Xanthophyll cycle mechanisms

Latowski, D., J. Grzyb, and K. Strzalka. 2004. The xanthophyll cycle—molecular mechanism and physiological significance. Acta Physiol. Plant. 26 197-212. 

S. Demers, S. Roy, R. Gagnon, C. Vignault (1991). Rapid light-induced changes in cell fluorescence and in xanthophyll-cycle pigments of Alexandrium excavatum (Dinophyceae) and Thalassiosira pseudonana (Bacillariophyceae) A photo-protection mechanism. Mar. Ecol Prog. Ser., 76,185-193. 

Central to elucidating the mechanism of qE is to understand the role of the xanthophyll cycle. Eirst it is necessary to obtain information on where within the LHCII system the active xanthophyll carotenoids are bound. Information on the structural features of violaxanthin and zeaxanthin that determine their binding to LHCII is needed, just as such information is vital to understanding other allosteric effectors in... 

If the xanthophyll binding sites effective in quenching are peripheral to the LHCII complexes it would be expected that carotenoids added to the complexes in vitro would exert effects on quenching. In fact, such effects are found, and yield the clearest information on the mechanism of action of the xanthophyll cycle. 

These results shown that the xanthophyll cycle is light dependent in etioplasts as it is in thylakoids. Therefore the de-epoxidation must take place in other stroma membranes like prothylakoids or plastid envelope which have no photosynthetic activity. This mechanism would be triggered in response to pH changes, suggesting the idea that the main role of the xanthophyll cycle may be the photoprotection of the chlorophyll by dissipating the energy that this pigment cannot process. 

A regulatory mechanism to dissipate excess of excitation energy during high light exposure has been proposed to partially protect leaves from photoinhibition of PS2 photochemistry <9, 10). This mechanism is the non-radiative energy dissipation of excess excitation energy where the xanthophylls cycle may play an Important role (11). 

Interestingly, even at a concentration where it is beneficial to growth, Cd can become toxic if the Zn concentration becomes severely limiting [37,38]. This presumably reflects a loss of activity caused by Cd substitution for Zn in some essential Zn enzymes ([36,37] and references therein). Other mechanisms of Cd toxicity include oxidative stress, as reviewed in [52] and inhibition of photosynthesis via interference with the xanthophyll cycle in the diatom Phaeodactylum tricornutum [53]. 

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