Abstract
Organic (e.g., sugars and amino acids) and inorganic (e.g., K+, Na+, PO42−, and SO42−) solutes are transported long-distance throughout plants. Lateral movement of these compounds between the xylem and the phloem, and vice versa, has also been reported in several plant species since the 1930s, and is believed to be important in the overall resource allocation. Studies of Arabidopsis thaliana have provided us with a better knowledge of the anatomical framework in which the lateral transport takes place, and have highlighted the role of specialized vascular and perivascular cells as an interface for solute exchanges. Important breakthroughs have also been made, mainly in Arabidopsis, in identifying some of the proteins involved in the cell-to-cell translocation of solutes, most notably a range of plasma membrane transporters that act in different cell types. Finally, in the future, state-of-art imaging techniques should help to better characterize the lateral transport of these compounds on a cellular level. This review brings the lateral transport of sugars and inorganic solutes back into focus and highlights its importance in terms of our overall understanding of plant resource allocation.
Highlights
In plants, organic and inorganic (e.g., K+, Na+, NH4 +, SO4 2−, PO4 2−, Ca2+ ) solutes are dissolved in a water-based mixture that is distributed throughout the plant
Since the existence of lateral movement of inorganic solutes was first reported in the 1930s, significant progress has been made in identifying several of the molecular actors involved in organic and inorganic solute exchanges
It has been discovered that the lateral transport of sugars is controlled by coordinated symplasmic and apoplasmic pathways, which constitutes a new paradigm that still needs to be confirmed for the lateral transfer of other solutes
Summary
Organic (e.g., organic acids, sugars, amino acids) and inorganic (e.g., K+ , Na+ , NH4 + , SO4 2− , PO4 2− , Ca2+ ) solutes are dissolved in a water-based mixture (i.e., phloem and xylem sap) that is distributed throughout the plant. Understanding how these nutrients are transported between organs and how this transport is coordinated is of major interest for the long-term goal of improving resource allocation in plants and in fine plant biomass production. The review will focus on the transport of photosynthates and on the transport of potassium, sodium, and phosphate as inorganic ions
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