Singular Features of the Bradyrhizobium-Lupinus Symbiosis

Abstract

Lupinus is a legume with great agronomic potential due to the high protein content of its seeds and its positive effect on soil fertility. It is able to fix atmospheric nitrogen through the establishment of a symbiosis with soil bacteria of the genus Bradyrhizobium. The process is carried out in a special subclass of indeterminate nodules known as lupinoid nodules. The Bradyrhizobium-Lupinus symbiosis has particular characteristics, which makes it different from other Rhizobium-legume symbioses. The entry of the bacteria into the plant root occurs at the junction between the root hair base and the adjacent epidermic cell and only sporadic “infection threads” have been observed. The involvement of mitogen-activated protein kinases (MAPKs) and aldehyde oxidase in infection and nodule morphogenesis have been reported for the first time in this symbiosis. The presence of nitric oxide synthase activity in plants has been detected for the first time, in roots and nodules of Lupinus albus. The unique lupin nodule cortical structure has contributed to the visualization and elucidation of the operational mechanisms of the oxygen diffusion barrier. Nodule senescence takes place in a similar way to that of determinate nodules, starting in the central area of the nodule. This symbiosis is especially resistant to abiotic stresses such as herbicides, nitrate, salinity and heavy metals. This unusual tolerance has permitted the use of inoculated lupin plants for the re-vegetation of degraded areas and as a pioneer plant to fight soil erosion and to reclaim eroded soils. The capability to accumulate Cd, Zn and others heavy metals in the nodulated roots has resulted in the consideration of this symbiosis as a potential phytoremediator