The NIN transcription factor coordinates CEP and CLE signaling peptides that regulate nodulation antagonistically

Abstract

Legumes tightly regulate nodule number to balance the cost of supporting symbiotic rhizobia with the benefits of nitrogen fixation. C-terminally Encoded Peptides (CEPs) and CLAVATA3like (CLE) peptides positively and negatively regulate nodulation, respectively, through independent systemic pathways, but how these regulations are coordinated remains unknown. Here, we show that rhizobia, Nod Factors, and cytokinins induce a symbiosisspecific CEP gene, MtCEP7, which positively regulates rhizobial infection. Via grafting and split root studies, we reveal that MtCEP7 increases nodule number systemically through the MtCRA2 receptor. MtCEP7 and MtCLE13 expression in rhizobia-inoculated roots rely on the MtCRE1 cytokinin receptor and on the MtNIN transcription factor. MtNIN binds and transactivates MtCEP7 and MtCLE13, and a NIN Binding Site (NBS) identified within the proximal MtCEP7 promoter is required for its symbiotic activation. Overall, these results demonstrate that a cytokinin-MtCRE1-MtNIN regulatory module coordinates the expression of two antagonistic, symbiosis-related, peptide hormones from different families to fine-tune nodule number.

No Home without Hormones: How Plant Hormones Control Legume Nodule Organogenesis

“…The maintenance of numerous positively and negatively acting pathways is likely to allow finer control over this integration. Both negative feedback through CLE/HAR1 and positive regulation through the CEP/CRA2 pathways can be induced by cytokinin and NIN ( Laffont et al., 2020 ) and appear to converge on a common regulator in miR2111 ( Figure 6 ; Laffont et al., 2019 ; Gautrat et al., 2020 ). HAR1-dependent CK synthesis in the shoot also appears to play a role in systemic nodule regulation ( Sasaki et al., 2014 ), although it has not yet been clearly established whether this also acts through the regulation of miR2111 or through additional, parallel pathways.…”

Shoot Extracts from Two Low Nodulation Mutants Significantly Reduce Nodule Number in Pea

“…The existence of this inhibitory substance makes these two mutants clearly distinct from cra2 mutants, the low nodulation phenotype of which is caused by the disruption of the positive CEP/CRA2 control of nodulation [6][7][8]. This raises a few questions, and an obvious one is to which of the two circuits making the AON pathway are E107 and E132 participating.…”

“…Earlier, it was considered solely as a negative feedback pathway, but now it is thought to be comprised of two complementary systemic pathways, e.g., [6,7]. In Medicago truncatula, the negative pathway acting through the CLE/SUNN module is counterweighed by the positive pathway acting through the CEP/CRA2 module [7,8]. Together, these two circuits regulate nodule development on a root system so that N 2 fixation and C demand are optimally balanced within the legume plant.…”

“…Together, these two circuits regulate nodule development on a root system so that N 2 fixation and C demand are optimally balanced within the legume plant. In short, upon recognition by the root of the rhizobial -Nod factors, the transcription factor NIN binds to the gene promoters of members of both the CEP and CLE families, activating the production of these two types of short peptides [4,8]. The CLE peptide is modified later, post-translationally, as it becomes arabinosylated by the enzyme encoded by MtRDN/LjPLENTY/PsNOD3 [5,9].…”

Nitrate inhibits nodule organogenesis through inhibition of cytokinin biosynthesis inLotus japonicus

“…This cytokinin signalling regulates expression of central components of nodulation signalling, including the transcription factor NIN (Nodule Inception; Schauser et al, 1999) via distal cis -elements in the Nin promoter (Liu et al, 2019). Cytokinin- and NIN-dependent signalling also initiates negative feedback of nodule organogenesis and infection via induction of CLE (CLAVATA3/ESR-related) peptides and the AON (Autoregulation of Nodulation) pathway (Soyano et al, 2014; Laffont et al, 2020; Ferguson et al, 2019). The AON pathway integrates signals from both prior nodulation events and soil nitrate availability to balance nodule development with plant resources.…”