Light induction of carotenoid biosynthesis genes in the green alga Haematococcus pluvialis: regulation by photosynthetic redox control.

The unicellular green alga Haematococcus pluvialis accumulates large amounts of the red ketocarotenoid astaxanthin when exposed to various stress situations such as salt stress and high light intensities. Here, the light regulation of Haematococcus carotenoid biosynthesis was examined. Isolation and characterization of the lycopene beta cyclase gene involved in carotenoid biosynthesis was carried out using a functional complementation approach. Subsequently, gene expression of lycopene cyclase, phytoene synthase, phytoene desaturase and carotenoid hydroxylase was analysed in green flagellate cells. All four genes revealed higher transcript levels in response to increased illumination. Not only the induction of astaxanthin biosynthesis but also carotenoid gene expression was found to be correlated with the redox state of the photosynthetic electron transport. In accordance with this result, increased transcript levels for carotenoid biosynthesis genes were detected under both blue and red light conditions. The application of different inhibitors of the photosynthetic electron flow indicated that the photosynthetic plastoquinone pool functions as the redox sensor for the up-regulation of carotenoid biosynthesis genes. These results suggested that in Haematococcus not only the specific astaxanthin pathway but also general carotenoid biosynthesis is subject to photosynthetic redox control.