4.3 Sustained quenching, qH
qH is a form of sustained and slowly reversible quenching that occurs in the peripheral antenna of PSII prior to photoinhibitory quenching (qI) (Malnoëet al. 2018). Bruet al. showed that the qH quenching site is a major trimeric antenna complex that consist of Lhcb1, Lhcb2, Lhcb3, which can form homotrimeric and heterotrimeric complexes (Bellottari et al 2012, Crepin et al 2018). Interestingly, Bru et al. also found specific isoforms of the major trimeric antenna complex that are not required for qH induction (Bruet al. 2021).
qH is known to be redox-regulated but independent of protonation of PsbS, accumulation of Zx, and phosphorylation by STN7 (Brookset al. 2013; Malnoë et al. 2018; Bru et al. 2021). It has been shown that qH is induced by oxidative stress conditions such as cold and high light (Levesque-Tremblay, Havaux & Ouellet 2009; Brooks et al. 2013; Malnoë et al.2018; Bru et al. 2021).
The molecular players of qH are SOQ1 (Brookset al. 2013; Malnoë et al. 2018; Malnoë 2018; Bru et al. 2021), plastid lipocalin (LCNP) (Brookset al. 2013; Malnoë et al. 2018; Bru et al. 2021), and relaxation of qH1 (ROQH1) (Amstutzet al. 2020) (Fig. 5). LCNP is a major component that induces qH. SOQ1 is a trans-membrane protein (see section 3.1) that inhibits LCNP, while ROQH1 relaxes qH. LCNP is a soluble lumenal protein that induces qH by binding to LHCII and thus induces a conformational change within LHCII causing it to undergo a transformation from a light-harvesting state to a dissipated state (Yuet al. 2022). In addition, the redox status of the six conserved cysteines within LCNP play a critical role in regulating LCNP’s activity (Malnoëet al. 2018).
Consequently, the reduced form of LCNP is inactive, while the oxidized form is active. Mechanistically, LCNP can be proposed to work as follows: SOQ1, transfers reducing equivalents to LCNP, which inhibits LCNP under non-stress conditions. However, under stress conditions (oxidative condition), SOQ1 no longer transfers reducing equivalents to LCNP, which keeps LCNP in an oxidized state and thus allows it to induce qH (Malnoë 2018). As a result, the function of LCNP and qH is being modulated by the redox state of the chloroplast (See Fig. 5).