Perception and response to ethylene and cytokinins in arbuscular mycorrhizal fungi ; Perception et réponse à l'éthylène et aux cytokinines chez les champignons mycorhiziens à arbuscules

The arbuscular mycorrhizal symbioses associates the roots of most plant species with microscopic soil fungi, and is an important contributor to plant mineral and water nutrition. Signals are exchanged between the fungal and plant symbionts at all stages of the interaction. Root-exuded strigolactones are well-known early symbiotic cues, but the role of other phytohormones as interkingdom signals has seldom been investigated. Here we focus on ethylene and cytokinins, for which candidate receptors containing all domains required for perception and signal transduction have been identified in the genome of the arbuscular mycorrhizal fungus Rhizophagus irregularis. Spore germination and hyphal morphology were not impacted upon exposure to exogenous ethylene treatment. Nevertheless, heterologous complementation of a Saccharomyces cerevisiae mutant strain with the candidate ethylene receptor gene RiHHK6 indicated that this protein can sense ethylene and transduce the signal through a functional histidine kinase domain. Accordingly, its N-terminal sensing domain expressed in Pichia pastoris displayed saturable binding to radiolabelled ethylene. Thus, RiHHK6 displays the expected characteristics of an ethylene receptor. Similarly to plant ethylene receptors, the histidine kinase domain of RiHHK6 is active in the absence of ethylene, and turned off upon ethylene perception. As for cytokinins, the germination of R. irregularis spores in vitro was stimulated by exogenous application of isopentenyladenine, N6-benzyl adenine or cis-zeatin. However, the candidate cytokinin receptor RiHHK7 did not complement the Saccharomyces cerevisiae mutant strain nor Medicago truncatula cytokinin receptor mutants. The N-terminal domain of RiHHK7 was also unable to bind isopentenyladenine or trans-zeatin. Thus, R. irregularis seems to possess a cytokinin receptor as it responds to these hormones, but this receptor is unlikely to be RiHHK7. Taken together, our results support the current hypothesis that arbuscular mycorrhizal fungi respond to ...