It's spelled phytochrome, not phytachrome. There's an overview on phytochrome on wiki. The guys that did the most work on it at UCB were Peter Quail and Lew Feldman. Quail worked on the molecular genetics and Feldman on the physiology.
Also after reading the above, phytochrome responds to the IR and far IR spectrum. I should have remembered this. We used special boxes that blocked out all light and then `irradiated' the seedlings with spectrum adjustable light sources trying to find the exact response curves for our particular cultivar (Lazy among others). The point here was that Lazy's gravitropic response was mediated by IR and far IR, turning it on and turning it off (more supraoptimal stuff)....
CG