An excess of light absorption will induce excessive excited state of chlorophyll (Chl) molecules. If the energy cannot be transferred out in time through conversion into chemical energy, the energy will be transferred to the surrounding oxygen molecules, resulting in reactive oxygen species (ROS), which will oxidize the photosynthetic membrane, cause its destruction, and inhibit photosynthesis [1, 2]. In order to protect the photosynthetic membrane from light damage, in the long-term evolution process, photosynthesis has developed a series of photoprotective mechanisms to reduce the damage of excess light energy. The heat dissipation is usually detected by non-photochemical quenching (NPQ) of Chl fluorescence, which depends on the the proton gradient (ΔpH) across thylakoid membrane and the xanthophyll cycle [3, 4].
There are five components of NPQ in total, defined as qE, qZ, qT, qI and qH respectively. Each component has its own induction and relaxation kinetics, and each has its own known dependable factors . The major component of NPQ is qE, which depends on three factors ΔpH, PsbS, and zeaxanthin . The Arabidopsis deficient in PsbS and zeaxanthin is termed as npq2 and npq4 respectively. The npq2 and npq4 are xanthophyll cycle mutants, closely associated with reversible conversion of violaxantion to zeaxathin . The npq 4, deficient of proton sensor protein PsbS, provides a quenching site of qE . Zeaxanthin-dependent quenching is called qZ, requires the formation of zeaxanthin but not ΔpH . qT depending on STN7 kinase is responsible for the state transition between PSI and PSII, due to the necessity that the phosphorylated antenna proteins need to move away from PSII [10, 11]. qI or photoinhibitory quenching is a slow-relaxing process, which accounts for the photoinhibitory quenching of PSII reaction centers . qH accounts for a slowly relaxing process in the peripheral antenna of PSII . In addition, there is also a special NPQ component called qM, which accounts for the fluorescence parameter due to chloroplast movements .
The thylakoid lumen is a narrow continuous space enclosed by thylakoid membrane. The lumenal proteins in higher plants have been revealed to have multiple functions for PSII . PSII could easily be photodamaged by high light intensity. The damaged PSII is disintegrated from grana thylakoid and moved to the stroma lamellae and rearranged there. During this process, PSII core protein D1 are degraded by lumenal and stroma deg/Fst protease and re-synthezed . The lumenal peripheral protein of PSII, PsbO is phosphorylated by a nucleoside diphosphate kinase NDPK3 and is dephosphorylated by lumenal acid phosphatase tlp18.3, which is involved in the phosphorylation signaling of PSII assembly [17, 18]. After reassembly, the intact PSII complexes return to the grana thylakoid.
Several lumenal enzymes have activity only in light when the pH of lumen is acidic duo to the formation of ΔpH . Through noncyclic electron transport chain, electrons are finally transferred to NADP+, the final electron acceptor, which is used for carbon fixation. With regard to the assembly of lumenal proteins for the photosystem complex, the electrons needed was transferred from chloroplast stroma thioredoxins (Trxs). HCF164 has been shown to be an electron donor in the lumen . HCF164 was firstly identified as a factor being involved in the assembly of Cyt b6f complex, through reducing FeS protein and Cyt b6. PSI-N was another target of HCF164 because of its Trx domain.
The suppressor of quenching 1 (SOQ1) in Arabidopsis had been introduced as a lumenal protein maintaining the efficiency of light harvesting . The SOQ1 protein is a negative regulator of qH, which is independent of zeaxanthin, PsbS, ΔpH, STN7, and D1 reparation. The SOQ1 spans the thylakoid membrane with a molecular weight (MW) of 104 kDa . The stroma-exposed region of SOQ1 contains a haloacid dehalogenase-like hydrolase (HAD) domain, and the lumen-exposed region contains a thioredoxin-like (Trx-like) and β-propeller NHL domain . It is the lumenal Trx-like not the stromal domain that is required to suppress qH . The formation of qH is prevented by SOQ1 reducing lumenal or lumen-exposed target proteins . Possible mechanism is that one or more quenching site(s) is formed in the peripheral and/or proximal PSII antenna in the absence of SOQ1, which decreases the lifetime of excited-state Chl . However, its reduced activity and target proteins remains unidentified.
The SOQ1 was originally characterized in a screening of quenching suppressor(s) in npq4 background, a qE-deficient line that cannot dissipate excess light energy absorbed . The npq4 was mutagenized again to produce soq1 npq4 double mutant, which shows high and slowly relaxing NPQ . Several qH players had been gradually characterized through this screening strategy. By using soq1 npq4 as the background material, two more players involved in photoprotection qH, chlide a oxygenase (CAO), responsible for the conversion of Chl a to Chl b, and a lumenal lipocalin (LCNP) was identified . In addition, ROQH1, an atypical short chain dehydrogenase/reductase is required to turn off qH, which is a sustained form of antenna quenching, and is induced by high light intensity and cold stress . The soq1 roqh1 double mutant displays a low fluorescence phenotype, indicative of possible constitutive NPQ. It has been pointed out lately that novel molecular players (suppressors and enhancers) involved in photoprotection qH could be identified through conducting soq1 npq4 double mutant and Chl fluorescence imaging . In addition, the SOQ1 was shown as a downstream factor of the chloroplast Trx system leading by the NADPH-dependent Trx reductase C (NTRC), which modulates photosynthesis depending on light intensity and leaf age . SOQ1 is supposed to accept electrons from NTRC. If the amount of NTRC enhanced, it would persistently activate SOQ1, and finally repressed the qH‐type NPQ.
It has been previously reported that a thylakoid lumenal protein, OsTLP27, is required for accumulation of photosynthetic proteins in rice . However, characterization of the lumenal proteins still remains largely insufficient in rice, especially the downstream/target proteins and the role in metabolism of OsSOQ1. Here, the function analysis of a homolog of Arabidopsis SOQ1, OsSOQ1, also a multi-domain protein in rice, is presented. It has been shown that OsSOQ1 is related to the drought response in rice . Homozygous mutants (ossoq1) were obtained by CRISPR/Cas9 to knockout OsSOQ1. The mutants showed significant lower plant height, tiller number, panicle length, effective panicle, and grain number per panicle relative to the WT plants. Western blot analysis showed that OsSOQ1 is a thylakoid membrane protein, with the Trx-like domain facing the lumen. Deficient in OsSOQ1 did not affect the protein level of PSII subunits, but down-regulated the level of a NPQ player PsbS, resulting in a low NPQ under high light intensity in the mutants. UPLC-MS/MS experiments showed that OsSOQ1 is involved in fatty acid metabolism in leaves. The Trx-like domain exhibited redox activity in vitro as shown by insulin assay; and in the yeast two-hybrid experiment, it was found that the Trx-like domain interacts with the chloroplast lipocalin OsLCNP, which usually transports lipid molecules. These critical findings revealed that the role of OsSOQ1 is to maintain the photochemical efficiency of PSII under high light intensity and regulate the metabolism of fatty acid in rice.