Solute transport across plant cell membranes

Abstract

The transport of solutes across cell membranes, including organic nutrients, such as sugar, osmolytes, ions or metabolic waste products, is of extreme importance in all living systems. Up to 14% of the genome of all organisms represents information for transport proteins, which reflects the importance of such process. Transporters are also involved in the transduction of environmental and endogenous signals. Several transport systems have been identified and fully characterised at both molecular and biophysical levels in a wide variety of living organisms, from bacteria to humans, with the bacterial lactose permease (LacY) being a good example of such successful studies. The majority of transporter proteins are very well conserved throughout living systems, and some of them, such as sugar transporters, belong to a large family (SP, Sugar Porter). In higher plants, the photoassimilated carbon is transported from mature leaves throughout the phloem, mainly in the form of sucrose, as in the grapevine, or mannitol, as in the olive tree, to heterotrophic organs such as developing leaves, flowers, fruits and roots, which rely on its supply for their growth and development. Thus, the unlocking of the mechanisms of photoassimilate transport into plant sink tissues, as well as their regulation, has an important basic and applied relevance. Moreover, as most living organisms, plants also face a continuous battle against adverse environmental factors like increasing soil salinity, heat and drought. In this context, solute transport also has a relevant role in plant defence. For instance, the efficient exclusion of Na+ excess from the cytoplasm and vacuolar Na+ accumulation are the most important steps towards the maintenance of ion homeostasis under salt stress. The production, transport and accumulation of compatible solutes like mannitol are also important plant responses to salinity and drought. Like animals, where important diseases such as depression and hypertension are commonly treated with drugs targeted to specific transporters, plants have also benefited from the extensive and ongoing study of membrane transport. The present review provides an overview on the investigation that has been conducted in our laboratory under the scope of this fascinating topic.