AbstractThe plant hormones, auxins and cytokinins, are involved in several stages of plant growth and development such as cell elongation, cell division, tissue differentiation, and apical dominance. The biosynthesis and the underlying mechanism of auxins and cytokinins action are subjects of intense investigation. Not only plants but also microorganisms can synthesize auxins and cytokinins. The role of phytohormone biosynthesis by microorganisms is not fully elucidated: in several cases of pathogenic fungi and bacteria these compounds are involved in pathogenesis on plants; auxin and cytokinin production may also be involved in root growth stimulation by beneficial bacteria and associative symbiosis.The genetic mechanism of auxin biosynthesis and regulation byPseudomonas, Agrobacterium, Rhizobium, Bradyrhizobium, andAzospirillum, are well studied; in these bacteria several physiological effects have been correlated to the bacterial phytohormones biosynthesis. The pathogenic bacteriaPseudomonasandAgrobacteriumproduce indole-3-acetic acid via the indole-3-acetamide pathway, for which the genes are plasmid borne. However, they do possess also the indole-3-pyruvic acid pathway, which is chromosomally encoded. In addition, they have genes that can conjugate free auxins or hydrolyze conjugated forms of auxins and cytokinins. InAgrobacteriumthere are also several genes, located near the auxin and cytokinin biosynthetic genes, that are involved in the regulation of auxins and cytokinins sensibility of the transformed plant tissue. Symbiotic bacteriaRhizobiumandBradyrhizobiumsynthesize indole-3-acetic acid via indole-3-pyruvic acid; also, the genetic determinants for the indole-3-acetamide pathway have been detected, but their activity has not been demonstrated. In the plant growth-promoting bacteriumAzospirillum, as inAgrobacteriumandPseudomonas, both the indole-3-pyruvic acid and the indole-3-acetamide pathways are present, although inAzospirillumthe indole-3-pyruvic acid pathway is of major significance. In addition, biochemical evidence for a tryptophan-independent indole-3-acetic acid pathway inAzospirillumhas been presented.