Plant Hormones Differentially Control the Sub-Cellular Localization of Plasma Membrane Microdomains during the Early Stage of Soybean Nodulation

Abstract

Phytohormones regulate the mutualistic symbiotic interaction between legumes and rhizobia, nitrogen-fixing soil bacteria, notably by controlling the formation of the infection thread in the root hair (RH). At the cellular level, the formation of the infection thread is promoted by the translocation of plasma membrane microdomains at the tip of the RH. We hypothesize that phytohormones regulate the translocation of plasma membrane microdomains to regulate infection thread formation. Accordingly, we treated with hormone and hormone inhibitors transgenic soybean roots expressing fusions between the Green Fluorescent Protein (GFP) and GmFWL1 or GmFLOT2/4, two microdomain-associated proteins translocated at the tip of the soybean RH in response to rhizobia. Auxin and cytokinin treatments are sufficient to trigger or inhibit the translocation of GmFWL1 and GmFLOT2/4 to the RH tip independently of the presence of rhizobia, respectively. Unexpectedly, the application of salicylic acid, a phytohormone regulating the plant defense system, also promotes the translocation of GmFWL1 and GmFLOT2/4 to the RH tip regardless of the presence of rhizobia. These results suggest that phytohormones are playing a central role in controlling the early stages of rhizobia infection by regulating the translocation of plasma membrane microdomains. They also support the concept of crosstalk of phytohormones to control nodulation.

Salicylic Acid in Root Growth and Development

“…This gives an idea of a possible SA contribution in the nodulation process, independent of plant immunity mechanisms. This is consistent with the recently described role of SA in the regulation of the subcellular localization of plasma membrane microdomains, which is essential during the early stage of nodulation in soybeans [265].…”

Structure and Development of the Legume-Rhizobial Symbiotic Interface in Infection Threads

“…These include enzymes such as the E3 ubiquitin ligase in L. japonicus [ 87 ], the putative E3 ubiquitin ligase with the Nodule specific RING finger domain (LjnsRING) in L. japonicus [ 163 ], the E3 ubiquitin ligase with ‘zinc finger’ type domain, Seven in absentia (MtSINA) in M. truncatula [ 164 ] and Cystathionine-β-synthase-like1 (MtCBS1) in M. truncatula [ 165 ]. Recently, using pea gibberellin-deficient and della -deficient mutants, it was shown that the phytohormone gibberellin suppresses the formation of infection threads [ 166 ], and its amount in infection threads is much lower than in bacteroids [ 167 ], on the contrary, phytohormones cytokinins and auxins play an important role in the development and propagation of infection threads, as well as in the release of bacteria from infection droplets [ 132 , 168 , 169 , 170 ]. A putative role of ethylene in infection thread maturation was also suggested [ 171 ].…”

Biological and Cellular Functions of the Microdomain-Associated FWL/CNR Protein Family in Plants

“…Plant hormones also play an important role in controlling the translocation of microdomains in specific regions of the plasma membrane. Treatment of transgenic soybean roots clearly revealed that GmFWL1 and GmFLOT2 are both translocated at the tip of the root hair cells in response to auxin and salicylic acid [71], hormones regulating nodulation and plant-microbe interactions [72][73][74][75][76]. Conversely, cytokinin inhibits this re-location [71] and the infection of root hair cell by rhizobia [77][78][79][80].…”

“…Treatment of transgenic soybean roots clearly revealed that GmFWL1 and GmFLOT2 are both translocated at the tip of the root hair cells in response to auxin and salicylic acid [71], hormones regulating nodulation and plant-microbe interactions [72][73][74][75][76]. Conversely, cytokinin inhibits this re-location [71] and the infection of root hair cell by rhizobia [77][78][79][80]. Consequently, it is tempting to speculate that plant hormones control plant-microbe interactions [81][82][83][84][85][86] by controlling the distribution of microdomains in the plasma membrane.…”