Endogenously synthesized plant hormones direct the ordered growth of tissues,organs,and the overall plant body plan,thereby integrating external information with internal metabolism.Because plant hormones can have such dramatic and systemic effects on plant morphology,hormone activity has been a target of selective breeding for desirable plant traits,whether consciously or otherwise.Most notably,the dwarf varieties of wheat and rice that underpinned the Green Revolution resulted from mutations that affect gibberellin signaling (Hedden,2002).Generally,plant hormones require a pathway for hormone biosynthesis,a mechanism for hormone perception and signal transduction,and a series of downstream effector components that bring about a cellular response.Conceptually,modification of any one of these steps-either by genetic or chemical means-could bring about changes in plant development.In an effort to rationally develop high-yield crops that will support anticipated population growth,substantial research effort continues to focus on a better understanding of plant hormones and of the ways to manipulate their activity.
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