Utilization and Transport of Mannitol in Olea europaea and Implications for Salt Stress Tolerance

Mannitol is one of the primary photosynthetic products and the major phloem-translocated carbohydrate in Olea europaea L., an important crop in the Mediterranean basin. Uptake of mannitol in heterotrophic cell suspensions of O. europaea was shown to be mediated by a 1 : 1 polyol : H+ symport system with a Km of 1.3 mM mannitol and a Vmax of 1.3 nmol min(-1) mg(-1) DW. Dulcitol, sorbitol and xylitol competed for mannitol uptake, whereas glucose and sucrose did not. Reverse transcription-PCR (RT-PCR) performed on mRNA extracted from cultured cells exhibiting high mannitol transport activity allowed the cloning of a partial O. europaea mannitol carrier OeMaT1. The Vmax of mannitol uptake and the amount of OeMaT1 transcripts increased along with polyol depletion from the medium, suggesting that the mannitol transport system may be regulated by its own substrate. Addition of 100-500 mM NaCl to cultured cells enhanced the capacity of the polyol : H+ symport system and the amount of OeMaT1 transcripts, whereas it strongly repressed mannitol dehydrogenase activity. Measurements of cell viability showed that mannitol-grown cells remained viable 24 h after a 250 and 500 mM NaCl pulse, whereas extensive loss of cell viability was observed in sucrose-grown cells. OeMaT1 transcripts increased throughout maturation of olive fruits, suggesting that an OeMaT is involved in the accumulation of mannitol during ripening of olive. Thus, mannitol transport and compartmentation by OeMaT are important to allocate this source of carbon and energy, as well as for salt tolerance and olive ripening.