Research - Institute of Plant Biology - Lendület - Laboratory for Molecular Photobioenergetics

Szilvia Zita TÓTH
senior research associate

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László KOVÁCS senior research associate
Valéria NAGY research associate
Soujanya KUNTAM research associate
Attila CSORBA Administrator Expert
André Manuel VIDAL MEIRELES junior research associate
Anna PODMANICZKI junior research associate
Dávid TÓTH Administrator Expert
Eszter SZÉLES Ph. D. Student

NOVEL ROLES OF ASCORBATE: FROM PHOTOSYNTHESIS TO PLANT PRODUCTIVITY AND ALGAL BIOHYDROGEN PRODUCTION

Ascorbate is an essential vitamin for humans, which is of plant origin and fulfills various roles both in mammalian and plant cells. Szilvia Z. Tóth and her group focus on the biosynthesis, transport and physiological roles of ascorbate in the plant. Their results may ultimately contribute to increasing the ascorbate contents of plants which may be of essential importance for several reasons: i) the naturally occurring ascorbate has a higher bioavailability than the chemically synthesized one, ii) ascorbate improves the post-harvesting properties of fruits and vegetables, and iii) it plays an important role under environmental stress conditions and thus may increase plant productivity.

Using a combination of biochemical, molecular biology and biophysical tools we aim to elucidate:

  • The regulation of Asc biosynthesis in higher plants and green algae Several Asc biosynthetic pathways have been identified in plants. Beside the principal route (the “Smirnoff-Wheeler” pathway) there are three other, alternative pathways in plants of which the physiological significance and contribution is yet to be clarified. In green algae, ascorbate is synthesized in the “Smirnoff-Wheeler” pathway; however, we have discovered that the regulation of ascorbate biosynthesis is markedly different in green algae and seed plants (Vidal-Meireles et al., 2017; Tóth et al., 2018).

  • Figure 1. The regulation of ascorbate biosynthesis in seed plants and green algae. Modified from Tóth et al. (2018).


  • The transport of ascorbate in the plant cell
    Ascorbate biosynthesis in plants take place in the mitochondria and ascorbate has to be transported to the various cell compartments, necessitating specific transport systems, since neither ascorbate nor its oxidized form, dehydroascorbate can easily diffuse through lipid bilayers. The functions of ascorbate in the cell highlight the need for transporters in the membranes of basically all cell compartments as well as in the plasma membrane. However, up to now, only one ascorbate transporter has been identified (called AtPHT4;4, Miyaji et al., 2015), but there must be several others (Fernie and Tóth 2015). Currently we are investigating the homologues of AtPHT4;4 in the green alga Chlamydomonas reinhardtii.

  • Figure 2. Putative and identified ascorbate transporters in the plant cell. Modified from Fernie and Tóth (2015)


  • The effects of ascorbate on the donor side of photosystem II
    When the oxygen-evolving complex of the photosynthetic apparatus becomes inactivated by heat stress, ascorbate donates electrons to photosystem II at significant rates, with halftimes typically between 20 and 50 ms, depending on the ascorbate content of the leaves (Tóth et al., 2009). We have also shown that by donating electrons to photosystem II, ascorbate slows down donor-side-induced photoinhibition (Tóth et al., 2011; Tóth et al., 2013,).

On the other hand, we have also discovered that at a high concentration, ascorbate may inactivate the oxygen-evolving complex of photosystem II. We have observed that upon sulphur deprivation, cellular ascorbate concentration dramatically increases, which leads to the inactivation of the oxygen evolving complex, anaerobiosis is established enabling the expression of hydrogenases and H2 production. By this means, ascorbate indirectly modulates algal H2 production (Nagy et al., 2012, Nagy et al., 2016, Nagy et al., 2018a).


Photobiological H2 production by green algae

Hydrogen is a highly efficient and clean energy source. Photobiological H2 production by algae has the potential of becoming a genuinely CO2-neutral energy source, because upon the combustion of H2, only water is produced. The [Fe-Fe]-type hydrogenases of green algae are probably the most active catalysts for H2 production, which, however, are highly sensitive to the O2 produced during photosynthesis. Resolving this antagonism has been a major hurdle for implementing a sustainable H2 production for the last 40 years.

Significant H2 production can be achieved by sulphur deprivation that results in the downregulation of photosystem II activity with a concomitant decrease of O2 evolution, thereby the establishment of anaerobiosis, which induces the expression of hydrogenases. However, this process is not sustainable and expensive thus cannot be applied on an industrial scale.

We recently demonstrated that hydrogenase expression and activity induced upon a short dark anaerobic treatment can be sustained by preventing the activation of the Calvin-Benson cycle via substrate limitation. This H2 production process is highly efficient, fully photoautotrophic and the cultures remain photosynthetically active. By the additional application of a highly efficient O2 absorbent, H2 production markedly increased and we have obtained H2 yields that are several-fold higher than in the case of the classical sulphur deprivation method. Thus, we have demonstrated that it is possible to sustainably use algal cells as whole-cell catalysts for H2 production, which enables industrial application of algal biohydrogen production (Nagy and Tóth 2017; Nagy et al., 2018b).


Figure 3. H2 production by green algae. Based on Nagy et al. (2018b).


Selected publications:

Nagy V, Vidal-Meireles A, Podmaniczki A, Szentmihályi K, Rákhely G, Zsigmond L, Kovács L, Tóth SZ (2018a) The mechanism of photosystem II inactivation during sulphur deprivation-induced H2 production in Chlamydomonas reinhardtii. Plant J 94: 548-561

Nagy V, Podmaniczki A, Vidal-Meireles A, Tengölics R, Kovács L, Rákhely G, Scoma A, Tóth SZ (2018b) Water-splitting-based, sustainable and efficient H2 production in green algae as achieved by substrate limitation of the Calvin-Benson-Bassham cycle. Biotechnol Biofuels 11: 69

Tóth SZ, Lőrincz T, Szarka A (2018) Concentration does matter: The beneficial and potentially harmful effects of ascorbate in humans and plants. Antiox Redox Signal 29: 1516-1533

Nagy V, Tóth SZ (2017) Photoautotrophic and sustainable production of hydrogen in algae. European Patent Application 17155168.2, priority date: 08.02.2017.

Vidal-Meireles A, Neupert J, Zsigmond L, Rosado-Souza L, Kovács L, Nagy V, Galambos A, Fernie AR, Bock R, Tóth SZ (2017) Regulation of ascorbate biosynthesis in green algae has evolved to enable rapid stress-induced response via the VTC2 gene encoding GDP-L-galactose phosphorylase. New Phytol 214: 668-681

Nagy V, Vidal-Meireles A, Tengölics R, Rákhely G, Garab G, Kovács L. and Tóth SZ (2016) Ascorbate accumulation during sulphur deprivation and its effects on photosystem II activity and H2 production of the green alga Chlamydomonas reinhardtii. Plant Cell Environ 39: 1460-1472.

Fernie AR, Tóth SZ (2015) Identification of the elusive chloroplast ascorbate transporter extends of the substrate specificity of the PHT family. Mol Plant 8: 674-676.

Tóth SZ, Schansker G, Garab G (2013) The physiological roles and metabolism of ascorbate in chloroplasts. Physiol Plantarum 148: 161-175

Nagy V, Tengölics R, Schansker G, Rákhely G, Kovács KL, Garab G, Tóth SZ (2012) Stimulatory effect of ascorbate, the alternative electron donor of photosystem II, on the hydrogen production of sulphur-deprived Chlamydomonas reinhardtii. Int J Hydrogen Energy 37: 8864-8871

Tóth SZ, Nagy V, Puthur JT, Kovács L, Garab G (2011) The physiological role of ascorbate as photosystem II electron donor: protection against photoinactivation in heat-stressed leaves. Plant Physiol 156: 382-392

Tóth SZ, Puthur JT, Nagy V, Garab G (2009) Experimental evidence for ascorbate-dependent electron transport in leaves with inactive oxygen-evolving complexes. Plant Physiol 149: 1568-1578