Spontaneous Root-Nodule Formation in the Model LegumeLotus japonicus: A Novel Class of Mutants Nodulates in the Absence of Rhizobia

Abstract

Root-nodule development in legumes is an inducible developmental process initially triggered by perception of lipochitin-oligosaccharide signals secreted by the bacterial microsymbiont. In nature, rhizobial colonization and invasion of the legume root is therefore a prerequisite for formation of nitrogen-fixing root nodules. Here, we report isolation and characterization of chemically induced spontaneously nodulating mutants in a model legume amenable to molecular genetics. Six mutant lines of Lotus japonicus were identified in a screen for spontaneous nodule development under axenic conditions, i.e., in the absence of rhizobia. Spontaneous nodules do not contain rhizobia, bacteroids, or infection threads. Phenotypically, they resemble ineffective white nodules formed by some bacterial mutants on wild-type plants or certain plant mutants inoculated with wild-type Mesorhizobium loti. Spontaneous nodules formed on mutant lines show the ontogeny and characteristic histological features described for rhizobia-induced nodules on wild-type plants. Physiological responses to nitrate and ethylene are also maintained, as elevated levels inhibit spontaneous nodulation. Activation of the nodule developmental program in spontaneous nodules was shown for the early nodulin genes Enod2 and Nin, which are both upregulated in spontaneous nodules as well as in rhizobial nodules. Both monogenic recessive and dominant spontaneous nodule formation (snf) mutations were isolated in this mutant screen, and map positions were determined for three loci. We suggest that future molecular characterization of these mutants will identify key plant determinants involved in regulating nodulation and provide new insight into plant organ development.

TRICOTencodes an AMP1-related carboxypeptidase that regulates root nodule development and shoot apical meristem maintenance inLotus japonicus

“…When grown under nitrogen-deprived conditions, snf2 plants readily formed spontaneous nodules (Fig. 1A-C) as described previously (Tirichine et al, 2006). By contrast, spontaneous nodule formation was substantially reduced in tco plants, which formed approximately one-tenth of the number found in snf2 plants; moreover, the nodules formed in tco plants had an underdeveloped structure (Fig.…”

Establishing symbiotic nitrogen fixation in cereals and other non-legume crops: The Greener Nitrogen Revolution

“…The Leguminosae nodulation seems to occur amongst the more specialized subfamilies. It is important to note that the host plant itself encodes the genetic developmental programme responsible for the development of the nodule tissues and for regulation of the process, with organogenesis being triggered by the rhizobial micro-symbiont [33]. This raises questions in regard to the means of cell entry in non-nodular and non-leguminous symbioses.…”

A Small GTPase of the Rab Family Is Required for Root Hair Formation and Preinfection Stages of the Common Bean–RhizobiumSymbiotic Association

“…Perception of Nod factor by the plant triggers different responses responsible for changes in young epidermal cells and, at the same time, is necessary for the molecular events that will lead to nodule organogenesis (Oldroyd and Downie, 2008). These responses can occur independently, as was shown by the phenotype of different mutants, in which bacterial infection, and related epidermal responses, is observed in the absence of nodule formation and vice versa (Gleason et al, 2006;Tirichine et al, 2006aTirichine et al, , 2006bTirichine et al, , 2007Murray et al, 2007). After the initial chemical communication, the physical association begins with the attachment of the bacteria to the surface of the root through plant lectins and polysaccharides present on the surface of the bacteria (Smit et al, 1992;Diaz et al, 1995).…”