Insights from Placing Photosynthetic Light Harvesting into Context

Jul 31, 2014 - ... of Ecology and Evolutionary Biology, University of Colorado, Boulder, ... and their costs and benefits, for increasing the maximal ...
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Insights from Placing Photosynthetic Light Harvesting into Context Barbara Demmig-Adams,* Jared J. Stewart, Tyson A. Burch, and William W. Adams III Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, Colorado 80309-0334, United States ABSTRACT: Solar-energy conversion through natural photosynthesis forms the base of virtually all food chains on Earth and provides fiber, materials, and fuels, as well as inspiration for the design of biomimetic energy-conversion systems. We summarize well-known as well as recently discovered feedback loops between natural light-harvesting systems and whole-organism function in natural settings. We propose that the low effective quantum yield of natural light-harvesting systems in high light is caused by downstream limitations rather than unavoidable intrinsic vulnerabilities. We evaluate potential avenues, and their costs and benefits, for increasing the maximal rate and photon yield of photosynthesis in high light in plants and photosynthetic microbes. By summarizing mechanisms observable only in complex systems (whole plants, algae, or, in some cases, intact leaves), we aim to stimulate future research efforts on reciprocal feedback loops between light harvesting and downstream processes in whole organisms and to provide additional arguments for the significance of research on photosynthetic light harvesting.

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here is a vast body of work, spanning multiple decades, on the photochemistry of photosynthesis and on mechanisms modulating light-harvesting efficiency from the viewpoint of physical chemistry. Recent communications on light harvesting have emphasized the utility of the development of “bioinspired materials”2 based on “lessons from nature about solar light harvesting”3,4 and have posed the question “Did nature think of it first?” for newly described energy-transfer processes.5 It is, furthermore, believed that an “understanding...of natural systems constitutes a step towards a blueprint for the constitution of artificial light-harvesting devices that can reproduce the efficacy of natural systems”6 (see also ref 7). We argue in the present Perspective that the low lightharvesting efficiency seen in all photosynthetic organisms during full-sun exposure in natural settings is not an intrinsic property of the light-harvesting and photochemical reactions but is instead caused by limitations in downstream reactions. We summarize interactions between light harvesting and the functioning of whole organisms in natural environments, including recent developments to which our group contributed, from an ecological and evolutionary viewpoint. We aim to stimulate future studies that further integrate the discipline of physical chemistry with those of ecology and evolution. Regulatory Plant Networks Linking Light Harvesting to WholePlant Function and Plant Response to the Environment. Multiple signaling networks, involving several products of photosynthesis as actual signaling components, exist between light harvesting and whole-plant function (Figure 1). These signaling networks not only coordinate plant growth, development, and reproduction but also allow the plant to respond to a large variety of physical and biological factors in its environment in a manner that optimizes the plant’s survival and productivity. The main products of photosynthesis involved in the above © XXXX American Chemical Society

We argue that the low effective quantum yield of photosynthetic light harvesting seen in all photosynthetic organisms during fullsun exposure in natural settings is not an intrinsic property of the light-harvesting and photochemical reactions but is instead caused by limitations in downstream reactions. signaling networks are (i) sugars on the one hand and (ii) reactive oxygen species (ROS) on the other.8 Links between Photosynthetic Sugar Production and WholePlant Sugar Consumption. Sunlight harnessed by the lightharvesting system is converted to the energy-rich intermediates nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP) used for the production of sugars, the majority of which are transported to the rest of the plant as a source of energy and/or to be stored (see the Abstract graphic). The degree to which the sugar products of photosynthesis are utilized by all growing and/or sugar-storing portions of the plant (sugar “sinks”) exerts feedback regulation on the leaves’ capacity for light harvesting and carbon fixation; sugar buildup in leaves leads to downregulation of photosynthesis, whereas efficient export and translocation of sugars from Received: May 28, 2014 Accepted: July 31, 2014

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capacity was best predicted by the total surface area of sugarloading cells per leaf area in plant species that load sugars via membrane-bound transporters.1,12,15 On the other hand, maximal photosynthetic capacity was predicted by the apparent total volume of sugar-loading cells per leaf area in plant species that load sugars via cytosolic enzymes trapping sugars in loading cells by converting photosynthetically produced sucrose to larger sugars unable to diffuse back into the photosynthesizing cells but capable of diffusing into the export tubes.1,16 Photosynthetic capacity per leaf area of all of these different species was closely correlated not only with anatomical and ultrastructural features of leaves’ sugar-loading complexes but also with leaf structural features that directly affect CO2 fixation capacity per leaf area, such as the number of chloroplasts per leaf area (determined by the leaf thickness15−17 and/or cell packing17). Photosynthesis and Cellular Redox-Signaling Networks. The harvesting of solar energy and its conversion into chemical energy by photosynthetic organisms not only provide energy carriers (such as sugars) but is also a major source of reactive oxygen.19 In addition to sugar-based signaling loops, a large number of signaling networks involve the cellular redox state, that is, the balance between oxidants and antioxidants. Although damaging in large quantities, reactive oxygen provides vital input into redox-dependent signaling networks controlling key plant functions including growth, development, reproduction, and defense against multiple environmental stresses.8,20−22 The light-harvesting system thus serves as an integrating gauge assisting in the orchestration of plant responses to the combination of all developmental and environmental factors, including physical (abiotic) factors, such as drought, cold, or heat, and biological (biotic) factors, such as pests and pathogens (Figure 1). Photosynthetic Organisms in Nature Deal with Excess Absorbed Light on a Daily Basis. While natural photosynthesis is capable of operating at a remarkable, near-unity maximal efficiency of light absorption and excitation energy transfer in the lightharvesting system (dawn and dusk in Figure 2A,C), the effective photon yield of natural photosynthetic systems in full sunlight is much lower (midday in Figure 2B,D).23,24 Most plants (and other photosynthetic organisms) utilize absorbed sunlight for photosynthetic electron transport highly efficiently during portions of the day with limited light availability (dawn and dusk in Figure 2). During midday in sunny locations, however, even plants with the highest photosynthesis rates fast-growing annual plants under environmental conditions favorable for plant growthutilize less than half of the full sunlight absorbed by their leaves (Figure 2A,B at noon), and slow-growing (perennial and evergreen) plants use even less (Figure 2C,D at noon).23,24 These responses are consistent with a regulation of light harvesting by downstream processes, including, for example, sink demand; fast-growing annuals with a high sink activity and a high maximal photosynthesis rate exhibit a lesser decrease of effective photon yield in high light (Figure 2A,B at noon), while slow-growing perennials and evergreens with lower sink activity and a lower maximal photosynthesis rate exhibit a more pronounced decrease of effective photon yield in high light (Figure 2C,D at noon). The resulting excess light (light that is absorbed but not utilized in photosynthesis) is largely safely dissipated from light-harvesting complexes as thermal energy through a regulated process,23−25 presumably to limit transfer

Figure 1. Schematic depiction of the multiple feedback and feedforward links (via regulatory sugar-based and redox-based signaling networks) between light harvesting in leaves and critical whole-plant processes (sugar translocation and partitioning, water transport and distribution, growth, development, reproduction) as well as key plant− environment interactions. LHC = light-harvesting complex; PSII = photosystem II.

leaves stimulates upregulation of photosynthesis through gene regulation of proteins serving in light harvesting and carbon fixation.9−11 The fact that efforts to unilaterally increase the levels of foliar CO2 fixation by genetic engineering, without addressing, for example, plant sink strength, have been disappointing (summarized in ref 12) suggests that efforts to engineer or breed increased plant productivity will depend on an integrated optimization of all relevant bottlenecks. Overexpression of the CO2 fixing capacity, for example, may only result in a more productive plant if combined with nonlimiting sink strength. Furthermore, while experimental decreases of whole-plant sink strength have been used to demonstrate a resulting downregulation of photosynthesis, a better understanding of the possible role of sink strength augmentation in increased plant productivity is needed. For example, manipulation of available sink strength alone did not have any effect on photosynthetic performance in an evergreen with a low density of foliar sugarexporting veins,13 suggesting that other factors need to be taken into consideration. The present review introduces as an additional bottleneck the capacity of leaves to load the sugar products of photosynthesis into conduits that export those sugars to the rest of the plant. Carbon-Export Capacity of Leaves As a Novel, Additional Bottleneck. It had been tacitly assumed that leaves possess an ample, unlimited capacity to export sugars. However, we recently demonstrated anatomical differences in the leaf’s sugarexport conduits (including different sizes and/or numbers of export tubes) not only across multiple plant species1,14−17 but also within species in response to different environmental conditions.1,12,14,17,18 The apparent total transport capacity of a leaf’s sugar-exporting veins predicted a leaf’s maximal (lightand CO2-saturated) photosynthetic capacity.1,12,15,16 Correlations between photosynthesis and sugar-export capacity were obtained for a large number of diverse plant species, irrespective of the particular mechanism of sugar loading into the leaf’s export system. Specifically, maximal photosynthetic 2881

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Figure 2. Schematic depiction of diurnal changes in (A,C) light absorption, estimated electron-transport rates, and (B,D) lightharvesting efficiency (effective photon yield) by leaves of an annual (sunflower) and an evergreen (periwinkle) plant growing naturally side-by-side in a full-sun-exposed site on a clear day. Red = absorbed photons not utilized for photosynthetic electron transport (i.e., dissipated thermally) and decreased light-harvesting efficiency under full sunlight at midday; green = utilization of absorbed photons in photosynthetic electron transport and high light-harvesting efficiency under low irradiance early and late in the day; yellow-to-orange = decreasing fraction of the absorbed photons utilized for photosynthetic electron transport and decreasing light-harvesting efficiency around noon. Replotted from ref 24; original data from ref 23.

Figure 3. (A) Maximal photosynthetic capacity and (B) effective photon yield under light- and CO2-saturating conditions at 25 °C in leaves of Arabidopsis thaliana (Swedish ecotype) with small-sized (warm-grown; multicolored columns) versus large-sized (cold-grown; green columns) carbon-export conduits. Leaf thickness covaried with the size of the carbon-export conduits. Plants were grown in growth chambers under 400 μmol photons m−2 s−1 at air temperatures of either 25 (Small) or 8 °C (Large) and were characterized for vein anatomy and photosynthetic capacity as described in refs 12 and 18. The effective photon yield was calculated from pulse-amplitudemodulated chlorophyll fluorescence using steady-state fluorescence (F) and maximal (Fm′) fluorescence in high light. The single asterisk above a column pair designates a significant difference at p < 0.05; the double asterisk above the column pair designates a significant difference at p < 0.01 (Student’s t-test).

of excess energy to oxygen and the resulting formation of potentially destructive reactive oxygen.8,19 Both photosynthetic capacity and the effective photon yield of natural photosynthesis in high light are presumably enhanced by a combination of high levels of CO2 fixation, a high foliar carbon-export capacity, and high levels of sugar utilization by sinks.10,11,26 This notion is supported by the data in Figure 3, showing a greater light- and CO2-saturated photosynthetic capacity (Figure 3A), coupled with a higher effective photon yield in high light (Figure 3B), in response to growing conditions augmenting foliar sugar-export conduits as well as increased leaf thickness.12,15 It should be noted that this concomitant increase in carbon-export capacity and photosynthetic capacity was demonstrated under cold growth temperatures, where these adjustments prevented the loss of photosynthetic activity and the putative loss of sugar-transporter activity that takes place in warm-grown plants suddenly transferred to cool conditions. These cold growth temperatures do reduce plant growth rates, and the observed adjustments in cold-temperature-acclimated plants merely avoid a more dramatic growth arrest. While the data reported in Figure 3 support a link between carbon-export capacity and photosynthetic capacity, the differences seen here likely do not represent the maximal extent of this effect. Future efforts to augment plant productivity should integrate CO2 fixation, foliar sugar export, and whole-plant sink strength. Moreover, carbon-based signaling networks will likely need to be integrated with redox-signaling networks. Trade-Of fs? From an evolutionary vantage point, it is important to assess costs and benefits (trade-offs) of adaptive features seen in organisms. If the carbon-export capacity of leaves is indeed a limiting factor for photosynthesis in plants, then there must be an evolutionary explanation as to why leaves

of different species, and/or under different environmental conditions, exhibit conduits with differing sugar-export capacities. A reason for the often limiting maximal carbonexport capacity of leaves could be the benefit that a plant derives from accumulating sugars in its leaves (as the polymer starch) at peak irradiance during the day; leaves are thereby able to provide a continuous stream of sugars (remobilized from diurnally stored starch) to the rest of the plant during the night in the absence of photosynthetic activity. It was indeed shown that leaves continuously export sugars over the 24 h day/night period27 and that starch accumulation in the leaves during the day is required to allow plants to grow under natural day/night patterns.28,29 It remains to be seen whether or not, and under what specific environmental conditions, augmentation of leaf carbon-export capacity may be a viable approach to enhance crop productivity. An additional trade-off may be related to the fact that the conduits serving in sugar export from leaves to the rest of the plant for growth, development, and reproduction are also the primary route for the spread of many pathogens throughout the plant.21,30,31 We have suggested8,14,22 that features of sugarexporting conduits favoring high carbon-export rates may simultaneously increase plant vulnerability to pathogens 2882

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photon yield of photosynthetic electron transport, energycarrier excreting cultures should have an increased overall photon yield of energy-carrier production. The likely interference of strong energy-carrier draining with cell division and further culture growth has the potential to allow establishment of long-lived, nondividing cultures that neither consume the majority of photosynthetically produced energy carriers for growth and cell division nor exhibit the photosynthetic inhibition seen in nonsecreting cells under growtharresting conditions. We have identified and recruited naturally occurring microalgae capable of continuous energy-carrier excretion under moderate saline conditions and shown that maximal light- and CO2-saturated photosynthetic capacity was higher (Figure 5A), along with a greater chlorophyll content per cell (Figure 5B) and a lesser susceptibility to high-lightinduced depressions in photon yield (Figure 5C), under moderately saline conditions with high levels of energy-carrier excretion versus nonsaline conditions with negligible energycarrier excretion rates. These are the features expected to result from a lessening of feedback limitation on light harvesting, that is, from a lessening of feedback-based repression of chlorophyll (a+b)-binding (CAB) proteins and carbon fixing proteins3 by removal of photosynthetically produced energy carriers. Energy-carrier excretion by photosynthetic microbes presumably evolved as a result of ecological advantages in certain environments. In nature, certain algae and photosynthetic bacteria occur in close, symbiotic association with nonphotosynthetic hosts that provide CO2, nitrogen and shelter in exchange for energy carriers excreted by the photosynthetic microbe.36,37 Humans could essentially step into the role of the nonphotosynthetic host by triggering energy-carrier excretion from algae and photosynthetic bacteria in order to use these energy carriers as precursors for biofuels and/or high-value chemicals. However, irrespective of the prospects for improving the effective photon yield of high-light-driven photosynthesis by eliminating downstream limitations, there does not seem to be a reason to assume that the low quantum efficiency of natural light-harvesting systems in high light is an intrinsic property of these systems. Whole-Plant-Based View of the Regulation of Light Harvesting by Thermal Energy Dissipation. As discussed above, the chloroplast plays a key role in determining a leaf’s cellular redox state. Changes in chloroplast redox state are, in turn, buffered by the safe disposal of excess absorbed light energy via thermal energy dissipation. From the above-discussed connections, it is clear that the chloroplast must navigate a balance between (i) allowing some level of reactive oxygen formation to orchestrate responses to environmental change (and/or changes in plant developmental state) via the redox-signaling network while simultaneously (ii) preventing damage from massive formation of ROS. Studying Thermal Energy Dissipation in Whole Plants under Natural Conditions. Photoprotective thermal dissipation of excess excitation energy is widely studied via nonphotochemical quenching of chlorophyll fluorescence (NPQ), and active debate continues about the nature of the dissipater (quencher) in NPQ.25 Studies with purified light-harvesting complexes (e.g., ref 38) or mutants with altered carotenoid pigment complements39 have revealed various xanthophylls (oxygencontaining carotenoids) with the principal ability to act as quenchers, including the constitutively present lutein as well as other xanthophylls formed only under excess light. The effective photon yield in full sunlight of a large number of

utilizing these same conduits as their means to spread from one region of the plant to another, thus resulting in a trade-off between plant productivity and pathogen defense. Because plant pathogen defenses involve redox-dependent signaling networks,32 future research on light harvesting in plants should also take these latter networks into consideration. For instance, efforts to upregulate photoprotective thermal energy dissipation capacity or antioxidant levels should include an evaluation of possible trade-offs between abiotic stress resistance and the ability of the plant to respond appropriately to other aspects of the environment.8,21,22 As an example of such a trade-off, a plant line engineered to be deficient in photoprotective energy dissipation possessed a greater pest resistance under field conditions.33 Determination of whether or not increased plant tolerance to abiotic stresses by enhanced photoprotection and/ or antioxidant levels (simultaneously causing photosynthesis to be “desensitized to environmental change”)21 increases vulnerability to pests or pathogens thus needs to be undertaken on a case-by-case basis.

We propose that triggering high levels of export of photosynthetically produced energy carriers from single-celled algae and photosynthetic bacteria may be a feasible and attractive approach to enhance the rate and effective photon yield of algal and bacterial photosynthesis in high light by eliminating feedback inhibition of photosynthesis as well as dramatically curtailing energycarrier consumption by the microbe’s own growth and cell division. Implications for Algal Biof uels. While further research is needed to evaluate the feasibility of increasing plant productivity by manipulations involving carbon export from leaves, draining of photosynthate from single-celled photosynthetic microbes via nondestructive removal of energy carriers may offer a tangible option for enhancing the microbial photosynthesis rate, effective photon yield, and high-light tolerance (Figure 4). Current algal biofuel production is largely based on internal accumulation of energy carriers under conditions that, as expected from feedback inhibition by photosynthate buildup, strongly decrease the rate and photon efficiency of photosynthesis34,35 (see Figure 4B). We propose that triggering high levels of export of photosynthetically produced energy carriers from single-celled algae and photosynthetic bacteria may be a feasible and attractive approach to enhance the rate and effective photon yield of algal and bacterial photosynthesis in high light by eliminating feedback inhibition of photosynthesis as well as dramatically curtailing energy-carrier consumption by the microbe’s own growth and cell division (as schematically depicted in Figure 4C). Excretion of energy carriers, furthermore, eliminates the time- and energy-consuming need to harvest, digest, and regrow microbial cultures. In addition to an increased effective 2883

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Figure 4. Schematic depiction of the relationship between algal/bacterial growth and cell division, the rate and photon yield of photosynthesis in high light (green = high; red = low), and energy-carrier excretion. Red arrows (bottom of funnel A) depict the inevitable loss of most absorbed light energy as unusable heat (as dictated by the laws of thermodynamics) during the myriad of metabolic reactions carried out by growing and dividing cells as well as (tops of funnels A and B) both regulated and unregulated thermal energy dissipation of excitation energy (as unusable heat) in the light-harvesting system under high light exposure (funnel A; the photon yield is much lower under high-light than that under low-light conditions) and especially under growth arrest without energy-carrier draining (funnel B). Funnel C depicts the projected increase in maximal photosynthetic rate and effective photon yield in high light from growth arrest in combination with high rates of continuous energy-carrier excretion/draining that may be able to remove all limitations to photosynthesis due to feedback downregulation when consumption of the products of photosynthesis falls behind the synthesis of those products. Encircled CO2, CO2 fixation in the chloroplast.

settings. It should be noted that the environmental conditions during plant growth have a strong impact on the extent and kinetics of NPQ as well as on chlorophyll, carotenoid, and the protein composition of the light-harvesting system.24,43 We suggest that growth light intensity and distribution (constant versus fluctuating light), as well as other parameters of the physical environment (see Figure 1), should routinely be listed as part of published Methods sections to facilitate evaluation of possible causes for contrasting observations among different studies. In addition, future research should address mechanisms of thermal energy dissipation in systems retaining the properties of whole organisms grown under conditions mimicking those of natural settings. The following observations may provide an avenue to reconcile diverging views on the involvement of different xanthophylls in thermal energy dissipation. Light-harvesting complexes possess several xanthophyll-binding sites, not all of which participate in thermal dissipation; molecules of constitutively present lutein proposed to act as quenchers in thermal dissipation may either occupy (e.g., in zeaxanthindeficient mutants) xanthophyll-binding sites normally occupied by zeaxanthin or be under the regulatory control of zeaxanthin. Lutein is apparently bound to typical zeaxanthin-binding sites in zeaxanthin-deficient mutants with increased lutein levels44 and in plants possessing a second high-light-driven xanthophyll cycle that converts lutein epoxide to lutein.45,46 Concerning a possible regulatory control by zeaxanthin of lutein-dependent quenching, the act of zeaxanthin binding to certain lightharvesting complexes in high light was proposed to control/ trigger an additional involvement in thermal dissipation of lutein bound to other sites.47

naturally growing plant species, on the other hand, falls on a single relationship with the leaves’ concentration of only those xanthophylls formed under excess light (Figure 6). Zeaxanthin and antheraxanthin are formed exclusively under excess light in the major xanthophyll cycle of plants, a set of interconvertible xanthophylls (violaxanthin, antheraxanthin, and zeaxanthin = VAZ).40 In other words, the ratio of the excess-light-induced zeaxanthin + antheraxanthin relative to the total pool of xanthophyll cycle pigments is able to predict the effective photon yield of naturally growing wild-type (nonmutant, nonengineered) leaves in high light, irrespective of plant species or environmental conditions (Figure 6).41

Humans could essentially step into the role of the nonphotosynthetic host by triggering energy-carrier excretion from algae and photosynthetic bacteria in order to use these energy carriers as precursors for biofuels and/or high-value chemicals. It thus appears that, in naturally growing wild-type leaves, only those xanthophylls formed exclusively under excess light act as either the actual quenchers24,40 and/or the regulators of quenching.40,42 This finding should be taken into consideration by a school of thought that focuses on quenching in isolated (purified) systems or mutants that is not associated with zeaxanthin and that may not occur in wild-type plants in natural 2884

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Figure 6. Relationship between the conversion of the (VAZ) xanthophyll cycle to high-light-induced zeaxanthin and antheraxanthin (de-epoxidation state as [Z + A]/[V + A + Z]) and the light-harvesting efficiency under midday exposure to natural sunlight (effective photon yield; see the legend of Figure 3) in 10 differently angled leaves of an evergreen shrub (blue) and leaves of 24 different plant species (red), all growing naturally in full-sun-exposed locations. Original data from ref 41. A = antheraxanthin; V = violaxanthin; Z = zeaxanthin. r2 = 0.869.

membrane versus slow-acting quenching (photoinhibitory quenching, qI) associated with changes in thylakoid protein composition (including degradation of photosystem II core proteins and upregulation of stress-associated members of the light-harvesting-protein family).43 It is widely assumed that these two quenching components are mechanistically unrelated. Placing these two (fast-acting and slow-acting) quenching processes into the context of the whole plant in its environment (see, e.g., ref 24) reveals a key role of pH-dependent qE under environmental conditions favorable for plant growth (Figure 7)

Figure 5. (A) Maximal (light- and CO2-saturated) photosynthetic capacity, (B) chlorophyll content, and (C) postillumination intrinsic light-harvesting efficiency (photosystem II efficiency from the ratio of variable [Fv] to maximal [Fm] chlorophyll fluorescence) of the green algal genus Chlamydomonas grown under conditions preventing (None; multicolored columns) or permitting (High; green columns) energy-carrier (glycerol) excretion at high rates. Algal cultures were maintained at 20 °C in 50 mL test tubes continuously bubbled with 5% CO2 in air under a light intensity of either 200 (A,B) or 1000 μmol photons m−2 s−1 (C). The photosynthetic capacity was measured at 20 °C as light- and CO2-saturated oxygen evolution rates in cell suspensions corresponding to 25 μg of chlorophyll per mL in a Hansatech DW2/2 liquid-phase oxygen electrode (Norfolk, England, U.K.). The single asterisk above the column pair denotes a significant difference at p < 0.05; the triple asterisk above the column pair denotes a significant difference at p < 0.001 (Student’s t-test).

Figure 7. Schematic depiction of the regulation of light-harvesting efficiency by pH-dependent thermal energy dissipation that occurs under conditions allowing continuous, or at least intermittent, CO2 fixation and plant growth. The kinetics of onset and relaxation of this form of energy dissipation can vary from seconds to hours.24,25,43,53,54 CAB = chlorophyll (a+b)-binding proteins; D1 = core protein of photosystem II; V = violaxanthin; Z = zeaxanthin.

Future research should address mechanisms of thermal energy dissipation in systems retaining the properties of whole organisms grown under conditions mimicking those of natural settings.

versus a key role of protein-composition-dependent qI under environmental conditions unfavorable for plant growth (Figure 8). The pH within the photosynthetic membrane monitors the ever-changing balance between the CO2 fixation rate and absorbed light in natural environments. Under conditions when photosynthetically produced sugars are consumed by the plant’s sinks, intramembrane pH is a measure of the ever-changing level of excess light and serves as the trigger for both zeaxanthin formation and the actual onset of thermal energy dissipation

What Causes Dif ferent Kinetics of the Onset and Relaxation of Thermal Energy Dissipation? The significance of two major NPQ processes with widely different kinetics of onset and relaxation (see, e.g., ref 48) continues to be debated, that is, fast-acting energy quenching (energy-dependent quenching, qE) regulated by the pH gradient across the photosynthetic 2885

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Figure 8. Schematic depiction of the regulation of light-harvesting efficiency by pH-independent, protein-composition-dependent thermal energy dissipation, which occurs under conditions preventing CO2 fixation and plant growth and thus causing sugar accumulation in leaves. The kinetics of onset and relaxation of this form of thermal energy dissipation can vary from days to seasons.24,25,43,50,53,55 CAB = chlorophyll (a+b)-binding proteins; D1 = core protein of photosystem II; ROS = reactive oxygen species; Z = zeaxanthin; qI = photoinhibitory quenching, that is, sustained depression in photosynthetic efficiency, or photoinhibition, that occurs most strongly in evergreen species. Schematic double helix = DNA in the chloroplast (e.g., the gene coding for D1) and in the nucleus (CAB genes, affected strongly in annuals but not evergreens), respectively.

(Figure 7).24,40,42 On the other hand, when photosynthetically produced sugars accumulate in leaves under environmental conditions arresting plant growth, sugar-based repression of the major light-harvesting proteins8 and of the photosystem II reaction center protein D149,50 is associated with an often drastic decrease in photosynthetic electron-transport capacity (Figure 8). D1 levels are repressed by accumulation of not only sugars but also of ROS,51,52 both of which increase particularly under sink limitation (Figure 8). Simultaneously, the kinetics of formation and removal of the quencher and/or quenching regulator zeaxanthin also vary dramatically in response to the environment and especially to whether or not plant growth is possible.24,53 Once again, plant growth conditions thus have a strong impact on the extent and kinetics of both NPQ and the VAZ cycle as well as on chlorophyll and protein composition of the light-harvesting system.24,43 It should not, therefore, be tacitly assumed that zeaxanthin cannot play a role in both of these quenching components (fast-acting, pH-dependent versus slow-acting, pH-independent). Dif ferences among Plant Species in the Response to Light Harvesting to Plant Sink Demand. While downregulation of the rates of electron transport and of carbon fixation in response to whole-plant sink limitation are apparently ubiquitous responses, the response of the light-harvesting system varies among species and can be dominated by either a preemptive decrease in light absorption or by continuously locked-in thermal energy dissipation of absorbed excess light. Under sink-limiting conditions, annual species exhibit apparent decreases in light absorption via decreased levels of chlorophyll and the lightharvesting proteins to which they are bound.9 In contrast, evergreen plant species, as their name indicates, do not lose much chlorophyll even when plant growth is completely arrested under unfavorable environmental conditions. Instead, for the duration of entire seasons precluding plant growth, evergreen species maintain light-harvesting complexes, continuously absorb large quantities of excessive light, continuously maintain high zeaxanthin levels day-and-night, and continuously dissipate the excess absorbed light energy via pHindependent, locked-in qI, associated with a continuously

depressed photon efficiency of electron transport.50,53,55 We have reported that the latter syndrome, photoinhibition as defined by a depressed photon efficiency of electron transport, is invariably associated with a buildup of high sugar and starch levels in leaves.50 One could argue that the depressed maximal rates of electron transport associated with decreased levels of Chl and CAB proteins in sink-limited annuals should also be viewed as a form of photoinhibition, albeit without continuously locked-in thermal dissipation (or the associated continuously low photon efficiency). We have proposed that photoinhibition in nature is the prerequisite for maintenance of green leaves or needles throughout growth-precluding seasons.24,50,53 The latter results obtained with whole plants have implications for future research. A large body of research (reviewed in ref 50) has thus far tacitly assumed that photoinhibition is limiting worldwide crop production and that the photochemical apparatus should therefore be targeted for engineering for improved high-light tolerance. The observation of carbohydrate buildup in photoinhibited leaves suggests that photoinhibited plants are not starved for carbon and that future research should evaluate the relationship between light harvesting and whole-plant responses to stresses that limit plant growth. Once again, the lasting inactivation of photochemistry and light harvesting in certain natural settings does not appear to be caused by an unalterable, intrinsic vulnerability of these primary systems to high light. This insight can provide directions for the engineering of natural systems as well as for the modeling of artificial light-harvesting systems after natural ones. Proposed Dual Control of Light Harvesting in Fluctuating versus Constantly Excessive Light Environments in Nature. Dual control of light-harvesting efficiency (consisting of quencher formation/removal as well as their engagement/disengagement in thermal energy dissipation) may optimize frequent switching from highly effective light harvesting under limiting light to highly effective photoprotective dissipation under excess light while allowing plants to retain a “biochemical memory”45 of light stress events in natural environments, where the position 2886

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of the sun as well as that of shade-casting objects/canopies create variable light environments in a recurring pattern day after day.24,53,54 We propose that such dual control of lightharvesting efficiency is provided in nature by the combination of (i) zeaxanthin formation via the (VAZ) xanthophyll cycle and (ii) engagement of zeaxanthin (and of any quenchers controlled by zeaxanthin) in thermal dissipation in lightharvesting complexes. Daily exposure to just a few minutes of moderately excessive light prompts leaves to continuously maintain some zeaxanthin and to exhibit a much faster onset of NPQ during a period of excess light, which we propose to occur via rapid engagement of zeaxanthin in thermal dissipation.24,53,54,56 This would allow actively growing plants to use changes in intrathylakoid pH (Figure 7) to modulate zeaxanthin formation and engagement over the course of the day (or to retain zeaxanthin and modulate only its engagement in a location with rapidly fluctuating light), while plants (especially evergreens) exposed to conditions arresting plant growth continuously maintain high levels of both zeaxanthin and thermal dissipation in a pH-independent way (Figure 8).24,43,50,53,55 In fact, because zeaxanthin is slowly formed (via biosynthesis from β-carotene) during growth in high light by virtually all photosynthetic organisms, including those lacking a xanthophyll cycle (see, e.g., ref 57), it is attractive to speculate that the evolution of the VAZ cycle in algae and plants may have been favored specifically by selective pressure for quencher removal (and return to highly efficient light harvesting) under conditions permitting rapid growth. In summary, we have shown that studies of whole plants in natural settings, including species with contrasting adaptive features, can contribute to further improvements in understanding photosynthetic light harvesting and that future work will benefit from studies at multiple levels of biological organization via integrating the disciplines of biophysics, chemistry, molecular biology, ecology, and evolution.

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export) features that contribute to the genetic adaptation and growth-condition-dependent acclimation of plants to different environments.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Science Foundation (Award Numbers IOS-0841546, DEB-1022236, and IOS1044552 to B.D.-A. and W.W.A.) and the University of Colorado at Boulder.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Barbara Demmig-Adams is a Professor at the University of Colorado. Her research focuses on photosynthesis, photoprotection (especially thermal energy dissipation, xanthophylls, and other antioxidants), and redox-signaling networks in a context of comparative eco-physiology, the study of varieties and species with differential genetic adaptations in contrasting environments. Website: http://www.colorado.edu/eeb/ facultysites/adams_demmig-adams/. Jared J. Stewart is a graduate student in the group of Profs. Barbara Demmig-Adams and William Adams at the University of Colorado. Jared’s research focuses on links among photosynthesis, leaf anatomy, plant growth and development, and plant defenses. Tyson A. Burch is a graduate student in the group of Profs. Barbara Demmig-Adams and William Adams at the University of Colorado. Tyson’s research focuses on links between algal photosynthesis and energy-carrier release as dependent on genetic and environmental factors. William W. Adams III is a Professor at the University of Colorado. His research focuses on leaf structural (particularly the vascular system) and functional (photosynthesis, photoprotection, sugar 2887

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