Cytoklepty in the plankton: A host strategy to optimize the bioenergetic machinery of endosymbiotic algae.

Endosymbioses have shaped the evolutionary trajectory of life and remain ecologically important. Investigating oceanic photosymbioses can illuminate how algal endosymbionts are energetically exploited by their heterotrophic hosts and inform on putative initial steps of plastid acquisition in eukaryotes. By combining three-dimensional subcellular imaging with photophysiology, carbon flux imaging, and transcriptomics, we show that cell division of endosymbionts (Phaeocystis) is blocked within hosts (Acantharia) and that their cellular architecture and bioenergetic machinery are radically altered. Transcriptional evidence indicates that a nutrient-independent mechanism prevents symbiont cell division and decouples nuclear and plastid division. As endosymbiont plastids proliferate, the volume of the photosynthetic machinery volume increases 100-fold in correlation with the expansion of a reticular mitochondrial network in close proximity to plastids. Photosynthetic efficiency tends to increase with cell size, and photon propagation modeling indicates that the networked mitochondrial architecture enhances light capture. This is accompanied by 150-fold higher carbon uptake and up-regulation of genes involved in photosynthesis and carbon fixation, which, in conjunction with a ca.15-fold size increase of pyrenoids demonstrates enhanced primary production in symbiosis. Mass spectrometry imaging revealed major carbon allocation to plastids and transfer to the host cell. As in most photosymbioses, microalgae are contained within a host phagosome (symbiosome), but here, the phagosome invaginates into enlarged microalgal cells, perhaps to optimize metabolic exchange. This observation adds evidence that the algal metamorphosis is irreversible. Hosts, therefore, trigger and benefit from major bioenergetic remodeling of symbiotic microalgae with potential consequences for the oceanic carbon cycle. Unlike other photosymbioses, this interaction represents a so-called cytoklepty, which is a putative initial step toward plastid acquisition.

Loss of CpFTSY Reduces Photosynthetic Performance and Affects Insertion of PsaC of PSI in Diatoms.

The chloroplast signal recognition particle (CpSRP) receptor (CpFTSY) is a component of the CpSRP pathway that post-translationally targets light-harvesting complex proteins (LHCPs) to the thylakoid membranes in plants and green algae containing chloroplasts derived from primary endosymbiosis. In plants, CpFTSY also plays a major role in the co-translational incorporation of chloroplast-encoded subunits of photosynthetic complexes into the thylakoids. This role has not been demonstrated in green algae. So far, its function in organisms with chloroplasts derived from secondary endosymbiotic events has not been elucidated. Here, we report the generation and characterization of mutants lacking CpFTSY in the diatom Phaeodactylum tricornutum. We found that this protein is not involved in inserting LHCPs into thylakoid membranes, indicating that the post-translational part of the CpSRP pathway is not active in this group of microalgae. The lack of CpFTSY caused an increased level of photoprotection, low electron transport rates, inefficient repair of photosystem II (PSII), reduced growth, a strong decline in the PSI subunit PsaC and upregulation of proteins that might compensate for a non-functional co-translational CpSRP pathway during light stress conditions. The phenotype was highly similar to the one described for diatoms lacking another component of the co-translational CpSRP pathway, the CpSRP54 protein. However, in contrast to cpsrp54 mutants, only one thylakoid membrane protein, PetD of the Cytb6f complex, was downregulated in cpftsy. Our results point to a minor role for CpFTSY in the co-translational CpSRP pathway, suggesting that other mechanisms may partially compensate for the effect of a disrupted CpSRP pathway.

Protection of photosystem I during sudden light stress depends on ferredoxin:NADP(H) reductase abundance and interactions.

Plant tolerance to high light and oxidative stress is increased by overexpression of the photosynthetic enzyme Ferredoxin:NADP(H) reductase (FNR), but the specific mechanism of FNR-mediated protection remains enigmatic. It has also been reported that the localization of this enzyme within the chloroplast is related to its role in stress tolerance. Here, we dissected the impact of FNR content and location on photoinactivation of photosystem I (PSI) and photosystem II (PSII) during high light stress of Arabidopsis (Arabidopsis thaliana). The reaction center of PSII is efficiently turned over during light stress, while damage to PSI takes much longer to repair. Our results indicate a PSI sepcific effect, where efficient oxidation of the PSI primary donor (P700) upon transition from darkness to light, depends on FNR recruitment to the thylakoid membrane tether proteins: thylakoid rhodanase-like protein (TROL) and translocon at the inner envelope of chloroplasts 62 (Tic62). When these interactions were disrupted, PSI photoinactivation occurred. In contrast, there was a moderate delay in the onset of PSII damage. Based on measurements of ΔpH formation and cyclic electron flow, we propose that FNR location influences the speed at which photosynthetic control is induced, resulting in specific impact on PSI damage. Membrane tethering of FNR therefore plays a role in alleviating high light stress, by regulating electron distribution during short-term responses to light.

Catalytic Reactions and Energy Conservation in the Cytochrome bc1 and b6f Complexes of Energy-Transducing Membranes.

This review focuses on key components of respiratory and photosynthetic energy-transduction systems: the cytochrome bc1 and b6f (Cytbc1/b6f) membranous multisubunit homodimeric complexes. These remarkable molecular machines catalyze electron transfer from membranous quinones to water-soluble electron carriers (such as cytochromes c or plastocyanin), coupling electron flow to proton translocation across the energy-transducing membrane and contributing to the generation of a transmembrane electrochemical potential gradient, which powers cellular metabolism in the majority of living organisms. Cytsbc1/b6f share many similarities but also have significant differences. While decades of research have provided extensive knowledge on these enzymes, several important aspects of their molecular mechanisms remain to be elucidated. We summarize a broad range of structural, mechanistic, and physiological aspects required for function of Cytbc1/b6f, combining textbook fundamentals with new intriguing concepts that have emerged from more recent studies. The discussion covers but is not limited to (i) mechanisms of energy-conserving bifurcation of electron pathway and energy-wasting superoxide generation at the quinol oxidation site, (ii) the mechanism by which semiquinone is stabilized at the quinone reduction site, (iii) interactions with substrates and specific inhibitors, (iv) intermonomer electron transfer and the role of a dimeric complex, and (v) higher levels of organization and regulation that involve Cytsbc1/b6f. In addressing these topics, we point out existing uncertainties and controversies, which, as suggested, will drive further research in this field.

A complex containing PGRL1 and PGR5 is involved in the switch between linear and cyclic electron flow in Arabidopsis.

During photosynthesis, two photoreaction centers located in the thylakoid membranes of the chloroplast, photosystems I and II (PSI and PSII), use light energy to mobilize electrons to generate ATP and NADPH. Different modes of electron flow exist, of which the linear electron flow is driven by PSI and PSII, generating ATP and NADPH, whereas the cyclic electron flow (CEF) only generates ATP and is driven by the PSI alone. Different environmental and metabolic conditions require the adjustment of ATP/NADPH ratios and a switch of electron distribution between the two photosystems. With the exception of PGR5, other components facilitating CEF are unknown. Here, we report the identification of PGRL1, a transmembrane protein present in thylakoids of Arabidopsis thaliana. Plants lacking PGRL1 show perturbation of CEF, similar to PGR5-deficient plants. We find that PGRL1 and PGR5 interact physically and associate with PSI. We therefore propose that the PGRL1-PGR5 complex facilitates CEF in eukaryotes.

Functional studies of CpSRP54 in diatoms show that the mechanism of thylakoid protein insertion differs from that in plants and green algae.

The chloroplast signal recognition particle 54 kDa (CpSRP54) protein is a member of the CpSRP pathway known to target proteins to thylakoid membranes in plants and green algae. Loss of CpSRP54 in the marine diatom Phaeodactylum tricornutum lowers the accumulation of a selection of chloroplast-encoded subunits of photosynthetic complexes, indicating a role in the co-translational part of the CpSRP pathway. In contrast to plants and green algae, absence of CpSRP54 does not have a negative effect on the content of light-harvesting antenna complex proteins and pigments in P. tricornutum, indicating that the diatom CpSRP54 protein has not evolved to function in the post-translational part of the CpSRP pathway. Cpsrp54 KO mutants display altered photophysiological responses, with a stronger induction of photoprotective mechanisms and lower growth rates compared to wild type when exposed to increased light intensities. Nonetheless, their phenotype is relatively mild, thanks to the activation of mechanisms alleviating the loss of CpSRP54, involving upregulation of chaperones. We conclude that plants, green algae, and diatoms have evolved differences in the pathways for co-translational and post-translational insertion of proteins into the thylakoid membranes.

Impaired photoprotection in Phaeodactylum tricornutum KEA3 mutants reveals the proton regulatory circuit of diatoms light acclimation.

Diatoms are successful phytoplankton clades able to acclimate to changing environmental conditions, including e.g. variable light intensity. Diatoms are outstanding at dissipating light energy exceeding the maximum photosynthetic electron transfer (PET) capacity via the nonphotochemical quenching (NPQ) process. While the molecular effectors of NPQ as well as the involvement of the proton motive force (PMF) in its regulation are known, the regulators of the PET/PMF relationship remain unidentified in diatoms. We generated mutants of the H+ /K+ antiporter KEA3 in the model diatom Phaeodactylum tricornutum. Loss of KEA3 activity affects the PET/PMF coupling and NPQ responses at the onset of illumination, during transients and in steady-state conditions. Thus, this antiporter is a main regulator of the PET/PMF coupling. Consistent with this conclusion, a parsimonious model including only two free components, KEA3 and the diadinoxanthin de-epoxidase, describes most of the feedback loops between PET and NPQ. This simple regulatory system allows for efficient responses to fast (minutes) or slow (e.g. diel) changes in light environment, thanks to the presence of a regulatory calcium ion (Ca2+ )-binding domain in KEA3 modulating its activity. This circuit is likely tuned by the NPQ-effector proteins, LHCXs, providing diatoms with the required flexibility to thrive in different ocean provinces.

Characterization of the Bubblegum acyl-CoA synthetase of Microchloropsis gaditana.

The metabolic pathways of glycerolipids are well described in cells containing chloroplasts limited by a two-membrane envelope but not in cells containing plastids limited by four membranes, including heterokonts. Fatty acids (FAs) produced in the plastid, palmitic and palmitoleic acids (16:0 and 16:1), are used in the cytosol for the synthesis of glycerolipids via various routes, requiring multiple acyl-Coenzyme A (CoA) synthetases (ACS). Here, we characterized an ACS of the Bubblegum subfamily in the photosynthetic eukaryote Microchloropsis gaditana, an oleaginous heterokont used for the production of lipids for multiple applications. Genome engineering with TALE-N allowed the generation of MgACSBG point mutations, but no knockout was obtained. Point mutations triggered an overall decrease of 16:1 in lipids, a specific increase of unsaturated 18-carbon acyls in phosphatidylcholine and decrease of 20-carbon acyls in the betaine lipid diacylglyceryl-trimethyl-homoserine. The profile of acyl-CoAs highlighted a decrease in 16:1-CoA and 18:3-CoA. Structural modeling supported that mutations affect accessibility of FA to the MgACSBG reaction site. Expression in yeast defective in acyl-CoA biosynthesis further confirmed that point mutations affect ACSBG activity. Altogether, this study supports a critical role of heterokont MgACSBG in the production of 16:1-CoA and 18:3-CoA. In M. gaditana mutants, the excess saturated and monounsaturated FAs were diverted to triacylglycerol, thus suggesting strategies to improve the oil content in this microalga.

A blue-light photoreceptor mediates the feedback regulation of photosynthesis.

In plants and algae, light serves both as the energy source for photosynthesis and a biological signal that triggers cellular responses via specific sensory photoreceptors. Red light is perceived by bilin-containing phytochromes and blue light by the flavin-containing cryptochromes and/or phototropins (PHOTs), the latter containing two photosensory light, oxygen, or voltage (LOV) domains. Photoperception spans several orders of light intensity, ranging from far below the threshold for photosynthesis to values beyond the capacity of photosynthetic CO2 assimilation. Excess light may cause oxidative damage and cell death, processes prevented by enhanced thermal dissipation via high-energy quenching (qE), a key photoprotective response. Here we show the existence of a molecular link between photoreception, photosynthesis, and photoprotection in the green alga Chlamydomonas reinhardtii. We show that PHOT controls qE by inducing the expression of the qE effector protein LHCSR3 (light-harvesting complex stress-related protein 3) in high light intensities. This control requires blue-light perception by LOV domains on PHOT, LHCSR3 induction through PHOT kinase, and light dissipation in photosystem II via LHCSR3. Mutants deficient in the PHOT gene display severely reduced fitness under excessive light conditions, indicating that the sensing, utilization, and dissipation of light is a concerted process that plays a vital role in microalgal acclimation to environments of variable light intensities.

Subcellular architecture and metabolic connection in the planktonic photosymbiosis between Collodaria (radiolarians) and their microalgae.

Photosymbiosis is widespread and ecologically important in the oceanic plankton but remains poorly studied. Here, we used multimodal subcellular imaging to investigate the photosymbiosis between colonial Collodaria and their microalga dinoflagellate (Brandtodinium). We showed that this symbiosis is very dynamic whereby symbionts interact with different host cells via extracellular vesicles within the colony. 3D electron microscopy revealed that the photosynthetic apparatus of the microalgae was more voluminous in symbiosis compared to free-living while the mitochondria volume was similar. Stable isotope probing coupled with NanoSIMS showed that carbon and nitrogen were stored in the symbiotic microalga in starch granules and purine crystals respectively. Nitrogen was also allocated to the algal nucleolus. In the host, low 13 C transfer was detected in the Golgi. Metal mapping revealed that intracellular iron concentration was similar in free-living and symbiotic microalgae (c. 40 ppm) and twofold higher in the host, whereas copper concentration increased in symbionts and was detected in the host cell and extracellular vesicles. Sulfur concentration was around two times higher in symbionts (chromatin and pyrenoid) than their host. This study improves our understanding on the functioning of this oceanic photosymbiosis and paves the way for more studies to further assess its biogeochemical significance.

Mixotrophic growth of the extremophile Galdieria sulphuraria reveals the flexibility of its carbon assimilation metabolism.

Galdieria sulphuraria is a cosmopolitan microalga found in volcanic hot springs and calderas. It grows at low pH in photoautotrophic (use of light as a source of energy) or heterotrophic (respiration as a source of energy) conditions, using an unusually broad range of organic carbon sources. Previous data suggested that G. sulphuraria cannot grow mixotrophically (simultaneously exploiting light and organic carbon as energy sources), its photosynthetic machinery being repressed by organic carbon. Here, we show that G. sulphuraria SAG21.92 thrives in photoautotrophy, heterotrophy and mixotrophy. By comparing growth, biomass production, photosynthetic and respiratory performances in these three trophic modes, we show that addition of organic carbon to cultures (mixotrophy) relieves inorganic carbon limitation of photosynthesis thanks to increased CO2 supply through respiration. This synergistic effect is lost when inorganic carbon limitation is artificially overcome by saturating photosynthesis with added external CO2 . Proteomic and metabolic profiling corroborates this conclusion suggesting that mixotrophy is an opportunistic mechanism to increase intracellular CO2 concentration under physiological conditions, boosting photosynthesis by enhancing the carboxylation activity of Ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) and decreasing photorespiration. We discuss possible implications of these findings for the ecological success of Galdieria in extreme environments and for biotechnological applications.

Embryonic Photosynthesis Affects Post-Germination Plant Growth.

Photosynthesis is the fundamental process fueling plant vegetative growth and development. The progeny of plants relies on maternal photosynthesis, via food reserves in the seed, to supply the necessary energy for seed germination and early seedling establishment. Intriguingly, before seed maturation, Arabidopsis (Arabidopsis thaliana) embryos are also photosynthetically active, the biological significance of which remains poorly understood. Investigating this system is genetically challenging because mutations perturbing photosynthesis are expected to affect both embryonic and vegetative tissues. Here, we isolated a temperature-sensitive mutation affecting CPN60α2, which encodes a subunit of the chloroplast chaperonin complex CPN60. When exposed to cold temperatures, cpn60α2 mutants accumulate less chlorophyll in newly produced tissues, thus allowing the specific disturbance of embryonic photosynthesis. Analyses of cpn60α2 mutants were combined with independent genetic and pharmacological approaches to show that embryonic photosynthetic activity is necessary for normal skoto- and photomorphogenesis in juvenile seedlings as well as long-term adult plant development. Our results reveal the importance of embryonic photosynthetic activity for normal adult plant growth, development, and health.

Energetic coupling between plastids and mitochondria drives CO2 assimilation in diatoms.

Diatoms are one of the most ecologically successful classes of photosynthetic marine eukaryotes in the contemporary oceans. Over the past 30 million years, they have helped to moderate Earth's climate by absorbing carbon dioxide from the atmosphere, sequestering it via the biological carbon pump and ultimately burying organic carbon in the lithosphere. The proportion of planetary primary production by diatoms in the modern oceans is roughly equivalent to that of terrestrial rainforests. In photosynthesis, the efficient conversion of carbon dioxide into organic matter requires a tight control of the ATP/NADPH ratio which, in other photosynthetic organisms, relies principally on a range of plastid-localized ATP generating processes. Here we show that diatoms regulate ATP/NADPH through extensive energetic exchanges between plastids and mitochondria. This interaction comprises the re-routing of reducing power generated in the plastid towards mitochondria and the import of mitochondrial ATP into the plastid, and is mandatory for optimized carbon fixation and growth. We propose that the process may have contributed to the ecological success of diatoms in the ocean.

The biosynthetic capacities of the plastids and integration between cytoplasmic and chloroplast processes.

Plastids are semiautonomous organelles derived from cyanobacterial ancestors. Following endosymbiosis, plastids have evolved to optimize their functions, thereby limiting metabolic redundancy with other cell compartments. Contemporary plastids have also recruited proteins produced by the nuclear genome of the host cell. In addition, many genes acquired from the cyanobacterial ancestor evolved to code for proteins that are targeted to cell compartments other than the plastid. Consequently, metabolic pathways are now a patchwork of enzymes of diverse origins, located in various cell compartments. Because of this, a wide range of metabolites and ions traffic between the plastids and other cell compartments. In this review, we provide a comprehensive analysis of the well-known, and of the as yet uncharacterized, chloroplast/cytosol exchange processes, which can be deduced from what is currently known about compartmentation of plant-cell metabolism.

Identification of the Arabidopsis Calmodulin-Dependent NAD+ Kinase That Sustains the Elicitor-Induced Oxidative Burst.

NADP(H) is an essential cofactor of multiple metabolic processes in all living organisms, and in plants, NADP(H) is required as the substrate of Ca2+-dependent NADPH oxidases, which catalyze a reactive oxygen species burst in response to various stimuli. While NADP+ production in plants has long been known to involve a calmodulin (CaM)/Ca2+-dependent NAD+ kinase, the nature of the enzyme catalyzing this activity has remained enigmatic, as has its role in plant physiology. Here, we used proteomic, biochemical, molecular, and in vivo analyses to identify an Arabidopsis (Arabidopsis thaliana) protein that catalyzes NADP+ production exclusively in the presence of CaM/Ca2+ This enzyme, which we named NAD kinase-CaM dependent (NADKc), has a CaM-binding peptide located in its N-terminal region and displays peculiar biochemical properties as well as different domain organization compared with known plant NAD+ kinases. In response to a pathogen elicitor, the activity of NADKc, which is associated with the mitochondrial periphery, contributes to an increase in the cellular NADP+ concentration and to the amplification of the elicitor-induced oxidative burst. Based on a phylogenetic analysis and enzymatic assays, we propose that the CaM/Ca2+-dependent NAD+ kinase activity found in photosynthetic organisms is carried out by NADKc-related proteins. Thus, NADKc represents the missing link between Ca2+ signaling, metabolism, and the oxidative burst.

Dynamic Changes between Two LHCX-Related Energy Quenching Sites Control Diatom Photoacclimation.

Marine diatoms are prominent phytoplankton organisms that perform photosynthesis in extremely variable environments. Diatoms possess a strong ability to dissipate excess absorbed energy as heat via nonphotochemical quenching (NPQ). This process relies on changes in carotenoid pigment composition (xanthophyll cycle) and on specific members of the light-harvesting complex family specialized in photoprotection (LHCXs), which potentially act as NPQ effectors. However, the link between light stress, NPQ, and the existence of different LHCX isoforms is not understood in these organisms. Using picosecond fluorescence analysis, we observed two types of NPQ in the pennate diatom Phaeodactylum tricornutum that were dependent on light conditions. Short exposure of low-light-acclimated cells to high light triggers the onset of energy quenching close to the core of photosystem II, while prolonged light stress activates NPQ in the antenna. Biochemical analysis indicated a link between the changes in the NPQ site/mechanism and the induction of different LHCX isoforms, which accumulate either in the antenna complexes or in the core complex. By comparing the responses of wild-type cells and transgenic lines with a reduced expression of the major LHCX isoform, LHCX1, we conclude that core complex-associated NPQ is more effective in photoprotection than is the antenna complex. Overall, our data clarify the complex molecular scenario of light responses in diatoms and provide a rationale for the existence of a degenerate family of LHCX proteins in these algae.

Role of cyclic and pseudo-cyclic electron transport in response to dynamic light changes in Physcomitrella patens.

Photosynthetic organisms support cell metabolism by harvesting sunlight and driving the electron transport chain at the level of thylakoid membranes. Excitation energy and electron flow in the photosynthetic apparatus is continuously modulated in response to dynamic environmental conditions. Alternative electron flow around photosystem I plays a seminal role in this regulation contributing to photoprotection by mitigating overreduction of the electron carriers. Different pathways of alternative electron flow coexist in the moss Physcomitrella patens, including cyclic electron flow mediated by the PGRL1/PGR5 complex and pseudo-cyclic electron flow mediated by the flavodiiron proteins FLV. In this work, we generated P. patens plants carrying both pgrl1 and flva knock-out mutations. A comparative analysis of the WT, pgrl1, flva, and pgrl1 flva lines suggests that cyclic and pseudo-cyclic processes have a synergic role in the regulation of photosynthetic electron transport. However, although both contribute to photosystem I protection from overreduction by modulating electron flow following changes in environmental conditions, FLV activity is particularly relevant in the first seconds after a light change whereas PGRL1 has a major role upon sustained strong illumination.

Global spectroscopic analysis to study the regulation of the photosynthetic proton motive force: A critical reappraisal.

In natural variable environments, plants rapidly adjust photosynthesis for optimum balance between photochemistry and photoprotection. These adjustments mainly occur via changes in their proton motive force (pmf). Recent studies based on time resolved analysis of the Electro Chromic Signal (ECS) bandshift of photosynthetic pigments in the model plant Arabidopsis thaliana have suggested an active role of ion fluxes across the thylakoid membranes in the regulation of the pmf. Among the different channels and transporters possibly involved in this phenomenon, we previously identified the TPK3 potassium channel. Plants silenced for TPK3 expression displayed light stress signatures, with reduced Non Photochemical Quenching (NPQ) capacity and sustained anthocyanin accumulation, even at moderate intensities. In this work we re-examined the role of this protein in pmf regulation, starting from the observation that both TPK3 knock-down (TPK3 KD) and WT plants display enhanced anthocyanin accumulation in the light under certain growth conditions, especially in old leaves. We thus compared the pmf features of young "green" (without anthocyanins) and old "red" (with anthocyanins) leaves in both genotypes using a global fit analysis of the ECS. We found that the differences in the ECS profile measured between the two genotypes reflect not only differences in TPK3 expression level, but also a modified photosynthetic activity of stressed red leaves, which are present in a larger amounts in the TPK3 KD plants.

Fine-tuned regulation of the K+ /H+ antiporter KEA3 is required to optimize photosynthesis during induction.

KEA3 is a thylakoid membrane localized K+ /H+ antiporter that regulates photosynthesis by modulating two components of proton motive force (pmf), the proton gradient (∆pH) and the electric potential (∆ψ). We identified a mutant allele of KEA3, disturbed proton gradient regulation (dpgr) based on its reduced non-photochemical quenching (NPQ) in artificial (CO2 -free with low O2 ) air. This phenotype was enhanced in the mutant backgrounds of PSI cyclic electron transport (pgr5 and crr2-1). In ambient air, reduced NPQ was observed during induction of photosynthesis in dpgr, the phenotype that was enhanced after overnight dark adaptation. In contrast, the knockout allele of kea3-1 exhibited a high-NPQ phenotype during steady state in ambient air. Consistent with this kea3-1 phenotype in ambient air, the membrane topology of KEA3 indicated a proton efflux from the thylakoid lumen to the stroma. The dpgr heterozygotes showed a semidominant and dominant phenotype in artificial and ambient air, respectively. In dpgr, the protein level of KEA3 was unaffected but the downregulation of its activity was probably disturbed. Our findings suggest that fine regulation of KEA3 activity is necessary for optimizing photosynthesis.

Arabidopsis thaliana plants challenged with uranium reveal new insights into iron and phosphate homeostasis.

Uranium (U) is a naturally occurring radionuclide that is toxic to plants. It is known to interfere with phosphate nutrition and to modify the expression of iron (Fe)-responsive genes. The transporters involved in the uptake of U from the environment are unknown. Here, we addressed whether IRT1, a high-affinity Fe2+ transporter, could contribute to U uptake in Arabidopsis thaliana. An irt1 null mutant was grown hydroponically in different conditions of Fe bioavailability and phosphate supply, and challenged with uranyl. Several physiological parameters (fitness, photosynthesis) were measured to evaluate the response to U treatment. We found that IRT1 is not a major route for U uptake in our experimental conditions. However, the analysis of irt1 indicated that uranyl interferes with Fe and phosphate homeostasis at different levels. In phosphate-sufficient conditions, the absence of the cation chelator EDTA in the medium has drastic consequences on the physiology of irt1, with important symptoms of Fe deficiency in chloroplasts. These effects are counterbalanced by U, probably because the radionuclide competes with Fe for complexation with phosphate and thus releases active Fe for metabolic and biogenic processes. Our study reveals that challenging plants with U is useful to decipher the complex interplay between Fe and phosphate.