A GRAS-Type Transcription Factor with a Specific Function in Mycorrhizal Signaling

SummaryLegumes establish mutualistic associations with mycorrhizal fungi and with nitrogen-fixing rhizobial bacteria. These interactions occur following plant recognition of Nod factor from rhizobial bacteria and Myc factor from mycorrhizal fungi [1, 2 and 3]. A common symbiosis signaling pathway is involved in the recognition of both Nod factor and Myc factor and is required for the establishment of these two symbioses [4, 5 and 6]. The outcomes of these associations differ, and therefore, despite the commonality in signaling, there must be mechanisms that allow specificity. In Nod factor signaling, a complex of GRAS-domain transcription factors controls gene expression downstream of the symbiosis signaling pathway. Here, we show that a GRAS-domain transcription factor, RAM1, functions in mycorrhizal-specific signaling. Plants mutated in RAM1 are unable to be colonized by mycorrhizal fungi, with a defect in hyphopodia formation on the surface of the root. RAM1 is specifically required for Myc factor signaling and appears to have no role in Nod factor signaling. RAM1 regulates the expression of RAM2, a glycerol-3-phosphate acyl transferase that promotes cutin biosynthesis to enhance hyphopodia formation. We conclude that mycorrhizal signaling downstream of the symbiosis-signaling pathway has parallels with nodulation-specific signaling and functions to promote mycorrhizal colonization by regulating cutin biosynthesis.Video Abstract To view the video inline, enable JavaScript on your browser. However, you can download and view the video by clicking on the icon belowHelp with MP4 filesOptionsDownload video (90093 K)

► A GRAS-type transcription factor that functions in the mycorrhizal association
► This transcription factor is not required for nodulation
► The work reveals specific modes of signaling for Myc factors compared to Nod factors
► One target of this transcription factor is the activation of cutin signaling