Abstract

The review summarizes some of our current knowledge on the phenomenon of exudation from the cut surface of detached roots with emphasis on results that were mostly established over the last fifty years. The phenomenon is quantitatively documented in the 18th century (by Hales in 1727). By the 19th century, theories mainly ascribed exudation to the secretion of living root cells. The 20th century favored the osmometer model of root exudation. Nevertheless, growing insights into the mechanisms of water transport and new or rediscovered observations stimulated the quest for a more adequate exudation model. The historical overview shows how understanding of exudation changed with time following experimental opportunities and novel ideas from different areas of knowledge. Later theories included cytoskeleton-dependent micro-pulsations of turgor in root cells to explain the observed water exudation. Recent progress in experimental biomedicine led to detailed study of channels and transporters for ion transport via cellular membranes and to the discovery of aquaporins. These universal molecular entities have been incorporated to the more complex models of water transport via plant roots. A new set of ideas and explanations was based on cellular osmoregulation by mechanosensitive ion channels. Thermodynamic calculations predicted the possibility of water transport against osmotic forces based on co-transport of water with ions via cation-chloride cotransporters. Recent observations of rhizodermis exudation, exudation of roots without an external aqueous medium, segments cut from roots, pulses of exudation, a phase shifting of water uptake and exudation, and of effects of physiologically active compounds (like ion channel blockers, metabolic agents, and cytoskeletal agents) will likely refine our understanding of the phenomenon. So far, it seems that more than one mechanism is responsible for root pressure and root exudation, processes which are important for refilling of embolized xylem vessels. However, recent advances in ion and water transport research at the molecular level suggest potential future directions to understanding of root exudation and new models awaiting experimental testing.

Highlights

  • Roots of many plant species show extrusion of liquid from their cut surface, typically when located in wet soil or submerged in distilled water or a solution

  • We briefly describe the models of root exudation (Figure 2; Table 2) which are mainly based on an osmotic explanation of exudation alone, starting from the earlier one membrane models with two compartments to more complicated two membrane models with three compartments

  • This could be used as an argument against the involvement of an mechanosensitive channels (MSC), and against explaining inhibition of exudation observed with gadolinium to less specific effects on membranes reported by [179]

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Summary

A Quest for Mechanisms of Plant Root Exudation Brings New

K.A. Timiriazev Institute of Plant Physiology RAS, 35 Botanicheskaya St., Moscow 127276, Russia. Max Planck Institute for the History of Science, Boltzmannstraße 22, 14195 Berlin, Germany.

Introduction
Root Pressure and the Ascent of Sap
Origin of Exudate
Initial Concepts and Further Models for Understanding Mechanisms of Root
Rhizodermis Exudation
Experimental setup setup for rhizodermis exudation:
Exudation of Root Segments without Root Apex and Stem Exudation
Effects of Chemical Agents on Exudation and Root Pressure
Ion Channel Blockers
Summary on the Effects of Chemicals on Exudation
Findings
50% (Figures
Full Text
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