Tissue-Specific Apocarotenoid Glycosylation Contributes to Carotenoid Homeostasis in Arabidopsis Leaves

Abstract

Attaining defined steady-state carotenoid levels requires balancing of the rates governing their synthesis and metabolism. Phytoene formation mediated by phytoene synthase (PSY) is rate limiting in the biosynthesis of carotenoids, whereas carotenoid catabolism involves a multitude of nonenzymatic and enzymatic processes. We investigated carotenoid and apocarotenoid formation in Arabidopsis (Arabidopsis thaliana) in response to enhanced pathway flux upon PSY overexpression. This resulted in a dramatic accumulation of mainly b-carotene in roots and nongreen calli, whereas carotenoids remained unchanged in leaves. We show that, in chloroplasts, surplus PSY was partially soluble, localized in the stroma and, therefore, inactive, whereas the membrane-bound portion mediated a doubling of phytoene synthesis rates. Increased pathway flux was not compensated by enhanced generation of long-chain apocarotenals but resulted in higher levels of C 13 apocarotenoid glycosides (AGs). Using mutant lines deficient in carotenoid cleavage dioxygenases (CCDs), we identified CCD4 as being mainly responsible for the majority of AGs formed. Moreover, changed AG patterns in the carotene hydroxylase mutants lutein deficient1 (lut1) and lut5 exhibiting altered leaf carotenoids allowed us to define specific xanthophyll species as precursors for the apocarotenoid aglycons detected. In contrast to leaves, carotenoid hyperaccumulating roots contained higher levels of b-carotene-derived apocarotenals, whereas AGs were absent. These contrasting responses are associated with tissue-specific capacities to synthesize xanthophylls, which thus determine the modes of carotenoid accumulation and apocarotenoid formation.

Carotenogenesis Is Regulated by 5′UTR-Mediated Translation of Phytoene Synthase Splice Variants

“…Upon pathway flux increase, such as in lines with higher PSY protein levels, steady-state carotenoid levels are maintained. This is only partially explained by increased CCD4-mediated cleavage into apocarotenoids (Lätari et al, 2015). This indicates that there are additional mechanisms capable of compensating flux overflow; the feedback control of carotenoid synthesis would represent the most sustainable solution.…”

“…S4). This compensation of flux increase is at least partially caused by CCD4-mediated carotenoid degradation leading to the formation of apocarotenoids (Lätari et al, 2015). In contrast, roots do respond with carotenoid accumulation upon flux increases (Maass et al, 2009;Welsch et al, 2010).…”

“…1, E and F). PSY activity was tested in the presence of norflurazon, which inhibits phytoene desaturation, resulting in the accumulation of phytoene (Beisel et al, 2010;Lätari et al, 2015). This revealed that phytoene synthesis was completely stalled during dark adaptation but became active upon illumination, concomitant with increased ASV2 expression and AtPSY protein levels.…”

Involvement of aerial organs on the ABA accumulation in roots of Citrus plants under water deficit

“…In this sense, dehydration could be triggering processes in roots which involve an extensive transformation of carotenoid pool. Indeed, under stress conditions, carotenoid-derived signaling compounds such as SLs or other volatile norisoprenoid compounds, with different molecular weight and structure and diverse functions in plants, are actually produced (Rubio-Moraga et al, 2014;Ha et al, 2014;Avendaño-Vázquez et al, 2014;Lätari et al, 2015;. Furthermore, the similar structure among carotenoids ( Fig.…”

“…Despite ABA, several carotenoid derivatives such strigolactones (Ha et al, 2014) (Lätari et al, 2015). In addition, carotenoids can quench molecules of ROS generated during stress counteracting their harmful effects on physiology (Ramel et al 2012a;Lado et al 2016) The oxidation of carotenoids renders different derivatives such as short-chain volatile aldehydes or ketones (β-ionone or β-cyclocitral) that could act as stress signals (Ramel et al 2012b).…”