Research interest

Nitrogen-fixing root nodule development in Rhizobium-legume symbiosis

Symbiosis between Rhizobium soil bacteria and legume plants leads to the development of root nodules where endosymbiotic Rhizobium bacteria, known as bacteroids, acquire the ability to fix nitrogen - converting atmospheric nitrogen into ammonia via the nitrogenase enzyme.

Each plant has specific Rhizobium partners, and each Rhizobium has its plant partners. This partnership is achieved through consecutive and reciprocal molecular communication. Initially, plant flavonoids induce the production of Nod factors in their Rhizobium partners, which initiate nodule organogenesis and enable Rhizobium infection through infection threads formed in growing root hair cells from which rhizobia are released in the nodule cells as an organelle-like structure, called symbiosomes. The fate of endosymbiotic rhizobia depends on the host plant, which can be reversible, as in soybean or the model legume Lotus japonicus, or irreversible and terminal, as in the model legume Medicago truncatula or pea, vetch, and all Inverted Repeat Lacking Clade (IRLC) legumes converting rhizobia to polyploid, uncultivable large bacteroids, with increased membrane permeablity and altered physiology (Mergaert et al., 2006).

In the indeterminate nodules of M. truncatula, cell proliferation persists in the nodule meristem, leading to constant nodule growth and differentiation along its length, resulting in distinct nodule zones (ZI, ZII, IZ, ZIII) where all stages of symbiotic cell development are sequentially present from the youngest to fully differentiated stages (Figure 1); ZI: meristem, ZII: infection zone where symbiosomes proliferate in the host cell, IZ: differentiation zone where bacteroids undergo extreme morphological and physiological alterations and ZIII: nitrogen fixing zone (Kondorosi et al., 2013). Differentiation of bacteroids is coordinated with development of the symbiotic nodule cells which undergo repeated endoreduplication cycles increasing the cell volume with duplication of the genome from 2C/4C to 32C/64C (C: haploid DNA content) and (Cebolla et al., 1999, Vinardell et al., 2003, Nagymihály et al., 2017).

We have demonstrated that terminal differentiation of bacteroids is host-controlled and the plant effectors are secreted symbiotic plant peptides (Mergaert et al., 2006; Van de Velde et al., 2010). Most of these peptides belong to the nodule-specific cysteine-rich (NCR) family (Mergaert et al., 2003; Silverstein et al., 2006) while the others are nodule-specific glycine-rich peptides (nodGRP) (Kevei et al., 2002, Alunni et al., 2007) (Figure 2). These peptides have only evolved in IRLC legumes where bacteroids undergo terminal differentiation and only expressed in the symbiotic cells in successive series (Montiel et al., 2017).

NCR peptides share a homologous signal peptide sequence and have a conserved pattern of 4 or 6 cysteine residues within highly divergent 30-50 amino acid long mature peptides (Mergaert et al., 2003; Montiel et al., 2017; Lima et al., 2020) that are directed to bacteroids through the secretory pathway (Van de Velde et al., 2010). In M. truncatula nodules, over 700 NCR peptides are expressed in symbiotic cells at various developmental stages (Mergaert et al., 2003; Maunoury et al., 2010; Limpens et al., 2013; Nallu et al., 2013; Guefrachi et al., 2014; Roux et al., 2014; Montiel et al., 2017). These NCRs exhibit distinct physicochemical properties (pI: 3.5-11.2, net charge: -11 to +9), which suggest a wide range of biological activities (Lima et al., 2022). The function of individual NCRs remains largely unexplored, with only a few investigated since their discovery (Roy et al., 2020). These NCRs have been shown to interact with the bacterial membranes and in the cytosol with various bacterial proteins and DNA, influencing transcription and translation, and altering ribosome composition (Farkas et al., 2014; Tiricz et al., 2013; Penterman et al., 2014). Hundreds of NCRs are expressed in the interzone (IZ), with several involved in arresting bacterial cell division (Tiricz et al., 2013; Farkas et al., 2014; Shabab et al., 2016; Lima et al., 2020), facilitating bacteroid maturation (Horváth et al., 2023, 2015), and maintaining the viability of symbiosomes within the host cell (Kim et al., 2015; Gao et al., 2023; Zhang et al., 2023).

The nodGRPs are also expressed in successive waves in the early, middle or late stages of the symbiotic cell development (Kevei et al., 2002; Alunni et al., 2007; Roux et al., 2014; Marx et al., 2016). NodGRPs contain a signal peptide and their mature sequence averages about 120 amino acids in length. NodGRPs were also identified in the proteome of bacteroids (Dürgő et al., 2015). In M. truncatula 38 nodGRP genes exist. While their functions remain unresolved, they likely play an integral and so far, unknown role in the molecular interactions between the plant and the bacterium.

 

References

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