Apocarotenoids: A New Carotenoid-Derived Pathway.

Carotenoids are precursors of carotenoid derived molecules termed apocarotenoids, which include isoprenoids with important functions in plant-environment interactions such as the attraction of pollinators and the defense against pathogens and herbivores. Apocarotenoids also include volatile aromatic compounds that act as repellents, chemoattractants, growth simulators and inhibitors, as well as the phytohormones abscisic acid and strigolactones. In plants, apocarotenoids can be found in several types of plastids (etioplast, leucoplast and chromoplast) and among different plant tissues such as flowers and roots. The structural similarity of some flower and spice isoprenoid volatile organic compounds (ß-ionone and safranal) to carotenoids has led to the recent discovery of carotenoid-specific cleavage oxygenases, including carotenoid cleavage dioxygenases and 9-cis-epoxydioxygenases, which tailor and transform carotenoids into apocarotenoids. The great diversity of apocarotenoids is a consequence of the huge amount of carotenoid precursors, the variations in specific cleavage sites and the modifications after cleavage. Lycopene, ß-carotene and zeaxanthin are the precursors of the main apocarotenoids described to date, which include bixin, crocin, picrocrocin, abscisic acid, strigolactone and mycorradicin.The current chapter will give rise to an overview of the biosynthesis and function of the most important apocarotenoids in plants, as well as the current knowledge about the carotenoid cleavage oxygenase enzymes involved in these biosynthetic pathways.

Identification and Characterization of the Heat-Induced Plastidial Stress Granules Reveal New Insight Into Arabidopsis Stress Response.

Plants exhibit different physiological and molecular responses to adverse changes in their environment. One such molecular response is the sequestration of proteins, RNAs, and metabolites into cytoplasmic bodies called stress granules (cSGs). Here we report that, in addition to cSGs, heat stress also induces the formation of SG-like foci (cGs) in the chloroplasts of the model plant Arabidopsis thaliana. Similarly to the cSGs, (i) cpSG assemble rapidly in response to stress and disappear when the stress ceases, (ii) cpSG formation is inhibited by treatment with a translation inhibitor (lincomycin), and (iii) cpSG are composed of a stable core and a fluid outer shell. A previously published protocol for cSG extraction was successfully adapted to isolate cpSG, followed by protein, metabolite, and RNA analysis. Analogously to the cSGs, cpSG sequester proteins essential for SG formation, dynamics, and function, also including RNA-binding proteins with prion-like domain, ATPases and chaperones, and the amino acids proline and glutamic acid. However, the most intriguing observation relates to the cpSG localization of proteins, such as a complete magnesium chelatase complex, which is involved in photosynthetic acclimation to stress. These data suggest that cpSG have a role in plant stress tolerance.