Contrasting Responses of Plastid Terminal Oxidase Activity Under Salt Stress in Two C(4)Species With Different Salt Tolerance

The present study reveals contrasting responses of photosynthesis to salt stress in two C(4)species: a glycophyteSetaria viridis(SV) and a halophyteSpartina alterniflora(SA). Specifically, the effect of short-term salt stress treatment on the photosynthetic CO(2)uptake and electron transport were investigated inSVand its salt-tolerant close relativeSA. In this experiment, at the beginning, plants were grown in soil then were exposed to salt stress under hydroponic conditions for two weeks.SVdemonstrated a much higher susceptibility to salt stress thanSA; while,SVwas incapable to survive subjected to about 100 mM,SAcan tolerate salt concentrations up to 550 mM with slight effect on photosynthetic CO(2)uptake rates and electrons transport chain conductance (g(ETC)). Regardless the oxygen concentration used, our results show an enhancement in the P(700)oxidation with increasing O(2)concentration for SV following NaCl treatment and almost no change for SA. We also observed an activation of the cyclic NDH-dependent pathway inSVby about 2.36 times upon exposure to 50 mM NaCl for 12 days (d); however, its activity inSAdrops by about 25% compared to the control without salt treatment. Using PTOX inhibitor (n-PG) and that of the Q(o)-binding site of Cytb(6)/f (DBMIB), at two O(2)levels (2 and 21%), to restrict electrons flow towards PSI, we successfully revealed the presence of a possible PTOX activity under salt stress forSAbut not forSV. However, by q-PCR and western-blot analysis, we showed an increase in PTOX amount by about 3-4 times forSAunder salt stress but not or very less forSV. Overall, this study provides strong proof for the existence of PTOX as an alternative electron pathway in C(4)species (SA), which might play more than a photoprotective role under salt stress.