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Cyclic electron flow provides acclimatory plasticity for the photosynthetic machinery under various environmental conditions and developmental stages.

Suorsa M - Front Plant Sci (2015)

Bottom Line: As a result, a transthylakoid proton gradient (ΔpH) is generated, leading to the production of ATP without concomitant production of NADPH, thus increasing the ATP/NADPH ratio within the chloroplast.At least two routes for CEF exist: a PROTON GRADIENT REGULATION5-PGRL1-and a chloroplast NDH-like complex mediated pathway.This review focuses on recent findings concerning the characteristics of both CEF routes in higher plants, with special emphasis paid on the crucial role of CEF in under challenging environmental conditions and developmental stages.

View Article: PubMed Central - PubMed

Affiliation: Molecular Plant Biology, Department of Biochemistry, University of Turku Turku, Finland.

ABSTRACT
Photosynthetic electron flow operates in two modes, linear and cyclic. In cyclic electron flow (CEF), electrons are recycled around photosystem I. As a result, a transthylakoid proton gradient (ΔpH) is generated, leading to the production of ATP without concomitant production of NADPH, thus increasing the ATP/NADPH ratio within the chloroplast. At least two routes for CEF exist: a PROTON GRADIENT REGULATION5-PGRL1-and a chloroplast NDH-like complex mediated pathway. This review focuses on recent findings concerning the characteristics of both CEF routes in higher plants, with special emphasis paid on the crucial role of CEF in under challenging environmental conditions and developmental stages.

No MeSH data available.


Hypothetical model describing upregulation of cyclic electron flow (CEF) under drought and cold stress. Drought stress induces stomatal closure and subsequent limitation in CO2 levels, whereas cold stress slows down the enzyme activity. Both conditions result in downregulation of the Calvin-Benson-Bassham cycle. (A) A hypothetical situation in which CEF would not function, and which would thus end up in severe over-reduction of the ETC and stroma, and finally to photodamage. (B) When the PGR5 and PGRL1 proteins receive electrons from ferredoxin (Fd) and trigger lumen acidification, electron flow toward PSI becomes, thus preventing ETC and stromal components from over-reduction and photodamage. In addition, lumen acidification leads to induction of non-photochemical quenching (NPQ; not drawn to the figure). (C) The NDH complex receives electrons from Fd, thus functioning as a safety valve for the excess of electrons. Furthermore, NDH likely functions as a proton pump, which participates in lumen acidification. Note that under most natural conditions, drought and cold likely induce upregulation of both CEF routes to some degree, but the preferential route depends on plant species. The illustrations describe only the hypothetical, not the actual position of the proteins and protein complexes. CBB, Calvin-Benson-Bassham cycle; PSI, photosystem; Fd, ferredoxin; e-, electron; H+, proton.
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Figure 1: Hypothetical model describing upregulation of cyclic electron flow (CEF) under drought and cold stress. Drought stress induces stomatal closure and subsequent limitation in CO2 levels, whereas cold stress slows down the enzyme activity. Both conditions result in downregulation of the Calvin-Benson-Bassham cycle. (A) A hypothetical situation in which CEF would not function, and which would thus end up in severe over-reduction of the ETC and stroma, and finally to photodamage. (B) When the PGR5 and PGRL1 proteins receive electrons from ferredoxin (Fd) and trigger lumen acidification, electron flow toward PSI becomes, thus preventing ETC and stromal components from over-reduction and photodamage. In addition, lumen acidification leads to induction of non-photochemical quenching (NPQ; not drawn to the figure). (C) The NDH complex receives electrons from Fd, thus functioning as a safety valve for the excess of electrons. Furthermore, NDH likely functions as a proton pump, which participates in lumen acidification. Note that under most natural conditions, drought and cold likely induce upregulation of both CEF routes to some degree, but the preferential route depends on plant species. The illustrations describe only the hypothetical, not the actual position of the proteins and protein complexes. CBB, Calvin-Benson-Bassham cycle; PSI, photosystem; Fd, ferredoxin; e-, electron; H+, proton.

Mentions: Similar to drought stress, also cold stress causes lowered carbon fixation, which in turn results in an excessive amount of reducing equivalents and thus imbalanced stromal redox state (Figure 1). Cold stress in combination with light illumination threatens particularly PSI (Sonoike and Terashima, 1994; Sonoike et al., 1995; Tjus et al., 1998; Kudoh and Sonoike, 2002), and similar to situation during fluctuating light, CEF plays an important role in protection of PSI also under low-temperature-caused stress. For example, treatment of spinach leaves with low temperatures has been shown to enhance CEF (Kou et al., 2013). In addition, a 3-day-treatment of maize plants with lowered temperature induced upregulation of particularly the PGR-mediated CEF (Savitch et al., 2011). In line with this, cold-acclimated Arabidopsis plants showed upregulation of PGR5–PGRL1-dependent CEF, while NDH complex abundancies rather decreased upon cold acclimation (Ivanov et al., 2012). On the other hand, rice mutants lacking the NDH complex showed a growth defect as a response to lowered temperatures (Yamori et al., 2011). Furthermore, the tobacco ndhb mutants exposed to a combination of low temperature and low light intensity showed disturbed regulation of electron transfer chain (ETC) as compared to WT (Li et al., 2004). It seems likely that the responses against cold stress in chilling-sensitive plants differ from those of tolerant species, which again highlights the broad variety in CEF responses in different species. Enhanced CEF has also been suggested to be involved in drastic modulations of ETC occurring in conifer needles during winter (Oquist and Huner, 2003), yet experimental evidence is still needed to verify these hypothesis.


Cyclic electron flow provides acclimatory plasticity for the photosynthetic machinery under various environmental conditions and developmental stages.

Suorsa M - Front Plant Sci (2015)

Hypothetical model describing upregulation of cyclic electron flow (CEF) under drought and cold stress. Drought stress induces stomatal closure and subsequent limitation in CO2 levels, whereas cold stress slows down the enzyme activity. Both conditions result in downregulation of the Calvin-Benson-Bassham cycle. (A) A hypothetical situation in which CEF would not function, and which would thus end up in severe over-reduction of the ETC and stroma, and finally to photodamage. (B) When the PGR5 and PGRL1 proteins receive electrons from ferredoxin (Fd) and trigger lumen acidification, electron flow toward PSI becomes, thus preventing ETC and stromal components from over-reduction and photodamage. In addition, lumen acidification leads to induction of non-photochemical quenching (NPQ; not drawn to the figure). (C) The NDH complex receives electrons from Fd, thus functioning as a safety valve for the excess of electrons. Furthermore, NDH likely functions as a proton pump, which participates in lumen acidification. Note that under most natural conditions, drought and cold likely induce upregulation of both CEF routes to some degree, but the preferential route depends on plant species. The illustrations describe only the hypothetical, not the actual position of the proteins and protein complexes. CBB, Calvin-Benson-Bassham cycle; PSI, photosystem; Fd, ferredoxin; e-, electron; H+, proton.
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Related In: Results  -  Collection

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Figure 1: Hypothetical model describing upregulation of cyclic electron flow (CEF) under drought and cold stress. Drought stress induces stomatal closure and subsequent limitation in CO2 levels, whereas cold stress slows down the enzyme activity. Both conditions result in downregulation of the Calvin-Benson-Bassham cycle. (A) A hypothetical situation in which CEF would not function, and which would thus end up in severe over-reduction of the ETC and stroma, and finally to photodamage. (B) When the PGR5 and PGRL1 proteins receive electrons from ferredoxin (Fd) and trigger lumen acidification, electron flow toward PSI becomes, thus preventing ETC and stromal components from over-reduction and photodamage. In addition, lumen acidification leads to induction of non-photochemical quenching (NPQ; not drawn to the figure). (C) The NDH complex receives electrons from Fd, thus functioning as a safety valve for the excess of electrons. Furthermore, NDH likely functions as a proton pump, which participates in lumen acidification. Note that under most natural conditions, drought and cold likely induce upregulation of both CEF routes to some degree, but the preferential route depends on plant species. The illustrations describe only the hypothetical, not the actual position of the proteins and protein complexes. CBB, Calvin-Benson-Bassham cycle; PSI, photosystem; Fd, ferredoxin; e-, electron; H+, proton.
Mentions: Similar to drought stress, also cold stress causes lowered carbon fixation, which in turn results in an excessive amount of reducing equivalents and thus imbalanced stromal redox state (Figure 1). Cold stress in combination with light illumination threatens particularly PSI (Sonoike and Terashima, 1994; Sonoike et al., 1995; Tjus et al., 1998; Kudoh and Sonoike, 2002), and similar to situation during fluctuating light, CEF plays an important role in protection of PSI also under low-temperature-caused stress. For example, treatment of spinach leaves with low temperatures has been shown to enhance CEF (Kou et al., 2013). In addition, a 3-day-treatment of maize plants with lowered temperature induced upregulation of particularly the PGR-mediated CEF (Savitch et al., 2011). In line with this, cold-acclimated Arabidopsis plants showed upregulation of PGR5–PGRL1-dependent CEF, while NDH complex abundancies rather decreased upon cold acclimation (Ivanov et al., 2012). On the other hand, rice mutants lacking the NDH complex showed a growth defect as a response to lowered temperatures (Yamori et al., 2011). Furthermore, the tobacco ndhb mutants exposed to a combination of low temperature and low light intensity showed disturbed regulation of electron transfer chain (ETC) as compared to WT (Li et al., 2004). It seems likely that the responses against cold stress in chilling-sensitive plants differ from those of tolerant species, which again highlights the broad variety in CEF responses in different species. Enhanced CEF has also been suggested to be involved in drastic modulations of ETC occurring in conifer needles during winter (Oquist and Huner, 2003), yet experimental evidence is still needed to verify these hypothesis.

Bottom Line: As a result, a transthylakoid proton gradient (ΔpH) is generated, leading to the production of ATP without concomitant production of NADPH, thus increasing the ATP/NADPH ratio within the chloroplast.At least two routes for CEF exist: a PROTON GRADIENT REGULATION5-PGRL1-and a chloroplast NDH-like complex mediated pathway.This review focuses on recent findings concerning the characteristics of both CEF routes in higher plants, with special emphasis paid on the crucial role of CEF in under challenging environmental conditions and developmental stages.

View Article: PubMed Central - PubMed

Affiliation: Molecular Plant Biology, Department of Biochemistry, University of Turku Turku, Finland.

ABSTRACT
Photosynthetic electron flow operates in two modes, linear and cyclic. In cyclic electron flow (CEF), electrons are recycled around photosystem I. As a result, a transthylakoid proton gradient (ΔpH) is generated, leading to the production of ATP without concomitant production of NADPH, thus increasing the ATP/NADPH ratio within the chloroplast. At least two routes for CEF exist: a PROTON GRADIENT REGULATION5-PGRL1-and a chloroplast NDH-like complex mediated pathway. This review focuses on recent findings concerning the characteristics of both CEF routes in higher plants, with special emphasis paid on the crucial role of CEF in under challenging environmental conditions and developmental stages.

No MeSH data available.