Phytochrome control of plastid gibberellin levels.

Previously published work has shown that phytochrome is present in association with etioplast-enriched organelle suspensions and that it mediates in vitro the levels of gibberellin-like substances (GA-LS) extractable from them into aqueous methanol. However, the technique used to isolate the organelle suspensions has recently been critically examined, both biochemically and with the use of the electron microscope and found to purify mitochondria to a similar extent to etioplasts. The validity of these results is therefore questionable. This thesis presents results of experiments designed to study further the presence of phytochrome in association with etioplast- enriched suspensions and also re-examines the role of phytochrome in the mediation of GA-LS levels. When etioplasts are prepared free of mitochondrial contamination (as judged by biochemical marker enzymes) phytochrome remains in association with the etioplasts and also mediates the levels of GA-LS extractable from them. Suspensions enriched with mitochondria also contain spectrophotometrically- detectable phytochrome but exhibit no phytochrome mediation of extractable GA-LS levels. It was previously hypothesized that photoconversion of etioplast- associated phytochrome to Pfr mediated a change in the permeability of the etioplast envelope membranes with respect to GA-LS thereby allowing an efflux of GA-LS out of the etioplasts into the surrounding medium. The results presented in this thesis show that a large proportion of the red light-mediated increase in GA-LS is retained within the etioplasts. The results suggest, therefore, that phytochrome photoconversion has an effect on the extractability of GA-LS from etioplasts possibly via conformational changes of the etioplast membranes. Results of preliminary experiments with chloroplast-enriched organelle suspensions are also presented and suggest that phytochrome is detectable in association with chloroplasts (namely the envelope membranes) and that it mediates the levels of GA-LS extractable from them.