The Control of Auxin Transport in Parasitic and Symbiotic Root–Microbe Interactions

Abstract

Most field-grown plants are surrounded by microbes, especially from the soil. Some of these, including bacteria, fungi and nematodes, specifically manipulate the growth and development of their plant hosts, primarily for the formation of structures housing the microbes in roots. These developmental processes require the correct localization of the phytohormone auxin, which is involved in the control of cell division, cell enlargement, organ development and defense, and is thus a likely target for microbes that infect and invade plants. Some microbes have the ability to directly synthesize auxin. Others produce specific signals that indirectly alter the accumulation of auxin in the plant by altering auxin transport. This review highlights root–microbe interactions in which auxin transport is known to be targeted by symbionts and parasites to manipulate the development of their host root system. We include case studies for parasitic root–nematode interactions, mycorrhizal symbioses as well as nitrogen fixing symbioses in actinorhizal and legume hosts. The mechanisms to achieve auxin transport control that have been studied in model organisms include the induction of plant flavonoids that indirectly alter auxin transport and the direct targeting of auxin transporters by nematode effectors. In most cases, detailed mechanisms of auxin transport control remain unknown.

Indole-3-Acetic Acid Is Produced by Emiliania huxleyi Coccolith-Bearing Cells and Triggers a Physiological Response in Bald Cells

“…Auxins can induce acidification in cell walls and an increase in the cell membrane potential. This can be through the transport of the lipophilic form of IAA (IAAH) into the cell which affects the membrane potential as it then dissociates into the anionic form (IAA − ) (Nelles, 1977 ; Cleland, 2010 ; Zhang and Van Duijn, 2014 ), which leads to the activation of plasma membrane H + -ATPases and potassium channels, inducing modifications to the cell wall and hyperpolarizing the membrane (Osakabe et al, 2013 ; Ng et al, 2015 ; Velasquez et al, 2016 ). This change in a cell membrane's pH gradient, and the subsequent modifications on the cell wall by the released enzymes, leads to the cleavage of load-bearing cell-wall crosslinks promoting the turgor pressure needed for cell expansion (Cleland, 2010 ; Velasquez et al, 2016 ).…”

Gibberellin antagonizes jasmonate‐induced defense against Meloidogyne graminicola in rice

“…Auxin is primarily produced in the shoot from where it is transported to the root by a family of influx (AUX/LAX) and efflux (PIN) transporter proteins (reviewed by Ng et al, 2015). Studies in Arabidopsis and tomato showed the positive role of the auxin transporter proteins PIN/AUX1/LAX in the initiation and development of nematode feeding sites (Goverse et al, 2000;Hammes et al, 2005;Grunewald et al, 2009;Lee et al, 2011;Kyndt et al, 2016).…”

Molecular players of auxin transport systems: advances in genomic and molecular events

“…The pH , of apoplastic cellular environment of IAA molecule is ranges in between 5 to 5.5 (Gout et al 1992;Pin Ng et al 2015) due to the presence of plasma membrane bound H + ATPases and at this pH , 83% of the IAA molecules remain in anionic (IAA − , dissociated) and 17% (IAA + , associated) in cationic form (Zažímalová et al 2010). The negative charge (anion) of the IAA − molecule prevents it to pass through the lipohilic plasma membrane and it only allows the protonated (cation) IAA + by passive diffusion (Zažímalová et al 2010;Pin Ng et al 2015). However, around 83% of the anionic IAA − cannot pass though the plasma membrane and thus it requires auxin influx carriers to transport these molecules inside the cells.…”

“…However, around 83% of the anionic IAA − cannot pass though the plasma membrane and thus it requires auxin influx carriers to transport these molecules inside the cells. Further, after entering the cytosol, IAA encounters with the alkaline environment (pH 7 to 7.5) of the cell (Gout et al 1992;Zažímalová et al 2010;Pin Ng et al 2015). The IAA remains in the anionic (IAA − ) form in the alkaline environment which makes it difficult to pass out of the cell, making the cell a weak anioinic chamber.…”