Function and evolution of aLotus japonicusAP2/ERF family transcription factor that is required for development of infection threads

Abstract

Legume-rhizobium symbiosis is achieved by two major events evolutionarily acquired: root hair infection and organogenesis. Infection thread (IT) development is a distinct element for rhizobial infection. Through ITs, rhizobia are efficiently transported from infection foci on root hairs to dividing meristematic cortical cells. To unveil this process, we performed genetic screening using Lotus japonicus MG-20 and isolated symbiotic mutant lines affecting nodulation, root hair morphology, and IT development. Map-based cloning identified an AP2/ERF transcription factor gene orthologous to Medicago truncatula ERN1. LjERN1 was activated in response to rhizobial infection and depended on CYCLOPS and NSP2. Legumes conserve an ERN1 homolog, ERN2, that functions redundantly with ERN1 in M. truncatula. Phylogenetic analysis showed that the lineages of ERN1 and ERN2 genes originated from a gene duplication event in the common ancestor of legume plants. However, genomic analysis suggested the lack of ERN2 gene in the L. japonicus genome, consistent with Ljern1 mutants exhibited a root hair phenotype that is observed in ern1/ern2 double mutants in M. truncatula. Molecular evolutionary analysis suggested that the nonsynonymous/synonymous rate ratios of legume ERN1 genes was almost identical to that of non-legume plants, whereas the ERN2 genes experienced a relaxed selective constraint.

NIN Acts as a Network Hub Controlling a Growth Module Required for Rhizobial Infection

“…NF-YA1-interacting subunits B/C involved in nodulation were discovered in M. truncatula, L. japonicus, and Phaseolus vulgaris (Zanetti et al, 2010;Soyano et al, 2013;Baudin et al, 2015). ERN1 encodes an AP2/ERF family transcription factor, expressed in root hairs undergoing rhizobial infection and in developing nodules of both M. truncatula and L. japonicus (Cerri et al, 2012(Cerri et al, , 2017Kawaharada et al, 2017;Yano et al, 2017). This expression is in part mirrored by ERN1's close homolog, ERN2, which has relatively lower but partially overlapping expression in root tissues undergoing rhizobial infection (Cerri et al, 2012;Roux et al, 2014).…”

Cytokinin Biosynthesis Promotes Cortical Cell Responses during Nodule Development

“…Initially, calcium-spiking events must be decoded by CALCIUM AND CALMODULIN-DEPENDENT KINASE (CCaMK; Lévy et al, 2004;Tirichine et al, 2006;Sieberer et al, 2009) and its phosphorylation target CYCLOPS (Singh et al, 2014). In the model legumes L. japonicus and M. truncatula, a single copy or duplicated copies, respectively, of the ERF REQUIRED FOR NODULATION (LjERN1, MtERN1, and ERN2) transcription factors are required for the activation of both epidermal and cortical transcriptional responses (Cerri et al, 2012(Cerri et al, , 2016(Cerri et al, , 2017Kawaharada et al, 2017;Yano et al, 2017). Downstream of cytokinin perception, nodule organogenesis in L. japonicus and M. truncatula requires Nodule Inception (Nin; Schauser et al, 1999;Marsh et al, 2007) and the GRAS transcription factors Nsp1 and Nsp2 (Kaló et al, 2005;Smit et al, 2005;Heckmann et al, 2006;Murakami et al, 2006;Hirsch et al, 2009).…”

Dynamics of Ethylene Production in Response to Compatible Nod Factor

“…Downstream targets include transcriptional regulators such as NIN (Schauser et al, 1999), which through the heterotrimeric NF-Y complex regulates nodule differentiation (Soyano et al, 2013;Laporte et al, 2014;Baudin et al, 2015;Hossain et al, 2016). Additionally, NIN and the AP2/ERF transcription factor ERN1 (Cerri et al, 2012(Cerri et al, , 2016(Cerri et al, , 2017Kawaharada et al, 2017;Yano et al, 2017) are transcriptionally activated by the GRAS-type transcription factors NSP1 and NSP2 (Kaló et al, 2005;Smit et al, 2005;Heckmann et al, 2006;Murakami et al, 2006;Hirsch et al, 2009). Nodulation signaling also induces cytokinin biosynthesis (Reid et al, 2017), and activation of the cytokinin receptor LHK1 is sufficient to induce nodule organogenesis (Tirichine et al, 2007;Murray et al, 2007).…”