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.