Dynamics of Ethylene Production in Response to Compatible Nod Factor

Abstract

Establishment of symbiotic nitrogen-fixation in legumes is regulated by the plant hormone ethylene, but it has remained unclear whether and how its biosynthesis is regulated by the symbiotic pathway. We established a sensitive ethylene detection system for Lotus japonicus and found that ethylene production increased as early as 6 hours after inoculation with Mesorhizobium loti. This ethylene response was dependent on Nod factor production by compatible rhizobia. Analyses of nodulation mutants showed that perception of Nod factor was required for ethylene emission, while downstream transcription factors including CYCLOPS, NIN, and ERN1 were not required for this response. Activation of the nodulation signaling pathway in spontaneously nodulating mutants was also sufficient to elevate ethylene production. Ethylene signaling is controlled by EIN2, which is duplicated in L. japonicus. We obtained a L. japonicus Ljein2a Ljein2b double mutant that exhibits complete ethylene insensitivity and confirms that these two genes act redundantly in ethylene signaling. Consistent with this redundancy, both LjEin2a and LjEin2b are required for negative regulation of nodulation and Ljein2a Ljein2b double mutants are hypernodulating and hyperinfected. We also identified an unexpected role for ethylene in the onset of nitrogen fixation, with the Ljein2a Ljein2b double mutant showing severely reduced nitrogen fixation. These results demonstrate that ethylene production is an early and sustained nodulation response that acts at multiple stages to regulate infection, nodule organogenesis, and nitrogen fixation in L. japonicus.

Nitrate inhibition of nodule formation in Medicago truncatula is mediated by ACC SYNTHASE 10

“…The skl mutant is hyper-infected by rhizobium and forms a multitude of nodules in distinct clusters along the root (Penmetsa and Cook, 1997). A similar phenotype is observed in Lotus japonicus , when both EIN2-encoding genes are mutated (Miyata et al, 2013; Reid et al, 2018), indicating an important role for ethylene signalling in the regulation of nodulation.…”

“…Several ethylene biosynthesis genes have been shown to be induced by LCO application or rhizobial inoculation, including ACS1 and ACS3 (Breakspear et al, 2014; Larrainzar et al, 2015; Van Zeijl et al, 2015). This corresponds to an increase in ethylene release measured after rhizobial inoculation in L. japonicus (Reid et al, 2018). This response is likely part of a negative feedback loop to restrict rhizobial infections after a successful interaction has been initiated (Van Zeijl et al, 2015).…”

“…The conversion of S -adenosylmethionine into ACC through ACS activity is generally considered rate limiting (Kende, 1993). Several ACS-encoding genes in Medicago truncatula are transcriptionally induced in response to LCO signalling (Larrainzar et al, 2015; Van Zeijl et al, 2015; Herrbach et al, 2017), and an increase in ethylene release has been reported for Lotus japonicus at six hours after inoculation (Reid et al, 2018). The sickle ( skl ) mutant in M. truncatula is ethylene insensitive due to a mutation in EIN2 , which encodes a central component in the ethylene signalling cascade (Penmetsa and Cook, 1997; Penmetsa et al, 2008).…”

Celebrating 20 Years of Genetic Discoveries in Legume Nodulation and Symbiotic Nitrogen Fixation

“…Mtskl exhibits massive proliferation of ITs and nodule primordia, although these rarely develop into mature nodules. This role of EIN2 is conserved in L. japonicus but requires two copies of EIN2 such that the double mutant ein2a ein2b does not fix nitrogen (Miyata et al, 2013;Reid et al, 2018). Not only is Mtskl hypersensitive to beneficial bacteria but it is also hypersusceptible to infection by fungal pathogens, suggesting that ethylene plays a crucial role in fine-tuning interactions with microbes (Penmetsa et al, 2008).…”

Natural variation identifies aPxygene controlling root vascular organization and formation of nodules and lateral roots inLotus japonicus

“…The simultaneous development of nodule organs is mediated by NSP1, NSP2, ERN1, NIN and NF-Ys transcription factors 41,42 that together with localized changes in plant hormone homeostasis (cytokinin, auxin and jasmonic acid) regulate initiation of infection thread formation and cell divisions leading to nodule organogenesis [43][44][45][46] . In contrast, ethylene and abscisic acid suppress nodule formation [47][48][49] .…”