NodD1 and NodD2 Are Not Required for the Symbiotic Interaction of <i>Bradyrhizobium</i> ORS285 with Nod-Factor-Independent <i>Aeschynomene</i> Legumes

<div><p>Photosynthetic <i>Bradyrhizobium</i> strain ORS285 forms nitrogen-fixing nodules on the roots and stems of tropical aquatic legumes of the <i>Aeschynomene</i> genus. Depending on the <i>Aeschynomene</i> species, this symbiotic interaction does or does not rely on the synthesis of Nod-factors (NFs). However, whether during the interaction of <i>Bradyrhizobium</i> ORS285 with NF-independent <i>Aeschynomene</i> species the <i>nod</i> genes are expressed and if the general regulator NodD plays a symbiotic role is unknown. Expression studies showed that in contrast to the interaction with the NF-dependent <i>Aeschynomene</i> species, <i>A</i>. <i>afraspera</i>, the <i>Bradyrhizobium</i> ORS285 <i>nod</i> genes are not induced upon contact with the NF-independent host plant <i>A</i>. <i>indica</i>. Mutational analysis of the two <i>nodD</i> genes present in ORS285, showed that deletion of <i>nod</i>D1 and <i>nod</i>D2 did not affect the symbiotic interaction between <i>Bradyrhizobium</i> ORS285 and <i>A</i>. <i>indica</i> whereas the deletions had an effect on the symbiotic interaction with <i>A</i>. <i>afraspera</i> plants. In addition, when the expression of <i>nod</i> genes was artificially induced by adding naringenin to the plant growth medium, the nodulation of <i>A</i>. <i>indica</i> by <i>Bradyrhizobium</i> ORS285 is delayed and resulted in lower nodule numbers.</p>