## 3 Microbial Interactions in Plant Microbiomes
### Key Roles of Beneficial Microbes in Maintaining Plant Health  
Plant-associated microorganisms influence plant growth in various ways and can be categorized as beneficial, deleterious, or neutral based on their interactions with the host [48]. Among these, plant growth-promoting microorganisms (PGPMs) play a crucial role in enhancing plant health and development [49; 50; 51; 52] (Figure 1). The majority of beneficial microbes are bacteria, commonly referred to as plant growth-promoting rhizbacteria (PGPR), which colonize plant roots and support growth through various direct and indirect mechanisms [53; 54]. First introduced by [PERSON] et al. (1978) [55], PGPR includes free-living, symbiotic, and endophytic bacteria that facilitate plant growth. These microbes can be further classified into symbiotic PGPR, such as _Rhizobium_ and _Franka_, which establish mutualistic associations with plants, and free-living PGPR, like _Pseudomonas_ and _Bacillus_, which inhabit the rhizosphere without forming specialized structures [56]. In addition to PGPR, other beneficial microbes, including arbuscular mycorrhizal fungi (AMF) and rhizobia, contribute to plant health by improving nutrient uptake and stress tol erance [57; 58], with AMF soil amendments commercially available. These microbes interact in various ways, ranging from antagonism to mutualism, with some species exhibiting positive co-occurrence while others display negative co-occurrence patterns [59; 60].  
PGPR promote plant growth through direct mechanisms, including nitrogen fixation, phosphate solubilization, and the production of phytohormones such as auxins, gibberellins, and cytokins [61; 62; 63; 64]. Some PGPR enhance nutrient availability by secreting siderophores that chelate iron, making it more accessible to plants [65; 66]. Additionally, these beneficial microbes modulate root architecture, enhance water uptake, and help plants cope with abiotic stress factors like salinity, drought, and heavy metals by producing osmoprotectants and antioxidant enzymes [58]. Well-documented plant-beneficial rhizosphere microbiomes include rhizobia symbiosis, which provides nitrogen, and mycorrhizal associations, which facilitate phosphorus acquisition [67; 68]. Several biotic and abiotic factors, such as temperature, soil moisture, pH, root exudate composition, and mineral concentrations, significantly influence the interaction between rhizobacteria and plant roots, thereby affecting colonization efficiency [69].  
Apart from direct benefits, PGPR and other beneficial microbes contribute to plant health through indirect mechanisms by suppressing pathogens and inducing plant defense  
Figure 1: Contrasting plant health outcomes in the presence and absence of beneficial microbes in response to biotic and abiotic stressors. PGPMs—plant growth-promoting microorganisms, PGPR—plant growth-promoting rhizobacteria, AMF—arbuscular mycorrhizal fungi, and ISR—induced systemic resistance (the image of the plant was generated using AI).  
responses [70, 71]. These microbes compete with phytopathogens for nutrients and space, produce antimicrobial compounds, and activate ISR in plants. Some PGPR synthesize lipopeptides and antibiotics that inhibit pathogen growth or disrupt quorum-sensing signals essential for virulence [46]. Additionally, PGPR volatile organic compounds (VOCs) enhance plant immunity and promote systemic resistance [72]. The colonization process of PGPR follows four key steps: (i) chemotactic signal recognition, (ii) attachment to the root surface, (iii) evasion of plant immune defenses, and (iv) biofilm formation on the root surface [73]. These microbes can also establish biofilm-like structures composed of multibacterial communities in the rhizosphere, further strengthening plant-microbe interactions and enhancing plant resilience and productivity.