In Arabidopsis, Traverso et al. (2013) have reexamined the subcellular localizations of members of the subtype II and III. By using a combination of predictive
software and in vitro assays, they showed that members of the subtype II (TRXh2,
h7, h8) and III (TRXh9, h10, atypical CxxS2) are subjected to N-terminal (Gly2
and Cys4) fatty acid acylations (N-myristoylation and S-palmitoylation). The role
of these posttranslational modifications is to target proteins to membranes. Accordingly, TRX::GFP fusions revealed that myristoylation targeted proteins to the
endomembrane system including the endoplasmic reticulum (ER) and Golgi, and
that partitioning between this membrane compartment and the cytosol correlated
with the catalytic efficiency of the N-myristoyltransferase acting at the N-terminus
of the TRX. Moreover, a palmitoylatable Cys residue (only present in subtype III)
flanking the N-myristoylation site was crucial to target proteins to the plasma
membrane (Traverso et al. 2013). Independently, Meng et al. (2010) showed that
N-myristoylation and S-palmitoylation of TRXh9 at Gly2/Cys4 residues is associated with the capacity of the protein to move from cell-to-cell, suggesting a role of
this TRXh isoform in intercellular communication and cell growth.
In spite of the presence of distinct TRXh isoforms in the endomembrane system,
their role is still unknown. While they might sustain other thiol oxidoreductases (i.e.,
protein disulfide isomerases, ER oxidases, etc.) in disulfide reduction/isomerization
in the ER, no putative target proteins have been identified yet in these compartments.
Concerning their reduction pathway, neither putative endomembrane nor plasma
membrane NTR have been identified so far. Alternatively, GSH of the ER could
constitute a physiological reducer of endomembrane TRX, as shown for subtype III
isoform TRXh9 in poplar (Gelhaye et al. 2003; Koh et al. 2008). Of note, an atypical
TRXh member of subtype III (CxxS2) is acting as a protein disulfide isomerase
in vitro, which might be physiologically relevant (Serrato et al. 2008).
Finally, an NRX1 isoform (GbNRX1) has recently been involved in the apoplast
immune response to the pathogenic fungus Verticillium dahliae. Here, GbNRX1
functions in apoplastic ROS scavenging after the ROS burst that occurs upon
recognition of the pathogen (Li et al. 2016).
4 Conclusions and Perspectives
In this review, we have discussed emerging topics of higher plant TRX. It is
becoming clear that TRX form an elaborate network in plants being important to
integrate metabolism, stress responses, development, and gene expression in a
fluctuating environment. The chloroplast contains light- and NADPH-dependent
TRX systems, which are combined via the redox balance of 2-Cys PRX to allow a
cooperative control of chloroplast functions and photoautotrophic growth in higher
plants. Extraplastidial TRX have been found to play critical and integrative roles in
controlling pathogen defense and abiotic stress tolerance of plants interfacing with
other signals such as ROS and phytohormones. Additional emerging fields in TRX
functions are the regulation of plant development and the interaction of TRX with
244
I. Thormählen et al.
software and in vitro assays, they showed that members of the subtype II (TRXh2,
h7, h8) and III (TRXh9, h10, atypical CxxS2) are subjected to N-terminal (Gly2
and Cys4) fatty acid acylations (N-myristoylation and S-palmitoylation). The role
of these posttranslational modifications is to target proteins to membranes. Accordingly, TRX::GFP fusions revealed that myristoylation targeted proteins to the
endomembrane system including the endoplasmic reticulum (ER) and Golgi, and
that partitioning between this membrane compartment and the cytosol correlated
with the catalytic efficiency of the N-myristoyltransferase acting at the N-terminus
of the TRX. Moreover, a palmitoylatable Cys residue (only present in subtype III)
flanking the N-myristoylation site was crucial to target proteins to the plasma
membrane (Traverso et al. 2013). Independently, Meng et al. (2010) showed that
N-myristoylation and S-palmitoylation of TRXh9 at Gly2/Cys4 residues is associated with the capacity of the protein to move from cell-to-cell, suggesting a role of
this TRXh isoform in intercellular communication and cell growth.
In spite of the presence of distinct TRXh isoforms in the endomembrane system,
their role is still unknown. While they might sustain other thiol oxidoreductases (i.e.,
protein disulfide isomerases, ER oxidases, etc.) in disulfide reduction/isomerization
in the ER, no putative target proteins have been identified yet in these compartments.
Concerning their reduction pathway, neither putative endomembrane nor plasma
membrane NTR have been identified so far. Alternatively, GSH of the ER could
constitute a physiological reducer of endomembrane TRX, as shown for subtype III
isoform TRXh9 in poplar (Gelhaye et al. 2003; Koh et al. 2008). Of note, an atypical
TRXh member of subtype III (CxxS2) is acting as a protein disulfide isomerase
in vitro, which might be physiologically relevant (Serrato et al. 2008).
Finally, an NRX1 isoform (GbNRX1) has recently been involved in the apoplast
immune response to the pathogenic fungus Verticillium dahliae. Here, GbNRX1
functions in apoplastic ROS scavenging after the ROS burst that occurs upon
recognition of the pathogen (Li et al. 2016).
4 Conclusions and Perspectives
In this review, we have discussed emerging topics of higher plant TRX. It is
becoming clear that TRX form an elaborate network in plants being important to
integrate metabolism, stress responses, development, and gene expression in a
fluctuating environment. The chloroplast contains light- and NADPH-dependent
TRX systems, which are combined via the redox balance of 2-Cys PRX to allow a
cooperative control of chloroplast functions and photoautotrophic growth in higher
plants. Extraplastidial TRX have been found to play critical and integrative roles in
controlling pathogen defense and abiotic stress tolerance of plants interfacing with
other signals such as ROS and phytohormones. Additional emerging fields in TRX
functions are the regulation of plant development and the interaction of TRX with
244
I. Thormählen et al.