1 INTRODUCTION
Soil salinity is a major environmental stress that adversely affects
crop productivity and harvest quality (Horie & Schroeder, 2004).
Approximately one fifth of the world’s cultivated area and about half of
the world’s irrigated lands are affected by salinization (Sairam &
Tyagi, 2004). Mechanisms of how plants respond and/or tolerate salt
stress are under intensive study (Zhu 2001; Munns & Tester, 2008). To
survive and overcome salt stress, plants respond and adapt with complex
mechanisms that include developmental, morphological, physiological and
biochemical strategies (Taji et al., 2004; Acosta-Motos et al., 2015),
which serve to modulate ion homeostasis, osmolyte biosynthesis,
compartmentation of toxic ions, and reactive oxygen species (ROS)
scavenging systems (Stepien & Klobus, 2005; Flowers & Colmer, 2008;
Stepien & Johnson, 2009). In this study, we report that a protein
involved in alternative electron transfer, PTOX, might be related to
salt tolerance in C4 plants.
The protein PTOX is a plastid-localized plastoquinol oxygen
oxido-reductase that was discovered in the so-called immutans ofArabidopsis thaliana which shows a variegated leaf phenotype
(Rédei, 1963; Wetzel et al., 1994, Carol et al., 1999; Wu et al., 1999;
Shahbazi et al., 2007). In chloroplasts, PTOX is located at the stroma
lamellae facing the stroma (Lennon et al., 2003) and it is essential for
the plastid development and carotenoid biosynthesis in plants (Carol et
al., 1999; Aluru et al., 2001). PTOX is also involved in photosynthetic
electron transport (Okegawa et al., 2010; Trouillard et al., 2012),
chlororespiration (Cournac et al., 2000), poising chloroplast redox
potential under dark (Nawrocki et al., 2015), and in stress response
(McDonald et al., 2011; Sun and Wen, 2011). Plants grown in moderate
light under non-stress conditions have low PTOX concentrations (about 1
PTOX protein per 100 PSII; Lennon et al., 2003); in contrast, elevated
PTOX levels have been reported in plants exposed to abiotic stresses
such as high temperatures, high light and drought (Quiles, 2006), high
salinity (Stepien & Johnson, 2009), low temperatures and high
intensities of visible light (Ivanov et al., 2012) and UV light (Laureau
et al., 2013).
In this study, in an effort to understand potential mechanism of how the
halophyte SA tolerate high salt stress, we show that compared to
a glycophyte species SV , under high salt stress (500 mM),SA showed increased expression of PTOX, which might have played a
critical role for the maintenance of photosynthetic physiology and hence
high photosynthetic efficiency of this species under salt stress.