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Manganese (Mn) is an essential metal for plant growth. The most important Mn-containing enzyme is the Mn4CaO5 cluster that catalyzes water oxidation in photosystem II (PSII). Mn deficiency primarily affects photosynthesis, whereas Mn excess is generally toxic. Here, we studied Mn excess and deficiency in the liverwort Marchantia polymorpha, an emerging model ideally suited for analysis of metal stress since it accumulates rapidly toxic substances due to the absence of well-developed vascular and radicular systems and a reduced cuticle. We established growth conditions for Mn excess and deficiency and analyzed the metal content in thalli and isolated chloroplasts. In vivo super-resolution fluorescence microscopy and transmission electron microscopy revealed changes in the organization of the thylakoid membrane under Mn excess and deficiency. Both Mn excess and Mn deficiency increased the stacking of the thylakoid membrane. We investigated photosynthetic performance by measuring chlorophyll fluorescence at room temperature and 77 K, measuring P700 absorbance, and studying the susceptibility of thalli to photoinhibition. Nonoptimal Mn concentrations changed the ratio of PSI to PSII. Upon Mn deficiency, higher non-photochemical quenching was observed, electron donation to PSI was favored, and PSII was less susceptible to photoinhibition. Mn deficiency seemed to favor cyclic electron flow around PSI, thereby protecting PSII in high light. The results presented here suggest an important role of Mn in the organization of the thylakoid membrane and photosynthetic electron transport.

Climate change is caused by the ever-increasing anthropogenic CO2 emissions and the resulting accumulation of carbon dioxide in the atmosphere, whose effects are so far mitigated by the oceanic and continental CO2 uptakes. The terrestrial sink is the most uncertain component of the global carbon cycle mainly because the gross primary production (GPP), which is the quantity of atmospheric carbon absorbed by plants through photosynthesis, is highly variable in time and space, and because the involved processes are complex. As photosynthesis is not directly measurable at a scale larger than the leaf, land surface modelers have been looking for large-scale proxies of GPP. Solar-induced chlorophyll fluorescence (SIF) estimates from satellite instruments have emerged as a promising resource to inform on the space-time distribution of GPP, and have been increasingly used over the last decade. However, numerous challenges remain to be addressed to acutely understand the SIF signal, and its relationship with plant photosynthesis, to be able to correctly exploit space-borne SIF retrievals to constrain GPP simulated by land surface models (LSMs). Notably, we still lack knowledge on the non-photochemical quenching (NPQ), which represents the third deactivation pathway of the light energy absorbed by chlorophyll pigments, alongside photochemistry and fluorescence. In this study, we focused on boreal evergreen needleleaf forests (BorENF), which cover 29% of the world's total forest area, and whose GPP budget is still debated. We took advantage of both passive and active fluorescence measurements to improve the representation of NPQ, SIF and GPP in the ORCHIDEE LSM. We first used active measurements taken at the Hyytiälä BorENF site on Pinus sylvestris trees, to separately model the sustained and reversible NPQ components. Indeed, it was previously documented that during winter such evergreen trees suppress photosynthesis and sustain molecular modifications of the photosynthetic chain allowing the dissipation of the excess energy absorbed as heat (sustained NPQ). The reversible NPQ occurs during growth season in response to environmental stress (e.g., excessive light or droughts). In a second step, we optimised several ORCHIDEE parameters related to NPQ, SIF and GPP representations, using data assimilation techniques. We performed a multi-variables and multi-sites approach, simultaneously assimilating in situ GPP estimates at nine BorENF FLUXNET sites and collocated SIF estimates from the TROPOMI satellite instrument. The improvements brought to SIF and GPP were evaluated at those sites over independent years (i.e. not used in assimilation) with positive results, and at the regional scale against the FLUXCOM and FLUXSAT GPP products, as well as against TROPOMI SIF data.

Cystathionine-β-synthase (CBS) domains are found in proteins of all living organisms and have been proposed to play a role as energy sensors regulating protein activities through their adenosyl ligand binding capacity. In plants, members of the CBSX protein family carry a stand-alone pair of CBS domains. In Arabidopsis (Arabidopsis thaliana), CBSX1 and CBSX2 are targeted to plastids where they have been proposed to regulate thioredoxins (TRXs). TRXs are ubiquitous cysteine thiol oxido-reductases involved in the redox-based regulation of numerous enzymatic activities as well as in the regeneration of thiol-dependent peroxidases. In Arabidopsis, 10 TRX isoforms have been identified in plastids and divided into five sub-types. Here, we show that CBSX2 specifically inhibits the activities of m-type TRXs toward two chloroplast TRX-related targets. By testing activation of NADP-malate dehydrogenase and reduction of 2-Cys peroxiredoxin, we found that TRXm1/2 inhibition by CBSX2 was alleviated in the presence of AMP or ATP. We also determined, by pull-down assays, a direct interaction of CBSX2 with reduced TRXm1 and m2 that was abolished in the presence of adenosyl ligands. In addition, we report that, compared with wild-type plants, the Arabidopsis T-DNA double mutant cbsx1 cbsx2 exhibits growth and chlorophyll accumulation defects in cold conditions, suggesting a function of plastidial CBSX proteins in plant stress adaptation. Together, our results show an energy-sensing regulation of plastid TRX m activities by CBSX, possibly allowing a feedback regulation of ATP homeostasis via activation of cyclic electron flow in the chloroplast, to maintain a high energy level for optimal growth.

Identifying traits that exhibit improved drought resistance is highly important to cope with the challenges of predicted climate change. We investigated the response of state transition mutants to drought. Compared with the wild type, state transition mutants were less affected by drought. Photosynthetic parameters in leaves probed by chlorophyll fluorescence confirmed that mutants possess a more reduced plastoquinone (PQ) pool, as expected due to the absence of state transitions. Seedlings of the mutants showed an enhanced growth of the primary root and more lateral root formation. The photosystem II inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea, leading to an oxidized PQ pool, inhibited primary root growth in wild type and mutants, while the cytochromeb6fcomplex inhibitor 2,5-dibromo-3-methyl-6-isopropylbenzoquinone, leading to a reduced PQ pool, stimulated root growth. A more reduced state of the PQ pool was associated with a slight but significant increase in singlet oxygen production. Singlet oxygen may trigger a, yet unknown, signaling cascade promoting root growth. We propose that photosynthetic mutants with a deregulated ratio of photosystem II to photosystem I activity can provide a novel path for improving crop drought resistance.Summary StatementArabidopsis thalianamutants affected in state transitions are more drought resistant.

Drought induces stomata closure and lowers the CO2 concentration in the mesophyll, limiting CO2 assimilation and favouring photorespiration. The photosynthetic apparatus is protected under drought conditions by a number of downregulation mechanisms like photosynthetic control and activation of cyclic electron transport leading to the generation of a high proton gradient across the thylakoid membrane. Here, we studied photosynthetic electron transport by chlorophyll fluorescence, thermoluminescence and P700 absorption measurements in spinach exposed to moderate drought stress. Chlorophyll fluorescence induction and decay kinetics were slowed down. Under drought conditions an increase of the thermoluminescence AG-band and a downshift of the maximum temperatures of both, the B-band and the AG-band was observed when leaves were illuminated under conditions that maintained the proton gradient. When leaves were frozen prior to the thermoluminescence measurements, the maximum temperature of the B-band was upshifted in drought-stressed leaves. This shows a stabilization of the QB /QB •- redox couple in accordance with the slower fluorescence decay kinetics. We propose that, during drought stress, photorespiration exerts a feedback control on photosystem II via the binding of a photorespiratory metabolite at the non-heme iron at the acceptor side of photosystem II. According to our hypothesis, an exchange of bicarbonate at the non-heme iron by a photorespiratory metabolite such as glycolate would not only affect the midpoint potential of the QA /QA •- couple, as shown previously, but also that of the QB /QB •- couple. This article is protected by copyright. All rights reserved.

International audience; Light capture by chlorophylls and photosynthetic electron transport bury the risk of the generation of reactive oxygen species (ROS) including singlet oxygen, superoxide anion radicals and hydrogen peroxide. Rapid changes in light intensity, electron fluxes and accumulation of strong oxidants and reductants increase ROS production. Superoxide is mainly generated at the level of photosystem I while photosystem II is the main source of singlet oxygen. ROS can induce oxidative damage of the photosynthetic apparatus, however, ROS are also important to tune processes inside the chloroplast and participate in retrograde signalling regulating the expression of genes involved in acclimation responses. Under most physiological conditions light harvesting and photosynthetic electron transport are regulated to keep the level of ROS at a non-destructive level. Photosystem II is most prone to photoinhibition but can be quickly repaired while photosystem I is protected in most cases. The size of the transmembrane proton gradient is central for the onset of mechanisms that protect against photoinhibition. The proton gradient allows dissipation of excess energy as heat in the antenna systems and it regulates electron transport. pH-dependent slowing down of electron donation to photosystem I protects it against ROS generation and damage. Cyclic electron transfer and photoreduction of oxygen contribute to the size of the proton gradient. The yield of singlet oxygen production in photosystem II is regulated by changes in the midpoint potential of its primary quinone acceptor. In addition, numerous antioxidants inside the photosystems, the antenna and the thylakoid membrane quench or scavenge ROS.