Phytochrome-initiated fast rhythm controlling developmental response

Abstract

To identify the steps controlling a developmental response, we must relate changes occurring soon after the inducing signal to responses many hours later. In higher plants exposure to red light shifts developmental patterns by converting phytochrome, a ubiquitous plant pigment, into its active form¹. Far red light converts phytochrome back to its original form and stops the response. Red light induces changes in electrical properties of plants in 15–30s (refs 2, 3) and within minutes ATP levels4, NADP transport⁵, and nitrate reductase activity⁶ change. It requires hours or even days for phytochrome effects on circadian rhyythms⁷, germination, de-aetiolation, and flowering responses to appear¹. Until now there has been no indication of the mechanism by which events immediately following phytochrome conversion determine the later development. It should be possible to probe the nature of the first controlling steps by perturbing these steps and thus affecting growth. If the steps directly following phytochrome conversion are sensitive to additional red light, then a second illumination will alter the overall growth. We have found that a brief flash of red light given to dark-grown bean seedlings starts oscillations in a temperature-compensated system. The interaction of this system with a second flash of red light showed that the oscillating system controls subsequent development.