Dissection of Nodule Development by Supplementation of Rhizobium leguminosarum biovar phaseoli Purine Auxotrophs with 4-Aminoimidazole-5-Carboxamide Riboside

Abstract

Purine auxotrophs of Rhizobium leguminosarum biovar phaseoli CFN42 elicit uninfected pseudonodules on bean (Phaseolus vulgaris L.). Addition of 4-aminoimidazole-5-carboxamide (AICA) riboside to the root medium during incubation of the plant with these mutants leads to enhanced nodule development, although nitrogenase activity is not detected. Nodules elicited in this manner had infection threads and anatomical features characteristic of normal nodules, such as peripheral vasculature rather than the central vasculature of the pseudonodules that were elicited without AICA riboside supplementation. Although 105 to 106 bacteria could be recovered from these nodules after full development, bacteria were not observed in the interior nodule cells. Instead, large cells with extensive internal membranes were present. Approximately 5% of the normal amount of leghemoglobin and 10% of the normal amount of uricase were detected in these nodules. To promote the development of true nodules rather than pseudonodules, AICA riboside was required no later than the second day through no more than the sixth day following inoculation. After this period, removal of AICA riboside from the root medium did not prevent the formation of true nodules. This observation suggests that there is a critical stage of infection, reached before nodule emergence, at which development becomes committed to forming a true nodule rather than a pseudonodule.

The Mesorhizobium loti purB Gene Is Involved in Infection Thread Formation and Nodule Development in Lotus japonicus

“…By contrast, it has been reported that 5-aminoimidazole-4-carboxamide (AICA) riboside, which is a nonphosphorylated derivative of the purine biosynthetic intermediate AICA ribonucleotide (AICAR), promoted the symbiosis of purine mutants. Similar findings have been reported in several other cases (10,15,16,17).…”

Infection of soybean and pea nodules by Rhizobium spp. purine auxotrophs in the presence of 5-aminoimidazole-4-carboxamide riboside

“…Potentially more revealing for understanding infection are deficiencies that allow nodules or pseudonodules to form but lead to poorly developed infection threads. This phenotype has been found mainly in polysaccharide-contain extensive internal membranes but appear to lack bacteroids (21). It seems, therefore, that AICA riboside restores almost complete infection thread development but cannot overcome a second limitation that prevents the proliferation of bacteroids.…”

Varying the Abundance of O Antigen in Rhizobium etli and Its Effect on Symbiosis with Phaseolus vulgaris

“…Microscopic examination reveals that the infection threads in these nodules are distended and cease development within the root hair or the subjacent cell layer (35). Closely resembling the development of nodules elicited by mutants that do not infect at all (46), the resulting nodule structure lacks most features of a normal mature nodule and has been described as a pseudonodule (27,35). Oantigen-deficient mutants of Rhizobium leguminosarum and Bradyrhizobium japonicum also exhibit defects in infection and cause aberrant development of nodules on their respective host legumes (4,12,22,23,37,38,39,44).…”