Legumes, such as Medicago truncatula, can fix atmospheric nitrogen by forming symbiotic associations with soil-borne bacteria collectively called rhizobia. As a result of this relationship, specific roots organs called nodules, are developed that houses rhizobia and where the nitrogen fixation process occurs. Nodule formation is tightly regulated by complex signalling mechanisms and environmental cues, such as nitrate availability. Molecular signals move between the site of infection and the cortex/pericycle to coordinate nodule organogenesis and also systemically along the vascular system to coordinate root and shoot responses. Despite recent progress in the identification of some of these signals very little is known about the pathways for intercellular transport.
In this project, the role of the cell-wall polysaccharide callose in the establishment of symbiotic interaction between Medicago truncatula and Sinorhizobium meliloti was addressed. Callose metabolism regulates transport through plasmodesmata: intercellular channels that form a symplastic path for transport. Using immuno-histochemistry we found that callose is downregulated as early as 16 hours post-bacterial inoculation. Concomitantly, the expression of a plasmodesmata located callose degrading enzyme (Medtr3g083580), identified using phylogeny, was induced. Roots constitutively expressing either Medtr3g083580 or its Arabidopsis orthologue PdBG1, showed reduced callose levels and a higher rate of infection and nodulation, even when grown in nitrate concentrations that inhibit nodulation. The effects were stronger when using a promoter active early after rhizobial infection and were mimicked, in high nitrate conditions, by the ectopic expression of a novel plasmodesmata receptor-like kinase (Medtr1g073320).
The results suggest an important role for callose in the control of nodulation, both under nitrate deprived or sufficient conditions, likely associated with the regulation of transport via plasmodesmata. The relevance of the findings is discussed in light of potential applications in crop improvement and in reducing the use of nitrogen fertilizers
