Review on Small RNA in Plant-Microbe interaction

Leaf imunity is also activated in response to root interaction with incompatible nitrogen fixing bacteria

Glycerol-3-phosphate mediates rhizobia-induced systemic signaling in soybean

The interaction with soild bacteria is not only limited to plant roots. Foliar immunity system is also involved. Compond synthesized in leaves is transport to root

Glycerol-3-phosphate (G3P) is a well-known mobile regulator of systemic acquired resistance (SAR)

G3P is needed for stran-scpecific exlcusion of non-desirable bacteria, and is associated with foliar pathogen imunity.

  • Some rhizobia inject effector by T3SS to induce nodulation in soybean (Okazaki et al, 2013), but the T3SS-dependent pathway is also genotype-specific. The exclusion of incompatible symbiotic bateria might be epalined by the G3P paper.

T3SS will complement the NF-NFR symbiosis pathway, Soybean mutated on nfr genes can still form nodules using T3SS system, but t3ss nfr mutant won’t form nodules Hijacking of leguminous nodulation signaling by the rhizobial type III secretion system

Nod genes is not necessary for legume symbiosis in some rhizobia

Legumes Symbioses: Absence of Nod Genes in Photosynthetic Bradyrhizobia

In two symbiotic, photosynthetic, Bradyrhizobium strains, BTAi1 and ORS278, canonical nodABC genes and typical lipochitooligosaccharidic Nod factors are not required for symbiosis in some legumes.

There is a possible alternative pathway, where a purine derivative may play a key role in triggering nodule formation

Is Nod Factors also required in intercellular invasion (including crack entry) symbiosis?

In A. indica and A. sensitiva NFs are neither required for invasion of the roots nor for nodule primordia formation (Giraud et al., 2007).

bradyrhizobia can nodulate Aeschynomene plants in two ways: one that depends on a functional T3SS and other NFs, and T3SS independent, which relies on a still unknown mechanism.(Okazaki et al., 2015)

Sesbania rostrata, intercellular invasion of the roots depends on NFs to induce cell death for infection pocket formation (Capoen et al., 2010). The colonization of the outer cortex depends on NFs but would be independent of calcium spiking. There is a less stringent requirement for NF structures for inducing and infecting the nodule primordia.

NFs are not necessary for invasion of Arachis hypogaea roots (Ibañez and Fabra, 2011).

peanut nodulation can also be achieved by rhizobia lacking nod genes, but the nodulation seems to be vestigial and non-significant in natural conditions.

Whether NF-independent nodulation of peanut depends on a functional rhizobial T3SS is still not known. And those determinants (‘entry key’) allowing NF-independent nodulation have not been identified.

  • These results indicate that peanut have have NF-dependent and -independent systems.

    In the NF- dependent program, these bacterial molecules are required for initiating the cellular divisions for nodule primordia development

  • For NF dependent system, there are two possible models for NF preception:
    1. NFs are required for intracellular infection of the basal cortical cell that forms the nodule primordia
    2. rhizobia can become intracellular without NFs but that these NFs are required in order to reinitiate meristematic activity of the infected cell.

NFR genes (NFR1 NFR5) in peanut have been identified (Ibáñez et al., 2015). But their epidermic or cortial localization has not been determined.

  • In our study, the NFR5 CRISPR knock out lead to non-nodulating phenotype (but not for NFR1), suggesting the NFR may play a lead function.

an appropriate bacterial cell surface (rhizobial exopolysaccharides, EPS) is important for peanut intercellular infection.

Peanut nodulation symbiosis

Root hair is not required for peanut nodulation (Maku et al., 2018)

Bradyrhizobia invaded the roots at epidermis, protoplasm of cortical cell, and cortical cells of the main root near the newly emerged lateral root in the absence of RoH.