Abstract
Nodulation is energetically costly to the host: legumes balance the nitrogen demand with the energy expense by limiting the number of nodules through long-distance signaling. A split root system was used to investigate systemic autoregulation of nodulation (AON) in Medicago truncatula and the role of the AON genes RDN1 and SUNN in the regulatory circuit. Developing nodule primordia did not trigger AON in plants carrying mutations in RDN1 and SUNN genes, while wild type plants had fully induced AON within three days. However, despite lacking an early suppression response, AON mutants suppressed nodulation when roots were inoculated 10 days or more apart, correlated with the maturation of nitrogen fixing nodules. In addition to correlation between nitrogen fixation and suppression of nodulation, suppression by extreme nutrient stress was also observed in all genotypes and may be a component of the observed response due to the conditions of the assay. These results suggest there is more than one systemic regulatory circuit controlling nodulation in M. truncatula. While both signals are present in wild type plants, the second signal can only be observed in plants lacking the early repression (AON mutants). RDN1 and SUNN are not essential for response to the later signal.
Highlights
When legumes set up a symbiosis with bacteria in the genus Rhizobia, the plants develop a new organ on the roots to house the bacteria
In order to determine if the excess nodule number phenotypes of the rdn1 and sunn mutants involve changes in autoregulation of nodulation (AON) timing, we investigated AON in wild type plants and plants carrying mutant alleles of
We compared the nodulation of Root B to the nodulation of Root A 21 days after the inoculation of root B, reporting the results as a percentage of the total nodules on Root A to account for the higher nodule numbers of AON mutants
Summary
When legumes set up a symbiosis with bacteria in the genus Rhizobia, the plants develop a new organ on the roots (a nodule) to house the bacteria. Our findings using measurements of nitrogen fixation, response to supplied nitrogen and inoculation with bacteria unable to fix nitrogen support the assertion that the second AON signal is not associated with early nodulation events but rather with the establishment of nitrogen fixing nodules and this signal is presumably masked by the CLE12/13-SUNN signaling in wild type plants, it is unaffected and detectable in the sunn and rdn AON mutants Both wild type and AON mutant plants suppress nodulation under severe nutrient stress, suggesting that the nitrate and stress signals are not affected in AON regulatory mutants
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