Unusual carotenoid composition and a new type of xanthophyll cycle in plants

Abstract

The capture of photons by the photosynthetic apparatus is the first step in photosynthesis in all autotrophic higher plants. This light capture is dominated by pigment-containing proteins known as light-harvesting complexes (LHCs). The xanthophyll-carotenoid complement of these LHCs (neoxanthin, violaxanthin, and lutein) is highly conserved, with no deletions and few, uncommon additions. We report that neoxanthin, considered an integral component of LHCs, is stoichiometrically replaced by lutein-5,6-epoxide in the parasitic angiosperm Cuscuta reflexa, without compromising the structural integrity of the LHCs. Lutein-5,6-epoxide differs from neoxanthin in that it is involved in a light-driven deepoxidation cycle similar to the deepoxidation of violaxanthin in the xanthophyll cycle, which is implicated in protection against photodamage. The absence of neoxanthin and its replacement by lutein-5,6-epoxide changes our understanding of the structure-function relationship in LHCs, has implications for biosynthetic pathways involving neoxanthin (such as the plant hormone abscisic acid), and identifies one of the early steps associated with the evolution of heterotrophy from autotrophy in plants.

Figure 1

Cuscuta species are parasitic on the above-ground parts of other plants. The parasites consist of a twining stem that lacks leaves and has no roots. They are dependent on the host plant for organic and inorganic solutes that they extract by means of haustoria, primarily from the host phloem (12). (A) C. reflexa growing on a Coleus host and (B) a close view of the host–parasite attachment with C. reflexa attached by haustoria to the stem of Coleus.

Figure 2

Carotenoid composition of higher plants. (A) HPLC separation of pigments from dark-adapted samples from S. oleracea (dark) and C. reflexa (dark), C. reflexa exposed to light at 1,200 μmol photons m⁻² s⁻¹ for 2–4 h [C. reflexa (light)]. The identification of the lutein-5,6-epoxide in C. reflexa was achieved by comparing its elution times and absorption spectrum (inset) with a pure sample of lutein-5,6-epoxide (standard); they were identical. N, neoxanthin; V, violaxanthin; Lx, lutein-5,6-epoxide; A, antheraxanthin; L, lutein; Z, zeaxanthin; a, chlorophyll a; b, chlorophyll b; C, β-carotene. (B) Biosynthetic pathway of carotenoids showing that neoxanthin is synthesized later than β-carotene-derived xanthophyll cycle carotenoids and distinct from α-carotene-derived lutein.

Figure 3

Light-driven carotenoid cycles in C. reflexa. (A) Time-course analysis of xanthophylls after exposure of C. reflexa filaments to high irradiance. Plants were dark adapted for 12 h before exposure to 1,200 μmol m⁻² s⁻¹. (B) Analysis of xanthophylls from C. reflexa over a diurnal period. Samples were taken predawn (predawn I), midday, and predawn the following day (predawn II). We propose that two distinct light-driven carotenoid cycles operate in C. reflexa (C): the xanthophyll cycle and a lutein-5,6-epoxide cycle.

Figure 4

Lutein-5,6-epoxide-containing and neoxanthin-containing thylakoids and LHCIIb. The absorption spectra of C. reflexa and S. oleracea thylakoids (A) and LHCIIb (B). The arrow indicates the accentuated absorption at around 490 nm. (C) The 77 K fluorescence spectra of LHCIIb from C. reflexa and S. oleracea (overlapping). (D) Similar preparations of LHCIIb followed by centrifugation through a sucrose concentration gradient to separate bands of trimers (Tr) from monomers (M) of LHCIIb and free pigment (Fp).