Hormonal Mediation of the Light Control of Internode Extension Using Pea Genotypes

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Light causes a pronounced decrease in stem growth rate. Dr. Davies proposes to investigate the kinetics of hormonal changes in relation to the growth kinetics following the transition from dark to red light in order to determine to what extent, and in what manner, changes in endogenous hormones may mediate light- induced changes in stem growth rate. Red light will be used to reduce the complexity inherent in white light. Dr. Davies will use a range of pea length genotypes whose gibberellin (GA) content is known. All of the genotypes grow faster in the dark than in the light but vary in the proportional reduction in their growth rate in the light. In some genotypes GAs are absent yet they are tall; in such plants he finds a very high indoleacetic acid (IAA) level. He has considerable evidence that GA may act via the induction of tryptophan racemase causing the production of D-tryptophan, which is then the precursor of IAA. The kinetics of growth are recorded by position transducers under darkness and illumination of known spectral composition, and following hormone and inhibitor applications. GAs, IAA and IAA conjugates, will be measured by gas chromatography-mass spectrometry and components of the IAA synthesis pathway assayed. This will show the point(s) in the GA and IAA metabolic pathways at which light acts, and whether a light induced change in GA levels in fact operates by altering the level of IAA. %%% Light causes a decrease in the growth of stems of plants previously grown in the dark. The mechanism by which this occurs is unknown. One possibility is that light alters the level of plant hormones. To determine whether plant hormones are involved one need to know the rate at which light induces a decrease in growth, and then examine the changes in the internal levels of the plant hormones to see if they change at the same time. Dr. Davies will measure the rate of growth change and hormonal content in pea varieties which have different stem lengths. The difference in stem lengths is genetically controlled, and has been shown to be due to changes in levels or responsiveness to plant hormones called gibberellins. There is also evidence that this hormone may interact in some way in the synthesis of another hormone, indoleacetic acid. Dr. Davies will measure the changes in the internal levels of gibberellin and indole acetic acid to determine if and how light causes a decrease in stem growth. This will improve our knowledge of how plants grow.