Abstract
E107 and E132 are pea mutants that nodulate poorly. Because they have a shoot-controlled nodulation phenotype, we asked if their mutated genes were implicated in the autoregulation of nodulation (AON), a mechanism which consists of two systemic circuits, the positive CEP/CRA2 and the negative CLE/SUNN, coordinated via NIN and miR2111. We further characterized the mutants’ phenotype by studying nodule distribution and nodulation efficiency. E107 was similar to wild-type (WT) in its nodule distribution, but E132 had an extended nodulation zone with nodules forming distally on its lateral roots. Moreover, we tested whether their shoots produced a compound inhibitory to nodulation. We made ethyl-acetate extracts of roots and shoots of both mutants and WT, which we applied to rhizobia-inoculated WT seedlings and to pure rhizobial cultures. Whereas free-living bacteria were unaffected by any of the extracts, WT treated with shoot extracts from either inoculated mutant had fewer nodules than that of control. E107 and E132 shoot extracts led to a 50% and a 35% reduction in nodule number, respectively. We propose that E107 and E132 belong to a new sub-class of AON mutants, i.e., hypo-nodulators, and that their respective gene products are acting in the AON descending branch, upstream of TML signaling.
Highlights
Our knowledge on autoregulation of nodulation (AON) has expanded greatly in the last few years [1,2,3]
Whereas E132 has a mycorrhizal phenotype similar to that of WT [24], E107 forms a low number of colonization units, with most fungal entry prohibited beyond the epidermis [20]
We have shown that neither of them is compromised in its symbiotic efficiency
Summary
Our knowledge on autoregulation of nodulation (AON) has expanded greatly in the last few years [1,2,3] This regulatory mechanism, highly conserved among legumes, e.g., [4,5], involves a complex and finely-tuned long-distance signaling. 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 N2 fixation and C demand are optimally balanced within the legume plant. The CLE peptide is modified later, post-translationally, as it becomes arabinosylated by the enzyme encoded by
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