## 3 Microbial Interactions in Plant Microbiomes
### The Rhizosphere: A Hotspot for Microbial Activity with the Plant  
The rhizosphere is the zone around plant roots where complex microbial communities inhabit the rhizplane and soil particles because of the nutritional values of root-released metabolites [7]. The rhizosphere is enriched with organic materials of high and low molecular weight from the plant and products from microbes and other fauna in this space (e.g., nematode and larval waste products). Certain compounds in root exudates, such as sugars, amino acids, and organic acids, serve not only as chemoattract that draw soil microbes to the root but also support microbiome formation [24]. The rhizosphere microbial density, up to \(10^{12}\) cells/g soil, and diversity, up to 30,000 distinct prokaryote species, are orders of magnitude higher than the surrounding bulk soil [25; 26]. Within the rhizosphere, microbes intensively compete for space, water, and nutrition. Plants may allocate up to 40% of their photosynthetically derived carbon as exudates into the rhizosphere, consisting primarily of various compounds such as amino acids, complex sugars, and organic acids [27]. The increased availability of nutrients through the dissolution of minerals in the soil promotes the growth of both microbes and the plant host [28; 29; 30; 31]. Plant growth may also be enhanced by microbial volatiles such as butanediol [28; 32; 33]. The production of auxins and cytokines or the control of ethylene levels can further dictate plant morphology and growth [34].  
The rhizosphere's chemical variety includes factors that regulate the morphology and function of both the plant and the microbes. Cell signaling factors that govern biofilm formation for the microbes are important. This vital structure offers high survival value to several stresses. Rhizosphere communication can be classified into three major types: microbe-to-microbe, plant-to-microbe, and microbe-to-plant signaling.  
Microbe-to-Microbe Signaling: Microorganisms communicate via quorum sensing (QS), using autoinducers like N-acyl homoserine lactones (AHLs) in Gram-negative bacteria and peptides in Gram-positive species [35]. QS regulates biofilm formation, chemotaxis, and virulence, facilitating coordinated microbial behavior [36]. Additionally, volatile organic compounds (VOCs), like alkanes, ketones, alkene, terpenoids, and sulfurs [37], and inorganic compounds (VICs), like nitric oxide (NO), hydrogen sulfide (H\({}_{2}\)S), ammonia, hydrogen cyanide (HCN), and carbon dioxide (CO\({}_{2}\)) [38], mediate long-distance interactions. However, their perception by microbial cells remains poorly understood [39]. Other signaling molecules, such as trehalose and thiamine, play roles in microbial symbioses, promoting survival in nutrient-limited environments [40].  
Plant-to-Microbe Signaling: Plants release a diverse array of root exudates, including flavonoids, strigolactones, and organic acids, which shape microbial communities and facilitate beneficial interactions [41]. For instance, strigolactones stimulate mycorrhizal colonization under nutrient deficiency [42], while flavonoids in legume exudates promote rhizbial infection and nitrogen fixation [43]. These signaling compounds also influence nutrient cycling, promoting phosphorus solubilization and microbial chemotaxis.  
Microbe-to-Plant Signaling: Rhizosphere microbes impact plant development, immunity, and stress responses through microbe-associated molecular patterns (MAMPs), QS molecules, and phytohormone production [44; 45]. MAMPs, such as lipopolysaccharidesaccharides, peptidoglycans, flagellin, and chitin, stimulate the systemic development of tolerance to abiotic and biotic stresses in a plant [46]. Beneficial microbes induce systemic resistance (ISR) and modulate plant hormonal balance, enhancing biotic and abiotic stress tolerance. Additionally, microbial VOCs can promote plant growth and nutrient uptake [47], though the underlying mechanisms require further study.