Knock-Down of the IFR1 Protein Perturbs the Homeostasis of Reactive Electrophile Species and Boosts Photosynthetic Hydrogen Production in Chlamydomonas reinhardtii.

Deepak Venkanna,Thomas Baier,Anant V Patel,Christian Südfeld,Sarah V Homburg,Olaf Kruse,Lutz Wobbe

doi:10.3389/fpls.2017.01347

Deepak Venkanna, Thomas Baier + Show 5 more

Open Access

https://doi.org/10.3389/fpls.2017.01347

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Abstract

The protein superfamily of short-chain dehydrogenases/reductases (SDR), including members of the atypical type (aSDR), covers a huge range of catalyzed reactions and in vivo substrates. This superfamily also comprises isoflavone reductase-like (IRL) proteins, which are aSDRs highly homologous to isoflavone reductases from leguminous plants. The molecular function of IRLs in non-leguminous plants and green microalgae has not been identified as yet, but several lines of evidence point at their implication in reactive oxygen species homeostasis. The Chlamydomonas reinhardtii IRL protein IFR1 was identified in a previous study, analyzing the transcriptomic changes occurring during the acclimation to sulfur deprivation and anaerobiosis, a condition that triggers photobiological hydrogen production in this microalgae. Accumulation of the cytosolic IFR1 protein is induced by sulfur limitation as well as by the exposure of C. reinhardtii cells to reactive electrophile species (RES) such as reactive carbonyls. The latter has not been described for IRL proteins before. Over-accumulation of IFR1 in the singlet oxygen response 1 (sor1) mutant together with the presence of an electrophile response element, known to be required for SOR1-dependent gene activation as a response to RES, in the promoter of IFR1, indicate that IFR1 expression is controlled by the SOR1-dependent pathway. An implication of IFR1 into RES homeostasis, is further implied by a knock-down of IFR1, which results in a diminished tolerance toward RES. Intriguingly, IFR1 knock-down has a positive effect on photosystem II (PSII) stability under sulfur-deprived conditions used to trigger photobiological hydrogen production, by reducing PSII-dependent oxygen evolution, in C. reinhardtii. Reduced PSII photoinhibition in IFR1 knock-down strains prolongs the hydrogen production phase resulting in an almost doubled final hydrogen yield compared to the parental strain. Finally, IFR1 knock-down could be successfully used to further increase hydrogen yields of the high hydrogen-producing mutant stm6, demonstrating that IFR1 is a promising target for genetic engineering approaches aiming at an increased hydrogen production capacity of C. reinhardtii cells.

Highlights

Among the most urgent challenges of our society today, are those associated to global warming, depletion of fossil fuels and a steady increase of the energy demand, which can pose a threat to economic and political stability (Organisation for Economic Co-operation and Development [OECD]/International Energy Agency [IEA], 2011)
In silico analyses performed with the amino acid sequence of IFR1 revealed that this protein represents an atypical member of the short-chain dehydrogenase/reductase (SDR) superfamily
A wide-scale bioinformatics study on SDRs in plant genomes suggested a distinct SDR family for isoflavone reductase (IFR), phenylcoumaran benzylic ether reductase (PCBER) and eugenol synthase and IFR1 was 1 of 15 C. reinhardtii proteins that could not be assigned to any SDR family during that study, a high homology of IFR1 to members of the SDR460A family was claimed, (Moummou et al, 2012)

Summary

Introduction

Among the most urgent challenges of our society today, are those associated to global warming, depletion of fossil fuels and a steady increase of the energy demand, which can pose a threat to economic and political stability (Organisation for Economic Co-operation and Development [OECD]/International Energy Agency [IEA], 2011). Photobiological hydrogen production has to be split into a two-stage process, which can be achieved by the experimental protocol proposed by Melis et al (2000) This protocol relies on biomass generation under sulfur-replete conditions in the first stage and subsequent withdrawal of sulfur to trigger photoinhibition of photosystem II, resulting in a continuous decline of photosynthetic oxygen evolution, while mitochondrial respiration remains relatively unaffected by the lack of sulfur in the medium. The C. reinhardtii mutant stm (Schönfeld et al, 2004) displays an enhanced hydrogen production capacity (Kruse et al, 2005) and its increased rate of mitochondrial oxygen consumption (Uhmeyer et al, 2017), was proposed to protect PSII during sulfur deprivation by accelerating the establishment of anaerobic conditions (Volgusheva et al, 2013), where irreversible, oxygen-dependent photoinhibition (Vass et al, 1992) cannot occur. In addition to its photobiological production, hydrogen can be produced under dark fermentative conditions in C. reinhardtii (Grossman et al, 2011)

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