Calvin-Benson cycle (CBC) enzymes (Buchanan 1980). It was observed that
chloroplast TRX act as electron carrier able to modulate enzyme activity by changing the redox state of the target proteins dependent on light. Thus, the previously
suggested idea that the increased CO 2 fixation under light is due to the provision of
the energy and redox carriers ATP and NADPH during photosynthesis, which might
facilitate a promoted protein synthesis of the CBC enzymes, could be ruled out.
Following the initial finding about TRX around 40 years ago, extensive in vitro
studies with isolated or recombinant TRX proteins were performed (reviewed by
Meyer et al. 2012; Geigenberger et al. 2017). These studies extended the list of
potential TRX targets and demonstrated distinct functions for the single isoforms.
TRX types f and m were found to be predominantly important for the activation of
anabolic pathways, like the CBC (Wolosiuk and Buchanan 1977; Breazale et al.
1978; Collin et al. 2003; Marri et al. 2009; Yoshida and Hisabori 2016b), the
biosynthesis of starch (Ballicora et al. 2000; Skryhan et al. 2015; Thormählen
et al. 2013) and lipids (Sasaki et al. 1997; Yamaryo et al. 2006), and for balancing
the energy and redox homeostasis in chloroplasts by participating in ATP synthesis
(McKinney et al. 1978) and the malate valve (Wolosiuk et al. 1977; Collin et al.
2003; Yoshida and Hisabori 2016b). Additionally, many other enzymes involved in
diverse chloroplast processes were observed to be redox regulated by TRXf and m
in vitro, like the oxidative pentose phosphate (Scheibe and Anderson 1981; Née et al.
2009) and shikimate pathway (Entus et al. 2002), the nitrogen assimilation (Lichter
and Häberlein 1998) or protein import processes (Bartsch et al. 2008). In vitro
studies investigating the regulation of the cyclic electron transport in the photosynthetic light reaction showed a preference for TRXm isoforms, but described so far
contrasting effects leading to activation (Hertle et al. 2013) or inhibition (Courteille
et al. 2013) of this process. TRXx, y, and z showed higher affinities for enzymes
involved in oxidative stress responses (Collin et al. 2003, 2004; Navrot et al. 2006;
Vieira Dos Santos et al. 2007; Chibani et al. 2011; Bohrer et al. 2012). The
TRX-dependent redox activation of starch degradation (Mikkelsen et al. 2005;
Sparla et al. 2006; Valerio et al. 2011; Seung et al. 2013; Silver et al. 2013) and
chlorophyll synthesis (Ikegami et al. 2007; Luo et al. 2012; Yoshida and Hisabori
2016b) presented no clear distinction in the preference to different TRX isoforms
until now. Additionally, it remains unclear how the starch degradation process in
leaves could be activated by TRX reduction, since mobilization of transient starch
occurs mainly during the night, when the photosynthetic reduction pathway of TRX
is inactive.
2.1.3 The Plastidial TRX System Shows Surprisingly Redundant
Activities In Vivo
Within the last decade, reverse genetic approaches were used to investigate
Arabidopsis mutant lines lacking or overexpressing single or multiple TRX, so
that in vivo evidences for the functional roles of the individual TRX in planta
could be addressed. Surprisingly, under normal light conditions most single mutant
228
I. Thormählen et al.
chloroplast TRX act as electron carrier able to modulate enzyme activity by changing the redox state of the target proteins dependent on light. Thus, the previously
suggested idea that the increased CO 2 fixation under light is due to the provision of
the energy and redox carriers ATP and NADPH during photosynthesis, which might
facilitate a promoted protein synthesis of the CBC enzymes, could be ruled out.
Following the initial finding about TRX around 40 years ago, extensive in vitro
studies with isolated or recombinant TRX proteins were performed (reviewed by
Meyer et al. 2012; Geigenberger et al. 2017). These studies extended the list of
potential TRX targets and demonstrated distinct functions for the single isoforms.
TRX types f and m were found to be predominantly important for the activation of
anabolic pathways, like the CBC (Wolosiuk and Buchanan 1977; Breazale et al.
1978; Collin et al. 2003; Marri et al. 2009; Yoshida and Hisabori 2016b), the
biosynthesis of starch (Ballicora et al. 2000; Skryhan et al. 2015; Thormählen
et al. 2013) and lipids (Sasaki et al. 1997; Yamaryo et al. 2006), and for balancing
the energy and redox homeostasis in chloroplasts by participating in ATP synthesis
(McKinney et al. 1978) and the malate valve (Wolosiuk et al. 1977; Collin et al.
2003; Yoshida and Hisabori 2016b). Additionally, many other enzymes involved in
diverse chloroplast processes were observed to be redox regulated by TRXf and m
in vitro, like the oxidative pentose phosphate (Scheibe and Anderson 1981; Née et al.
2009) and shikimate pathway (Entus et al. 2002), the nitrogen assimilation (Lichter
and Häberlein 1998) or protein import processes (Bartsch et al. 2008). In vitro
studies investigating the regulation of the cyclic electron transport in the photosynthetic light reaction showed a preference for TRXm isoforms, but described so far
contrasting effects leading to activation (Hertle et al. 2013) or inhibition (Courteille
et al. 2013) of this process. TRXx, y, and z showed higher affinities for enzymes
involved in oxidative stress responses (Collin et al. 2003, 2004; Navrot et al. 2006;
Vieira Dos Santos et al. 2007; Chibani et al. 2011; Bohrer et al. 2012). The
TRX-dependent redox activation of starch degradation (Mikkelsen et al. 2005;
Sparla et al. 2006; Valerio et al. 2011; Seung et al. 2013; Silver et al. 2013) and
chlorophyll synthesis (Ikegami et al. 2007; Luo et al. 2012; Yoshida and Hisabori
2016b) presented no clear distinction in the preference to different TRX isoforms
until now. Additionally, it remains unclear how the starch degradation process in
leaves could be activated by TRX reduction, since mobilization of transient starch
occurs mainly during the night, when the photosynthetic reduction pathway of TRX
is inactive.
2.1.3 The Plastidial TRX System Shows Surprisingly Redundant
Activities In Vivo
Within the last decade, reverse genetic approaches were used to investigate
Arabidopsis mutant lines lacking or overexpressing single or multiple TRX, so
that in vivo evidences for the functional roles of the individual TRX in planta
could be addressed. Surprisingly, under normal light conditions most single mutant
228
I. Thormählen et al.