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Abstmct:In this review we will focus on two areas in which the accumulated evidence for a critical physiological role of polyamines is becoming compelling, i.e. reproductive activity and response to abiotic stress. Regarding reproductive behavior, it seems clear that polyamines are members of the array of internal compounds required for flower initiation, normal floral organ morphogenesis. fruit growth and fruit ripening in particular plant species. Abiotic stresses such as osmotic stress can "turn on" arginine decarboxylase (ADC) genes, resulting in a rapid increase in their mRNA levels. Localization of ADC enzyme in the chloroplast suggests a role of PAS in the maintenance of photosynthetic activity during senescence responses induced by osmotic stress. We envisage that the use of transgenic plants and improved molecular probes will resolve in the near future the physiological significance of stress-induced changes in PA metabolism as well as the role of these compounds in reproductive activity.

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Several lines of experimental evidence indicate a close connection between polyamines (PAs) and reproductive development in Arabidopsis thaliana. (l) Measurement of the titers of endogenous spermidine (SPD) and putrescine (PUT), extracted from various organs of two ecotypes and a genetic line of Arabidopsis, revealed that flowers had the highest titers of both PAs, with SPD predominating. (2) In aseptic cultures of whole plants of the ecotype Columbia, the application of appropriate enzyme inhibitors lowered SPD titer while almost completely preventing bolting and flowering. When the plants were removed to an inhibitor‐free medium, bolting and flowering resumed. (3) SPD added to the medium of aseptically cultured plants of Columbia growing under short‐day (SD) conditions, where flowering is naturally delayed, increased the SPD titer and augmented the rate and extent of flowering. Under long‐day (LD) conditions, where flowering is already rapid and abundant, it did not promote flowering any further. (4) Enzyme inhibitors of SPD synthesis given shortly before the transition from SD conditions to LD conditions prevented flowering. (5) In a delayed‐flowering mutant (CS 3123), the addition of SPD significantly accelerated flowering.

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