β-Cyclocitral Marked as Bioactive Compound in Plants: Comparison
Please note this is a comparison between Version 2 by Jessie Wu and Version 1 by Mohammad Faizan.

β-cyclocitral (βCC), a main apocarotenoid of β-carotene, increases plants’ resistance against stresses. It has recently appeared as a novel bioactive composite in a variety of organisms from plants to animals. In plants, βCC marked as stress signals that accrue under adverse ecological conditions. βCC regulates nuclear gene expression through several signaling pathways, leading to stress tolerance. 

  • β-carotene
  • plant
  • apocarotenoid

1. Biosynthesis of β-CCyclocitral

The formation of β-cyclocitral (βCC) occurs either by direct oxidation of β-carotene through ROS (1O2) or by an enzymatic pathway. A family of non-heme iron-dependent enzymes in plants catalyzes the carotenoids by an enzymatic cleavage via 9-cis-epoxycarotenoid cleavage dioxygenases (NCEDs) and carotenoid cleavage dioxygenases (CCDs), resulting in apocarotenoids, an oxidation product [34,35][1][2]. The first step in abscisic acid (ABA) production is catalyzed by NCED enzymes cleaving the 11, 12 (11′, 12′) double bond of 9-cis-violaxanthin or 9-cisneoxanthin [36][3]. Furthermore, CCD enzymes and NCED enzymes do not share cleavage specificities. In Arabidopsis, there are four CCDs (CCD1, CCD4, CCD7, and CCD8). It is unknown whether one of these CCDs creates βCC from carotene in plant leaves. In each of the four CCDs in Arabidopsis deficient mutants, the accumulation of βCC was not affected, which suggests that β-carotene oxidation mediated by CCD in this species is not a major source of this apocarotenoid [37][4], despite the fact that between 4 CCDs functional redundancy cannot be ruled out. This is similar in cyanobacteria, where βCC formation aided by CCD was not found [38][5]. Unlike CCDs that are plastidial, cytosolic CCD1 cleaves the double bonds of 9, 10 (9′, 10′) to produce varying volatiles and apocarotenoids of extensive acyclic or monocyclic apocarotenoids and carotenoids.
The strigolactones biosynthesis is dependent on CCD8 and CCD7 [39][6]. Since CCD4 has a specific cleavage activity at 9, 10 (9′, 10′) and 5, 6 (5′, 6′) double bond, it does not generate βCC [34,35][1][2]. Furthermore, in high light conditions, CCD4 is highly downregulated, which activates the accumulation of βCC [37][4]. However, the cleavage of β-carotene in citrus from the location 7, 8 (7′, 8′), CCD4b is reported under CCD4 enzyme, which results in the production of βCC [8][7]. Similarly, another CCD4c in the Crocus stigma from CCD4 can cleave carotenoids at 9-10 (9′, 10′), resulting into β-ionone and produces βCC with low efficiency at 7 and 8 (7′, 8′) [40][8]. For the production of βCC, CCD4b gene in Vitis vinifera in the carotenoid-accumulating yeast strain is also reported [41][9]. Another way for the oxidation of carotenoids can be provided by lipoxygenase [9][10]. Similarly, in leaves of Solanum lycopersicum and Arabidopsis, knockout mutants for 13-lipoxygenase LOX2 were reported to have low levels of βCC [26][11]. On the other hand, in the βCC accumulation under high light and 1O2 stresses, it is unknown if this enzyme is involved despite the fact that LOX2 is induced under these circumstances [37][4]. Eventually, from the fungus Lepista irina, extracellular fluid purified a peroxidase which produces βCC and other unstable apocarotenoids from the cleavage of β-carotene [42][12].
When compared to photosystem II, it is thought that photosystem I does not produce considerable amounts of 1O2. Auto-oxidation of β-carotene can also produce βCC, especially when attacked by the reactive specie 1O2 [31][13]. Carotenoids quench 1O2 through a physical mechanism that involves energy transfer from 1O2 to the carotenoid, followed by the excited quencher’s thermal decay [43][14]. However, carotene can be oxidised by 1O2, allowing 1O2 to be chemically quenched. 1O2 is an electrophilic molecule that has a strong affinity for double bonds in carotenoid molecules, oxidizing them and creating a range of apocarotenoids, including βCC [10][15]. In microalgae, the principal oxidation products of β-carotene are β-ionone and βCC, which release large amounts of these chemicals during summer blooms [44][16].


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