Besides the major five hormones, polyamines can also exert regulatory control over growth and development at micromolar concentrations. They are widespread and probably occur in all cells. However, there is some controversy as to whether polyamines should be classified as hormones. Polyamines have a wide range of effects on plant development. It appears polyamines are present in all cells and are not confined to the specific site where they are synthesised. Whether polyamines are classified as plant hormones or not is immaterial as they definitely control plant growth and development to a great extent and are therefore, major plant growth regulators.