Nodulation: A Symbiotic Tale

Surge 173

Nitrogen is the most limiting macronutrient for plant growth. Rhizobia engage in a symbiotic relationship with legumes and trigger the formation of nodules within which the bacteria are housed and nitrogen fixation occurs. The bacterial nod genes are induced by flavonoids secreted by plants experiencing nitrogen stress, which results in the production of Nod factor (NF), a critical signal triggering nodule formation. In α-rhizobia, nodD encodes a constitutively expressed regulatory protein sensitive to these flavonoids and induces the expression of the nod genes. It is thus far believed that β-rhizobia function similarly to α-rhizobia, however few studies have addressed the regulatory mechanisms in β-rhizobia. To understand the nodulation pathway in B. tuberum (a β-rhizobium), I am investigating the role and regulation of the bacterial nod genes by looking at nod gene mutants. B. tuberum deletion mutants were made in nodD1, nodD2 and nodC. The ΔnodD1 mutant is still able to nodulate. However, the ΔnodC mutant, which encodes the backbone of NF, forms no nodules. This implies that other regulatory factors (possibly nodD2) are managing the expression of nod genes, which are required for nodulation. This analysis shows that there is conservation between the nodulation pathway in α- and β-rhizobia.