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Microorganisms 2018, 6(3), 95; https://doi.org/10.3390/microorganisms6030095

Rhizophagy Cycle: An Oxidative Process in Plants for Nutrient Extraction from Symbiotic Microbes

1
Department of Plant Biology, Rutgers University, New Brunswick, NJ 08901, USA
2
Centre of Advanced Study in Botany, Banaras Hindu University, Varanasi, UP 221005, India
3
U.S. Geological Survey, Great Lakes Science Center, 1451 Green Road, Ann Arbor, MI 48105-2807, USA
*
Author to whom correspondence should be addressed.
Received: 22 August 2018 / Revised: 1 September 2018 / Accepted: 5 September 2018 / Published: 17 September 2018
(This article belongs to the Special Issue Plant Control of Symbiotic Microbe Behavior and Reproduction)
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Abstract

In this paper, we describe a mechanism for the transfer of nutrients from symbiotic microbes (bacteria and fungi) to host plant roots that we term the ‘rhizophagy cycle.’ In the rhizophagy cycle, microbes alternate between a root intracellular endophytic phase and a free-living soil phase. Microbes acquire soil nutrients in the free-living soil phase; nutrients are extracted through exposure to host-produced reactive oxygen in the intracellular endophytic phase. We conducted experiments on several seed-vectored microbes in several host species. We found that initially the symbiotic microbes grow on the rhizoplane in the exudate zone adjacent the root meristem. Microbes enter root tip meristem cells—locating within the periplasmic spaces between cell wall and plasma membrane. In the periplasmic spaces of root cells, microbes convert to wall-less protoplast forms. As root cells mature, microbes continue to be subjected to reactive oxygen (superoxide) produced by NADPH oxidases (NOX) on the root cell plasma membranes. Reactive oxygen degrades some of the intracellular microbes, also likely inducing electrolyte leakage from microbes—effectively extracting nutrients from microbes. Surviving bacteria in root epidermal cells trigger root hair elongation and as hairs elongate bacteria exit at the hair tips, reforming cell walls and cell shapes as microbes emerge into the rhizosphere where they may obtain additional nutrients. Precisely what nutrients are transferred through rhizophagy or how important this process is for nutrient acquisition is still unknown. View Full-Text
Keywords: endobiome; endophyte; nutrient transport; reactive oxygen; rhizosphere; symbiosis endobiome; endophyte; nutrient transport; reactive oxygen; rhizosphere; symbiosis
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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White, J.F.; Kingsley, K.L.; Verma, S.K.; Kowalski, K.P. Rhizophagy Cycle: An Oxidative Process in Plants for Nutrient Extraction from Symbiotic Microbes. Microorganisms 2018, 6, 95.

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