Recent Publications:
Wu G, Ortiz-Flores G, Ortiz-Lopez A, Ort DR (2007) A Point Mutation in atpC1 Raises the Redox Potential of the Arabidopsis Chloroplast ATP Synthase {gamma}-Subunit Regulatory Disulfide above the Range of Thioredoxin Modulation. J Biol Chem 282: 36782-36789
The light-dependent regulation of chloroplast ATP synthase activity depends on an intricate but ill defined interplay between the proton electrochemical potential across the thylakoid membrane and thioredoxin-mediated redox modulation of a cysteine bridge located on the ATP synthase {gamma}-subunit. The abnormal light-dependent regulation of the chloroplast ATP synthase in the Arabidopsis thaliana cfq (coupling factor quick recovery) mutant was caused by a point mutation (G to A) in the atpC1 gene, which caused an amino acid substitution (E244K) in the vicinity of the redox modulation domain in the {gamma}-subunit of ATP synthase. Equilibrium redox titration revealed that this mutation made the regulatory sulfhydryl group energetically much more difficult to reduce relative to the wild type (i.e. raised the Em7.9 by 39 mV). Enzymatic studies using isolated chloroplasts showed significantly lower light-induced ATPase and ATP synthase activity in the mutant compared with the wild type. The lower ATP synthesis capacity in turn restricted overall rates of leaf photosynthesis in the cfq mutant under low light. This work provides in situ validation of the concept that thioredoxin-dependent reduction of the {gamma}-subunit regulatory disulfide modulates the proton electrochemical potential energy requirement for activation of the chloroplast ATP synthase and that the activation state of the ATP synthase can limit leaf level photosynthesis
This paper was featured in JBC's "Papers of the Week" for December 20, 2007
Grennan AK, Ort DR (2007) Cool temperatures interfere with D1 synthesis in tomato by causing ribosomal pausing. Photosynth Res 94: 375-385 (DOI : 10.1007/s11120-007-9169-x)
Photodamage occurs when leaves are exposed to light in excess of what can be used for photosynthesis and in excess of the capacity of ancillary photoprotective as well as repair mechanisms. An important site of photodamage is the chloroplast encoded D1 protein, a component of the photosystem II (PSII) reaction-center. Even under optimal growth irradiance, D1 is photodamaged necessitating rapid turnover to prevent the accumulation of photodamaged PSII reaction centers and consequent inhibition of photosynthesis. However, this on-going process of D1 turnover and replacement was impeded in the chilling-sensitive tomato (Solanum lycopersicum) plants when exposed to high growth light at cool temperature. The decrease in D1 turnover and replacement was found not to be due to changes in the steady-state level of the psbA message. While the recruitment of ribosomes to psbA transcript, initiation of D1 translation, and the association of polysomes with the thylakoid membrane occurred normally, chilling temperatures caused ribosomal pausing during D1 peptide elongation in tomato. The pause locations were non-randomly located on the D1 transcript. The interference with translation caused by ribosomal pausing allowed photodamaged PSII centers to accumulate leading to the consequent inhibition of photosynthesis.
Stomatal responses to atmospheric change have been well documented through a range of laboratory- and field-based experiments. Increases in atmospheric concentration of CO2 ([CO2]) have been shown to decrease stomatal conductance (gs) for a wide range of species under numerous conditions. Less well understood, however, is the extent to which leaf level responses translate to changes in ecosystem evapotranspiration (ET). Since many changes at the soil, plant and canopy microclimate level may feed back on ET, it is not certain that a decrease in gs will decrease ET in rain-fed crops. To examine the scaling of the effect of elevated [CO2] on gs at the leaf to ecosystem ET, soybean (Glycine max) was grown in field conditions under control (ca 375 umol CO2 mol-1 air) and elevated [CO2] (ca 550 umol mol-1) using Free Air CO2 Enrichment (FACE). ET was determined from the time of canopy closure to crop senescence using a residual energy balance approach over four growing seasons. Elevated [CO2] caused ET to decrease between 9 and 16% depending on year and despite large increases in photosynthesis and seed yield. Ecosystem ET was linked with gs of the upper canopy leaves when averaged across the growing seasons such that a 10% decrease in gs results in a 8.6% decrease in ET; this relationship was not altered by growth at elevated [CO2]. The findings are consistent with model and historical analyses which suggest that, despite system feedbacks, decreased gs of upper canopy leaves at elevated [CO2] results in decreased transfer of water vapor to the atmosphere.
Bernacchi CJ, Leakey ADB, Heady LE, Morgan PB, Dohleman FG, McGrath JM, Gillespie KM, Wittig VE, Rogers A, Long SP, Ort DR (2006) Hourly and seasonal variation in photosynthesis and stomatal conductance of soybean grown at future CO2 and ozone concentrations for 3 years under fully open-air field conditions. Plant Cell and Environ 29 (11): 2077-2090
It is anticipated that enrichment of the atmosphere with CO2 will increase photosynthetic carbon assimilation in C3 plants. Analysis of controlled environment studies conducted to date indicates that plant growth at concentrations of carbon dioxide ([CO2]) anticipated for 2050 (similar to 550 mu mol mol(-1)) will stimulate leaf photosynthetic carbon assimilation (A) by 20 to 40%. Simultaneously, concentrations of tropospheric ozone ([O-3]) are expected to increase by 2050, and growth in controlled environments at elevated [O-3] significantly reduces A. However, the simultaneous effects of both increases on a major crop under open-air conditions have never been tested. Over three consecutive growing seasons > 4700 individual measurements of A, photosynthetic electron transport (J(PSII)) and stomatal conductance (g(s)) were measured on Glycine max (L.) Merr. (soybean). Experimental treatments used free-air gas concentration enrichment (FACE) technology in a fully replicated, factorial complete block design. The mean A in the control plots was 14.5 mu mol m(-2) s(-1). At elevated [CO2], mean A was 24% higher and the treatment effect was statistically significant on 80% of days. There was a strong positive correlation between daytime maximum temperatures and mean daily integrated A at elevated [CO2], which accounted for much of the variation in CO2 effect among days. The effect of elevated [CO2] on photosynthesis also tended to be greater under water stress conditions. The elevated [O-3] treatment had no statistically significant effect on mean A, g(s) or J(PSII) on newly expanded leaves. Combined elevation of [CO2] and [O-3] resulted in a slightly smaller increase in average A than when [CO2] alone was elevated, and was significantly greater than the control on 67% of days. Thus, the change in atmospheric composition predicted for the middle of this century will, based on the results of a 3 year open-air field experiment, have smaller effects on photosynthesis, g(s) and whole chain electron transport through photosystem II than predicted by the substantial literature on relevant controlled environment studies on soybean and likely most other C3 plants.
Under normal field growth conditions, canola (Brassica napus) seeds produce chloroplasts during early seed development and then catabolize the photosynthetic machinery during seed maturation, producing mature seeds at harvest that are essentially free of chlorophyll (Chl). However, frost exposure early in canola seed development disrupts the normal programming of Chl degradation, resulting in green seed at harvest and thereby significantly devaluing the crop. Pheophorbide a oxygenase (PaO), a key control point in the overall regulation of Chl degradation, was affected by freezing. Pheophorbide a, the substrate of PaO, accumulated during late stages of maturation in seeds that had been exposed to freezing during early seed development. Freezing interfered with the induction of PaO activity that normally occurs in the later phases of canola seed development when Chl should be cleared from the seed. Moreover, we found that the induction of PaO activity in canola seed was largely posttranslationally controlled and it was at this level that freezing interfered with PaO activation. The increased accumulation of PaO transcript and protein levels during seed development was not altered by the freezing episode, and the increase in PaO protein was small compared to the increase in PaO activity. We found that PaO could be phosphorylated and that phosphorylation decreased with increasing activity, implicating PaO dephosphorylation as an important posttranslational control mechanism for this enzyme. Two PaO genes, BnPaO1 and BnPaO2, were identified in senescing canola leaves and during early seed development, but only BnPaO2 was expressed in maturing, degreening seeds.
Leakey ADB, Bernacchi CJ, Ort DR, Long SP (2006) Long-term growth of soybean at elevated [CO2] does not cause acclimation of stomatal conductance under fully open-air conditions. Plant Cell Environ 29: 1794-1800
Accurately predicting plant function and global biogeochemical cycles later in this century will be complicated if stomatal conductance (g(s)) acclimates to growth at elevated [CO2], in the sense of a long-term alteration of the response of g(s) to [CO2], humidity (h) and/or photosynthetic rate (A). If so, photosynthetic and stomatal models will require parameterization at each growth [CO2] of interest. Photosynthetic acclimation to long-term growth at elevated [CO2] occurs frequently. Acclimation of g(s) has rarely been examined, even though stomatal density commonly changes with growth [CO2]. Soybean was grown under field conditions at ambient [CO2] (378 mu mol mol(-1)) and elevated [CO2] (552 mu mol mol(-1)) using free-air [CO2] enrichment (FACE). This study tested for stomatal acclimation by parameterizing and validating the widely used Ball et al. model (1987, Progress in Photosynthesis Research, vol IV, 221-224) with measurements of leaf gas exchange. The dependence of g(s) on A, h and [CO2] at the leaf surface was unaltered by long-term growth at elevated [CO2]. This suggests that the commonly observed decrease in g(s) under elevated [CO2] is due entirely to the direct instantaneous effect of [CO2] on g(s) and that there is no longer-term acclimation of g(s) independent of photosynthetic acclimation. The model accurately predicted g(s) for soybean growing under ambient and elevated [CO2] in the field. Model parameters under ambient and elevated [CO2] were indistinguishable, demonstrating that stomatal function under ambient and elevated [CO2] could be modelled without the need for parameterization at each growth [CO2].
Prior SA, Torbert HA, Runion GB, Rogers HH, Ort DR, Nelson RL (2006) Free-air carbon dioxide enrichment of soybean: Influence of crop variety on residue decomposition. J Environ Qualit 35: 1470-1477
Elevated atmospheric CO2 can result in larger plants returning greater amounts of residue to the soil. However, the effects of elevated CO2 on carbon (C) and nitrogen (N) cycling for different soybean varieties have not been examined. Aboveground residue of eight soybean [Glycine mar (L.) Merr.] varieties was collected from a field study where crops had been grown under two different atmospheric CO2 levels [370 mu mol mol(-1) (ambient) and 550 mu mol mol(-1) (free-air carbon dioxide enrichment, FACE)]. Senesced residue material was used in a 60-d laboratory incubation study to evaluate potential C and N mineralization. In addition to assessing the overall effects of CO2 level and variety, a few specific variety comparisons were also made. Across varieties, overall residue N concentration was increased by FACE, but residue C concentration was only slightly increased. Overall residue C to N ratio was lower under FACE and total mineralized N was increased by FACE, suggesting that increased N-2 fixation impacted residue decomposition; total mineralized C was also slightly increased by FACE. Across CO2 levels, varietal differences were also observed with the oldest variety having the lowest residue N concentration and highest residue C to N ratio; mineralized N was lowest in the oldest variety, illustrating the influence of high residue C to N ratio. It appears (based on our few specific varietal comparisons) that the breeding selection process may have resulted in some varietal differences in residue quality which can result in increased N or C mineralization under elevated CO2 conditions. This limited number of varietal comparisons indicated that more work investigating varietal influences on soil C and N cycling under elevated CO2 conditions is required.
Long SP, Ainsworth EA, Leakey ADB, Nosberger J, Ort DR (2006) Food for thought: lower than expected crop yield stimulation with rising carbon dioxide concentrations. Science 312:1918-1921 - this paper can be accessed from the SoyFACE publications page
Model projections suggest that although increased temperature and decreased soil moisture will act to reduce global crop yields by 2050, the direct fertilization effect of rising carbon dioxide concentration ([CO2]) will offset these tosses. The CO2 fertilization factors used in models to project future yields were derived from, enclosure studies conducted approximately 20 years ago. Free-air concentration enrichment (FACE) technology has now facilitated large-scale trials of the major grain crops at elevated [CO2] under fully open-air field conditions. In those trials, elevated [CO2] enhanced yield by similar to 50% less than in enclosure studies. This casts serious doubt on projections that rising [CO2] will fully offset losses due to climate change.
Rogers A, Gibon Y, Stitt M, Morgan PB, Bernacchi CJ, Ort DR, Long SP (2006) Increased C availability at elevated carbon dioxide concentration improves N assimilation in a legume. Plant Cell Environ 29: 1651-1658
Plant growth is typically stimulated at elevated carbon dioxide concentration ([CO2]), but a sustained and maximal stimulation of growth requires acquisition of additional N in proportion to the additional C fixed at elevated [CO2]. We hypothesized that legumes would be able to avoid N limitation at elevated [CO2]. Soybean was grown without N fertilizer from germination to final senescence at elevated [CO2] over two growing seasons under fully open-air conditions, providing a model legume system. Measurements of photosynthesis and foliar carbohydrate content showed that plants growing at elevated [CO2] had a c. 25% increase in the daily integral of photosynthesis and c. 58% increase in foliar carbohydrate content, suggesting that plants at elevated [CO2] had a surplus of photosynthate. Soybeans had a low leaf N content at the beginning of the season, which was a further c. 17% lower at elevated [CO2]. In the middle of the season, ureide, total amino acid and N content increased markedly, and the effect of elevated [CO2] on leaf N content disappeared. Analysis of individual amino acid levels supported the conclusion that plants at elevated [CO2] overcame an early-season N limitation. These soybean plants showed a c. 16% increase in dry mass at final harvest and showed no significant effect of elevated [CO2] on leaf N, protein or total amino acid content in the latter part of the season. One possible explanation for these findings is that N fixation had increased, and that these plants had acclimated to the increased N demand at elevated [CO2].
Gong P, Wu G, Ort DR (2006) Slow dark deactivation of Arabidopsis chloroplast ATP synthase caused by a mutation in a nonplastidic SAC domain protein. Photosynth Res 88: 133-142
Coupling factor slow recovery (cfs) is a recessive mutant of Arabidopsis with anomalous ATP synthase activation/deactivation characteristics as well as a distinct growth phenotype. The most significant feature of this mutant is that the dark-adapted deactivation of ATP synthase is a very slow relative to the wild type, indicating interference with ATP synthase regulation. Physical mapping of the mutation delimited it to a region in a pair of bacterial artificial chromosome clones. Examination of T-DNA insertion lines of all 34 putative genes located in this region identified two homozygous T-DNA insertion lines of the same gene, At3g59770, possessing phenotypes indistinguishable from the cfs mutant. At3g59770 had been previously identified as suppressor of actin 9 (SAC9), a protein with a SAC domain, a protein-protein interaction module containing two conserved tryptophans known as a WW domain, and an ATP/GTP-binding site motif A. Sequence analysis of cfs revealed a point mutation of G to A resulting in an amino acid substitution from tryptophan to STOP, thereby coding a truncated protein. Real-time-PCR amplification of the gene specific fragments showed that the T-DNA mutants did not have full-length transcripts whereas the cfs mutant transcribed a full-length mutated transcript. Further investigation of SAC9 RNA expression levels in different tissues of wild-type plants by RT-PCR revealed the highest expression in leaves. SAC 9 dysfunction interferes with ATP synthase deactivation, possibly by an alteration in phosphoinositide signaling inducing a stress mimicry response.
Long SP, Zhu X-G, Naidu SL, Ort DR (2006) Can improvement in photosynthesis increase crop yields? Plant Cell Environ 29: 315-330
The yield potential (Y-p) of a grain crop is the seed mass per unit ground area obtained under optimum growing conditions without weeds, pests and diseases. It is determined by the product of the available light energy and by the genetically determined properties: efficiency of light capture (epsilon(i)), the efficiency of conversion of the intercepted light into biomass (epsilon(c)) and the proportion of biomass partitioned into grain (eta). Plant breeding brings eta and epsilon(i) close to their theoretical maxima, leaving epsilon(c), primarily determined by photosynthesis, as the only remaining major prospect for improving Y-p. Leaf photosynthetic rate, however, is poorly correlated with yield when different genotypes of a crop species are compared. This led to the viewpoint that improvement of leaf photosynthesis has little value for improving Y-p. By contrast, the many recent experiments that compare the growth of a genotype in current and future projected elevated [CO2] environments show that increase in leaf photosynthesis is closely associated with similar increases in yield. Are there opportunities to achieve similar increases by genetic manipulation? Six potential routes of increasing epsilon(c) by improving photosynthetic efficiency were explored, ranging from altered canopy architecture to improved regeneration of the acceptor molecule for CO2. Collectively, these changes could improve epsilon(c) and, therefore, Y-p by c. 50%. Because some changes could be achieved by transgenic technology, the time of the development of commercial cultivars could be considerably less than by conventional breeding and potentially, within 10-15 years.
While increasing temperatures and altered soil moisture arising from climate change in the next 50 years are projected to decrease yield of food crops, elevated CO2 concentration ([CO2]) is predicted to enhance yield and offset these detrimental factors. However, C-4 photosynthesis is usually saturated at current [CO2] and theoretically should not be stimulated under elevated [CO2]. Nevertheless, some controlled environment studies have reported direct stimulation of C-4 photosynthesis and productivity, as well as physiological acclimation, under elevated [CO2]. To test if these effects occur in the open air and within the Corn Belt, maize (Zea mays) was grown in ambient [CO2] (376 mu mol mol(-1)) and elevated [CO2] (550 mmol mol(-1)) using Free-Air Concentration Enrichment technology. The 2004 season had ideal growing conditions in which the crop did not experience water stress. In the absence of water stress, growth at elevated [CO2] did not stimulate photosynthesis, biomass, or yield. Nor was there any CO2 effect on the activity of key photosynthetic enzymes, or metabolic markers of carbon and nitrogen status. Stomatal conductance was lower (-34%) and soil moisture was higher (up to 31%), consistent with reduced crop water use. The results provide unique field evidence that photosynthesis and production of maize may be unaffected by rising [CO2] in the absence of drought. This suggests that rising [CO2] may not provide the full dividend to North American maize production anticipated in projections of future global food supply.
Back to top- Zhu X-G, Govindjee, Baker NR, De Sturler E, Ort DR, Long SP (2005) Chlorophyll a fluorescence induction kinetics in leaves predicited from a model describing each discrete step of excitation energy and electron transfer associated with photosystem II. Planta 223: 114-133
- Zuniga-Feest A, Ort DR, Gutierrezc A, Gidekel M, Bravo LA, Corcuera JL (2005) regulation of sucrose-phosphate synthase acitivity in the grass Deschampsia antarctica. Photosyn Res 83:75-86
- Bernacchi CJ, Morgan PB, Long SP, Ort DR (2005) The growth of soybean under free air concentration enrichment (FACE) stimulates photosynthesis while decreasing apparent in vivo Rubisco capacity. Planta 220: 434-446
- Morgan PB, Bernacchi CJ, Ort DR, Long SP (2004) An in vivo analysis of the effect of season-long open-air elevation of ozone to anticipated 2050 levels on photosynthesis in soybean. Plant Physiol 135: 2348-2357
- Zhu X-G, Ort DR, Whitmarsh J, Long SP (2004) The slow reversibility of photosystem II thermal energy dissipation on transfer from high to low light may cause large losses in carbon gain by crop canopies. A theoretical analysis. J Exp Bot 55: 1167-1175
- Loreto F, Baker NR, Ort DR (2004) Environmental constraints: Chloroplast to leaf. In: Photosynthetic Adaptation Chloroplast to Landscape (Smith WK, Vogelman TC, Critchley, C, eds) Springer, pp 231-261
- Baker NR, Ort DR, Harbinson J, Whitmarsh, J (2004) Light processing: Chloroplats to leaf. In Photosynthetic Adaptation Chloroplast to Landscape (Smith WK, Vogelman TC, Critchley, C, eds) Springer, pp 89-104
- Long SP, Ainsworth EA, Rogers A, Ort DR (2004) Rising Atmospheric Carbon Dioxide: Plants FACE the future. Ann Rev Plant Biol 55: 591-628
- Leakey ADB. Bernacchi CJ, Dohleman FG, Long SP, Ort DR (2004) Will photosynthesis of maize (Zea mays) in the U.S. Corn Belt increase in future [CO2] rich atmospheres? An analysis of diurnal courses of CO2 uptake under Free-Air Concentration Enrichment (FACE). Global Change Biol 10: 951-962
- Schrader SM, Wise RR, Wacholtz WF, Ort DR, Sharkey TD (2004) High leaf temperature limits photosynthesis in Pima cotton II. Thylakoid membrane responses to moderate heat stress. Plant Cell Environ 27: 725-7351
- Rogers A, Allen DJ, Davey PA, Morgan PB, Ainsworth EA, Bernacchi CJ, Cornic G, Dermody O, Heaton A, Mahoney J, Zhu X-H, DeLucia E, Ort DR, Long SP (2004) Leaf photosynthesis and carbohydrate dynamics of soybeans grown throughout their life-cycle under Free-Air Carbon dioxide Enrichment. Plant Cell Environ 27: 449-458
- Tucker DE, Allen DJ, Ort DR (2004) Control of nitrate reductase circadian and diurnal rhythms in tomato. Planta 219: 277-285
- Singsaas E, Ort DR, DeLucia E (2004) Elevated CO2 effects on mesophyll conductance and its consequences for interpreting photosynthetic physiology. Plant Cell Environ 27, 41-50
- Ort DR (2003) Contrasting mechanisms responsible for chilling-induced inhibition of photosynthesis in warm climate crops. Indian J Plant Physiol 2003 (Special Issue), 1-10
- Ort DR, Long SP (2003) Converting Solar Energy into Crop Production. (eds M.J. Chrispeels and D.E. Sadava), pp. 240-269. American Society of Plant Biologists/Jones and Bartlett, Boston
- Ort DR, Baker NR (2002) Photoprotection: The role of electron sinks. Curr Opin Plant Biol 5: 193-198
- Tucker DE, Ort DR (2002) Low temperature induces expression of nitrate reductase in tomato that temporarily overrides circadian regulation of activity. Photosynth Res 72: 285-293
- Ort DR (2002) Chilling-induced limitations on photosynthesis in warm climate plants: Contrasting mechanisms. Environ Control Biol 40: 7-18
- Allen DJ, Ort DR (2001) Impacts of chilling temperatures on photosynthesis in warm-climate crops. Trends Plant Sci 6: 36-42
- Ort DR (2001) When there is too much light. Plant Physiol 125: 29-32
- Singsaas E, Ort DR, DeLucia E (2000) Variation in measured values of photosynthetic quantum yield in ecophysiological studies. Oecologia 128: 15-23
- Hutchison RS, Groom Q, Ort DR (2000) Differential effects of chilling-induced photooxidation on the redox regulation of photosynthetic enzymes. Biochemistry 39: 6679-6688
