Suppression of hypernodulation in soybean by a leaf‐extracted, NARK‐ and Nod factor‐dependent, low molecular mass fraction

Abstract

Summary• Legumes regulate the number of nodules they form via a process called autoregulation of nodulation (AON). This involves a shoot-derived inhibitor (SDI) molecule that is synthesized in the shoots and is transported down to the roots where it inhibits further nodule development.• To characterize SDI, we developed a novel feeding bioassay. This involved feeding aqueous leaf extracts directly into the petiole of hypernodulating and supernodulating nark mutant plants of Glycine max (soybean). These mutants normally exhibit an increased nodulation phenotype because SDI is not produced and thus AON is nonfunctional.• Feeding wild-type leaf extracts presumed to contain SDI was successful in suppressing the increased nodulation phenotype, whereas feeding with Gmnark leaf extracts did not. Suppression activity was inoculation-dependent, Nod factordependent, required GmNARK activity, and was heat-, proteinase K-and ribonuclease A-resistant. Wild-type extracts maintained suppressive activity even at a ninefold dilution. Sinorhizobium meliloti-inoculated Medicago truncatula leaf extracts from wild-type, but not from supernodulating mutant Mtsunn, suppressed hypernodulation in soybean.• Our results demonstrate that the petiole feeding bioassay is an efficient and effective technique to introduce aqueous extracts into plants. They also demonstrate that SDI is a small compound with an apparent molecular mass of < 1000 Da and is unlikely to be a protein or an RNA molecule.

Autoregulation of Nodulation Interferes with Impacts of Nitrogen Fertilization Levels on the Leaf-Associated Bacterial Community in Soybeans

“…These findings support the hypothesis that the systems that autoregulate nodulation are responsible for the shifts seen in the community of symbiotic bacteria here and in previous studies (21), because, under the field conditions examined, autoregulation was activated only in the wildtype Enrei. Furthermore, leaf tissue is currently believed to be the site of production of SDF, which suppresses rhizobial infection of roots (26,34,47). Therefore, the effects of SDF could be reflected more directly in the community structures of leaf-associated bacteria than in those of stem-associated bacteria.…”

“…These findings led to the question of whether soybean-asso-ciated bacteria are more tightly regulated in leaves than in stems, since the shoot-derived factor (SDF) for the autoregulation of nodulation is thought to be produced there (26,47). If this is the case, then the observation of more direct and clear effects of autoregulation systems on soybean-associated bacteria, including endophytes and epiphytes, could be expected in the leaves than in the stems.…”

Soybean miR172c Targets the Repressive AP2 Transcription Factor NNC1 to Activate ENOD40 Expression and Regulate Nodule Initiation

“…This perception results in the production of a shootderived inhibitor (SDI), which travels to the roots to inhibit further nodule development (Delves et al, 1986;Lin et al, 2010;Sasaki et al, 2014). Excitingly, recent evidence has revealed that cytokinins, whose production is activated by Lj-HAR1 perception of AON CLE peptides in the shoot, may function as an SDI to systemically suppress nodulation in L. japonicus (Sasaki et al, 2014).…”

The receptor-like kinase KLAVIER mediates systemic regulation of nodulation and non-symbiotic shoot development in Lotus japonicus

“…Grafting experiments have shown that HAR1 and its orthologs function in the shoot to control root nodule number. Thus, HAR1 is likely to be involved, directly or indirectly, in the generation of a shoot-derived mobile signal that inhibits further nodulation (Krusell et al, 2002;Nishimura et al, 2002;Jiang and Gresshoff, 2002;Buzas and Gresshoff, 2007;Lin et al, 2010).…”