The role of ion fluxes in Nod factor signalling in Medicago sativa

Abstract

SummaryUsing ion-selective microelectrodes, the basis of Nod factor-induced changes in the plasma membrane potential was analysed by measuring the extracellular free concentrations of Ca 2⍣ , K ⍣ , H ⍣ and Cl -in the root hair zone of alfalfa. After addition of the Rhizobium meliloti Nod factor NodRm-IV(C16:2,S) at a concentration of 0.1 µM, a decrease in [Ca 2⍣ ] was observed first, which was followed after a few seconds by an increase of [Cl -], by an alkalinization, and then by a delayed increase of [K ⍣ ], all of which were transient changes. Simultaneously with the appearance of Cl -ions in the root hair zone, a decrease in cytosolic [Cl -] was measured. It was concluded that the depolarization was caused by temporary short-circuiting of the proton pump through the rapid release of Cl -ions along their steep electrochemical gradient. Since under resting conditions the driving force for K ⍣ ions was inwardly directed, their release was delayed until their driving force was inverted. This indicates that K ⍣ serves as a charge balance that eventually stops depolarization and initiates repolarization. Since the decrease in [Ca 2⍣ ] was observed seconds before the increase in [Cl -] and the depolarization, it is argued that Ca 2⍣ entering into the cell does not cause the depolarization directly, but might initiate it by triggering the activation of an anion channel that then releases the chloride ions. The observations that the Ca 2⍣ ionophore A23187 mimicks the Nod factor response, and that the Ca 2⍣ channel antagonist nifedipine inhibits this response, support the idea that Ca 2⍣ plays a primary role in the transduction of the Nod signal in alfalfa.

Deep Sequencing of the Medicago truncatula Root Transcriptome Reveals a Massive and Early Interaction between Nodulation Factor and Ethylene Signals

“…Myriad structural and functional alterations occur within the plant root in response to Nod factors, implicating a diverse repertoire of genes and proteins. The earliest known plant response to Nod factors involves depolarization of the root hair plasma membrane (Ehrhardt et al, 1992), a consequence of tipfocused ion fluxes (Felle et al, 1998). Such changes proceed and are likely causal to a transient cessation and subsequent reactivation of root hair polar growth, leading to the formation of a characteristic root hair curl (the shepherd's crook), entrapping bacteria that will ultimately colonize the nodule organ.…”

“…Bacteria enter the root hair following localized cell wall hydrolysis (Xie et al, 2012) within an inward-growing channel known as the infection thread. At the biochemical level, Nod factors elicit a complex series of changes, including ion fluxes at the root hair tip (Felle et al, 1998), periodic increases in intracellular calcium within and around the nucleus (Ehrhardt et al, 1996;de Ruijter et al, 1999;Wais et al, 2000), rearrangement of the root hair cytoskeleton (Cárdenas et al, 1998;Timmers et al, 1999), changes in phosphorylation profiles , modification of membrane protein dynamics (Haney et al, 2011), and induction of cortical cell divisions (Ehrhardt et al, 1992;Timmers et al, 1999, and refs. therein).…”

Self-Incompatibility-Induced Programmed Cell Death in Field Poppy Pollen Involves Dramatic Acidification of the Incompatible Pollen Tube Cytosol

“…Light-adapted cells respond to a decrease in light intensity with a rapid transient cytosolic acidification by approximately 0.3 pH units (Felle et al, 1986). Addition of nodulation factors resulted in an increase of 0.2 pH units in root hairs (Felle et al, 1998), and abscisic acid increased the [pH] cyt of guard cells by 0.3 pH units (Blatt and Armstrong, 1993). Changes in [pH] cyt are thought to activate stress responses (Felle, 2001).…”

Root-knot nematodes and bacterial Nod factors elicit common signal transduction events in Lotus japonicus

“…The specificity of particular rhizobial-legume associations is largely mediated at the root surface via specific lipochitooligosaccharide signaling molecules (2) synthesized by the bacterium and collectively termed Nod factors (NF). Invasion of the host by rhizobia is a highly orchestrated process, beginning with ion movements (3)(4)(5) and changes in cytoskeletal organization (6 -8). The resultant signals lead to the first morphologically visible step, which is the disruption of the normal tip-growing pattern and curling of epidermal root hair cells 2-4 h after initial exposure and before bacterial entry (9).…”