Mannitol transport and mannitol dehydrogenase activities are coordinated in olea europaea under salt and osmotic stresses

Abstract

The intracellular accumulation of organic compatible solutes functioning as osmoprotectants, such as polyols, is an important response mechanism of several plants to drought and salinity. In Olea europaea a mannitol transport system (OeMaT1) was previously characterised as a key player in plant response to salinity. In the present study, heterotrophic sink models, such as olive cell suspensions and fruit tissues, and source leaves were used for analytical, biochemical and molecular studies. The kinetic parameters of mannitol dehydrogenase (MTD) determined in mannitol-growing cells, at 25 °C and pH 9.0, were as follows: Km, 54.5 mM mannitol and Vmax, 0.47 μmol h-1 mg-1 protein. The corresponding cDNA was cloned and named OeMTD1. OeMTD1 expression was correlated with MTD activity, OeMaT1 expression and carrier-mediated mannitol transport, in mannitol- and sucrose-growing cells. Furthermore, sucrosegrowing cells displayed only residual OeMTD activity, even though high levels of OeMTD1 transcription were observed. There is evidence OeMTD is regulated at both transcriptional and post-transcriptional levels. MTD activity and OeMTD1 expression were repressed after Na+, K+ and PEG treatments, both in mannitol- and sucrose-growing cells. In contrast, salt and drought significantly increased mannitol transport activity and OeMaT1 expression. Altogether, these studies support that olive tree copes with salinity and drought by coordinating mannitol transport with intracellular metabolism.