air around a plant stimulates considerably isoprene emission, as also shown in this paper. In the green house experiments where the warming corresponded with high stress conditions, as confirmed by higher activities of the main antioxidant enzymes, we found that isoprene uncoupled from photosynthesis at a certain stage of the warming treatment and that even when photosynthesis approached to zero isoprene emission was still ongoing. In the field experiment, in a typical cold-limited environment, warming did not affect isoprene emission whereas it increased significantly CO2 assimilation. Our findings suggest that the increase of isoprene could be a good marker of heat stress, whereas the decrease of isoprene a good marker of accelerated foliar senescence, two hypotheses that should be better investigated in the future.

The impact of winter flooding with saline water on foliar carbon uptake and the volatile fraction of leaves and fruits of lemon (Citrus × limon) trees

We investigated the consequences of recurrent winter flooding with saline water on a lemon (Citrus×limon (L.) Burm.f.) orchard, focussing on photosynthesis limitations and emission of secondary metabolites (isoprenoids) from leaves and fruits. Measurements were carried out immediately after flooding (December), at the end of winter (April) and after a dry summer in which plants were irrigated with optimal quality water (September). Photosynthesis was negatively affected by flooding. The effect was still visible at the end of winter, whereas the photosynthetic rate was fully recovered after summer, indicating an unexpected resilience capacity of flooded plants. Photosynthesis inhibition by flooding was not due to diffusive limitations to CO2 entry into the leaf, as indicated by measurements of stomatal conductance and intercellular CO2 concentration. Biochemical and photochemical limitations seemed to play a more important role in limiting the photosynthesis of flooded plants. In young leaves, characterised by high rates of mitochondrial respiration, respiratory rates were enhanced by flooding. Flooding transiently caused large and rapid emission of several volatile isoprenoids. Emission of limonene, the most abundant compound, was stimulated in the leaves, and in young and mature fruits. Flooding changed the blend of emitted isoprenoids, but only few changes were observed in the stored isoprenoids pool.

Isoprene emission aids recovery of photosynthetic performance in transgenic Nicotiana tabacum following high intensity acute UV-B exposure.

Isoprene emission by terrestrial plants is believed to play a role in mitigating the effects of abiotic stress on photosynthesis. Ultraviolet-B light (UV-B) induces damage to the photosynthetic apparatus of plants, but the role of isoprene in UV-B tolerance is poorly understood. To investigate this putative protective role, we exposed non-emitting (NE) control and transgenic isoprene emitting (IE) Nicotiana tabacum (tobacco) plants to high intensity UV-B exposure. Methanol emissions increased with UV-B intensity, indicating oxidative damage. However, isoprene emission was unaffected during exposure to UV-B radiation, but declined in the 48 h following UV-B treatment at the highest UV-B intensities of 9 and 15 Wm(-2). Photosynthesis and the performance of photosystem II (PSII) declined to similar extents in IE and NE plants following UV-B exposure, suggesting that isoprene emission did not ameliorate the immediate impact of UV-B on photosynthesis. However, after the stress, photosynthesis and PSII recovered in IE plants, which maintained isoprene formation, but not in NE plants. Recovery of IE plants was also associated with elevated antioxidant levels and cycling; suggesting that both isoprene formation and antioxidant systems contributed to reinstating the integrity and functionality of cellular membranes and photosynthesis following exposure to excessive levels of UV-B radiation.

Ozone Effects on the Metabolism and the Antioxidant System of Poplar Leaves at Different Stages of Development

During Leaf Development Significant Metabolic Changes Occur. Leaf Development In Its First Stages Has Been Associated With An Increase In Photosynthetic Activity And In The Oxidative Damage, Which May Be Exacerbated By Exposure To Ozone. In This Work, Poplar Saplings Were Subjected To A High But Realistic Ozone Level (100 Ppb) For 10 Days. H2O2 And Lipid Peroxidation Levels, Photosynthesis And Photosynthetic Metabolites (Glucose, Pyruvate), Antioxidants (Phenolics, Ascorbic Acid) And Some Antioxidant Enzyme Activities Were Measured In Control And Ozone-Treated Poplar Leaves At Two Stages Of Development. Young Control Leaves Showed A Lower Photosynthesis And A Higher Concentration Of All Studied Metabolites Than Mature Control Leaves. The Latter Finding Supports The Theory That Developing, More Vulnerable, Leaves Need More Chemical Defence. However, Developing Ozone-Treated Leaves Presented Higher Photosynthesis And Lower Levels Of Metabolites Than Developing Control Leaves. This May Indicate A Faster Development Of Ozone Treated Leaves. In Fully Expanded (Mature) Leaves, As A Response To Ozone, The Activities Of Antioxidant Enzymes (Ascorbate Peroxidase And Catalase), And The Concentration Of Phenolics Increased, Whereas Ascorbate Concentration Did Not Change. This May Indicate That Both Induction Of Phenolics And The Action Of The Ascorbate-Glutathione Cycle Play A Significant Role In Protecting Fully Developed Poplar Leaves Against Ozone. In The Last Case, The Higher Activity Seems To Be Controlled By The Enzyme Activities And Not By The Concentration Of Ascorbic Acid.

Regulation of Isoprene and Monoterpene Emission

Isoprene and monoterpenes are synthesized and emitted into the atmosphere by many plant species, constitutively and/or after induction by environmental changes. Volatile isoprenoids are involved in defence against biotic and abiotic stresses. It is known that changes of the emission of volatile isoprenoids can be explained by the regulation of the activity of the corresponding synthases (isoprene or monoterpene synthases) or by substrate availability, but there are still gaps in the understanding of regulatory mechanisms controlling the emissions. Short-term variations in isoprene and monoterpene emissions depend basically on light and temperature, but control of monoterpene emission from plants that do not store terpenes is different from that of plants having specialized structures for their storage. Long-term and seasonal variations, however, are explained by developmental processes and by environmental factors, such as temperature and water stress.