Protection against strong-light-induced photodamage of the photosynthetic apparatus and entire organisms is a vital activity in plants and is also realized at the molecular level of the antenna complexes. Reported recently, the regulatory mechanisms which operate in the largest plant antenna complex, LHCII (light-harvesting complex II), based on light-driven processes, are briefly reviewed and discussed. Among those processes are the light-induced twisting of the configuration of the LHCII-bound neoxanthin, the light-induced configurational transition of the LHCII-bound violaxanthin, the light-induced trimer–monomer transition in LHCII and the blue-light-induced excitation quenching in LHCII. The physiological importance of the processes reviewed is also discussed with emphasis on the photoprotective excitation quenching and on possible involvement in the regulation of the xanthophyll cycle.
Collecting light quanta to power photosynthetic reactions is a primary task of plants and therefore several regulatory mechanisms, operating at all of the organizational levels of organisms, act to maximize captured energy. Among those regulatory mechanisms are the phototropic reactions of entire plants , chloroplast phototranslocation within a cell  and the phosphorylation-controlled migration of antenna complexes within the thylakoid membranes, referred to as the state I–state II transition . On the other hand, overexcitation of the photosynthetic apparatus can cause generation of the chlorophyll triplet states followed by formation of singlet oxygen and free radicals and in consequence can result in photodamage of the photosynthetic apparatus. The same physiological regulatory mechanisms which act to maximize captured light energy are also specialized in protecting the photosynthetic apparatus of plants against lethal photodegradation. Interestingly, the regulation acting to maintain a fine balance between collecting light energy and photoprotection is also realized at the molecular level, within separate pigment–protein antenna complexes. Light-induced regulatory mechanisms in the largest plant antenna complex, LHCII (light-harvesting complex II), which are a part of the overall response of the photosynthetic apparatus to light stress conditions, are briefly reviewed in the present paper.
Light-driven mechanisms in LHCII
The largest photosynthetic pigment–protein light-harvesting complex of plants, LHCII (Figure 1), apart from chlorophyll a and chlorophyll b (eight and six molecules per protein monomer respectively), comprises four molecules of xanthophylls: two molecules of lutein, one of neoxanthin and one of violaxanthin . Interestingly, the complex appears in a trimeric form in its native state [4,5]. Single-molecule fluorescence lifetime analysis has shown that the chlorophyll a fluorescence lifetime in the monomeric LHCII is essentially shorter compared with the trimeric complex . A sufficiently long lifetime of singlet excited states in the accessory pigment network seems to be a crucial requirement for effective long-range excitation transfer and therefore the formation of the LHCII trimeric structures can be interpreted as a molecular mechanism acting to improve photosynthetic capacity of the complex. Interestingly, under strong light illumination, a process of light-induced trimer–monomer transition in LHCII has been observed and interpreted in terms of a thermo-optic mechanism . The LHCII-bound violaxanthin photo-isomerization has been observed under similar conditions . The fact that violaxanthin is a pigment which is specifically localized in the trimeric LHCII at the borders of the individual monomers  (Figure 1B) suggests that both of these light-dependent mechanisms are related to each other. Very recently, a reversible light-induced change in the molecular configuration of the LHCII-bound xanthophyll, tentatively assigned to violaxanthin, was revealed on the basis of the resonance Raman study . The light-induced photo-isomerization and the molecular configuration transition of violaxanthin can be a direct photochemical reaction, but it can be also a thermally assisted process, under conditions of enhanced thermal energy dissipation in overexcited LHCII .
Molecular structure of LHCII (A) and trimeric organization of the complex (bottom view, B)
The resonance Raman analysis revealed also the light-induced twist in configuration of the LHCII-bound xanthophyll neoxanthin . This mechanism has also been identified to be active in isolated chloroplasts and entire plants and has been correlated with photoprotective high-energy excitation quenching referred to as qE . A molecular mechanism has been proposed, on the basis of neoxanthin-configuration-related change in the conformation of LHCII, which opens a channel of excessive energy dissipation by transfer to one of the protein-bound lutein molecules .
Several regulatory physiological processes in plants are controlled by blue light . Very recently, the FLIM (fluorescence lifetime imaging microscopy) study on isolated LHCII has revealed that blue light specifically induces a protein transformation from the state characterized by a relatively long chlorophyll fluorescence lifetime to a state characterized by a shorter lifetime . Shortening of the chlorophyll fluorescence lifetime reflects more efficient singlet excitation thermal dissipation and therefore the light-dependent process reported can be discussed in terms of a photoprotective activity within LHCII. The fact that this process is driven by blue light (absorbed both by carotenoids and chlorophylls) and is not active while illuminating with red light (absorbed exclusively by chlorophylls)  implies that the LHCII-bound xanthophylls are active as photoreceptors. A detailed molecular mechanism responsible for driving this light-dependent process remains to be elucidated. It is very likely that the light-driven transformation of the molecular configuration of the LHCII-bound xanthophylls and the light-induced trimer–monomer transition in LHCII, discussed above, are directly and causatively related to the light-induced shortening of the fluorescence lifetimes. This is owing to the fact that the chlorophyll a fluorescence lifetimes in the trimeric LHCII are longer, on average, than in the monomeric complex [6,13]. It is therefore possible that the light-driven chlorophyll fluorescence quenching, observed in isolated LHCII [14–18], reflects the process of light-induced monomerization of trimeric structures and shortening of fluorescence lifetimes reported.
Another interesting issue, related to the light-driven molecular processes in LHCII, seems to be the regulation of the xanthophyll cycle . Operation of the xanthophyll cycle in the photosynthetic apparatus requires violaxanthin to be freely available within the lipid phase of the thylakoid membrane, to complete the two steps of the enzymatic de-epoxidation to zeaxanthin . Violaxanthin is a xanthophyll relatively weakly bound to the protein environment of LHCII and the process of light-driven change of molecular configuration of this pigment can result in its uncoupling from the protein and transfer to the lipid environment of the membrane. Certainly, the light-dependent LHCII monomerization makes it easier, or even possible, for violaxanthin to migrate from the protein to the lipid environment. Violaxanthin in the all-trans fully relaxed configuration is a specific substrate of the de-epoxidase enzyme , and the pigment tends to adopt such configuration, after the light-driven transformations, because of the energy-minimization process . This makes the reaction of light-driven molecular configuration change of violaxanthin relevant and important from the physiological point of view.
A model of photoprotection, realized at the molecular level in the plant antenna complex LHCII, emerges from the analysis of the light-driven mechanisms discussed above (Figure 2). Upon overexcitation, the antenna complex LHCII, remaining in the trimeric form under physiological conditions, undergoes a transition to the monomeric state. Such a transition results in effective thermal energy dissipation. The dissipation can be even more efficient upon formation of aggregated structures of LHCII, which are composed of monomers (without trimeric sub-organization) [6,13,21]. It is worth mentioning that very recent examination of molecular organization of LHCII, on the basis of FLIM, revealed that violaxanthin stabilized the trimeric organization of the complex, in contrast with zeaxanthin which promoted the monomeric state of LHCII (W.I. Gruszecki, M. Zubik, R. Luchowski, E. Janik, W. Grudzinski, M. Gospodarek, J. Goc, L. Fiedor, Z. Gryczynski and I. Gryczynski, unpublished work).
Model of light-induced transformation of trimeric LHCII, leading to photoprotection realized via enhanced thermal energy dissipation
The results of the recent research overviewed above emphasize a central role of violaxanthin in sensing overexcitation conditions and in mediating photoprotection response at the level of LHCII. The rate of excitation energy transfer from violaxanthin to chlorophylls in LHCII is extremely low  and therefore light energy absorbed by violaxanthin may be utilized to drive configurational transformation of the pigment.
Experimental Plant Biology: Why Not?!: 4th Conference of Polish Society of Experimental Plant Biology, an Independent Meeting held at Jagiellonian University, Krakow, Poland, 21–25 September 2009. Organized and Edited by Kazimierz Strzałka (Jagiellonian University, Krakow, Poland).
I thank the many co-authors of my original publications who have helped me gain insight into the interesting topic of light-dependent dynamics of LHCII.
The Ministry of Science and Higher Education of Poland is acknowledged for financial support [grant number N N303 285034].