proteins consisting of two or three TRX-like domains fused to a Cys-rich C-terminal
domain. In Zea mays and Arabidopsis, characterized NRX isoforms share a dual
cytosolic/nuclear localization and harbor putative nuclear localization signals
(Laughner et al. 1998; Marchal et al. 2014). On the other hand, a cotton NRX
orthologue (GbNRX1) was preferentially found in the secretory pathway and the
apoplast (see below) (Li et al. 2016). Both NRX isoforms (NRX1 and 2) found in
Arabidopsis exhibit disulfide reduction activities. While NRX1 can be reduced by
NTR in vitro, constituting a functional TRX reduction pathway in the nucleus
(Marchal et al. 2014), the NRX2 physiological reducer is still unknown. Genetic
studies have assigned a function of NRX1 in plant fertility. It is required for pollen
tube navigation through the pistil, guiding it to the ovule (Qin et al. 2009; Marchal
et al. 2014). Still it is not yet established, if this function is dependent on the nuclear
or the cytosolic localization of NRX1 and what is the pathway involved.
Recent works have involved NRX1 in plant immunity responses (Kneeshaw et al.
2017). NRX1 gene expression is responsive to salicylic acid in Arabidopsis and
poplar and is compromised in the SAR induced by pathogenic bacterial strains of
Pseudomonas syringae (Kneeshaw et al. 2017). Upon pathogen trigger, NRX1 is
induced and binds target enzymes of major hydrogen peroxide scavenging pathways, including catalases. Mutant nrx1 plants displayed reduced catalase activity
and are hypersensitive to oxidative stress. Remarkably, catalase is maintained in a
reduced state by interaction with NRX1, a process necessary for its hydrogen
peroxide scavenging activity. Thus, the works by Kneeshaw et al. (2017) conclude
that hydrogen peroxide scavenging enzymes, like catalase, experience oxidative
distress in reactive oxygen species (ROS)-induced environments and require reductive protection from NRX1 for optimal activity.
A pea orthologue of mitochondrial o-type TRX (PsTRXo1) shows a dual mitochondrial and nuclear localization (Martí et al. 2009). Recent works by Calderón
et al. (2017) show that PsTRXo1 interacts with proliferating cell nuclear antigen
(PCNA) and that its overexpression in cell suspension affects cell growth and cell
cycle progression. While coinciding with an upregulation of PCNA protein, the
effect of PsTRXo1 expression on cell growth is not yet fully established (Calderón
et al. 2017).
TRXh isoforms accumulate in the nuclear compartment in tissues subjected to
oxidative stress or genotoxic conditions (Serrato et al. 2001; Serrato and Cejudo
2003; Sarkar et al. 2005; Pulido et al. 2009) or during seed germination in which the
NTR-TRXh system serves as a source of reducing power to regenerate 1-Cys (Pulido
et al. 2009).
3.3 Mitochondrial Thioredoxin System
Although TRX and TRX reductase activities were reported in plant mitochondrial
extracts two decades ago (Konrad et al. 1996; Banze and Follmann 2000), the first
mitochondrial TRX system has been identified in Arabidopsis by Laloi et al. (2001).
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I. Thormählen et al.
domain. In Zea mays and Arabidopsis, characterized NRX isoforms share a dual
cytosolic/nuclear localization and harbor putative nuclear localization signals
(Laughner et al. 1998; Marchal et al. 2014). On the other hand, a cotton NRX
orthologue (GbNRX1) was preferentially found in the secretory pathway and the
apoplast (see below) (Li et al. 2016). Both NRX isoforms (NRX1 and 2) found in
Arabidopsis exhibit disulfide reduction activities. While NRX1 can be reduced by
NTR in vitro, constituting a functional TRX reduction pathway in the nucleus
(Marchal et al. 2014), the NRX2 physiological reducer is still unknown. Genetic
studies have assigned a function of NRX1 in plant fertility. It is required for pollen
tube navigation through the pistil, guiding it to the ovule (Qin et al. 2009; Marchal
et al. 2014). Still it is not yet established, if this function is dependent on the nuclear
or the cytosolic localization of NRX1 and what is the pathway involved.
Recent works have involved NRX1 in plant immunity responses (Kneeshaw et al.
2017). NRX1 gene expression is responsive to salicylic acid in Arabidopsis and
poplar and is compromised in the SAR induced by pathogenic bacterial strains of
Pseudomonas syringae (Kneeshaw et al. 2017). Upon pathogen trigger, NRX1 is
induced and binds target enzymes of major hydrogen peroxide scavenging pathways, including catalases. Mutant nrx1 plants displayed reduced catalase activity
and are hypersensitive to oxidative stress. Remarkably, catalase is maintained in a
reduced state by interaction with NRX1, a process necessary for its hydrogen
peroxide scavenging activity. Thus, the works by Kneeshaw et al. (2017) conclude
that hydrogen peroxide scavenging enzymes, like catalase, experience oxidative
distress in reactive oxygen species (ROS)-induced environments and require reductive protection from NRX1 for optimal activity.
A pea orthologue of mitochondrial o-type TRX (PsTRXo1) shows a dual mitochondrial and nuclear localization (Martí et al. 2009). Recent works by Calderón
et al. (2017) show that PsTRXo1 interacts with proliferating cell nuclear antigen
(PCNA) and that its overexpression in cell suspension affects cell growth and cell
cycle progression. While coinciding with an upregulation of PCNA protein, the
effect of PsTRXo1 expression on cell growth is not yet fully established (Calderón
et al. 2017).
TRXh isoforms accumulate in the nuclear compartment in tissues subjected to
oxidative stress or genotoxic conditions (Serrato et al. 2001; Serrato and Cejudo
2003; Sarkar et al. 2005; Pulido et al. 2009) or during seed germination in which the
NTR-TRXh system serves as a source of reducing power to regenerate 1-Cys (Pulido
et al. 2009).
3.3 Mitochondrial Thioredoxin System
Although TRX and TRX reductase activities were reported in plant mitochondrial
extracts two decades ago (Konrad et al. 1996; Banze and Follmann 2000), the first
mitochondrial TRX system has been identified in Arabidopsis by Laloi et al. (2001).
242
I. Thormählen et al.