Elucidating a novel apocarotenoid signal controlling root development in plants

Abstract

Carotenoids are organic pigments that are essential for animal and human health. They provide a precursor for Vitamin A synthesis and are required for the prevention of eye diseases and certain types of cancer. In plants, they facilitate photosynthesis and photoprotection. Carotenoids can undergo specific enzymatic cleavage by carotenoid cleavage dioxygenases (CCD's) to produce phytohormones (e.g. strigolactone and abscisic acid) and apocarotenoids (e.g. B-ionone), of which some are generated by non-enzymatic oxidative cleavage (e.g. B-cyclocitral). Apocarotenoids such as B-cyclocitral act as signalling molecules to not only maintain cellular homeostasis in response to photo-oxidative stress, but also promote cell divisions in root meristems, stimulating root branching and growth. In addition to B-carotene, cis-carotenes have also been proposed to act as substrates for the generation of apocarotenoid metabolites that control shoot development and plastid biogenesis. The carotenoid-derived substrates and apocarotenoid signalling pathways that control root development remain to be elucidated. Interestingly, it was noted that the loss-in-function of the CAROTENOID ISOMERASE (CRTISO) gene causes a reduction in the primary root length and number of lateral roots in Arabidopsis mutants as compared to wild-type. The mutants (ccr2) lack CRTISO activity and show an early anchor root formation. Root tissues of the mutant grown on artificial media under fluorescent light were observed to accumulate high levels of B-carotene, an altered xanthophyll composition and trace amounts of cis-carotenes. I hypothesized that an apocarotenoid signal generated from either B-carotene or an upstream cis-carotene can control root architecture in Arabidopsis. A forward genetics approach using chemical mutagenesis successfully generated six to twelve independent revertant ccr2 (rccr2) mutant lines that reproducibly (rccr2#3, 6, 7, 11, 14, 18, 25, 26, 32, 33, 35, 37) displayed a longer primary root, varied lateral and anchor root formation compared to ccr2. Quantification of carotenoid content in leaves and roots of ccr2 and rccr2 mutant lines using HPLC verified reduced lutein, altered xanthophyll compositions and the accumulation of cis-carotenes compared to WT. The first objective of the study was to determine why the ccr2 primary roots were shorter. The next question that I addressed was if the carotenoid substrate required enzymatic cleavage to generate an apocarotenoid signal. In conclusion, with this study, I have ruled out the likelihood of a cis-carotene produced in ccr2 roots as a substrate for the production of a cleavable signal. Rather, I provide evidence that a B-carotene derived cleavage product generated in ccr2 roots may be regulating root cell expansion in the elongation zone and the timing of anchor root formation.

Date of Award	2018
Original language	English