Engineering topics
Kramer, David M.
Publications and source records attributed to Kramer, David M..
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.
Temporal Dynamics of Growth and Photosynthesis Suppression in Response to Jasmonate Signaling
Biotic stress constrains plant productivity in natural and agricultural ecosystems. Repression of photosynthetic genes is a conserved plant response to biotic attack, but how this transcriptional reprogramming is linked to changes in photosynthesis and the transition from growth- to defense-oriented metabolism is poorly understood. Here, we used a combination of noninvasive chlorophyll fluorescence imaging technology and RNA sequencing to determine the effect of the defense hormone jasmonate (JA) on the growth, photosynthetic efficiency, and gene expression of Arabidopsis (Arabidopsis thaliana) rosette leaves. High temporal resolution was achieved through treatment with coronatine (COR), a high-affinity agonist of the JA receptor. We show that leaf growth is rapidly arrested after COR treatment and that this effect is tightly correlated with changes in the expression of genes involved in growth, photosynthesis, and defense. Rapid COR-induced expression of defense genes occurred concomitantly with the repression of photosynthetic genes but was not associated with a reduced quantum efficiency of photosystem II. These findings support the view that photosynthetic capacity is maintained during the period in which stress-induced JA signaling redirects metabolism from growth to defense. Chlorophyll fluorescence images captured in a multiscale time series, however, revealed a transient COR-induced decrease in quantum efficiency of photosystem II at dawn of the day after treatment. Physiological studies suggest that this response results from delayed stomatal opening at the nightday transition. These collective results establish a high-resolution temporal view of how a major stress response pathway modulates plant growth and photosynthesis and highlight the utility of chlorophyll fluorescence imaging for revealing transient stress-induced perturbations in photosynthetic performance.
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. In this paper, 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.
The site of regulation of light capture in Symbiodinium: Does the peridinin–chlorophyll a–protein detach to regulate light capture?
Dinoflagellates from the genus Symbiodinium form symbiotic associations with cnidarians including corals and anemones. The photosynthetic apparatuses of these dinoflagellates possess a unique photosynthetic antenna system incorporating the peridinin–chlorophyll a–protein (PCP). It has been proposed that the appearance of a PCP-specific 77 K fluorescence emission band around 672–675 nm indicates that high light treatment results in PCP dissociation from intrinsic membrane antenna complexes, blocking excitation transfer to the intrinsic membrane-bound antenna complexes, chlorophyll a–chlorophyll c2–peridinin–protein-complex (acpPC) and associated photosystems (Reynolds et al., 2008 Proc Natl Acad Sci USA 105:13674–13678).We have tested this model using time-resolved fluorescence decay kinetics in conjunction with global fitting to compare the time-evolution of the PCP spectral bands before and after high light exposure. Here, our results show that no long-lived PCP fluorescence emission components appear either before or after high light treatment, indicating that the efficiency of excitation transfer from PCP to membrane antenna systems remains efficient and rapid even after exposure to high light. The apparent increased relative emission at around 675 nm was, instead, caused by strong preferential exciton quenching of the membrane antenna complexes associated with acpPC and reaction centers. This strong non-photochemical quenching (NPQ) is consistent with the activation of xanthophyll-associated quenching mechanisms and the generally-observed avoidance in nature of long-lived photoexcited states that can lead to oxidative damage. The acpPC component appears to be the most strongly quenched under high light exposure suggesting that it houses the photoprotective exciton quencher.
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 CO 2 to sugar, photorespiration, and NO 3 - 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 CO 2 and oxygen.Measurements of CEF were made under low and high light and compared with ATP/NADPH demand estimated from CO 2 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 CO 2 ), consistent with a primary role in energy balancing. However, nitrogen form had little impact on rates of CEF under high or low light. Finally, 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.
The Effect of Diel Temperature and Light Cycles on the Growth of Nannochloropsis oculata in a Photobioreactor Matrix
A matrix of photobioreactors integrated with metabolic sensors was used to examine the combined impact of light and temperature variations on the growth and physiology of the biofuel candidate microalgal species Nannochloropsis oculata. The experiments were performed with algal cultures maintained at a constant 20 °C versus a °15 C to 25 °C diel temperature cycle, where light intensity also followed a diel cycle with a maximum irradiance of 1920 mmol photons m -2 s -1 . No differences in algal growth (Chlorophyll a) were found between the two environmental regimes; however, the metabolic processes responded differently throughout the day to the change in environmental conditions. The variable temperature treatment resulted in greater damage to photosystem II due to the combined effect of strong light and high temperature. Cellular functions responded differently to conditions before midday as opposed to the afternoon, leading to strong hysteresis in dissolved oxygen concentration, quantum yield of photosystem II and net photosynthesis. Overnight metabolism performed differently, probably as a result of the temperature impact on respiration. Our photobioreactor matrix has produced novel insights into the physiological response of Nannochloropsis oculata to simulated environmental conditions. This information can be used to predict the effectiveness of deploying Nannochloropsis oculata in similar field conditions for commercial biofuel production.
Increasing Phosphatidylinositol (4,5)-Bisphosphate Biosynthesis Affects Basal Signaling and Chloroplast Metabolism in Arabidopsis thaliana
One challenge in studying the second messenger inositol(1,4,5)-trisphosphate (InsP 3 ) is that it is present in very low amounts and increases only transiently in response to stimuli. To identify events downstream of InsP 3 , we generated transgenic plants constitutively expressing the high specific activity, human phosphatidylinositol 4-phosphate 5-kinase Iα (HsPIPKIα). PIP5K is the enzyme that synthesizes phosphatidylinositol (4,5)-bisphosphate (PtdIns(4,5)P2); this reaction is flux limiting in InsP 3 biosynthesis in plants. Plasma membranes from transgenic Arabidopsis expressing HsPIPKIα had 2–3 fold higher PIP5K specific activity, and basal InsP 3 levels in seedlings and leaves were >2-fold higher than wild type. Although there was no significant difference in photosynthetic electron transport, HsPIPKIα plants had significantly higher starch (2–4 fold) and 20% higher anthocyanin compared to controls. Starch content was higher both during the day and at the end of dark period. In addition, transcripts of genes involved in starch metabolism such as SEX1 (glucan water dikinase) and SEX4 (phosphoglucan phosphatase), DBE (debranching enzyme), MEX1 (maltose transporter), APL3 (ADP-glucose pyrophosphorylase) and glucose-6-phosphate transporter (Glc6PT) were up-regulated in the HsPIPKIα plants. Our results reveal that increasing the phosphoinositide (PI) pathway affects chloroplast carbon metabolism and suggest that InsP 3 is one component of an inter-organelle signaling network regulating chloroplast metabolism.
The Energy Budget of Steady State Photosynthesis
Progress is reported in addressing these questions: Why do hcef mutants have increased CEF1? Is increased CEF1 caused by elevated expression or altered regulation of CEF1 components? Which metabolic pools can be regulators of CEF1? Do metabolites influence CEF1 directly or indirectly? Which CEF1 pathways are activated in high CEF1 mutants? Is PQR a proton pump? Is elevated CEF1 activated by state transitions?