Abstract

The plant hormone indole-3-acetic acid (IAA) is the main auxin in plants. IAA controls several physiological processes such as cell elongation and division, tissue differentiation and responses to light and gravity. IAA concentration is regulated in plants. Interestingly, bacteria can modulate IAA levels. The most widely distributed biosynthetic IAA routes in bacteria are indole-3-pyruvate (IPA) and indole-3-acetamide (IAM) anabolic pathways. These metabolic pathways use tryptophan as precursor. IPA pathway has been described mainly in plant growth-promoting bacteria (PGPB), whereas the IAM pathway is present in phytopathogenic bacteria. PGPB stimulate plant growth through various mechanisms including the production of plant hormones. Studies on PGPB that are capable to produce IAA indicate that this phytohormone plays a crucial role for promoting plant growth