8 - Roots as an integrated part of the translocation pathway

Abstract

The growing root tip is at the end of the translocation pathway and is highly dependent on the import of solutes for its continued growth and other metabolism. The delivery of organic solutes into growing root cells has been considered to occur by two completely different mechanisms: uptake from the apoplast across membranes or symplastic delivery through plasmodesmatally connected domains. The pathways have very different hardware and fundamentally different driving forces. Apoplastic uptake of solutes requires membrane transporters or channels, whereas in symplastically connected cells solutes and water can enter together without crossing a membrane. This chapter studies the relationship between root extension and solute deposition, reviews the evidence for the existence of symplastic and apoplastic pathways within the growing zone, and assesses the consequences of these two pathways for solute transport to growing sinks. Because qualitative changes in the pathway for solute delivery within the root-growing zone predict associated differences in the presence of specific transporters for both solutes and water, some speculations are offered as to the location of specific transporters within the root tip. To help link studies of solute delivery and growth in roots with studies on other tissues, parallels are drawn between the transport processes occurring in source, sink, and pathway.