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

The capture of photons by the photosynthetic apparatus is the first step in photosynthesis in all autotrophic higher plants. This light capture is dom inated by pigment-containing proteins known as light-harvesting complexes ( LHCs), The xanthophyll-carotenoid complement of these LHCs (neoxanthin, vio laxanthin, and lutein) is highly conserved, with no deletions and few, unco mmon additions. We report that neoxanthin, considered an integral component of LHCs, is stoichiometrically replaced by lutein-5,6 epoxide in the paras itic angiosperm Cuscuta reflexa, without compromising the structural integr ity of the LHCs, Lutein-5,6 epoxide differs from neoxanthin in that it is i nvolved 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 lute in-5,6-epoxide changes our understanding of the structure-function relation ship in LHCs, has implications for biosynthetic pathways involving neoxanth in (such as the plant hormone abscisic acid), and identifies one of the ear ly steps associated with the evolution of heterotrophy from autotrophy in p lants.