A Genetic Screen to Identify New Molecular Players Involved in Photoprotection qH in Arabidopsis thaliana

Abstract

Photosynthesis is a biological process which converts light energy into chemical energy that is used in the Calvin–Benson cycle to produce organic compounds. An excess of light can induce damage to the photosynthetic machinery. Therefore, plants have evolved photoprotective mechanisms such as non-photochemical quenching (NPQ). To focus molecular insights on slowly relaxing NPQ processes in Arabidopsis thaliana, previously, a qE-deficient line—the PsbS mutant—was mutagenized and a mutant with high and slowly relaxing NPQ was isolated. The mutated gene was named suppressor of quenching 1, or SOQ1, to describe its function. Indeed, when present, SOQ1 negatively regulates or suppresses a form of antenna NPQ that is slow to relax and is photoprotective. We have now termed this component qH and identified the plastid lipocalin, LCNP, as the effector for this energy dissipation mode to occur. Recently, we found that the relaxation of qH1, ROQH1, protein is required to turn off qH. The aim of this study is to identify new molecular players involved in photoprotection qH by a whole genome sequencing approach of chemically mutagenized Arabidopsis thaliana. We conducted an EMS-mutagenesis on the soq1 npq4 double mutant and used chlorophyll fluorescence imaging to screen for suppressors and enhancers of qH. Out of 22,000 mutagenized plants screened, the molecular players cited above were found using a mapping-by-sequencing approach. Here, we describe the phenotypic characterization of the other mutants isolated from this genetic screen and an additional 8000 plants screened. We have classified them in several classes based on their fluorescence parameters, NPQ kinetics, and pigment content. A high-throughput whole genome sequencing approach on 65 mutants will identify the causal mutations thanks to allelic mutations from having reached saturation of the genetic screen. The candidate genes could be involved in the formation or maintenance of quenching sites for qH, in the regulation of qH at the transcriptional level, or be part of the quenching site itself.

Current Knowledge on Mechanisms Preventing Photosynthesis Redox Imbalance in Plants

“…However, the down-regulation of SOQ1 under drought stress and the inability to reverse the electrophoretic mobility of LCNP with DTT in soq1 mutants, argues against it, suggesting rather an increased activity of LCNP due to the decrease in SOQ1 levels under stress conditions [ 82 ]. Nevertheless, the finding of SOQ1 as a possible NTRC interactor [ 54 ] and the recent identification of LTO1, in a genetic screen for suppressors of soq1 npq4 by Bru and co-workers (2020), has raised again the question of a possible redox regulation of LCNP [ 90 ]. The participation of NTRC in the down-regulation of qH has recently been proposed [ 91 ].…”

An energy-dissipative state of the major antenna complex of plants

“…The slowly reversible NPQ, or sustained energy dissipation, includes several mechanisms such as qZ (zeaxanthin-dependent, ∆pH-independent ( 14)), qH (see below), and qI (D1 photoinactivation (15)). We have recently uncovered, using chemical mutagenesis and genetic screens in Arabidopsis thaliana, several molecular players regulating qH (16)(17)(18)(19). qH requires the plastid lipocalin, LCNP (17), is negatively regulated by suppressor of quenching 1, SOQ1 (16), and is turned OFF by relaxation of qH 1, ROQH1 (18).…”

“…quenching, of chlorophyll (Chl) fluorescence through qT (due to state transition, movement of antenna phosphorylated by the kinase STN7 (Quick & Stitt, 1989)). We have recently uncovered, using chemical mutagenesis and genetic screens in Arabidopsis thaliana, several molecular players regulating qH (Brooks et al, 2013, Amstutz et al, 2020, Bru et al, 2020. qH requires the plastid lipocalin, LCNP (Malnoë et al, 2018), is negatively regulated by suppressor of quenching 1, SOQ1 (Brooks et al, 2013), and is turned OFF by relaxation of qH 1, ROQH1 (Amstutz et al, 2020).…”

Roles of OsSOQ1 in photoprotection and fatty acid metabolism of rice

“…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 [12]. It is the lumenal Trx-like not the stromal domain that is required to suppress qH [22]. The formation of qH is prevented by SOQ1 reducing lumenal or lumen-exposed target proteins [23].…”

“…The soq1 roqh1 double mutant displays a low uorescence 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 identi ed through conducting soq1 npq4 double mutant and Chl uorescence imaging [22]. 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 [25].…”