IAA (Indole-3-Acetic Acid) is the abundant and well known plant hormone of the auxin class. IAA is predominantly produced in the apical bud of and young leaves of plants and is known to be an inducer of cell division and elongation. IAA, as well as most other auxins, are also known to act in concert with, or in opposition to, other plant hormones. For example, the ratio of auxin to cytokinin in certain plant tissues determines initiation of root versus shoot buds. The auxin concentration varies depending on the part of the plant and the various concentrations are strictly controlled by the plant. The regulation of auxin concentration forms a pattern-like system that helps determine plant growth and development. IAA is often used in horticulture to promote emergent root growth and is used commercially to create root stem cuttings and promote uniform fruit and flower development.
At elevated levels, IAA can cause physical damage. For example, IAA stimulates the production of ethylene, which inhibits plant growth (Xie et al., 1996). It has also been implicated in plant pathogenesis, as bacterial symbionts have the ability to take up the plant biosynthesis pathway (Yamada et al., 1985; Yamada, 1993) and produce it in the plant host (Patton and Glick, 2002; Gravel et al., 2006). Capacity. al., 2007). Pathogenic bacteria can alter host metabolic processes for their own benefit, such as by regulating the production of IAA, which produces uncontrolled cellular division leading to gall disease in plants (Escobar and Dandekar, 2003). A well-studied example of this pathogenic strategy is crown gall disease caused by Agrobacterium tumefaciens, which involves gall formation on the lower stems and roots of infected plants.