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Open AccessReview

A Quest for Mechanisms of Plant Root Exudation Brings New Results and Models, 300 Years after Hales

by 1,2,*,† and 3,*,†
1
Department of Plant Sciences, College of Agricultural and Environmental Sciences, University of California, Davis, CA 95616, USA
2
K.A. Timiriazev Institute of Plant Physiology RAS, 35 Botanicheskaya St., Moscow 127276, Russia
3
Max Planck Institute for the History of Science, Boltzmannstraße 22, 14195 Berlin, Germany
*
Authors to whom correspondence should be addressed.
Both Authors contributed equally to the text.
Plants 2021, 10(1), 38; https://doi.org/10.3390/plants10010038
Received: 16 November 2020 / Revised: 20 December 2020 / Accepted: 21 December 2020 / Published: 25 December 2020
(This article belongs to the Special Issue Water and Ion Transport in Plants: New and Older Trends Meet Together)
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. View Full-Text
Keywords: root pressure; exudation; xylem embolism; mechanosensitive ion channels; ion transporters; aquaporins; water transport root pressure; exudation; xylem embolism; mechanosensitive ion channels; ion transporters; aquaporins; water transport
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MDPI and ACS Style

Volkov, V.; Schwenke, H. A Quest for Mechanisms of Plant Root Exudation Brings New Results and Models, 300 Years after Hales. Plants 2021, 10, 38. https://doi.org/10.3390/plants10010038

AMA Style

Volkov V, Schwenke H. A Quest for Mechanisms of Plant Root Exudation Brings New Results and Models, 300 Years after Hales. Plants. 2021; 10(1):38. https://doi.org/10.3390/plants10010038

Chicago/Turabian Style

Volkov, Vadim; Schwenke, Heiner. 2021. "A Quest for Mechanisms of Plant Root Exudation Brings New Results and Models, 300 Years after Hales" Plants 10, no. 1: 38. https://doi.org/10.3390/plants10010038

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