The rhizosphere, the soil in contact with plant roots, supports a diverse microbial community, including plant growth-promoting rhizobacteria (PGPR), which enhance plant growth through direct and indirect mechanisms. PGPR modify root system architecture by producing phytohormones and other signals, leading to increased lateral root branching and root hair development. They also improve root functioning, plant nutrition, and whole-plant physiology. Recent studies have provided insights into how PGPR signals trigger these responses, although the specific molecular pathways involved remain largely unknown. Ecologically, PGPR form functional groups influenced by various abiotic and biotic factors in natural and agricultural soils, which can modulate their effects on roots. This paper reviews the modes of action and signals of PGPR, recent progress in understanding the links between plant morphological and physiological effects induced by PGPR, and the importance of considering the size, diversity, and gene expression patterns of PGPR assemblages in the rhizosphere to better understand their impact on plant growth and functioning. Integrating mechanistic and ecological knowledge on PGPR populations will be crucial for developing sustainable agricultural management strategies.The rhizosphere, the soil in contact with plant roots, supports a diverse microbial community, including plant growth-promoting rhizobacteria (PGPR), which enhance plant growth through direct and indirect mechanisms. PGPR modify root system architecture by producing phytohormones and other signals, leading to increased lateral root branching and root hair development. They also improve root functioning, plant nutrition, and whole-plant physiology. Recent studies have provided insights into how PGPR signals trigger these responses, although the specific molecular pathways involved remain largely unknown. Ecologically, PGPR form functional groups influenced by various abiotic and biotic factors in natural and agricultural soils, which can modulate their effects on roots. This paper reviews the modes of action and signals of PGPR, recent progress in understanding the links between plant morphological and physiological effects induced by PGPR, and the importance of considering the size, diversity, and gene expression patterns of PGPR assemblages in the rhizosphere to better understand their impact on plant growth and functioning. Integrating mechanistic and ecological knowledge on PGPR populations will be crucial for developing sustainable agricultural management strategies.