Catalogue Search | MBRL
Are you sure you want to remove the book from the shelf?
{{itemTitle}}
836
result(s) for
"Kramer, David M."
Sort by:
Nickelodeon pandemonium! 1, Channeling fun
Collects eleven stories featuring the stars of Nickelodeon, including Sanjay and Craig, Pig, Boat, Banana, and Cricket, and Fee and Foo.
Book
Rewiring of jasmonate and phytochrome B signalling uncouples plant growth-defense tradeoffs
Plants resist infection and herbivory with innate immune responses that are often associated with reduced growth. Despite the importance of growth-defense tradeoffs in shaping plant productivity in natural and agricultural ecosystems, the molecular mechanisms that link growth and immunity are poorly understood. Here, we demonstrate that growth-defense tradeoffs mediated by the hormone jasmonate are uncoupled in an
Arabidopsis
mutant (
jazQ phyB
) lacking a quintet of Jasmonate ZIM-domain transcriptional repressors and the photoreceptor phyB. Analysis of epistatic interactions between
jazQ
and
phyB
reveal that growth inhibition associated with enhanced anti-insect resistance is likely not caused by diversion of photoassimilates from growth to defense but rather by a conserved transcriptional network that is hardwired to attenuate growth upon activation of jasmonate signalling. The ability to unlock growth-defense tradeoffs through relief of transcription repression provides an approach to assemble functional plant traits in new and potentially useful ways.
Plant immune responses are often associated with reduced growth. Here, the authors show that combining mutations in transcriptional repressors of the defense and light perception pathways can confer both robust growth and strong herbivore defense, demonstrating that growth-defense tradeoffs can be uncoupled.
Journal Article
Comparing Photosynthetic and Photovoltaic Efficiencies and Recognizing the Potential for Improvement
Comparing photosynthetic and photovoltaic efficiencies is not a simple issue. Although both processes harvest the energy in sunlight, they operate in distinctly different ways and produce different types of products: biomass or chemical fuels in the case of natural photosynthesis and nonstored electrical current in the case of photovoltaics. In order to find common ground for evaluating energy-conversion efficiency, we compare natural photosynthesis with present technologies for photovoltaic-driven electrolysis of water to produce hydrogen. Photovoltaic-driven electrolysis is the more efficient process when measured on an annual basis, yet short-term yields for photosynthetic conversion under optimal conditions come within a factor of 2 or 3 of the photovoltaic benchmark. We consider opportunities in which the frontiers of synthetic biology might be used to enhance natural photosynthesis for improved solar energy conversion efficiency.
Journal Article
Improved photosynthetic capacity and photosystem I oxidation via heterologous metabolism engineering in cyanobacteria
Cyanobacteria must prevent imbalances between absorbed light energy (source) and the metabolic capacity (sink) to utilize it to protect their photosynthetic apparatus against damage. A number of photoprotective mechanisms assist in dissipating excess absorbed energy, including respiratory terminal oxidases and flavodiiron proteins, but inherently reduce photosynthetic efficiency. Recently, it has been hypothesized that some engineered metabolic pathways may improve photosynthetic performance by correcting source/sink imbalances. In the context of this subject, we explored the interconnectivity between endogenous electron valves, and the activation of one or more heterologous metabolic sinks. We coexpressed two heterologous metabolic pathways that have been previously shown to positively impact photosynthetic activity in cyanobacteria, a sucrose production pathway (consuming ATP and reductant) and a reductant-only consuming cytochrome P450. Sucrose export was associated with improved quantum yield of phtotosystem II (PSII) and enhanced electron transport chain flux, especially at lower illumination levels, while cytochrome P450 activity led to photosynthetic enhancements primarily observed under high light. Moreover, coexpression of these two heterologous sinks showed additive impacts on photosynthesis, indicating that neither sink alone was capable of utilizing the full “overcapacity” of the electron transport chain. We find that heterologous sinks may partially compensate for the loss of photosystem I (PSI) oxidizing mechanisms even under rapid illumination changes, although this compensation is incomplete. Our results provide support for the theory that heterologous metabolism can act as a photosynthetic sink and exhibit some overlapping functionality with photoprotective mechanisms, while potentially conserving energy within useful metabolic products that might otherwise be “lost.”
Journal Article
Plastidial transporters KEA1, -2, and -3 are essential for chloroplast osmoregulation, integrity, and pH regulation in Arabidopsis
Multiple K ⁺ transporters and channels and the corresponding mutants have been described and studied in the plasma membrane and organelle membranes of plant cells. However, knowledge about the molecular identity of chloroplast K ⁺ transporters is limited. Potassium transport and a well-balanced K ⁺ homeostasis were suggested to play important roles in chloroplast function. Because no loss-of-function mutants have been identified, the importance of K ⁺ transporters for chloroplast function and photosynthesis remains to be determined. Here, we report single and higher-order loss-of-function mutants in members of the cation/proton antiporters-2 antiporter superfamily KEA1 , KEA2 , and KEA3 . KEA1 and KEA2 proteins are targeted to the inner envelope membrane of chloroplasts, whereas KEA3 is targeted to the thylakoid membrane. Higher-order but not single mutants showed increasingly impaired photosynthesis along with pale green leaves and severely stunted growth. The pH component of the proton motive force across the thylakoid membrane was significantly decreased in the kea1kea2 mutants, but increased in the kea3 mutant, indicating an altered chloroplast pH homeostasis. Electron microscopy of kea1kea2 leaf cells revealed dramatically swollen chloroplasts with disrupted envelope membranes and reduced thylakoid membrane density. Unexpectedly, exogenous NaCl application reversed the observed phenotypes. Furthermore, the kea1kea2 background enables genetic analyses of the functional significance of other chloroplast transporters as exemplified here in kea1kea2Na ⁺/H ⁺ antiporter1 (nhd1) triple mutants. Taken together, the presented data demonstrate a fundamental role of inner envelope KEA1 and KEA2 and thylakoid KEA3 transporters in chloroplast osmoregulation, integrity, and ion and pH homeostasis.
Journal Article
Ion antiport accelerates photosynthetic acclimation in fluctuating light environments
Many photosynthetic organisms globally, including crops, forests and algae, must grow in environments where the availability of light energy fluctuates dramatically. How photosynthesis maintains high efficiency despite such fluctuations in its energy source remains poorly understood. Here we show that
Arabidopsis thaliana
K
+
efflux antiporter (KEA3) is critical for high photosynthetic efficiency under fluctuating light. On a shift from dark to low light, or high to low light,
kea3
mutants show prolonged dissipation of absorbed light energy as heat. KEA3 localizes to the thylakoid membrane, and allows proton efflux from the thylakoid lumen by proton/potassium antiport. KEA3’s activity accelerates the downregulation of pH-dependent energy dissipation after transitions to low light, leading to faster recovery of high photosystem II quantum efficiency and increased CO
2
assimilation. Our results reveal a mechanism that increases the efficiency of photosynthesis under fluctuating light.
Plants must respond rapidly to unpredictable variations in light intensity to maximize photosynthetic efficiency. Here Armbruster
et al.
identify a potassium antiporter that is critical for accelerating proton fluxes across thylakoid membranes and minimizing energy loss in fluctuating light conditions.
Journal Article
Optimization of ATP Synthase c–Rings for Oxygenic Photosynthesis
The conversion of sunlight into useable cellular energy occurs
the proton-coupled electron transfer reactions of photosynthesis. Light is absorbed by photosynthetic pigments and transferred to photochemical reaction centers to initiate electron and proton transfer reactions to store energy in a redox gradient and an electrochemical proton gradient (proton motive force,
), composed of a concentration gradient (ΔpH) and an electric field (Δ
), which drives the synthesis of ATP through the thylakoid F
F
-ATP synthase. Although ATP synthase structure and function are conserved across biological kingdoms, the number of membrane-embedded ion-binding
subunits varies between organisms, ranging from 8 to 17, theoretically altering the H
/ATP ratio for different ATP synthase complexes, with profound implications for the bioenergetic processes of cellular metabolism. Of the known
-ring stoichiometries, photosynthetic
-rings are among the largest identified stoichiometries, and it has been proposed that decreasing the c-stoichiometry could increase the energy conversion efficiency of photosynthesis. Indeed, there is strong evidence that the high H
/ATP of the chloroplast ATP synthase results in a low ATP/nicotinamide adenine dinucleotide phosphate (NADPH) ratio produced by photosynthetic linear electron flow, requiring secondary processes such as cyclic electron flow to support downstream metabolism. We hypothesize that the larger
subunit stoichiometry observed in photosynthetic ATP synthases was selected for because it allows the thylakoid to maintain
in a range where ATP synthesis is supported, but avoids excess Δ
and ΔpH, both of which can lead to production of reactive oxygen species and subsequent photodamage. Numerical kinetic simulations of the energetics of chloroplast photosynthetic reactions with altered
-ring size predicts the energy storage of
and its effects on the photochemical reaction centers strongly support this hypothesis, suggesting that, despite the low efficiency and suboptimal ATP/NADPH ratio, a high H
/ATP is favored to avoid photodamage. This has important implications for the evolution and regulation of photosynthesis as well as for synthetic biology efforts to alter photosynthetic efficiency by engineering the ATP synthase.
Journal Article
The Response of Cyclic Electron Flow around Photosystem I to Changes in Photorespiration and Nitrate Assimilation
Photosynthesis captures light energy to produce ATP and NADPH. These molecules are consumed in the conversion of CO₂ to sugar, photorespiration, and NO₃⁻ assimilation. The production and consumption of ATP and NADPH must be balanced to prevent photoinhibition or photodamage. This balancing may occur via cyclic electron flow around photosystem I (CEF), which increases ATP/NADPH production during photosynthetic electron transport; however, it is not clear under what conditions CEF changes with ATP/NADPH demand. Measurements of chlorophyll fluorescence and dark interval relaxation kinetics were used to determine the contribution of CEF in balancing ATP/NADPH in hydroponically grown Arabidopsis (Arabidopsis thaliana) supplied different forms of nitrogen (nitrate versus ammonium) under changes in atmospheric CO₂ and oxygen. Measurements of CEF were made under low and high light and compared with ATP/NADPH demand estimated from CO₂ gas exchange. Under low light, contributions of CEF did not shift despite an up to 17% change in modeled ATP/NADPH demand. Under high light, CEF increased under photorespiratory conditions (high oxygen and low CO₂), consistent with a primary role in energy balancing. However, nitrogen form had little impact on rates of CEF under high or low light. We conclude that, according to modeled ATP/NADPH demand, CEF responded to energy demand under high light but not low light. These findings suggest that other mechanisms, such as the malate valve and the Mehler reaction, were able to maintain energy balance when electron flow was low but that CEF was required under higher flow.
Journal Article
Regulation of cyclic electron flow in Chlamydomonas reinhardtii under fluctuating carbon availability
The chloroplast must rapidly and precisely adjust photosynthetic ATP and NADPH output to meet changing metabolic demands imposed by fluctuating environmental conditions. Cyclic electron flow (CEF) around photosystem I is thought to contribute to this adjustment by providing ATP in excess of that supplied by linear electron low, balancing chloroplast energy budget when relative demand for ATP is high. We assessed the kinetics and energy production of CEF activation in
Chlamydomonas reinhardtii
under rapid changes of organic and inorganic carbon availability. Comparisons of transient electric field and chlorophyll fluorescence measurements indicated CEF was activated under conditions where ATP demand is expected to be high, consistent with a role in balancing the cellular ATP/NADPH budget under fluctuating environmental or metabolic conditions. CEF activation was not correlated with antenna state transitions, both in wild-type and the state transition mutant
stt7
-
9
, suggesting that CEF is rapidly regulated by allosteric or redox modulators. Comparing the CEF under ambient and high CO
2
conditions suggests an increase in required energy output of approximately 1ATP/CO
2
fixed, nearly sufficient to power proposed mechanistic models for the carbon-concentrating mechanism. Additionally, we see three-fold higher CEF rates in cells under steady-state conditions than cells under similar conditions with inhibited photosystem II, and up to five times higher in cells with severe depletion of inorganic carbon, implying that CEF has larger energetic capacity than predicted from some previous work.
Journal Article