lines deficient in plastidial TRX showed no or only minor visible phenotypes as
compared to the wild type (Pulido et al. 2010; Thormählen et al. 2013, 2017; Laugier
et al. 2013), despite the diverse roles previously proposed for these TRX based on
analyses in vitro. For example, TRXf was known to be the only TRX type, which
activates the redox regulated CBC enzymes fructose 1,6-bisphosphatase (FBPase) or
glyceraldehyde 3-phosphate dehydrogenase (Collin et al. 2003; Marri et al. 2009).
However, growth under moderate light conditions and CO 2 fixation rates were not
impaired in the Arabidopsis trxf1 single mutant, showing a strong deficiency in
TRXf protein level in leaves (Thormählen et al. 2013; Naranjo et al. 2016a). Even
the double mutant trxf1 trxf2 with complete lack of TRXf presented only slightly
decreased rosette fresh weights under short day conditions (Naranjo et al. 2016a).
The low impact of single TRX deficiencies on growth is most probably due to
underestimated redundant functions of the other TRX isoforms or of additional
enzymes in the plastidial redox regulatory network, like NTRC (see below). There
is, however, one exception. The recently discovered Arabidopsis trxz single mutant
presented an albino phenotype with strongly impaired growth compared to wild type
plants (Arsova et al. 2010). Here, obviously, other TRX isoforms were not able to
compensate for the deficiency in TRXz.
Some of the observed metabolic phenotypes in Arabidopsis mutant lines devoid
of TRXf or m isoforms were confirmatory of the in vitro results mentioned above.
The redox activation state of ADP-glucose pyrophosphorylase (AGPase), which
catalyzes the first committed step in starch synthesis, was attenuated in leaves of
trxf1 mutants during the day, accompanied by a reduced accumulation of starch
(Thormählen et al. 2013) in agreement with previous in vitro results (Ballicora et al.
2000; Geigenberger et al. 2005). However, leaves of Nicotiana tabacum
overexpression lines with elevated levels of TRXf accumulated higher levels of
starch, while no change in AGPase redox activation was observed (Sanz-Barrio et al.
2013). Plants lacking TRXf isoforms additionally showed an impaired lightdependent activation of the CBC enzyme FBPase (Thormählen et al. 2015; Yoshida
et al. 2015; Naranjo et al. 2016a), and the ribulose 1,5-bisphosphate carboxylase/
oxigenase (Rubisco) activase (Naranjo et al. 2016a). Okegawa and Motohashi
(2015) stated that the TRXm isoforms might be the predominant redox regulators
of the CBC enzymes, since Arabidopsis triple mutant lines simultaneously deficient
in TRXm1, m2, and m4 showed attenuated CO 2 fixation rates, smaller rosettes, and
impaired redox activation states of FBPase and sedoheptulose 1,7-bisphosphatase
(SBPase) as well as the malate valve enzyme NADP
+
-dependent malate dehydrogenase (NADP
+
-MDH). Since the plastidial FBPase is not activated by TRXm
isoforms in vitro (Collin et al. 2003; Yoshida and Hisabori 2016b), the in vivo
observation on the impaired FBPase redox activation (Okegawa and Motohashi
2015) might not be due to direct TRX effects, but most probably due to secondary
reasons, like an inhibited electron flow to downstream metabolic processes. The
latter suggestion would be in agreement with additional in vivo studies, which found
TRXm impacts on processes involved in the photosynthetic light reaction, like an
impaired biogenesis of photosystem (PS) II in Arabidopsis mutant lines simultaneously lacking TRXm1, m2, and m4 (Wang et al. 2013) or the inhibition of cyclic
On the Elaborate Network of Thioredoxins in Higher Plants
229
compared to the wild type (Pulido et al. 2010; Thormählen et al. 2013, 2017; Laugier
et al. 2013), despite the diverse roles previously proposed for these TRX based on
analyses in vitro. For example, TRXf was known to be the only TRX type, which
activates the redox regulated CBC enzymes fructose 1,6-bisphosphatase (FBPase) or
glyceraldehyde 3-phosphate dehydrogenase (Collin et al. 2003; Marri et al. 2009).
However, growth under moderate light conditions and CO 2 fixation rates were not
impaired in the Arabidopsis trxf1 single mutant, showing a strong deficiency in
TRXf protein level in leaves (Thormählen et al. 2013; Naranjo et al. 2016a). Even
the double mutant trxf1 trxf2 with complete lack of TRXf presented only slightly
decreased rosette fresh weights under short day conditions (Naranjo et al. 2016a).
The low impact of single TRX deficiencies on growth is most probably due to
underestimated redundant functions of the other TRX isoforms or of additional
enzymes in the plastidial redox regulatory network, like NTRC (see below). There
is, however, one exception. The recently discovered Arabidopsis trxz single mutant
presented an albino phenotype with strongly impaired growth compared to wild type
plants (Arsova et al. 2010). Here, obviously, other TRX isoforms were not able to
compensate for the deficiency in TRXz.
Some of the observed metabolic phenotypes in Arabidopsis mutant lines devoid
of TRXf or m isoforms were confirmatory of the in vitro results mentioned above.
The redox activation state of ADP-glucose pyrophosphorylase (AGPase), which
catalyzes the first committed step in starch synthesis, was attenuated in leaves of
trxf1 mutants during the day, accompanied by a reduced accumulation of starch
(Thormählen et al. 2013) in agreement with previous in vitro results (Ballicora et al.
2000; Geigenberger et al. 2005). However, leaves of Nicotiana tabacum
overexpression lines with elevated levels of TRXf accumulated higher levels of
starch, while no change in AGPase redox activation was observed (Sanz-Barrio et al.
2013). Plants lacking TRXf isoforms additionally showed an impaired lightdependent activation of the CBC enzyme FBPase (Thormählen et al. 2015; Yoshida
et al. 2015; Naranjo et al. 2016a), and the ribulose 1,5-bisphosphate carboxylase/
oxigenase (Rubisco) activase (Naranjo et al. 2016a). Okegawa and Motohashi
(2015) stated that the TRXm isoforms might be the predominant redox regulators
of the CBC enzymes, since Arabidopsis triple mutant lines simultaneously deficient
in TRXm1, m2, and m4 showed attenuated CO 2 fixation rates, smaller rosettes, and
impaired redox activation states of FBPase and sedoheptulose 1,7-bisphosphatase
(SBPase) as well as the malate valve enzyme NADP
+
-dependent malate dehydrogenase (NADP
+
-MDH). Since the plastidial FBPase is not activated by TRXm
isoforms in vitro (Collin et al. 2003; Yoshida and Hisabori 2016b), the in vivo
observation on the impaired FBPase redox activation (Okegawa and Motohashi
2015) might not be due to direct TRX effects, but most probably due to secondary
reasons, like an inhibited electron flow to downstream metabolic processes. The
latter suggestion would be in agreement with additional in vivo studies, which found
TRXm impacts on processes involved in the photosynthetic light reaction, like an
impaired biogenesis of photosystem (PS) II in Arabidopsis mutant lines simultaneously lacking TRXm1, m2, and m4 (Wang et al. 2013) or the inhibition of cyclic
On the Elaborate Network of Thioredoxins in Higher Plants
229