Abstract
Plants protect themselves from excess absorbed light energy through thermal dissipation, which is measured as nonphotochemical quenching of chlorophyll fluorescence (NPQ). The major component of NPQ, qE, is induced by high transthylakoid DeltapH in excess light and depends on the xanthophyll cycle, in which violaxanthin and antheraxanthin are deepoxidized to form zeaxanthin. To investigate the xanthophyll dependence of qE, we identified suppressor of zeaxanthin-less1 (szl1) as a suppressor of the Arabidopsis thaliana npq1 mutant, which lacks zeaxanthin. szl1 npq1 plants have a partially restored qE but lack zeaxanthin and have low levels of violaxanthin, antheraxanthin, and neoxanthin. However, they accumulate more lutein and alpha-carotene than the wild type. szl1 contains a point mutation in the lycopene beta-cyclase (LCYB) gene. Based on the pigment analysis, LCYB appears to be the major lycopene beta-cyclase and is not involved in neoxanthin synthesis. The Lhcb4 (CP29) and Lhcb5 (CP26) protein levels are reduced by 50% in szl1 npq1 relative to the wild type, whereas other Lhcb proteins are present at wild-type levels. Analysis of carotenoid radical cation formation and leaf absorbance changes strongly suggest that the higher amount of lutein substitutes for zeaxanthin in qE, implying a direct role in qE, as well as a mechanism that is weakly sensitive to carotenoid structural properties.
Highlights
Light is required for photosynthesis in plants, but the quantity of light in natural environments is highly variable
The suppressor was named suppressor of zeaxanthin-less1, and the szl1 npq1 plant was backcrossed to the npq1 parent three times and crossed to the wild type to isolate an szl1 single mutant, which was able to synthesize zeaxanthin upon exposure to high light (Figure 3B)
The wild-type, npq1, szl1, and szl1 npq1 plants were grown under low light (LL) conditions (150 mmol photons m22 s21)
Summary
Light is required for photosynthesis in plants, but the quantity of light in natural environments is highly variable. There are several components of NPQ in higher plants, pH-dependent energy dissipation ( called qE) accounts for the major part of NPQ and results in deexcitation of 1Chl* and the thermal dissipation of excess absorbed light energy in the lightharvesting antenna of PSII (Muller et al, 2001). Because it involves the deexcitation of 1Chl*, qE can be measured as a decrease in the maximum yield of chlorophyll fluorescence in intact leaves or isolated chloroplast membranes (Muller et al, 2001)
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