phytochrome apoprotein, a protein that together with a prosthetic group forms a particular biochemical molecule such as a hormone or enzyme, is synthesized in the Pr form. Upon binding the chromophore, the holoprotein, an apoprotein combined with its prosthetic group, becomes sensitive to light. If it absorbs red light it will change conformation to the biologically active Pfr form. The Pfr form can absorb far red light and switch back to the Pr form. The Pfr promotes and regulates photomorphogenesis in response to FR light, whereas Pr regulates de-etiolation in response to R light. Most plants have multiple phytochromes encoded by different genes. The different forms of phytochrome control different responses but there is also redundancy so that in the absence of one phytochrome, another may take on the missing functions. There are five genes that encode phytochromes in the Arabidopsis thaliana genetic model, PHYA-PHYE. PHYA is involved in the regulation of photomorphogenesis in response to far-red light. PHYB is involved in regulating photoreversible seed germination in response to red light. PHYC mediates the response between PHYA and PHYB. PHYD and PHYE mediate elongation of the internode and control the time in which the plant flowers. Molecular analyses of phytochrome and phytochrome-like genes in higher plants (ferns, mosses, algae) and photosynthetic bacteria have shown that phytochromes evolved from prokaryotic photoreceptors that predated the origin of plants. Takuma Tanada observed that the root tips of barley adhered to the sides of a beaker with a negatively charged surface after being treated with red light, yet released after being exposed to far-red light. For mung bean it was the opposite, where far-red light exposure caused the root tips to adhere, and red light caused the roots to detach. This effect of red and far-red light on root tips is now known as the Tanada effect. Blue light Plants contain multiple blue light photoreceptors which have different functions. Based on studies with action spectra, mutants and molecular analyses, it has been determined that higher plants contain at least 4, and probably 5, different blue light photoreceptors. Cryptochromes were the first blue light receptors to be isolated and characterized from any organism, and are responsible for the blue light reactions in photomorphogenesis. The proteins use a flavin as a chromophore. The cryptochromes have evolved from microbial DNA-photolyase, an enzyme that carries out light-dependent repair of UV damaged DNA. There are two different forms of