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Legumes establish symbiotic associations with nitrogen-fixing rhizobia, which, hosted within newly formed root nodules, have the optimized environment for fixing atmospheric dinitrogen for the plant. The formation of these unique organs requires close coordination between bacterial infection, initiated in the root epidermis, and the nodule organogenesis program initiated in parallel in the underlying root tissues. Although integration between these processes is necessary for nodule development, the signaling mechanisms underlying this coordination is largely unknown.

In this study, researchers have discovered the importance of intercellular (symplastic) communication in plants for the formation of nitrogen-fixing root nodules, during the rhizobium-legume symbiosis. The authors show that symbiotic bacteria promote symplastic communication of root tissues where nodules are initiated, and this involves the regulation of callose (β-1,3-glucan) turnover at plasmodesma sites (see box), which is orchestrated by the callose-degrading MtBG2 β-1,3-glucanase.

The MtBG2 β-1,3-glucanase promotes the creation of symplastic domains in developing nodules

The gradual expression of MtBG2 from dividing cells of the nodule primordia to those undergoing infection progressively creates symplastic domains necessary for nodule formation and colonization. By monitoring the transport of fluorescent molecules in vivo, researchers demonstrated the creation of symplastic domains in tissues colonized by the symbiotic bacteria.

Symplastic connectivity is necessary for the development and colonization of root nodules

Ectopic expression of β-1,3-glucanases (including MtBG2) increases the formation of nodules and their colonization. Conversely, blocking symplastic connections by MtBG2 RNA interference or by the overproduction of callose at palsmodesmata sites strongly reduce the formation of nodules and their colonization. This symplastic pathway is also crucial for the regulated spatial expression of the key symbiotic transcription factors NODULE INCEPTION (NIN) and NUCLEAR FACTOR-YA1 (NF-YA1).

This research demonstrates that symplastic intercellular communication is proactively enhanced by rhizobia, and this is necessary for appropriate coordination of bacterial infection and nodule development.