Introduction relative enzymes.
1. Introduction relative enzymes.
Light was a fundamental source of energy but is also
2. Functions of Xanthophyll Cycle
potentially harmful to organisms [1]. Plants had In 1962, Yamamoto et al. [9] firstly discovered the evolved a variety of regulatory mechanisms to respond presence of xanthophyll cycle in higher plants. Untill to the naturally varying light conditions [2]. 1987, Demmig et al. [10] found that Z might be related Xanthophyll cycle is now recognized as a key regulator with the dissipation of excess light energy, then and photoprotective mechanism found ubiquitously in xanthophyll cycle became attractive. Xanthophyll plants [3, 4]. Xanthophyll cycle, also known as cycle had multiple functions, such as thermal violaxanthin cycle, is that violaxanthin (V) transforms dissipation, protection against oxidative stress caused into anthers yellow mass (A) and zeaxanthin (Z) by light, modulation of the structure of thylakoid occuring de-epoxidation reaction in the bright light, membrane, involving in blue light signal transduction and that Z re-transforms into V occuring epoxidation in and regulating the synthesis of ABA. low light or dark [5, 6].
Xanthophyll cycle was wide spread in higher plants,
2.1 Thermal Dissipation
ferns, mosses and some algae, located in the thylakoid Xanthophyll cycle-dependent thermal dissipation membrane. There was a difference that the was believed to be the primary mechanism widely violaxanthin cycle was in higher plants, and that the occurring in plants [11-15]. Thermal dissipation was dinoflagellates flavin silicon cycle was in some algae, believed to be the primary photoprotective mechanism but the two loops were both formed Z or A [7]. There against excessive light energy in plants [11, 12]. Under was neither violaxanthin cycle nor dinoflagellates normal circumstances, 10% of the light energy in PSII flavin silicon cycle in Cyanobacteria, in which transformed Chl*, which could be consumed by 3 β-carotene become into Z in bright light [8]. We Xanthophyll, while 100% in separated LHCII. In outlined the functions of xanthophylls cycle and its midday under field conditions, when light was the only
Corresponding author: Ming-An Liao, Ph.D., professor, stress, 75% of the light energy that plant leaves research field: plant physiology. E-mail: [email protected].
absorbed was quenched through thermal dissipation
Xanthophyll Cycle and Its Relative Enzymes
[16]. The mutant of lack of effective thermal temperature, thereby could increase the tolerance of dissipation performed more sensitive to strong light,
photosynthetic apparatus against light and temperature especially in strong light with other stress factors.
[22]. However, the PSⅡstability decreased in an A. Numerous evidences showed that in photosynthetic
thaliana mutant with high Z and no V [27]. VDE apparatus of higher plants, the amount of Z and A was
enzyme existed in the etiolated Phaseolus vulgaris related to the extent of thermal dissipation, measured as
leaves, the activity of which was inhibited by DTT. NPQ [17, 18]. NPQ was inhibited by 80 % or more
This indicated VDE enzyme appeared before the when leaves were fed with DTT, an inhibitor of VDE
thylakoid and photosynthetic activity did. [17-19]. So far, most of the experiments support the
2.4 Involving in Blue Light Signal Transduction and view that thermal dissipation quenching was dependent
Regulating the Synthesis of ABA
on xanthophyll cycle. In normal photosynthesis, thylakoid lumen pH
Xanthophyll cycle was involved in blue signal light should be maintained at 5.5 or so. And generally
transduction in plants and Z was considered to be a speaking, the thermal dissipation of the main limiting
photoreceptor. In the dark, the cotton coleoptile factor was the formation of Z.
accumulating V but lacking of Z was not bent when induced by blue light pulse. But the degree of bending
2.2 Protection Against Lipid Peroxidation of the one containing different levels of Z was
The chlorophyll of thylakoid membrane was a type positively correlated with the increase of Z when of photosensitive material. Havaux et al. [20] and Sarry
induced by blue light pulse. The formation of Z and the [21] showed when Z production was inhibited by DTT,
side of openness of the thylakoid membrane matrix light stress would strongly enhance lipid degradation,
caused by blue light pulse, which was associated with and caused the peroxidation of unsaturated fatty acids
the concentration of Z, were inhibited by DTT. in the thylakoid membrane. It was thus suggested that
Somebody proposed Z was an intermediate pathway xanthophyll cycle might protect lipids from oxidative
of the synthesis of ABA. So regulating the synthesis of stress. In fact, some other experiments had shown that
ABA may be another function of xanthophyll cycle. In lutein and Z could quench the singlet state oxygen [22,
excessive light energy, VDE maybe reduce the 23].
synthesis of ABA. ZE was involved both in xanthophyll cycle and the synthesis of ABA precursors
2.3 Modulation of the Structure of the Thylakoid
Membranes