Method and Movie Information

Seedlings were grown on vertical 1% agar Petri plates for approximately 2 days in darkness until they were approximately 5 mm tall. They were then gently transferred to a fresh plate to form a row of developmentally uniform seedlings and this plate with seedlings was then placed vertically in a mount that held the seedlings horizontal and perpendicular to the optical path of a charge-coupled device (CCD) camera equipped with a close-focus zoom lens and interfaced with a computer. Backlighting the seedlings with diffuse irradiation from a single infrared light-emitting diode (peak output at 948 nm - plants can not detect this irradiation and so they act as if they are growing in darkness) produced the images that were acquired by the computer at user-specified time intervals before and during treatment from above with actinic blue light from a bank of high-output light-emitting diodes. This imaging system is capable of achieving a resolution of 5 microns per pixel.

The QuickTime movie that can be activated in the righthand panel is a time-lapsed compilation of separate infrared images taken at 5 minute intervals over a period of 12 hours. The Arabidopsis seedling on the left is wild type, while the one on the right is a mutant that is missing the cryptochrome1 photoreceptor. This mutation causes these seedlings to be weakly sensitive to blue light so that their growth is not inhibited as well by this light compared to the wild type. Once you start the movie, you will see that the wild type and mutant are growing similarly to each other in darkness. The appearance of a momentary black dot marks the 2-hour point when the seedlings are now continuously irradiated with actinic blue light. You will not see a change in the appearance of the image exposure at this time because a special filter is used with the camera that only allows infrared irradiation and longer wavelengths to pass through. Note, that very shortly after the blue light comes on, both seedlings will stop growing. The wild-type growth will remain inhibited, but the mutant will start to resume growing in a relatively uninhibited manner and will pop off of the surface of the agar a couple of times as it continues to grow. The observation that the mutant shows a small degree of inhibition before growing faster permitted us to conclude that another photoreceptor, in addition to cryptochrome1, is required for normal growth inhibition by blue light (Parks et al., 1998). We now know that this other photoreceptor is phototropin1, the same photoreceptor responsible for phototropism in plants (Folta and Spalding, 2001). If you clicked on either of these references, then click here to bring the movie link back into the frame at right.