Symbiotic hemoglobin (Hb) proteins and genes have been isolated and characterized from parasponia (Appleby et al., 1983; Landsmann et al., 1986). Parasponia andersonii has been shown to have a single Hb gene expressed in both nodules and non-nodular roots, suggesting that symbiotic and non-symbiotic roles result from a single gene (Landsmann et al., 1986, 1988; Bogusz et al., 1988). Its oxygen-binding properties and cellular location in young rhizobial-infected cells coincide with a role in oxygen transport to rhizobia in root nodules (Wittenberg et al., 1986; Gibson et al., 1989; Trinick et al., 1989). In addition, a comparative analysis of the promoter`s activity in transgenic casuarina revealed that, unlike the lack of preservation of P-specific cell expression. andersonii hemoglobin promoter in transgenic legume nodes (Bogusz et al., 1990) it maintains its specific expression in bacteria-infected cells of actinorhiza nodules (Franche et al., 1998). As mentioned above, it is interesting to note that the nodules of P. andersonii and C. glauca have the same origin and structure as the lateral roots. This suggests that in Rosales, beyond a similar nodule structure and ontogenesis, there is a non-legume symbiotic keygene that has identical regulatory mechanisms (Franche et al., 1998). In addition to legume-rhizobium and actinorhiza symbioses, parasponia is a key species for studies on the absorption of symbiotic bacteria in plant cells. A comparative analysis of these three symbiotic systems should help define strategies for the transfer of nitrogen fixation capacity to non-legumes.
Non-legumes are plants of other plant families, with the exception of the legume family. Simply put, non-legumes are not legumes. Similar to legumes, some non-legumes carry nodules that contain nitrogen-fixing bacteria. Nitrogen-fixing bacteria come from the genus Frankia. These are actinomycetes. These plants can also fix nitrogen. Several examples of non-nitrogen-fixing plants from legumes are alders and shrubs (Alnus sp.), laurel and sweet storm (Myrica sp.) and sweet fern (Comptonia peregrina). In addition, they have arbuscular mycorrhizal fungi that live in symbiosis with them. However, unlike legumes, phosphorus requirements are lower for non-legumes. In addition, they contain less nitrogen than legumes.
Understanding the molecular mechanism of BNF outside of legume-rhizobia symbiosis could have important agronomic effects and reduce or even avoid the use of nitrogen fertilizers. In the short term, a better understanding could lead to a more sustainable use of the biodiversity of nitrogen-fixing organisms and, in the longer term, to the transfer of endosymbiotic nitrogen fixation capacities to important non-legumes. Legumes belong to the family Fabaceae or Leguminosae. They bear a dehiscendant fruit called a pod or legume. Non-legumes come from other plant families. They produce different types of fruit. Legumes and non-legumes have root nodules. They contain nitrogen-fixing bacteria. Nitrogen-fixing bacteria in legumes belong to the genus Rhizobium. In contrast, the nitrogen-fixing bacteria in non-legumes belong to the genus Franks.
In addition to the sequences involved in the Frankia signaling pathway, the Ag12 and Cg12 genes, which encode for subtilisin-like serine proteases in A. glutinosa and C. glauca respectively, have also been studied (Ribeiro et al., 1995; Laplaze et al., 2000b). Subtilases are a superfamily of proteases that are thought to play a role in various aspects of plant development, including lateral root initiation and pathogen responses. Using transgenic Casuarinaceae, which contain fusions of Cg12 promoter reporter genes, Cg12 expression has been shown to begin very early in the symbiotic process and is specifically induced in Frankia-infected cells (Svistoonoff et al., 2003). As Cg12 reporter gene fuses in legume M. truncatula, a similar pattern of expression has been observed during the knotting process with Mesorhizobium meliloti (Svistoonoff et al., 2004). Preservation of the expression profile in M. truncatula suggests that a Nod factor-independent signaling pathway conserved between the two systems is specifically activated in cells infected with symbiotic bacteria. The mechanisms of intracellular infection in legumes and actinorhisal plants have been further investigated by introducing the promoter Enod11 of M.
truncatula into C. glauca (Svistoonoff et al., 2010). In M. Truncatula, the expression of the mtEnod11 gene, has been shown to correlate with pre-infection and infection events during knotweed formation (Journet et al., 2001). At C. Glauca, activation of the ProMtEnod11::gus reporter has been shown to correlate with Frankia infection in root hairs, prenuplets and nodules. These results suggest a high maintenance of regulatory pathways between legumes and actinorhizal plants in cells involved in bacterial infection and hosting. However, ProMtEnod11 is not activated in transgenic casuarina prior to infection when frankia signals are perceived, suggesting that the pre-infectious stage differs between actinorhofizal and legume-rhizobium symbioses (Svistoonoff et al., 2010). Legumes or Fabaceae are a family of flowering plants (legumes). In fact, it is the third largest family of flowering plants. It is also known by the name of the pea family or the legume family.
There are more than 18,000 species in this family. This family of plants is characterized by its compound leaves, which are feathered, and a typical fruit called legume or pod. Most legumes are dehiscent fruits. These dried fruits divide along two seams to release seeds into the environment. The creation of artificial symbioses or associations between nitrogen-fixing microorganisms and plants of great agricultural importance is a main objective in agriculture to reduce the need for chemical nitrogen fertilizers. Since much of the groundwork and important breakthroughs have been made on model legumes, strategies to increase genetic capacity to fix nitrogen in symbiosis are currently based on this knowledge (Charpentier and Oldroyd, 2010; Beatty & Gut, 2011). Recent advances in understanding endosymbiotic and endophytic nitrogen fixation with non-leguminous plants could represent new, original and alternative ways to develop nitrogen-fixing plants without legumes. Legumes are consumed and used to make food all over the world. Some of the most popular legumes are chickpeas (also known as garbanzo beans), which are a staple of Mediterranean and Middle Eastern cuisine. Hummus is made from chickpeas.
One of the most versatile legumes is soybeans, which are used to make tofu, soy sauce, vegetable oil and other products marketed as “vegetables.” Like soybeans, black-eyed peas are eaten by humans and used to feed livestock (hence their other name, cow beans) and planted to improve the soil. As with most legume-rhizobia symbiosis, parasponia-rhizobium symbiosis depends on nodding factors. Rhizobial nodding factors are lipochitooligosaccharides consisting of an acylated chitinoligomeric spine with different substitutions of functional groups at the level of terminal or non-terminal residues. These nodding factors are important symbiotic signals and are indispensable in the specific host-rhizobial interaction and in later stages of the infection process and tuberous organogenesis (Oldroyd and Downie, 2008). Recently, it has been shown that a single gene closely related to proteins in the lysine motif domain (LysM), which are involved in the perception of nod factor in legumes, is necessary for knötulation and mycorrhization in P. andersonii (Op den Camp et al., 2011). It has also been shown that the common “SYM” signaling pathway described for AM, legume-rhizobium and actinorhiza symbioses is activated during organogenesis of the P. andersonii tuber. Taken together, these data support the hypothesis of a common genetic ancestor of the node with a genetic predisposition to knoblation (Soltis et al., 1995). Although considerable information is available on the role of certain flavonoids in rhizobium-legume symbiosis, their role is not known during the different stages of parasponia nodule formation.
As this is a nods-dependent symbiotic interaction, it is likely that the role of flavonoids produced by parasponics is similar to that of legume nodulation.
