electron transport by TRXm4 orthologues in N. tabacum (Courteille et al. 2013).
Very recently Arabidopsis trxm1 and trxm2 single and double mutants were compared to wild type plants with respect to the transient activation of the NADP
+
-MDH
after the onset of light (Thormählen et al. 2017). It was demonstrated that the lightdependent redox activation of the NADP
+
-MDH decreased upon combined deficiencies of TRXm1 and m2, which corresponded to an impaired acclimation to
fluctuating light intensities during the day. In vitro experiments revealed the ability
of TRXf and at least partially TRXm to reductively activate the NADP
+
-MDH
(Collin et al. 2003; Yoshida and Hisabori 2016b). However, this contrasts with
illuminated leaves of trxf Arabidopsis mutant lines exhibiting even higher NADP
+
-
MDH activation states (Thormählen et al. 2015). Further studies are necessary to
investigate the distinct involvements of TRXf and m isoforms in redox regulated
processes playing a role in the fluctuating light response.
More recently, the findings about TRX-dependent redox regulation of the
chlorophyll biosynthesis pathway were extended in vivo. Transgenic pea plants
with simultaneously silenced TRXf and m genes were impaired in redox activation
of Mg-chelatase subunits (Luo et al. 2012). Da et al. (2017) demonstrated in
Arabidopsis that the combined silencing of TRXm1, m2, and m4 impairs the
chlorophyll biosynthesis pathway by attenuating the redox control of the
Mg-protoporphyrin methyl transferase (CHLM). Furthermore, the in vivo insights
about TRXm3 revealed functions in meristem development by modulating
symplastic protein transport (Benitez-Alfonso et al. 2009). In response to oxidative
stress, mutant lines lacking TRXy2 showed attenuated protein repair mechanisms in
leaves due to impaired redox activation of methionine sulfoxide reductases (MSR)
(Laugier et al. 2013). TRXz was demonstrated to be essential for autotrophic growth
by influencing plastidial gene expression as structural component of the plastidencoded RNA polymerase (Arsova et al. 2010), although this effect seems to be
independent of TRXz redox activity (Wimmelbacher and Börnke 2014). Taken
together, the diverse functions of typical TRX in plastidial processes investigated
within the last decades illustrate a complex redox regulatory system, which is still
not fully elucidated.
2.1.4 Plastids Contain Additional Atypical TRX with Mainly
Unresolved Functions
Contrary to the TRX described above, the members of the atypical TRX are not yet
well analyzed in their functional roles of plastidial metabolism (Meyer et al. 2012;
Belin et al. 2015). These unusual TRX have different protein properties such as
atypical active site motifs, multiple TRX domains, or combinations of TRX and
other domains. The proteins of the atypical Cys/His-rich TRX (ACHT) family
consist of six isoforms carrying diverse non-canonical active site motifs (Dangoor
et al. 2009; Meyer et al. 2012; Belin et al. 2015). The chloroplastic drought-induced
stress protein CDSP32 contains two TRX domains and several additional conserved
Cys (Rey et al. 1998). One of the TRX domains is functional with an unusual redox230
I. Thormählen et al.
Very recently Arabidopsis trxm1 and trxm2 single and double mutants were compared to wild type plants with respect to the transient activation of the NADP
+
-MDH
after the onset of light (Thormählen et al. 2017). It was demonstrated that the lightdependent redox activation of the NADP
+
-MDH decreased upon combined deficiencies of TRXm1 and m2, which corresponded to an impaired acclimation to
fluctuating light intensities during the day. In vitro experiments revealed the ability
of TRXf and at least partially TRXm to reductively activate the NADP
+
-MDH
(Collin et al. 2003; Yoshida and Hisabori 2016b). However, this contrasts with
illuminated leaves of trxf Arabidopsis mutant lines exhibiting even higher NADP
+
-
MDH activation states (Thormählen et al. 2015). Further studies are necessary to
investigate the distinct involvements of TRXf and m isoforms in redox regulated
processes playing a role in the fluctuating light response.
More recently, the findings about TRX-dependent redox regulation of the
chlorophyll biosynthesis pathway were extended in vivo. Transgenic pea plants
with simultaneously silenced TRXf and m genes were impaired in redox activation
of Mg-chelatase subunits (Luo et al. 2012). Da et al. (2017) demonstrated in
Arabidopsis that the combined silencing of TRXm1, m2, and m4 impairs the
chlorophyll biosynthesis pathway by attenuating the redox control of the
Mg-protoporphyrin methyl transferase (CHLM). Furthermore, the in vivo insights
about TRXm3 revealed functions in meristem development by modulating
symplastic protein transport (Benitez-Alfonso et al. 2009). In response to oxidative
stress, mutant lines lacking TRXy2 showed attenuated protein repair mechanisms in
leaves due to impaired redox activation of methionine sulfoxide reductases (MSR)
(Laugier et al. 2013). TRXz was demonstrated to be essential for autotrophic growth
by influencing plastidial gene expression as structural component of the plastidencoded RNA polymerase (Arsova et al. 2010), although this effect seems to be
independent of TRXz redox activity (Wimmelbacher and Börnke 2014). Taken
together, the diverse functions of typical TRX in plastidial processes investigated
within the last decades illustrate a complex redox regulatory system, which is still
not fully elucidated.
2.1.4 Plastids Contain Additional Atypical TRX with Mainly
Unresolved Functions
Contrary to the TRX described above, the members of the atypical TRX are not yet
well analyzed in their functional roles of plastidial metabolism (Meyer et al. 2012;
Belin et al. 2015). These unusual TRX have different protein properties such as
atypical active site motifs, multiple TRX domains, or combinations of TRX and
other domains. The proteins of the atypical Cys/His-rich TRX (ACHT) family
consist of six isoforms carrying diverse non-canonical active site motifs (Dangoor
et al. 2009; Meyer et al. 2012; Belin et al. 2015). The chloroplastic drought-induced
stress protein CDSP32 contains two TRX domains and several additional conserved
Cys (Rey et al. 1998). One of the TRX domains is functional with an unusual redox230
I. Thormählen et al.