Distribution and geometric isomerism of neoxanthin in oxygenic phototrophs: 9'-Cis, a sole molecular form

Abstract

The distribution and geometric form (cis or trans) of neoxanthin, one of the major carotenoids of oxygenic phototrophs, were systematically examined. The 9'-cis form of neoxanthin, but not the all-trans form, was found in chloroplasts of seed plants, ferns, mosses and green algae, all of which contain chlorophylls a and b. In contrast, neoxanthin was not found in other algal classes, such as Heterokontophyta, Rhodophyta and oxygenic phototrophic prokaryotes. Consequently, with regard to phylogeny, the appearance of neoxanthin appears to be associated with the appearance of chlorophyll b. In non-photosynthetic organs, such as petals and fruits, the presence of neoxanthin was classified into four types; those having only the 9'-cis form, those with only the all-trans form, those with both forms, and those without either form. Thus only 9'-cis neoxanthin is found in chloroplasts, and the all-trans neoxanthin is found only in non-photosynthetic organs. Because the absorption spectra of both forms are almost identical, their functions in photosynthesis might be similar. 9'-Cis neoxanthin is not involved in the xanthophyll-cycle, whereas it is a suitable substrate for abscisic acid synthesis. Differences in geometric isomerism are discussed in relation to abscisic acid synthesis.