Molecular Microbiology Research 2024, Vol.14, No.3, 153-161 http://microbescipublisher.com/index.php/mmr 154 2 Overview of Non-Rhizobial Endophytic Microbes 2.1 Definition and classification Non-rhizobial endophytic microbes are bacteria that reside within plant tissues without causing any apparent harm to their host. These microbes are distinct from rhizobial bacteria, which are typically associated with root nodules in legumes. Non-rhizobial endophytes can be found in various plant parts, including roots, stems, and leaves, and they often contribute to plant growth and health by promoting nutrient uptake, enhancing stress resistance, and protecting against pathogens (Saha et al., 2016; Lata et al., 2018). 2.2 Differences from rhizobial endophytes While rhizobial endophytes are primarily known for their ability to fix nitrogen in symbiotic relationships with leguminous plants, non-rhizobial endophytes exhibit a broader range of plant growth-promoting traits. Non-rhizobial endophytes can solubilize phosphate, produce siderophores, and synthesize plant hormones such as indole-3-acetic acid (IAA). They also exhibit antifungal activities and can enhance the plant's defense mechanisms against pathogens (Verma et al., 2017; Hernández et al., 2021). Unlike rhizobial endophytes, which are often limited to leguminous plants, non-rhizobial endophytes can colonize a wide variety of plant species, including rice (Hardoim et al., 2012; Jha et al., 2019). 2.3 Common non-rhizobial endophytes in rice Several non-rhizobial endophytes have been identified in rice plants. Pseudomonas is known for its plant growth-promoting properties, including nitrogen fixation and phosphate solubilization (Sahu et al., 2020). Bacillus is effective in promoting plant growth and inducing systemic resistance against pathogens such as Rhizoctonia solani. Azotobacter is renowned for its nitrogen-fixing ability and has been shown to increase rice yields under both greenhouse and field conditions. Sphingomonas is found in rice seeds and can promote root and shoot growth while protecting seedlings from soil pathogens. Enterobacter is known for producing IAA (Indole-3-acetic acid) and solubilizing phosphate, contributing to plant growth promotion (Zhang et al., 2022). These non-rhizobial endophytes play a crucial role in enhancing the growth and health of rice plants by improving nutrient uptake, promoting stress resistance, and protecting against diseases. 3 Mechanisms of Action 3.1 Nutrient uptake enhancement 3.1.1 Nitrogen fixation by non-rhizobial endophytes Non-rhizobial endophytic microbes play a significant role in enhancing nitrogen uptake in rice plants. For instance, a consortium of endophytic microbes isolated from Typha angustifolia has been shown to improve nitrogen metabolism in rice. These endophytes, predominantly nitrogen-fixing, significantly increased biomass, shoot length, and chlorophyll content in rice plants under both nitrogen-sufficient and nitrogen-deficient conditions. The upregulation of nitrogen uptake and assimilation genes in treated plants suggests that horizontal gene transfer of the dinitrogen reductase gene within the consortium is a key mechanism for nitrogen fixation (Kakar et al., 2016). Additionally, strains such as Bacillus siamensis and Priestia megateriumhave demonstrated biological nitrogen fixation capacity, contributing to increased agronomic parameters in rice (Rios-Ruiz et al., 2023). Furthermore, Azotobacter sp. strain Avi2 has been shown to enhance nitrogen fixation, leading to better vegetative and reproductive growth in rice. 3.1.2 Phosphorus solubilization Phosphorus solubilization is another critical mechanism by which non-rhizobial endophytes enhance nutrient uptake in rice. For example, Bacillus siamensis TUR07-02b has been identified for its ability to solubilize phosphate-Ca, which is crucial for improving phosphorus availability to rice plants (Figure 1) (Rios-Ruiz et al., 2023). Endophytic bacteria isolated from rice roots have shown significant phosphate solubilizing capabilities, with higher diversity and population density in root tissues compared to leaf tissues (Shofiyah et al., 2023). The endophytic fungus Phomopsis liquidambari also plays a role in optimizing phosphorus activation in paddy soil, thereby enhancing nutrient turnover and availability (Tang et al., 2019).
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