Molecular Soil Biology 2024, Vol.15, No.1, 8-16 http://bioscipublisher.com/index.php/msb 9 directions. By systematically reviewing the current knowledge on PGPR and their interactions with crop roots, this paper aims to provide a comprehensive understanding of their role in sustainable agriculture and identify areas for future research and development. 1 Molecular Mechanisms of PGPR-Root Interactions 1.1 Signal molecules and receptors Root exudates play a crucial role in attracting plant growth-promoting rhizobacteria (PGPR) to the rhizosphere. These exudates consist of a variety of organic compounds, including sugars, amino acids, and secondary metabolites, which serve as chemical signals to beneficial microbes. For instance, the study by (Yi et al., 2018) highlights the importance of root exudates in modulating the transcriptomic response of Bacillus mycoides to potato root exudates, indicating that these exudates are pivotal in establishing beneficial plant-microbe interactions. Similarly (Palma et al., 2020), discusses how root exudates influence the expression of bacterial genes involved in microbe-plant interactions, further emphasizing their role in attracting and selecting specific microbial populations. Specific signal molecules such as flavonoids and strigolactones are key players in the communication between plants and PGPR. Flavonoids, for example, have been shown to be involved in the biosynthetic pathways that regulate plant-microbe interactions (Thomas et al., 2019). These molecules not only attract beneficial bacteria but also modulate their behavior to enhance colonization and symbiosis. Strigolactones, another class of signal molecules, are known to play a role in the establishment of symbiotic relationships with mycorrhizal fungi and potentially with PGPR as well (Baysal and Silme, 2019). PGPR possess specific receptors that recognize and respond to plant-derived signals. These receptors enable the bacteria to detect and move towards the root exudates, facilitating colonization. The study by (Wheatley and Poole, 2018) reviews the molecular mechanisms governing bacterial attachment to roots, highlighting the role of specific receptors in recognizing plant signals. Additionally (Mark et al., 2005), identifies genes in Pseudomonas aeruginosa that are regulated in response to root exudates, suggesting the presence of specialized receptors that mediate these interactions. 1.2 Colonization and biofilm formation The initial attachment of PGPR to root surfaces is a critical step in the colonization process. This attachment is often mediated by bacterial surface structures such as pili and flagella, which facilitate close contact with the root epidermis (Wheatley and Poole, 2018). The biphasic mechanism of root attachment, as described in (Wheatley and Poole, 2018), involves an initial reversible phase followed by a more stable, irreversible attachment, ensuring effective colonization. Biofilm formation is a significant aspect of PGPR colonization, providing a protective environment for the bacteria and enhancing their ability to persist in the rhizosphere. Biofilms facilitate nutrient exchange and protect the bacteria from environmental stresses and antimicrobial compounds. The study by (Yi et al., 2018) underscores the importance of biofilm formation in the context of plant-microbe interactions, as it allows for sustained colonization and interaction with the host plant. Several factors influence the effectiveness of PGPR colonization, including the composition of root exudates, soil conditions, and the presence of other microbial communities. For instance (Somenahally, 2017), discusses how soil conditions such as moisture stress and pH can impact root-microbe interactions, thereby affecting colonization efficiency. Additionally, the presence of deleterious microorganisms in the rhizosphere can compete with PGPR for resources, influencing their colonization success (Schippers et al., 1987). 1.3 Modulation of root architecture PGPR can significantly influence root development and architecture, promoting root growth and branching. This modulation is often mediated by the production of plant hormones and other growth-promoting substances. For example (Thomas et al., 2019), identifies differentially expressed genes in rice roots during interactions with
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