Logan, David Calder; (1993) Physiology and biochemistry of seed germination in Striga hermonthica Del. (Benth.). Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Seeds of the plant parasitic angiosperm Striga hermonthica (Del.) Benth. possess a specialised dormancy mechanism operating in three sequential phases. Seeds require an after-ripening period followed by an extended period of imbibition (conditioning) before they acquire the potential for maximum germination. However, this potential is not realised unless the seed is in contact with compounds present in the root exudates of the host plant. Although a variety of compounds can substitute for the host derived signal the mechanism through which these act is unknown. This thesis examines the physiological and biochemical nature of both the imbibition and germination phases in the life-cycle of S. hermonthica. An inhibitor of ethylene biosynthesis, aminoethoxyvinyl glycine (AVG) was found to inhibit germination while addition of an intermediate in ethylene biosynthesis, 1-aminocyclopropane-l-carboxylic acid (ACC) was found to override this inhibition and to act as a substitute for the host derived signal. 2,5-norbornadiene (NDE), an inhibitor of ethylene action, also inhibits germination. Ethylene is rapidly produced by Striga seeds after treatment with host root exudates. These results are consistent with a model for Striga seed germination in which host derived signals and other compounds act by eliciting the synthesis of ethylene and in which ethylene itself initiates the biochemical changes leading to germination. Further studies using inhibitors of the ethylene biosynthetic pathway together with the cytokinin-active urea derivative thidiazuron, suggest that stimulation of the enzyme ACC synthase is necessary for natural host stimulated germination. Analyses of the effect of various combinations of stimulants and inhibitors on the germination of S. hermonthica prior to conditioning suggest that this phase is necessary, not for germination itself, but rather for the seed to react to the external host derived stimulus by initiation of ethylene production. Time-course analysis of polypeptide changes in seeds triggered to germinate by various compounds using 2-D SDS-PAGE allowed certain polypeptides to be identified as important to the germination process and likely to be involved in the primary events of ethylene biosynthesis or action.

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