Photosynthesis Research (v.104, #1)

Introduction by Jean-Pierre Jacquot; Nicolas Rouhier (1-3).

Isoprene emission protects photosynthesis in sunfleck exposed Grey poplar by Katja Behnke; Maaria Loivamäki; Ina Zimmer; Heinz Rennenberg; Jörg-Peter Schnitzler; Sandrine Louis (5-17).
In the present study, we combined transient temperature and light stress (sunfleck) and comparably analyzed photosynthetic gas exchange in Grey poplar which has been genetically modified in isoprene emission capacity. Overall, we demonstrate that for poplar leaves the ability to emit isoprene is crucial to maintain photosynthesis when exposed to sunflecks. Net CO2 assimilation and electron transport rates were strongly impaired in sunfleck-treated non-isoprene emitting poplars. Similar impairment was not detected when the leaves were exposed to high light (lightflecks) only. Within 10 h non-isoprene emitting poplars recovered from sunfleck-related impairment as indicated by chlorophyll fluorescence and microarray analysis. Unstressed leaves of non-isoprene emitting poplars had higher ascorbate contents, but also higher contents of malondialdehyde than wild-type. Microarray analyses revealed lipid and chlorophyll degradation processes in the non-isoprene emitting poplars. Thus, there is evidence for an adjustment of the antioxidative system in the non-isoprene emitting poplars even under normal growth conditions.
Keywords: Isoprene; Poplar; Sunfleck; Thermal stress; Microarray; Gas exchange

To examine the role of acclimation versus adaptation on the temperature responses of CO2 assimilation, we measured dark respiration (R n) and the CO2 response of net photosynthesis (A) in Populus balsamifera collected from warm and cool habitats and grown at warm and cool temperatures. R n and the rate of photosynthetic electron transport (J) are significantly higher in plants grown at 19 versus 27°C; R n is not affected by the native thermal habitat. By contrast, both the maximum capacity of rubisco (V cmax) and A are relatively insensitive to growth temperature, but both parameters are slightly higher in plants from cool habitats. A is limited by rubisco capacity from 17–37°C regardless of growth temperature, and there is little evidence for an electron-transport limitation. Stomatal conductance (g s) is higher in warm-grown plants, but declines with increasing measurement temperature from 17 to 37°C, regardless of growth temperature. The mesophyll conductance (g m) is relatively temperature insensitive below 25°C, but g m declines at 37°C in cool-grown plants. Plants acclimated to cool temperatures have increased R n/A, but this response does not differ between warm- and cool-adapted populations. Primary carbon metabolism clearly acclimates to growth temperature in P. balsamifera, but the ecotypic differences in A suggest that global warming scenarios might affect populations at the northern and southern edges of the boreal forest in different ways.
Keywords: Acclimation; Gas exchange; Homeostasis; Mesophyll conductance

Effects of flooding on leaf development, transpiration, and photosynthesis in narrowleaf cottonwood, a willow-like poplar by Stewart B. Rood; Julie L. Nielsen; Leslee Shenton; Karen M. Gill; Matthew G. Letts (31-39).
The narrowleaf cottonwood, Populus angustifolia, occurs in occasionally flooded, low elevation zones along river valleys near the North American Rocky Mountains. This small poplar has narrow leaves and fine branching and thus resembles willows, which are commonly flood-tolerant. We investigated the flood response of narrowleaf cottonwoods and a related native hybrid, jackii cottonwood (P. × jackii = P. balsamifera × P. deltoides), by studying saplings of 24 clones in a greenhouse, with some pots being inundated to provide the flood treatment. Flooding slightly reduced leaf numbers (−10%), and leaf sizes were reduced by about 21% in female P. angustifolia versus a 50% reduction in the female hybrids. Flooding-reduced stomatal conductance and net photosynthetic rate, and reduced transpiration particularly in P. × jackii. The effects on foliar gas exchange declined over a 5-week interval, suggesting compensation. The moderate impact of flooding supports the hypothesis that narrowleaf cottonwoods are flood-tolerant, and we anticipate that these trees could provide traits to increase the flood tolerance of fast-growing hybrid poplars. The results further indicate that female cottonwoods may be more flood-tolerant than males, and females could be more successful in lower, flood-prone sites.
Keywords: Flood tolerance; Hybrid poplar; Photosynthesis; Populus angustifolia ; Populus × jackii ; Riparian; Stress tolerance; Transpiration

Photosynthetic and respiratory changes in leaves of poplar elicited by rust infection by Ian T. Major; Marie-Claude Nicole; Sébastien Duplessis; Armand Séguin (41-48).
Poplars are challenged by a wide range of pathogens during their lifespan, and have an innate immunity system that activates defence responses to restrict pathogen growth. Large-scale expression studies of poplar–rust interactions have shown concerted transcriptional changes during defence responses, as in other plant pathosystems. Detailed analysis of expression profiles of metabolic pathways in these studies indicates that photosynthesis and respiration are also important components of the poplar response to rust infection. This is consistent with our current understanding of plant pathogen interactions as defence responses impose substantive demands for resources and energy that are met by reorganization of primary metabolism. This review applies the results of poplar transcriptome analyses to current research describing how plants divert energy from plant primary metabolism for resistance mechanisms.
Keywords: Melampsora spp.; Defense response; Resource mobilization; Transcriptome

Effects of fosmidomycin on plant photosynthesis as measured by gas exchange and chlorophyll fluorescence by Malcolm Possell; Annette Ryan; Claudia E. Vickers; Philip M. Mullineaux; C. Nicholas Hewitt (49-59).
In higher plants, many isoprenoids are synthesised via the chloroplastic 1-deoxy-d-xylulose 5-phosphate/2-C-methyl-d-erythritol 4-phosphate (MEP) pathway. Attempts to elucidate the function of individual isoprenoids have used the antibiotic/herbicidal compound fosmidomycin (3-[N-formyl-N-hydroxy amino] propyl phosphonic acid) to inhibit this pathway. Examination of the effect of fosmidomycin on the major components of photosynthesis in leaves of white poplar (Populus alba) and tobacco (Nicotiana tabacum) was made. Fosmidomycin reduced net photosynthesis in both species within 1 h of application, but only when photosynthesis was light-saturated. In P. alba, these reductions were confounded by high light and fosmidomycin inducing stomatal patchiness. In tobacco, this was caused by significant reductions in PSII chlorophyll fluorescence and reductions in V cmax and J max. Our data indicate that the diminution of photosynthesis is likely a complex effect resulting from the inhibition of multiple MEP pathway products, resulting in photoinhibition and photo-damage. These effects should be accounted for in experimental design and analysis when using fosmidomycin to avoid misinterpretation of results as measured by gas exchange and chlorophyll fluorescence.
Keywords: Fosmidomycin; Photosynthesis; Populus alba ; Nicotiana tabacum ; Isoprenoids; Isoprene

Volatile emissions and phenolic compound concentrations along a vertical profile of Populus nigra leaves exposed to realistic ozone concentrations by Silvano Fares; Elina Oksanen; Mika Lännenpää; Riitta Julkunen-Tiitto; Francesco Loreto (61-74).
Plants are exposed to increasing levels of tropospheric ozone concentrations. This pollutant penetrates in leaves through stomata and quickly reacts inside leaves, thus making plants valuable ozone sinks, but at the same time triggers oxidation processes which lead to leaf injuries. To counteract these negative effects, plants produce an array of antioxidants which react with ozone and reactive molecules which ozone generates in the leaf tissues. In this study, we measured the effect of an ozone concentration which is likely to be attained in many areas of the world in the near future (80 ppb) on leaves of the vertical profile of the widespread agroforestry species Populus nigra. Changes in (1) physiological parameters (photosynthesis and stomatal conductance), (2) ozone uptake, (3) emission of volatile organic compounds (VOCs, i.e. isoprene, methanol and other oxygenated compounds), (4) concentration of antioxidant surface compounds, and (5) concentration of phenolic compounds were assessed. The aim was to assess whether the defensive pathways leading to isoprenoids and phenolics formation were induced when a moderate and chronic increment of ozone is not able to damage photosynthesis. No visual injuries and minor changes in physiology and ozone uptake were observed. The emission of isoprene and oxygenated six-carbon (C6) volatiles were inhibited by ozone, whereas methanol emission was increased, especially in developing leaves. We interpret these results as suggesting an ontogenetic shift in ozone-treated leaves, leading to a slower development and a faster senescence. Most surface and phenolic compounds showed a declining trend in concentration from the youngest to the fully expanded leaves. Ozone reduced the concentrations of chlorogenic acid derivatives at the leaf surface, whereas in total leaf extracts a metabolic shift towards few phenolics with higher antioxidant capacity was observed.
Keywords: Populus nigra ; Ozone uptake; Volatile organic compounds; Surface compounds; Phenolic compounds

The post-translational modification consisting in the formation/reduction of disulfide bonds has been the subject of intense research in plants since the discovery in the 1970s that many chloroplastic enzymes are regulated by light through dithiol–disulfide exchange reactions catalyzed by oxidoreductases called thioredoxins (Trxs). Further biochemical and proteomic studies have considerably increased the number of target enzymes and processes regulated by these mechanisms in many sub-cellular compartments. Recently, glutathionylation, a modification consisting in the reversible formation of a glutathione adduct on cysteine residues, was proposed as an alternative redox regulation mechanism. Glutaredoxins (Grxs), proteins related to Trxs, are efficient catalysts for deglutathionylation, the opposite reaction. Hence, the Trxs- and Grxs-dependent pathways might constitute complementary and not only redundant regulatory processes. This article focuses on these two multigenic families and associated protein partners in poplar and on their involvement in the regulation of some major chloroplastic processes such as stress response, carbohydrate and heme/chlorophyll metabolism.
Keywords: Chloroplast; Glutaredoxin; Photosynthesis; Stress; Thioredoxin