4.1 Energy-dependent quenching, qE
Energy-dependent quenching (qE) is the most rapidly-responding form of NPQ. It is triggered by the acidification of the lumen (proton gradient (∆pH) component of proton motive force (pmf ) ) which results in the protonation of PsbS as well as the activation of violaxanthin de-epoxidase (VDE), which catalyzes the conversion of violaxanthin (Vx) to zeaxanthin (Zx). Zx subsequently dissipates excessive light energy and induces qE along with PsbS (Niyogiet al. 1998).
Recent studies have shown that both the proton gradient (∆pH) and also the redox status of the thiol-disulfides within in VDE play a role in controlling its function (Arnoux, Morosinotto, Saga, Bassi & Pignol 2009; Yu et al. 2014; Hallinet al. 2015; Arnouxet al. 2009; Simionato et al. 2015). Specifically, the proton gradient (∆pH) facilitates a structural change in VDE causing it to change from a monomer to a homo-dimer complex upon lumen acidification (Arnouxet al. 2009). In addition, to VDE undergoing a structural conformational change, VDE activity is also regulated in a thiol-disulfide-dependent manner (Hallet al. 2010; Kieselbach 2013; Yu et al. 2014; Simionatoet al. 2015; Hallin et al. 2015). This elaborate regulation is achieved due to the unique structure of VDE. The VDE protein consists of three domains: a Cys-rich N-terminal domain, a lipocalin-like domain (predicted to bind violaxanthin) (Sagaet al. 2010) and a glutamate-rich C-terminal domain. VDE has a total of 13 conserved Cys residues, 12 of which are at the N-terminal and these Cys residues play a major role in protein function. Consequently, VDE is active only when fully oxidized, with six disulfide bonds, is VDE active (Simionatoet al. 2015), and this corresponds to a more compact, rigid, thermostable form of the protein (Hallinet al. 2015).
It has been proposed that the electron donor for VDE is Trx-m through CcdA and HCF164 (Motohashi & Hisabori 2006, 2010) and the oxidase is LTO1(Luet al. 2013, 2015; Yu et al. 2014; Wu et al. 2021) (Fig. 4). Coincidently, an lto1-2 mutant line showed a lower de-epoxidation state of xanthophyll cycle pigment de-epoxidase index (DEI), suggesting that de-epoxidation of Vx to Zx was suppressed in thelto1-2 mutant line(Yu et al. 2014; Lu et al. 2015).
Another component that can affect the kinetics of qE is K+ efflux antiporter3 (KEA3), a potassium/proton antiporter. KEA3 exchanges protons (out of the lumen) and potassium ions (into the lumen) (Armbrusteret al. 2014). Consequently, when light transitions from high to low, the KEA3 antiporter function, helps relax qE. Furthermore, it has been recently reported that KEA3 has five Cys residues, but only one Cys residue is located at the N-terminal which extends into the lumen (Wanget al. 2017a). The location of this Cys residue suggests that KEA3 may be regulated by a redox mechanism, acting as adimer. However, the possible electron donors and oxidases involved in regulating KEA3 need further investigation.