Phytochrome regulation of phytochrome A mRNA levels in the model short-day-plant Pharbitis nil

The exposure of dark-grown Pharbitis nil seedlings to continuous R induces a rapid decrease in PHYA mRNA abundance with a half-life of about 2 h, A 5 min R pulse also induces this decline, and the effect is partially reversib le by subsequent FR irradiation, confirming that the regulation of expressi on is mediated via the Pfr form of a phytochrome, When de-etiolated seedlin gs are returned to darkness after a W photoperiod, PHYA mRNA slowly reaccum ulates from 20% to 50% of the dark level within 24 h, The rate of reaccumul ation is greatly accelerated by the removal of Pfr with a FR pulse, resulti ng in reaccumulation to 100% within approximately 11 h. Without FR irradiat ion PHYA mRNA expression remains fully repressed for at least 11 h after th e end of the photoperiod, suggesting that the controlling Pfr is highly sta ble.