Contrasting Responses of Plastid Terminal Oxidase Activity Under Salt Stress in Two C4 Species With Different Salt Tolerance

Abstract

The present study reveals contrasting responses of photosynthesis to salt stress in two C₄ species: a glycophyte <i>Setaria viridis</i> (<i>SV</i>) and a halophyte <i>Spartina alterniflora</i> (<i>SA</i>). Specifically, the effect of short-term salt stress treatment on the photosynthetic CO₂ uptake and electron transport were investigated in <i>SV</i> and its salt-tolerant close relative <i>SA</i>. In this experiment, at the beginning, plants were grown in soil then were exposed to salt stress under hydroponic conditions for two weeks. <i>SV</i> demonstrated a much higher susceptibility to salt stress than <i>SA</i>; while, <i>SV</i> was incapable to survive subjected to about 100 mM, <i>SA</i> can tolerate salt concentrations up to 550 mM with slight effect on photosynthetic CO₂ uptake rates and electrons transport chain conductance (<i>g_ETC</i> ). Regardless the oxygen concentration used, our results show an enhancement in the P₇₀₀ oxidation with increasing O₂ concentration for SV following NaCl treatment and almost no change for SA. We also observed an activation of the cyclic NDH-dependent pathway in <i>SV</i> by about 2.36 times upon exposure to 50 mM NaCl for 12 days (d); however, its activity in <i>SA</i> drops by about 25% compared to the control without salt treatment. Using PTOX inhibitor (<i>n-PG</i>) and that of the Q_o-binding site of Cytb₆/f (DBMIB), at two O₂ levels (2 and 21%), to restrict electrons flow towards PSI, we successfully revealed the presence of a possible PTOX activity under salt stress for <i>SA</i> but not for <i>SV</i>. However, by q-PCR and western-blot analysis, we showed an increase in PTOX amount by about 3-4 times for <i>SA</i> under salt stress but not or very less for <i>SV</i>. Overall, this study provides strong proof for the existence of PTOX as an alternative electron pathway in C₄ species (<i>SA</i>), which might play more than a photoprotective role under salt stress.