Inhibition of lycopene cyclase results in accumulation of chlorophyll precursors

Abstract

Free porphyrins and their magnesium complexes, including chlorophylls, are potent photo-sensitizers. Plants usually accumulate these compounds bound to proteins together with protective compounds like carotenoids. Besides their protective role, carotenoids can play a structural role in these complexes. To analyze the effect of impaired carotenogenesis on plastid membranes we applied to barley seedlings the bleaching herbicide 2-(4-chlorophenylthio)triethylamine (CPTA) as a specific inhibitor for the cyclization of lycopene. To avoid interference with photo-oxidation, the essential experiments were performed on seedlings grown in darkness. While the amount of total carotenoids decreased, we found accumulation of more 6-carotene than lycopene in darkness clearly showing that CPTA inhibits the lycopene beta-cyclase more effectively than the lycopene epsilon-cyclase. The CPTA treatment resulted in accumulation of non-photoactive protochlorophyllide a; the amount of photoactive protochlorophyllide and NADPH:protochlorophyllide oxidoreductase remained constant. Further, the level of Mg protophorphyrin and its monomethyl ester increased to an extent similar to that obtained by application of 5-aminolevulinic acid (ALA). The perturbation of the ultrastructure of etioplast inner membranes, observed after CPTA-treatment, was not found after ALA-treatment; this excluded the accumulated tetrapyrroles as responsible for the perturbation. By contrast, the down-regulation of Lhcb and RbcS genes found after CPTA-treatment was compatible with the presumed role of Mg protophorphyrin as "plastid signal" for regulation of nuclear gene expression. Possible mechanisms for enhancement of tetrapyrrole accumulation by non-cyclic carotenoids are discussed.

Mutations of genes in synthesis of the carotenoid precursors of ABA lead to pre‐harvest sprouting and photo‐oxidation in rice

“…The accumulation of lycopene may be explained by the presumption that lycopene ε-cyclase and lycopene β-cyclase constitute an enzyme complex in the thylakoid membranes of plant chloroplasts; in the absence of β-cyclase, the complex is destabilized, resulting in the loss of both ε- and β-cyclase activity and so lycopene accumulates ( Cunningham and Gantt, 1998 ). The results are consistent with 2-(4-chlorophenylthio)-triethylamine hydrochloride (CPTA)-treated barley seedlings (CPTA is a specific inhibitor of the cyclization of lycopene, especially lycopene β-cyclase; La Rocca et al , 2007 ). In addition, PSY mRNA is virtually absent in rice endosperm, which does not provide the substrate for these downstream enzymes and is consequently unable to form downstream products ( Schaub et al , 2005 ).…”

Carotenoids and Photosynthesis - Regulation of Carotenoid Biosyntesis by Photoreceptors

“…In addition, the psy gene, the fundamental gene that controls the biosynthesis of carotenoids, is co-expressed with photosynthetic genes that codify for plastoquinone, NAD(P)H deshydrogenase, tiorredoxin, plastocianin and ferredoxin (Meier et al, 2011). Moreover, according to the induction of carotenogenic genes during de-etiolation, chloroplyll genes are also induced (Woitsh & Römer, 2003) and the inhibition of lycopene cyclase with 2-(4 chlorophenylthiotriethyl-amine (CPTA) leads to accumulation of non-photoactive protochlorophyllide a (La Rocca et al, 2007). Also, PIF1 has been shown to bind to the promoter of PORC gene encoding Pchilide oxidoreductase whose activity is to convert Pchlide into chlorophylls (Moon et al, 2008).…”

BYPASS1: How a Tiny Mutant Tells a Big Story about Root‐to‐shoot Signaling

“…Alternatively, because the inhibitors induce photooxidation, the data could also indicate that the bps1 signal is synthesized in plastids. Although the inhibitor studies were carried out under low light conditions, inhibition of carotenoid biosynthesis might still have led to root plastid photooxidation (La Rocca et al 2007). Identification of the mobile signaling molecule is a high priority of future research.…”