The capability to use sunlight to provide the energy to sustain living processes was a powerful driving force in evolution; in oxygenic photosynthesis, the photooxidation of water provides a limitless supply of chemical energy. At the biochemical, physiological and ecological levels, algae and plants display a wide range of features which maximise the efficiency of collection and conversion of solar energy. Indeed the forms of the aerial parts of plants are shaped by adaptation for light collection, as is the molecular machinery within the chloroplasts, the plant cells organaelles specialised for photosynthesis. However, absorbing, storing and converting solar energy creates a problem, because the intensity of sunlight is highly variable – fluctuations of a 100-fold occur rapidly due to changes cloud cover and shading. Photosynthesis begins with light absorption, the “light reactions” providing the energy to drive the linked chemical reactions in the cell; because these “dark reactions” have a finite capacity they become progressively saturated when the light intensity increases and/or when their capacity is restricted (e.g. by low temperature, lack of water or C02). Light is absorbed by the green chlorophyll pigments in the chloroplasts, these molecules being bound to proteins to form light harvesting complexes, which are major constituents of the internal membranes of the chloroplast. Excited states of chlorophyll in the light harvesting complexes are rapidly transferred to photosynthetic reaction centres which utilise the excited states in the primary photochemical reactions of photosynthesis. However, as the dark reactions become saturated, more and more of the chlorophyll molecules remain in the excited state for longer (because the energy can not be used in the photochemical reactions) – it is said that the excited state lifetime increases. An important consequence of this longer lifetime is that the probability increases that excited chlorophyll molecules will set off deleterious chemical reactions that lead to serious damage to other constituents of the chloroplast and surrounding cell, ultimately leading to death of the cell, organ and even plant. The potential for photodamage is enhanced by the oxygen-rich micro-environment of the chloroplast and the highly oxidising conditions needed for the crucial photosynthetic reaction - the removal oxygen from water. If left unabated, plants could not survive under natural sunlight due to these deleterious effects of light.

Plants have evolved a number of lines of defence against photodamage. There are constitutive processes, which help ameliorate the harmful reactions, (eg the ubiquitous presence of carotenoids in photosynthetic organisms inhibits the major channel by which excited state chlorophylls cause damage) or repair any damage done (eg the replacement of damaged photosystem II reaction centre proteins). In addition, there are a variety of processes which are reversibly switched on when light saturation approaches, switching off again as light becomes limiting. At one level, light absorption can be controlled – alteration in leaf angle or distribution of chloroplasts in the leaf cell can modulate the amount of incident light absorbed. But, it is at the molecular level that the most profound switching process occurs [1]. In this, under light-saturating conditions a new channel is rapidly formed in the light harvesting complexes so that “excess” excited states are dissipated harmlessly by non-radiative conversion to heat, so shortening the excited state lifetime. Because excited states are quenched by a process not involving the photochemical reactions of photosynthesis, this process is widely known as nonphotochemical quenching or NPQ. NPQ is a major mechanism of photo protection, vital for plant survival [2].

Whilst NPQ is an integrated function of the chloroplast membranes, its fundamental mechanism is found inbuilt in the molecular structure of the light harvesting complexes known as LHCII. These remarkable complexes, which bind 50% of all the chlorophyll in plants, are very flexible – they can rapidly and reversibly switch by conformational change between structurally very similar but functionally very different states [3]. In one mode (the light harvesting state), they are optimally configured to efficiently transfer absorbed light energy to the photosynthetic reaction centres. In contrast, in the dissipative (quenched) state, greater than 90% of absorbed light energy is rapidly dissipated as heat. These contrasting states of LHCII can be observed in vitro in protein complexes purified from chloroplast membranes. Whilst the differences in the conformation of the protein in the two modes have yet to be deciphered, spectroscopic analysis has identified specific configuration changes in certain domains of the bound pigments, suggesting how the new dissipative channel is formed [4]. Of crucial importance in this channel are the three xanthophyll molecules bound to the complex, called neoxanthin, lutein and zeaxanthin [5,6]. These pigments have been used as “flags” that have allowed the spectroscopic measurement of the LHCII switch when NPQ is formed in whole leaves.

In vivo, in the chloroplast membranes, the proportion of LHCII molecules in the light harvesting and dissipative states is regulated in response to the degree of saturation of the photosynthetic dark reactions. The extent of light saturation is signalled by the pH gradient across the chloroplast membrane - the ∆pH is a product of the light reactions, and is consumed by the dark reactions, hence its magnitude signals the balance between them [7]. The increase in ∆pH is the trigger for a change in LHCII conformation involved in switching to the dissipative mode. This type of dynamic regulation occurs in many biological contexts and investigations have revealed that NPQ has many features in common with the well-established mechanisms controlling enzyme function [8]. Thus, the sensitivity of the switch to the ∆pH is modulated allosterically an additional factor, in this case by the extent of enzymatic conversion between two xanthophylls, violaxanthin and zeaxanthin. Intriguingly, this ratio is itself determined by the ∆pH, through the activity of the xanthophyll cycle [9], providing a high degree of amplification in the control mechanism. This latter effect of ∆pH is much slower than the LHCII switch and provides a type of memory of the overall patterns of the fluctuations in light conditions. The dynamics of LHCII are also controlled by the interactions between neighbouring molecules – the chloroplast membrane shows a high degree of long range structural order in the form of various LHCII-PSII supercomplexes, which appear to be crucial ingredients of optimal NPQ function [10]. One particular protein, called PsbS, has an especially vital role in promoting the best organisation of LHCII [11]. Thus, NPQ depends upon a rather complicated set of molecules and conditions. This is not surprising, since it is important that it occurs at the right time, to the right extent and at the right speed – if not, either too much light energy would be dissipated, reducing photosynthetic efficiency, or too little photoprotection would be given. This degree of optimisation and fine-tuning of NPQ to the physiological requirements of the whole organism obviously requires complexity.

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References

1. Horton P, Ruban AV, Walters RG (1996) Regulation of light harvesting in green plants. Annu Rev Plant Physiol Plant Mol Biol 47: 655-684
2. Külheim C, Ågren J, Jansson S (2002) Rapid regulation of light harvesting and plant fitness in the field. Science 297: 91-93
3. Horton P, Wentworth M, Ruban AV (2005) Control of the light harvesting function of chloroplast membranes: the LHCII-aggregation model for non-photochemical quenching, FEBS Lett 579: 4201-4206
4. Pascal AA, Liu Z, Broess K, van Oort B, van Amerongen H, Wang C, Horton P, Robert B, Chang W, Ruban A (2005) Molecular basis of photoprotection and control of photosynthetic light-harvesting. Nature 436: 134-137
5. Ruban AV, Berera R, Ilioaia C, van Stokkum IHM, Kennis JTM, Pascal AA, van Amerongen H, Robert B, Horton P, van Grondelle R (2007) Identification of a mechanism of photoprotective energy dissipation in higher plants. Nature 450: 575-578
6. Ahn TK, Avenson T, Ballottari M, Cheng YC, Niyogi KK, Bassi R, Fleming GR (2008) Architecture of a Charge-Transfer State Regulating Light Harvesting in a Plant Antenna Protein. Science 320: 794-797
7. Kramer DM, Cruz J A, Kanazawa A (2003) Balancing the central roles of the thylakoid proton gradient. Trends Plant Sci 8:27-32
8. Horton P, Ruban AV, Wentworth M (2000) Allosteric regulation of the light harvesting system of photosystem II. Phil Trans Roy Soc Lond B 355: 1361-1370
9. Demmig-Adams B, Adams III W (1996) The role of xanthophyll cycle carotenoids in the protection of photosynthesis. Trends Plant Sci 1: 21-26
10. Horton P, Johnson MP, Pérez-Bueno ML, Kiss AZ, Ruban AV (2008) Photosynthetic acclimation: does the dynamic structure and macro-organisation of photosystem II in higher plant grana membranes regulate light harvesting states? FEBS J 275: 1069-1079
11. Li XP, Bjorkman O, Shih C, Grossman AR, Rosenquist M, Jansson S, Niyogi KK (2000) A pigment-binding protein essential for regulation of photosynthetic light harvesting. Nature 403: 391-395