We found that two key enzymes of polyamine biosynthesis, S-adenosylmethionine decarboxylase (SAMDC; EC 4.1.1.50) and arginine decarboxylase (ADC, 4.1.1.19) in leaves of Pharbitis nil increased dramatically at lights-on and then gradually decreased in the light. The temperature and humidity in the darkness before lights-on had a marked effect on the photoinduction of SAMDC activity in leaves. When the temperature and relative humidity were high in the dark period before lights-on, less SAMDC activity was induced, but these conditions which had no effect on the induction of ADC activity after illumination were also effective on photoperiodic flowering of Pharbitis nil. While SAMDC activity was photoinducible in leaves in all plant species examined, ADC activity in leaves of long-day plants seem to be less induced than that of short-day or day-neutral plants. Cycloheximide inhibited the photoinduction of SAMDC and ADC activity, and the half-life of photoinduced SAMDC and ADC proteins of synthesis de novo was 30-40 min, respectively.

 

The level of mRNA for SAMDC in leaves of Pharbitis nil exposed to light began to increase after 15 min exposure, peaked by 45 min and was followed by a rapid decline. The half-life of SAMDC mRNA was estimated by using actinomycin D to be approximately 30 min, which partly accounts for the rapid decline in the mRNA level after the peak of light induction is reached. The mRNA level for SAMDC gene increased after light exposure from red, green, blue or UV light, but not after fat-red light exposure. The short irradiation of red light increased the expression of SAMDC gene and this induction was reverted by subsequent far-red light irradiation. The immediate blue light illumination after the initial red light exposure resulted in further increase of SAMDC mRNA level. The transcription of SAMDC gene was also shown to be under circadian control. The signal transduction processes involved in the regulation of SAMDC gene expression by blue and red light, were examined using pharmacological inhibitors of signaling pathways. Calcium and calmodulin regulated positively SAMDC gene expression in red light, whereas in blue light they regulated negatively. Both signal transduction pathways also require new protein synthesis.