Potential implications for epigenetic regulation of carotenoid biosynthesis during root and shoot development

Abstract

Major regulators of carotenoid biosynthesis have remained rather elusive even though the flux through the branch in the carotenoid pathway can affect plant development in response to environmental stimuli, such as light. Our recent investigations demonstrated that the production of the most abundant carotenoid in plants, lutein, is regulated by carotenoid isomerase (CRTISO) activity at a rate-limiting step of this branch point in carotenoid biosynthesis. CRTISO is required to isomerase cis-carotenes, such as tetra-cis-lycopene to all-trans-lycopene. In order to maintain permissive transcriptional regulation of CRTISO, active marks of histone lysine methylation are targeted to the promoter region by the SET DOMAIN GROUP8 (SDG8) methyltransferase. Mutants of SDG8 (ccr1) and CRTISO (ccr2) show an increase in shoot branching, which may be partly explained by limiting synthesis of the carotenoid-derived branching hormone, strigolactone. In this addendum, we demonstrate new functions for SDG8 in mediating branching in Arabidopsis roots. The roles that carotenoids and SDG8 play in root and shoot development begins to open new doors for investigating the regulation of carotenoid composition in response to epigenetic events.

Keywords: SDG8; arabidopsis; carotenoid; chromatin; epigenetic; histone; lutein; mRNA; methylation; regulation; root development; shoot branching; transcription.

Figure 1

Schematic diagram of the carotenoid biosynthetic pathway showing key regulatory steps. The bottleneck in the pathway is the first committed step in the synthesis phytoene from geranylgeranyl pyrophosphate and is catalyzed by the light responsive enzyme, phytoene synthase (PSY). The branch point in the pathway involves the isomerisation of tetra-cis-lycopene to all-trans-lycopene by the carotenoid isomerase (CRTISO), which is regulated by a histone methyltransferase, SET DOMAIN GROUP8 (SDG8). Lycopene undergoes further modifications by εLCY (epsilon cyclase) and β-LCY (beta-cyclase) to produce α- and β-carotene, respectively, which serve as substrates for the production of lutein, phytohormones (abscisic acid and strigolactones) and an unknown signaling compound mediated by BYPASS (BPS). ccr: carotenoid and chloroplast regulation, lut2: lutein deficient mutant, CCD: CAROTENOID CLEAVAGE DIOXYGENASE.

Figure 2

Root growth and branching in the carotenoid and chloroplast regulatory mutant ccr1. Seed was sterilized, plated onto Murashige and Skoog media (4.4 g/L MS salts, 1x MS salts, 0.5 g/L sucrose, 0.8% agar) and incubated in the dark for two days at 4°C before transferring plates to a growth chamber maintained at 21°C and illuminated by 150 µE of light for 16 hours. (A) Images of 17 day old plantlets from wild type Columbia (Col) and ccr1-4. (B) Average root length after 7, 10, 12 and 17 days of growth. (C) The average number of secondary root branches along the primary root after 12 and 17 days of growth. Standard error of 18 to 24 individual seedlings are displayed. Statistically significant data are indicated by a star (TTEST, 2 tailed, unpaired, p < 0.005).