Strigolactones enhance physiological and biochemical responses to salinity stress in tomato

Salinity stress is increasingly affecting plant crops, including vegetables. Strigolactones (SLs) are involved in modulating plant responses to osmotic stress. To unequivocally demonstrate the role of endogenous SLs under salt stress, we compared the responses of tomato plants silenced for the SL biosynthetic gene CCD7 (CAROTENOID CLEAVAGE DIOXYGENASE7) with the relative wild-type and tested the effect of the specific SL analogue enantiomer GR24^5DS in stressed plants. Salt application increased the substrate electrical conductivity and leaf and root Na⁺ concentration, and decreased stem water potential. Salinity also restrained growth, reduced stomatal conductance, increased content of leaf proline, and enhanced activity of ROS-scavenging enzymes. SL-depleted plants were more susceptible to stress, showing stronger reduction of shoot growth than wild-type plants, and lower leaf concentration of proline, K⁺ and Mg²⁺. Leaf MDA concentration was higher in SL-depleted plants. Stomatal conductance and leaf soluble sugar concentration under stress were not affected by genetic SL depletion, but they respectively decreased and increased in leaves treated with GR24^5DS. Activity of ROS-scavenging enzymes in leaves was also modulated by GR24^5DS treatment. Our results unambiguously demonstrate that endogenous SLs contribute to improving tolerance to salt stress in tomato by affecting differential accumulation of ions and organic solutes, as well as responses to oxidative stress.