Interestingly, the TRXh5 isoform is specifically involved in pathogen response
pathways. TRXh5 is a major actor of plant immunity, regulating the oligomeric state
of the non-pathogenesis-related protein expressor NPR1, a master regulator of the
systemic acquired resistance (SAR). NPR1 normally exists in the cytosol as a thiolbound oligomer. Upon pathogen challenge or treatment with salicylic acid, TRXh5
is upregulated (Laloi et al. 2004; Tada et al. 2008). TRXh5 then reduces NPR1
oligomers resulting in monomer release, translocation to the nucleus, and regulation
of defense gene expression associated with both local and systemic resistance.
Consequently, a trxh5 knock-out mutant is impaired in establishing a SAR and is
therefore more susceptible to pathogen infection (Tada et al. 2008). More recently,
Kneeshaw et al. (2014) have shown that TRXh5 acts in the SAR signaling by
selective NPR1 protein S-denitrosylation. TRXh acting as denitrosylase has been
additionally shown in ntra mutants in which higher levels of S-nitrosylated proteins
were found (Tada et al. 2008).
TRXh5 is also involved in the plant tolerance to the necrotrophic fungal pathogen
Cochliobolus victoriae. The redox mechanism involves binding of victorin, the virulent
effector of C. victoriae, to the first Cys residue in the TRXh5 active site (Sweat and
Wolpert 2007; Lorang et al. 2012). Victorin binding to TRXh5 activates the locus
orchestrating victorin effects protein LOV1, an Arabidopsis susceptibility protein, and
elicits a resistance-like response that confers disease susceptibility. Lorang et al. (2012)
proposed that victorin mimics a conventional pathogen virulence effector and confers
virulence to C. victoriae solely because it incites plant defense mechanisms.
Specifically, in self-incompatible species, TRXh isoforms (called THL in Brassica
napus) have been involved in recognition and rejection of self-incompatible pollen
through redox regulation of a key actor (S-locus receptor kinase) of the selfincompatible signaling pathway (Bower et al. 1996; Cabrillac et al. 2001; Ivanov
and Gaude 2009). While this specific regulatory mechanism has been questioned
(Yamamoto and Nasrallah 2009), works in other self-incompatible species (i.e.,
Solanaceae and Poaceae) have also established a role of TRXh isoforms in
self-incompatible pollen recognition, while acting by other ways (Li et al. 1996;
Juárez-Díaz et al. 2006).
A function of cytosolic TRX isoforms in thermotolerance has been suggested.
When overexpressed in plants, TRXh3 and the multidomain TRX tetratricoredoxin
(TDX) enhance heat stress tolerance due to their chaperone activities (Park et al.
2009; Lee et al. 2009). Interestingly, TDX was shown to interact with the singlestranded DNA binding protein Ssb2, a yeast heat-shock protein 70 chaperone,
suggesting that the TDX-dependent heat stress tolerance might be related to a
co-chaperone activity (Vignols et al. 2003). Of note, none of the trxh3 and tdx
knock-out mutants does exhibit a heat stress sensitive phenotype, suggesting that this
function is compensated by other proteins sharing chaperone activities.
3.2 Nuclear Thioredoxin System
Although several TRX isoforms have been identified in the nucleus, evidence of
their functions in the nucleus is still scarce. Nucleoredoxins (NRX) are multidomain
On the Elaborate Network of Thioredoxins in Higher Plants
241
pathways. TRXh5 is a major actor of plant immunity, regulating the oligomeric state
of the non-pathogenesis-related protein expressor NPR1, a master regulator of the
systemic acquired resistance (SAR). NPR1 normally exists in the cytosol as a thiolbound oligomer. Upon pathogen challenge or treatment with salicylic acid, TRXh5
is upregulated (Laloi et al. 2004; Tada et al. 2008). TRXh5 then reduces NPR1
oligomers resulting in monomer release, translocation to the nucleus, and regulation
of defense gene expression associated with both local and systemic resistance.
Consequently, a trxh5 knock-out mutant is impaired in establishing a SAR and is
therefore more susceptible to pathogen infection (Tada et al. 2008). More recently,
Kneeshaw et al. (2014) have shown that TRXh5 acts in the SAR signaling by
selective NPR1 protein S-denitrosylation. TRXh acting as denitrosylase has been
additionally shown in ntra mutants in which higher levels of S-nitrosylated proteins
were found (Tada et al. 2008).
TRXh5 is also involved in the plant tolerance to the necrotrophic fungal pathogen
Cochliobolus victoriae. The redox mechanism involves binding of victorin, the virulent
effector of C. victoriae, to the first Cys residue in the TRXh5 active site (Sweat and
Wolpert 2007; Lorang et al. 2012). Victorin binding to TRXh5 activates the locus
orchestrating victorin effects protein LOV1, an Arabidopsis susceptibility protein, and
elicits a resistance-like response that confers disease susceptibility. Lorang et al. (2012)
proposed that victorin mimics a conventional pathogen virulence effector and confers
virulence to C. victoriae solely because it incites plant defense mechanisms.
Specifically, in self-incompatible species, TRXh isoforms (called THL in Brassica
napus) have been involved in recognition and rejection of self-incompatible pollen
through redox regulation of a key actor (S-locus receptor kinase) of the selfincompatible signaling pathway (Bower et al. 1996; Cabrillac et al. 2001; Ivanov
and Gaude 2009). While this specific regulatory mechanism has been questioned
(Yamamoto and Nasrallah 2009), works in other self-incompatible species (i.e.,
Solanaceae and Poaceae) have also established a role of TRXh isoforms in
self-incompatible pollen recognition, while acting by other ways (Li et al. 1996;
Juárez-Díaz et al. 2006).
A function of cytosolic TRX isoforms in thermotolerance has been suggested.
When overexpressed in plants, TRXh3 and the multidomain TRX tetratricoredoxin
(TDX) enhance heat stress tolerance due to their chaperone activities (Park et al.
2009; Lee et al. 2009). Interestingly, TDX was shown to interact with the singlestranded DNA binding protein Ssb2, a yeast heat-shock protein 70 chaperone,
suggesting that the TDX-dependent heat stress tolerance might be related to a
co-chaperone activity (Vignols et al. 2003). Of note, none of the trxh3 and tdx
knock-out mutants does exhibit a heat stress sensitive phenotype, suggesting that this
function is compensated by other proteins sharing chaperone activities.
3.2 Nuclear Thioredoxin System
Although several TRX isoforms have been identified in the nucleus, evidence of
their functions in the nucleus is still scarce. Nucleoredoxins (NRX) are multidomain
On the Elaborate Network of Thioredoxins in Higher Plants
241