Hydroxytyrosol increases salt tolerance of maize and wheat by expanding the pore diameter on root cell wall

Ascorbic (ASA) and Jasmonic acids (JA) have been commonly used experimentally to increase plant salt tolerance. Nevertheless, their application in farming is not cost-effective. Here we investigated the antioxidant effect of Hydroxytyrosol (HT), a phenolic compound found in olive mill wastewater (OMW), on salt tolerance in maize and wheat. The growth rate of wheat and maize roots and shoots under salinity conditions was reduced when compared to the control plants. However, the addition of HT improved the growth rate. Similar results were obtained when an equivalent concentration of ASA and JA was used. Whereas salt treatment dramatically reduced root cell wall pore diameter, ASA (a known effective molecule) and HT restored the root pore diameters. The salt treatment also significantly inhibited the transpiration rate. In addition, HT and ASA were found to expand the root pores and accelerate the root water uptake, consequently enhancing the transpiration rates. More specifically, extraction of a HT-rich fraction from OMW caused a similar physiological cascade of events, revealing significant plant growth increases. This was the first time that the effect of HT and ASA on the root cell wall pore size was reported. Consequently, our findings may shed light on a novel mechanism by which salinity and HT affect the water uptake potential via the pores. Therefore, we propose a method of disposing of OMW by maximizing its utilization in agriculture.