In Arabidopsis, two NTR isoforms are localized in the mitochondria (NTRA and
NTRB). Both of them reduce TRXo1, one of the two o-type TRX isoforms in
Arabidopsis, which is targeted to the mitochondrial matrix through a cleavable
N-terminal signal (Laloi et al. 2001; Reichheld et al. 2005). Mitochondrial localization of TRXo1 was later confirmed in other plants including poplar and pea (Gelhaye
et al. 2004; Martí et al. 2009). Moreover, in poplar and Arabidopsis, the TRXh2
isoform has been identified in mitochondria, although its submitochondrial localizations have not been addressed yet (Gelhaye et al. 2004; Meng et al. 2010).
Proteomic affinity approaches have been used to identify potential mitochondrial
TRX using purified mitochondrial protein extracts (Balmer et al. 2004; Martí et al.
2009; Marchand et al. 2010; Yoshida et al. 2013). These approaches have been
highly valuable to identify putative TRX mitochondrial targets and open the field for
further validation. Nevertheless, a few enzymatic and genetic evidences have further
validated those interactions yet. In pea, TRXo1 has been shown to be an efficient
reducer of the PRXII-F, suggesting that TRXo is involved in mitochondrial ROS
metabolism (Barranco-Medina et al. 2008). Pea TRXh2 and Arabidopsis TRXo1
were also shown to affect in vitro the dimerization state of the alternative oxidase
(AOX) by reducing a heterodimeric disulfide (Cys78–Cys78), and further restoring
the AOX activity (Gelhaye et al. 2004; Yoshida et al. 2013). Nevertheless, evidence
is lacking to clearly favor the interpretation of a regulatory function over that of a
maintenance function, i.e., to avoid Cys over-oxidation upon ROS exposure. Moreover, genetic evidences are lacking to support this redox regulation in vivo.
Both proteomic and in vitro works have pinpointed a potent TRX-dependent
regulation of tricarboxylic acid (TCA) cycle enzymes (Balmer et al. 2004; Martí
et al. 2009; Yoshida et al. 2013; Schmidtmann et al. 2014). Thiol switching of
mitochondrial citrate synthase and isocitrate dehydrogenase activity were reported
in vitro (Schmidtmann et al. 2014; Yoshida and Hisabori 2014), while thiol
switching of malate dehydrogenase activity was found unchanged (Yoshida and
Hisabori 2016a; Huang et al. 2017). In order to further address the function of TRX
on the TCA cycle in vivo, Daloso et al. (2015) have characterized Arabidopsis
knock-out mutants for mitochondrial TRXo1 (trxo1) and mitochondrial NTR (double mutant ntra ntrb). Both mutants were affected in mitochondrial metabolic
fingerprints and flux patterns, suggesting a role of the matrix TRX system in
regulating metabolism in vivo. In a complementary approach, Daloso et al. (2015)
evaluated in vitro activities of TCA cycle and associated enzymes in mutants
deficient in mitochondrial TRX. Among TCA cycle enzyme activities affected in
the mutants, both succinate dehydrogenase (SDH) and fumarase (FUM) were proposed to be deactivated by reduced TRXo1, suggesting that TRXo is acting as a
direct regulator of carbon flow through the TCA cycle (Daloso et al. 2015).
3.4 Endomembrane Thioredoxin System
While TRXh were previously described as cytosolic isoforms, more recent works
have established alternative cellular localizations for some members of this family.
On the Elaborate Network of Thioredoxins in Higher Plants
243
NTRB). Both of them reduce TRXo1, one of the two o-type TRX isoforms in
Arabidopsis, which is targeted to the mitochondrial matrix through a cleavable
N-terminal signal (Laloi et al. 2001; Reichheld et al. 2005). Mitochondrial localization of TRXo1 was later confirmed in other plants including poplar and pea (Gelhaye
et al. 2004; Martí et al. 2009). Moreover, in poplar and Arabidopsis, the TRXh2
isoform has been identified in mitochondria, although its submitochondrial localizations have not been addressed yet (Gelhaye et al. 2004; Meng et al. 2010).
Proteomic affinity approaches have been used to identify potential mitochondrial
TRX using purified mitochondrial protein extracts (Balmer et al. 2004; Martí et al.
2009; Marchand et al. 2010; Yoshida et al. 2013). These approaches have been
highly valuable to identify putative TRX mitochondrial targets and open the field for
further validation. Nevertheless, a few enzymatic and genetic evidences have further
validated those interactions yet. In pea, TRXo1 has been shown to be an efficient
reducer of the PRXII-F, suggesting that TRXo is involved in mitochondrial ROS
metabolism (Barranco-Medina et al. 2008). Pea TRXh2 and Arabidopsis TRXo1
were also shown to affect in vitro the dimerization state of the alternative oxidase
(AOX) by reducing a heterodimeric disulfide (Cys78–Cys78), and further restoring
the AOX activity (Gelhaye et al. 2004; Yoshida et al. 2013). Nevertheless, evidence
is lacking to clearly favor the interpretation of a regulatory function over that of a
maintenance function, i.e., to avoid Cys over-oxidation upon ROS exposure. Moreover, genetic evidences are lacking to support this redox regulation in vivo.
Both proteomic and in vitro works have pinpointed a potent TRX-dependent
regulation of tricarboxylic acid (TCA) cycle enzymes (Balmer et al. 2004; Martí
et al. 2009; Yoshida et al. 2013; Schmidtmann et al. 2014). Thiol switching of
mitochondrial citrate synthase and isocitrate dehydrogenase activity were reported
in vitro (Schmidtmann et al. 2014; Yoshida and Hisabori 2014), while thiol
switching of malate dehydrogenase activity was found unchanged (Yoshida and
Hisabori 2016a; Huang et al. 2017). In order to further address the function of TRX
on the TCA cycle in vivo, Daloso et al. (2015) have characterized Arabidopsis
knock-out mutants for mitochondrial TRXo1 (trxo1) and mitochondrial NTR (double mutant ntra ntrb). Both mutants were affected in mitochondrial metabolic
fingerprints and flux patterns, suggesting a role of the matrix TRX system in
regulating metabolism in vivo. In a complementary approach, Daloso et al. (2015)
evaluated in vitro activities of TCA cycle and associated enzymes in mutants
deficient in mitochondrial TRX. Among TCA cycle enzyme activities affected in
the mutants, both succinate dehydrogenase (SDH) and fumarase (FUM) were proposed to be deactivated by reduced TRXo1, suggesting that TRXo is acting as a
direct regulator of carbon flow through the TCA cycle (Daloso et al. 2015).
3.4 Endomembrane Thioredoxin System
While TRXh were previously described as cytosolic isoforms, more recent works
have established alternative cellular localizations for some members of this family.
On the Elaborate Network of Thioredoxins in Higher Plants
243