Mannitol accumulation as a mechanism for salt stress tolerance in Olea europaea

Abstract

Resource allocation is an important and highly integrated process in higher plants involving the transport of photoassimilates from source to sink organs. Mannitol and sucrose are the primary photosynthetic products and the major phloem-translocated carbohydrates in Olea europaea L., an important and widespread crop in the Mediterranean basin. In suspension cultured cells of O. europaea, we have previously identified and characterized a H+-dependent polyol transport system whose activity was correlated with OeMaT1 (Olea europaea Mannitol Transporter) expression. The addition of 100-500 mM NaCl to cultured cells led to an increase of polyol carrier capacity and OeMaT1 transcription, as well as a strong repression of mannitol dehydrogenase activity. To further understand the function of mannitol as a compatible solute, cells of O. europaea were grown in sucrose and mannitol as the sole carbon and energy source in the absence and presence of NaCl. Results showed that mannitol was preferred over sucrose, the following values for specific growth rates obtained in the absence of salt: 0.180 d-1 and 0.076 d-1, respectively. Cells grown in the presence of up to 200 mM NaCl exhibited a marked decrease in specific growth rate along with a drastic increase of intracellular Na+ concentration. Cell viability, as assessed by fluorescein diacetate and propidium iodide, decreased along with the increase of NaCl concentration in the culture medium. Furthermore, the addition of 500 mM NaCl showed that mannitol-grown cells remained viable 24h after the salt pulse; sucrose-grown cells were not able to display green fluorescence after the same treatment, and exhibited extensive loss of cell viability as evaluated by the red fluorescence. These results suggest that mannitol behaves as an important osmolyte in O. europaea, showing an enhanced intracellular accumulation and thus compensating for the decrease of external water activity.