Microorganisms can promote plant growth by both direct and
indirect means. Production of phytohormones, such as auxins
or gibberellins, is a direct mechanism that is used by plant
growth-promoting microorganisms. Auxins are a group of
compounds having an indole ring, that have a positive effect
on plants by stimulating seed germination, root initiation,
cell elongation, and seedling growth (El-Tarabily 2008). Indole-3-acetic
acid (IAA), a plant growth hormone classified
as an indole derivative of the auxin family, has been widely
studied and found to be a dominant type of auxin in plants.
Several important roles that IAA has been shown to play in
plants include stimulation of cell division, cell elongation,
cell differentiation, light and gravitational responses, and regulation
of leaf fall and fruit ripening (Trotsenko et al. 2001;
Teale et al. 2006). Different levels of IAA production have
been reported in several groups of bacteria and yeasts.
As well as phytohormone production, microorganisms d