Varietas Delectat: Exploring Natural Variations in Nitrogen-Fixing Symbiosis Research

Abstract

The nitrogen-fixing symbiosis between leguminous plants and soil bacteria collectively called rhizobia plays an important role in the global nitrogen cycle and is an essential component of sustainable agriculture. Genetic determinants directing the development and functioning of the interaction have been identified with the help of a very limited number of model plants and bacterial strains. Most of the information obtained from the study of model systems could be validated on crop plants and their partners. The investigation of soybean cultivars and different rhizobia, however, has revealed the existence of ineffective interactions between otherwise effective partners that resemble gene-for-gene interactions described for pathogenic systems. Since then, incompatible interactions between natural isolates of model plants, called ecotypes, and different bacterial partner strains have been reported. Moreover, diverse phenotypes of both bacterial mutants on different host plants and plant mutants with different bacterial strains have been described. Identification of the genetic factors behind the phenotypic differences did already and will reveal novel functions of known genes/proteins, the role of certain proteins in some interactions, and the fine regulation of the steps during nodule development.

Keywords: legume ecotypes/cultivars; partner dependent mutation manifestation; rhizobium strains; symbiotic incompatibility; symbiotic nitrogen fixation.

FIGURE 1

Natural variations affecting early nodule development and infection. (A) Nodule formation and intercellular infection in the absence of Nod factors and NF recognition/perception requires either an unknown mechanism or the activity of Type III secretion systems for the translocation of effector proteins in bradyrhizobia as well as the common symbiosis signaling pathway and other unknown proteins in Aeschynomene species and in soybean plants. (B) Nod Factor production of rhizobia and NF recognition/perception are required for the early symbiotic events such as root hair curling. (C) Initiation and progression of bacterial invasion through ITs are strictly controlled. The structure and amount of NFs determine whether NF receptors allow the initiation of IT development. Similarly, the presence of surface polysaccharides with correct structure is checked by the plants with help of receptors and unknown mechanisms. Unknown mechanisms and stress from the plants also might inhibit infection unless bacteria can deal with them, for example, by expressing the TypA stress protein. Bacterial effector molecules (Nops) resembling pathogen virulence factors transported into the plant cells and Effector Triggered Immunity often led to the restriction of infection, however, there are cases, when they have a positive effect on nodulation. Those bacterial macromolecules, whose lack or incorrect structure led to the arrest of the interaction with certain partners, and those plant proteins, which restrict the bacterial mutants or whose lack can be overturned, are shown in red. Plant molecules, as well as bacterial proteins/molecules, whose lack or presence (in parenthesis), that are able to overturn these defects, are shown in green. Those bacterial and corresponding plant proteins, that are responsible for incompatibility and a defect in either of them leads to compatibility, are shown in blue. If the compatibility needs both the plant and bacterial factors, the proteins are shown in brown. (??) denotes that the given factor has not been identified yet.

FIGURE 2

Natural variations affecting bacteroid development, persistence, and functioning. After the release of rhizobia from the ITs, bacteria differentiate into nitrogen-fixing bacteroids of unmodified (U), elongated (E), or spherical (S) morphotypes. The NCR peptides affecting bacterial membrane and intracellular functions are delivered into bacteria by peptide transporters and contribute to the terminal differentiation of E and U morphotype bacteroids. Bacterial outer membrane composition, peptidases and independently evolved transporters affect the success of bacteroid development. The NCR peptides are also involved in strain discrimination. The level of stress and the available carbon sources in the plants affect how bacteria with mutations in stress related genes or carbon metabolism perform. Bacteria also might induce and endure plant defense reactions and sense signals from certain plants that affect gene expression. Colors are the same as in Figure 1.