chloroplast structure (Perez-Ruiz et al. 2006; Lepistö et al. 2009). In addition, plants
devoid of NTRC have been reported to be hypersensitive to different abiotic stresses,
such as salinity and drought (Serrato et al. 2004), prolonged darkness (Perez-Ruiz
et al. 2006), and heat (Chae et al. 2013), and also to biotic stress (Ishiga et al. 2012,
2016). These phenotypic characteristics of the ntrc mutant might be explained by the
antioxidant function of NTRC as an efficient reductant of 2-Cys PRX. In this regard,
it is worth mentioning that the NTRC-2-Cys PRX system has also been reported to
exist in other photosynthetic organisms including green algae, such as Chlorella
(Machida et al. 2012), and cyanobacteria, such as Anabaena (Pascual et al. 2011).
Indeed, an Anabaena mutant strain devoid of NTRC (ΔntrC) is more sensitive to
oxidative stress, especially to treatments with hydrogen peroxide, and the redox
balance of 2-Cys PRX is impaired, more oxidized, in the mutant (Sanchez-Riego
et al. 2016).
Beside the phenotypic features related with the response to stress, which might be
due to the antioxidant activity of the NTRC-2-Cys PRX system, it should be noted
that the Arabidopsis ntrc mutant has a rather pleiotropic phenotype, suggesting
additional functions for the enzyme. This notion was further supported by the
phenotype of an Arabidopsis mutant (Δ2cp) with severely decreased levels of
2-Cys PRX (Pulido et al. 2010), which is almost like the wild type.
It is well known that AGPase, a key regulatory enzyme of the starch biosynthesis
pathway, is subject to allosteric regulation but also to sugar and light-dependent
redox regulation (Geigenberger et al. 2005). On the other hand, as stated above,
NTRC uses as source of reducing power NADPH, which can be generated from
sugars during the night by the oxidative pentose phosphate pathway. These results
suggested the possibility that NTRC participates in the redox regulation of starch
biosynthesis, a notion further supported by the lower content of starch in roots and
leaves of the ntrc mutant. In vitro and in vivo analyses confirmed the participation of
NTRC in the redox regulation of AGPase (Michalska et al. 2009; Lepistö et al. 2013)
and, thus, in starch biosynthesis. Interestingly, the redox state of the AGPase was
found to be altered not only in leaf chloroplasts but also in roots, suggesting a role of
NTRC in non-photosynthetic tissues (Michalska et al. 2009), in line with the
localization of the enzyme in any type of plastids. However, Arabidopsis transgenic
plants expressing NTRC exclusively in green tissues, by expression of the NTRC
cDNA under the control of the RbcS gene promoter in the ntrc background, was
sufficient to rescue the wild type phenotype with respect to growth and pigmentation, while expression exclusively in roots had no effect (Kirchsteiger et al. 2012).
These results show the essential function of chloroplasts for the growth of photosynthetic and non-photosynthetic organs.
The characteristic pale green leaf phenotype of the ntrc mutant, which is due to
lower chlorophyll content than in the leaves of the wild type (Serrato et al. 2004;
Lepistö et al. 2009), suggested the participation of NTRC in chlorophyll biosynthesis. Several reports have shown the NTRC-dependent redox regulation of different
enzymes involved in the tetrapyrrole biosynthesis pathway including the glutamyltransfer RNA reductase GluTR1, CHLM (Richter et al. 2013) and the Mg-chelatase
subunit CHLI (Perez-Ruiz et al. 2014).
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I. Thormählen et al.
devoid of NTRC have been reported to be hypersensitive to different abiotic stresses,
such as salinity and drought (Serrato et al. 2004), prolonged darkness (Perez-Ruiz
et al. 2006), and heat (Chae et al. 2013), and also to biotic stress (Ishiga et al. 2012,
2016). These phenotypic characteristics of the ntrc mutant might be explained by the
antioxidant function of NTRC as an efficient reductant of 2-Cys PRX. In this regard,
it is worth mentioning that the NTRC-2-Cys PRX system has also been reported to
exist in other photosynthetic organisms including green algae, such as Chlorella
(Machida et al. 2012), and cyanobacteria, such as Anabaena (Pascual et al. 2011).
Indeed, an Anabaena mutant strain devoid of NTRC (ΔntrC) is more sensitive to
oxidative stress, especially to treatments with hydrogen peroxide, and the redox
balance of 2-Cys PRX is impaired, more oxidized, in the mutant (Sanchez-Riego
et al. 2016).
Beside the phenotypic features related with the response to stress, which might be
due to the antioxidant activity of the NTRC-2-Cys PRX system, it should be noted
that the Arabidopsis ntrc mutant has a rather pleiotropic phenotype, suggesting
additional functions for the enzyme. This notion was further supported by the
phenotype of an Arabidopsis mutant (Δ2cp) with severely decreased levels of
2-Cys PRX (Pulido et al. 2010), which is almost like the wild type.
It is well known that AGPase, a key regulatory enzyme of the starch biosynthesis
pathway, is subject to allosteric regulation but also to sugar and light-dependent
redox regulation (Geigenberger et al. 2005). On the other hand, as stated above,
NTRC uses as source of reducing power NADPH, which can be generated from
sugars during the night by the oxidative pentose phosphate pathway. These results
suggested the possibility that NTRC participates in the redox regulation of starch
biosynthesis, a notion further supported by the lower content of starch in roots and
leaves of the ntrc mutant. In vitro and in vivo analyses confirmed the participation of
NTRC in the redox regulation of AGPase (Michalska et al. 2009; Lepistö et al. 2013)
and, thus, in starch biosynthesis. Interestingly, the redox state of the AGPase was
found to be altered not only in leaf chloroplasts but also in roots, suggesting a role of
NTRC in non-photosynthetic tissues (Michalska et al. 2009), in line with the
localization of the enzyme in any type of plastids. However, Arabidopsis transgenic
plants expressing NTRC exclusively in green tissues, by expression of the NTRC
cDNA under the control of the RbcS gene promoter in the ntrc background, was
sufficient to rescue the wild type phenotype with respect to growth and pigmentation, while expression exclusively in roots had no effect (Kirchsteiger et al. 2012).
These results show the essential function of chloroplasts for the growth of photosynthetic and non-photosynthetic organs.
The characteristic pale green leaf phenotype of the ntrc mutant, which is due to
lower chlorophyll content than in the leaves of the wild type (Serrato et al. 2004;
Lepistö et al. 2009), suggested the participation of NTRC in chlorophyll biosynthesis. Several reports have shown the NTRC-dependent redox regulation of different
enzymes involved in the tetrapyrrole biosynthesis pathway including the glutamyltransfer RNA reductase GluTR1, CHLM (Richter et al. 2013) and the Mg-chelatase
subunit CHLI (Perez-Ruiz et al. 2014).
236
I. Thormählen et al.