Contrasting Responses of Photosynthesis to Salt Stress in the Glycophyte Arabidopsis and the Halophyte Thellungiella: Role of the Plastid Terminal Oxidase as an Alternative Electron Sink

The effects of short-term salt stress on gas exchange and the regulation of photosynthetic electron transport were examined in Arabidopsis (Arabidopsis thaliana) and its salt-tolerant close relative Thellungiella (Thellungiella halophila). Plants cultivated on soil were challenged for 2 weeks with NaCl. Arabidopsis showed a much higher sensitivity to salt than Thellungiella; while Arabidopsis plants were unable to survive exposure to greater than 150 mm salt, Thellugiella could tolerate concentrations as high as 500 mm with only minimal effects on gas exchange. Exposure of Arabidopsis to sublethal salt concentrations resulted in stomatal closure and inhibition of CO₂ fixation. This lead to an inhibition of electron transport though photosystem II (PSII), an increase in cyclic electron flow involving only PSI, and increased nonphotochemical quenching of chlorophyll fluorescence. In contrast, in Thellungiella, although gas exchange was marginally inhibited by high salt and PSI was unaffected, there was a large increase in electron flow involving PSII. This additional electron transport activity is oxygen dependent and sensitive to the alternative oxidase inhibitor n-propyl gallate. PSII electron transport in Thellungiella showed a reduced sensitivity to 2′-iodo-6-isopropyl-3-methyl-2′,4,4′-trinitrodiphenylether, an inhibitor of the cytochrome b₆f complex. At the same time, we observed a substantial up-regulation of a protein reacting with antibodies raised against the plastid terminal oxidase. No such up-regulation was seen in Arabidopsis. We conclude that in salt-stressed Thellungiella, plastid terminal oxidase acts as an alternative electron sink, accounting for up to 30% of total PSII electron flow.Salinity in soils is a major global problem and is one that is of growing importance (Pitman and Läuchli, 2002). Research in this area has been limited by the lack of a suitable salt-tolerant genetic model (Bressan et al., 2001; Flowers and Colmer, 2008). A salt-tolerant Arabidopsis (Arabidopsis thaliana) relative, Thellungiella (Thellungiella halophila), is now promising to help in salt stress tolerance research (Volkov et al., 2003; Amtmann et al., 2005). Salinity tolerance is a complex phenomenon, brought about by adaptations in a range of physiological processes. Plants have developed a complex defense system, including ion homeostasis, osmolyte biosynthesis, compartmentation of toxic ions, and reactive oxygen species (ROS) scavenging systems (Hasegawa et al., 2000; Mittova et al., 2004; Stepien and Klobus, 2005; Flowers and Colmer, 2008). Of paramount importance is the process of photosynthesis that is well established as a primary target of many forms of environmental stress, including salinity (Garcia-Sanchez et al., 2002; Liska et al., 2004; Stepien and Klobus, 2006).Soil salt prevents plants from taking up water, exposing them to drought stress. To conserve water, they close their stomata. This simultaneously restricts the entry of CO₂ into the leaf, reducing photosynthesis. At higher concentrations, NaCl may also directly inhibit photosynthesis. When such inhibition occurs, the plant is liable to suffer from oxidative stress. Absorption of sunlight leads to ROS formation, mainly in the chloroplast, either via photoreduction of O₂ to form superoxide (the Mehler reaction) or through the interaction of triplet-excited chlorophyll to form singlet excited oxygen (Asada, 2000; Foyer et al., 2002). ROS are highly reactive and can cause widespread damage to membranes, proteins, and DNA. To prevent such damage, there are a number of enzymatic processes in chloroplast to scavenge ROS (Asada, 2000). These are energetically demanding, requiring the synthesis of high concentrations of antioxidants and enzymes.An alternative strategy, placing less of a metabolic burden on plants, would be to avoid the production of ROS. This can be achieved by regulation of photosynthetic electron transport (Johnson, 2005). Although the role of salt stress in inducing oxidative damage has been widely studied (Hernàndez et al., 2001; Bor et al., 2003; Stepien and Klobus, 2005), the extent to which regulatory processes are induced under such conditions, and the extent to which variation in their capacity determines the degree of damage incurred by plants exposed to salt have not been widely investigated.Studies so far reported using Thellungiella as a model for salt tolerance have focused on short-term responses to salinity, in particular examining changes in gene expression (Inan et al., 2004; Kant et al., 2006; Wong et al., 2006). Fewer studies have examined the physiology of salt tolerance in this plant and none the effects of salt on leaf physiology. Here, we describe an investigation into the effects of salt stress on the regulation of photosynthesis in Arabidopsis and Thellungiella. We show that these plants respond to salt stress in highly contrasting ways. We discuss the implications of these results for our understanding of salt tolerance.