Field Crop 2024, Vol.7, No.2, 70-78 http://cropscipublisher.com/index.php/fc 72 2.2 Production of plant hormones PGPMs are known to produce various plant hormones that promote growth. For example, Azospirillum amazonense produces auxins, which are crucial for root development and overall plant growth. This bacterium has been shown to increase grain dry matter accumulation and the number of panicles in rice plants, primarily through the production of phytohormones. Similarly, other PGPRs produce indole-3-acetic acid (IAA) and other growth-promoting substances that enhance plant growth under stress conditions (Desoky et al., 2020). 2.3 Biological nitrogen fixation Biological nitrogen fixation (BNF) is a critical mechanism by which PGPMs contribute to plant growth. Azospirillum amazonense, for instance, has been shown to fix atmospheric nitrogen, thereby providing an essential nutrient for rice plants. The BNF contribution of A. amazonense was measured using the 15N isotope dilution technique, revealing significant nitrogen accumulation in rice grains (Rodrigues et al., 2008). This process not only reduces the need for chemical nitrogen fertilizers but also promotes sustainable agricultural practices. 2.4 Disease suppression PGPMs also play a vital role in suppressing plant diseases, thereby enhancing plant health and yield. Certain PGPRs exhibit antagonistic activity against plant pathogens such as Pyricularia oryzae, the causative agent of rice blast disease. These beneficial microbes produce antimicrobial compounds and lytic enzymes that inhibit the growth of pathogenic species, thereby protecting rice plants from infections (Meena et al., 2017). The mutual interactions between beneficial fungi and pathogenic microbes further contribute to disease suppression and improved plant health. 2.5 Enhancement of stress tolerance PGPMs enhance the stress tolerance of rice plants by improving their antioxidant defense systems and reducing oxidative stress. For example, PGPRs such as Bacillus cereus and Pseudomonas aeruginosa have been shown to alleviate salinity stress in wheat plants by enhancing antioxidant activities and reducing oxidative stress biomarkers (Desoky et al., 2020). These mechanisms are likely to be effective in rice as well, helping the plants to withstand various abiotic stresses such as salinity and drought. Additionally, PGPRs improve the physiological attributes of rice plants, including photosynthetic efficiency and water content, thereby enhancing their overall stress tolerance (Desoky et al., 2020). By leveraging these mechanisms, PGPMs offer a sustainable and eco-friendly approach to enhancing rice productivity and resilience against various biotic and abiotic stresses. 3 PGPM and rice productivity 3.1 Growth promotion Plant growth-promoting microorganisms (PGPM) have shown significant potential in enhancing the growth of rice plants. Studies have demonstrated that the application of PGPM, such as Pantoea ananatis and Piriformospora indica, can lead to substantial increases in various growth parameters. For instance, the tiller number per hill, leaf area index, and biomass dry weight were observed to increase by 9.0%~27.2%, 11.7%~45.4%, and 11.1%~24.7%, respectively, when compared to control treatments (Bakhshandeh et al., 2017). Additionally, co-inoculation of Bacillus velezensis and Brevundimonas diminuta has been found to significantly promote rice growth by enhancing the complexity of the microbial network in the rhizosphere, which in turn improves nitrogen absorption and overall plant growth (Figure 1) (Wang et al., 2023). In the experiment conducted by Wang et al. (2023), rice plants were subjected to different microbial treatments. A visual comparison of the rice plants after treatment showed significant differences in plant vigor and biomass among the four groups. The co-inoculation with B. velezensis FH-1 and B. diminuta NYM-3(FN) demonstrated the best results, significantly promoting the growth and nutrient absorption of the rice. This finding suggests that microbial consortia may be more effective in agricultural applications, potentially leading to better crop yields and sustainability. The study highlights the potential benefits of using specific microbial combinations to enhance plant growth and nutrient efficiency.
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