Sugar and polyol carriers expression during fruit ripening 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. Mannitol transport activity and OeMaT1 transcription increased under salt stress conditions, suggesting that the polyol behaves as an important osmolyte in O. europaea. In this work, RT-PCR performed on RNA extracted from olive fruit allowed the cloning of the monosaccharide transporter OeMST2 and the sucrose transporter OeSUT1. To understand key steps in long distance transport, and particularly partitioning and accumulation of polyol and sugars in olive fruit, the expression of OeMaT1, OeMST2 and OeSUT1, as well as sugar and polyol content were studied during the ripening process. Expression analysis showed an increased level of transcripts in the latter stages, suggesting that polyol and sugar carriers have a major role in the massive accumulation of photoassimilates that occurs during olive fruit maturation.