Rhizobial-Host Interactions and Symbiotic Nitrogen Fixation in Legume Crops Toward Agriculture Sustainability

Abstract

Symbiotic nitrogen fixation (SNF) process makes legume crops self-sufficient in nitrogen (N) in sharp contrast to cereal crops that require an external input by N-fertilizers. Since the latter process in cereal crops results in a huge quantity of greenhouse gas emission, the legume production systems are considered efficient and important for sustainable agriculture and climate preservation. Despite benefits of SNF, and the fact that chemical N-fertilizers cause N-pollution of the ecosystems, the focus on improving SNF efficiency in legumes did not become a breeder's priority. The size and stability of heritable effects under different environment conditions weigh significantly on any trait useful in breeding strategies. Here we review the challenges and progress made toward decoding the heritable components of SNF, which is considerably more complex than other crop allelic traits since the process involves genetic elements of both the host and the symbiotic rhizobial species. SNF-efficient rhizobial species designed based on the genetics of the host and its symbiotic partner face the test of a unique microbiome for its success and productivity. The progress made thus far in commercial legume crops with relevance to the dynamics of host-rhizobia interaction, environmental impact on rhizobial performance challenges, and what collectively determines the SNF efficiency under field conditions are also reviewed here.

Keywords: BNF; QTLs; SNF; environmental factors; host-specificity; rhizobia; symbiosis.

FIGURE 1

Comparison of carbon footprint generated by per unit serving of different food types. The areas in the triangles are approximate to carbon footprint values (http://css.umich.edu/factsheets/carbon-footprint-factsheet).

FIGURE 2

Genetic diversity of rhizobia associated with different crops for biological nitrogen fixation. (A) Common bean. (B) Soybean. (C) Chickpea. (D) Pea. The phylogenetic trees of 16S ribosomal RNA of the rhizobial species known to nodulate the indicated crop species were constructed in MEGA X (Kumar et al., 2018) using the neighbor-joining method. Caballeronia zhejiangensis HE983367 displayed in red font in A, C, and D was used as a reference sequence in the analysis; this rhizobial species does not forge symbiosis with these crops.

FIGURE 3

Prominent factors affecting the symbiosis and efficiency of biological nitrogen fixation.