S4). Taken together, the levels of lutein and zeaxanthin inz2leaves were consistently lower regardless of permissive and restrictive conditions, indicating that low levels of these carotenoid pigments are not a critical factor for leaf variegation and1O2production inz2mutants. the dark periods under SD, but not under CL conditions. Its accumulation was also positively correlated with1O2levels generated during the light period, which consequently altered the expression of1O2-responsive and cell death-related genes in the variegatedz2leaves. Taking these results together, we propose that thez2leaf variegation can be largely attributed to photoperiodic accumulation of tetra-cis-lycopene and TAS-114 generation TAS-114 of excessive1O2under natural USP39 day-night conditions. Keywords:carotenoid isomerase, rice, singlet oxygen, tetra-cis-lycopene,zebra2 == INTRODUCTION == Carotenoids are among the most abundant phytochemicals in nature, and contain a C40isoprenoid backbone with polyene side chains. Carotenoids contribute to the yellow, orange, and red colors of fruits, vegetables, flowers, and autumn leaves. In all photosynthetic organisms, carotenoids play important roles in harvesting light energy and also in protecting cells from the harmful effects of high light intensity. In addition, carotenoids are also important for human beings and animals because they are involved in vitamin A activity (Rock, 1997), antioxidant activity (Hadley et al., 2002), and preservation of eye health (Fraser and Bramley, 2004). Carotenoids are essential in plants, both to increase the efficiency of photosynthesis and to protect the photosynthetic apparatus from the effects of excess light. Some carotenoids are incorporated into the light-harvesting complexes (LHCs) to transfer excitation energy to chlorophyll with a high efficiency (Grossman et al., 1995). Three kinds of xanthophylls (antheraxanthin, violaxanthin, and zeaxanthin) are involved in the xanthophyll cycle, which dissipates excess energy by non-photochemical quenching (NPQ) in LHCs to avoid photo-oxidative damage (Jahns and Holzwarth, 2011;Li et al., 2009;Muller et al., 2001). Lutein also can quench the excitation energy generated from excited chlorophyll. Mutant analysis revealed that lutein can compensate for the absence of zeaxanthin and allow NPQ function (Li et al., 2009). In the reaction center of Photosystem II, -carotene is essential for deactivation of excited singlet oxygen (1O2) generated by excess light irradiance (Telfer, 2002). During last two decades, the mechanisms of carotenoid metabolism have been elucidated by identification of enzymes in this pathway (Lu and Li, 2008). In higher plants, carotenoid biosynthesis begins with the conversion of geranylgeranyl pyrophosphate into 15-cis-phyotene by phyotene synthase (Giuliano et al., 1993). To synthesize lycopene, phytoene is dehydrogenated by two desaturases, phytoene desaturase and -carotene desaturase (Dong et al., 2007;Norris et al., 1995), which introduce a series of four double bonds in acis-configuration. Thecis-bonds are isomerized into all-trans-conformations by carotenoid isomerase (CRTISO) (Isaacson et al., 2002;Park et al., 2002) and Z-ISO (Chen et al., 2010;Li et al., 2007). All-trans-lycopene is a branch point in the carotenoid biosynthesis pathway. It is cycled by -cyclase (LCYE) or -cyclase (LCYB) to form cyclic carotenes. LCYE plays a crucial role in determining the ratio of -carotene/-carotene (Harjes et al., 2008). Both – and -carotene are then subjected to a series of oxygenation reactions to produce lutein and xanthophylls, respectively. Further modifications of xanthophylls lead to production of abscisic acid (Cornish and Zeevaart, 1988;DellaPenna and Pogson, 2006). In higher TAS-114 plants, the carotenoid species derived from all-trans-lycopene are associated with photosystem proteins (Grossman et al., 1995), and all-trans-lycopene by itself is the most abundant carotenoid in tomato fruits (Isaacson et al., 2002). Although the physiological and biochemical functions of most carotenoids have been intensively investigated, few studies have examined the possible effects of carotenoid intermediates on plant cells. In recent years, it has been revealed that some chlorophyll intermediates.