Shedding light on how photoisomerisation of cis-carotenes can rapidly regulate nuclear gene expression in plants

Alagoz Y, Anantanawat K, Papanicolaou A and Cazzonelli C

Western Sydney University, Hawkesbury Institute for the Environment, 2753 Richmond Sydney Australia.

Carotenoids are organic pigments that are naturally synthesized by plants, algae, and some photosynthetic bacteria. The biosynthesis of carotenoids consists of a multistep pathway regulated by key environmental factors such as like light and temperature. Unlike bacteria, plants have evolved four additional enzymes that generate cis-carotene intermediates, mostly in fruits and in dark grown tissues. The light-mediated conversion of cis-carotenes to their cis/trans geometric isomers is an enigmatic phenomenon referred as photoisomerisation. The in-vivo conversion of prolycopene to all-trans-lycopene is a rate-limiting step before the branching of the pathway to ε- and β-carotenoids that drive photosynthetic and photoprotective functions. In-vitro photoisomerisation has been shown to be catalysed by hours of light and is reversible by higher temperatures. Here we have developed a novel in-vivo bioassay to shed real light on the photoisomerisation mechanism by exposing tomato mutant defective in the CAROTENOID ISOMERASE to different intensities of light. We demonstrate that high light can mediate photoisomerisation of z-carotene to neurosporene (orange) and tetra-cis-lycopene to all-trans-lycopene (pinkish-red pigment) rapidly. Our preliminary analysis also reveals rapid changes in nuclear gene expression. We hypothesise that cis-carotenes can act like photo-switches and oxidative cleavage of cis-carotenes can generate apocarotenoid signals (ACS) in the chloroplast to enable the synergistically feedback control over nuclear gene expression. As such the phytochromes, cryptochromes and apocarotenoid signalling molecules generated during/after photoisomerisation of cis-carotenes can mediate the early steps of photomorphogenesis.