Plants have developed fine tuned photoreceptor system to convert external light into biological signals. Phytochromes are one of the photoreceptors responsible for red/far-red light. Because they are thought to have only serine/threonine kinase activity, they transmit the light signals through other interacting factors such as PIFs, FHY, LAF1, ELF and so on. However, currently known phytochrome interacting proteins are not enough to explain whole light signaling.
To extend understanding about phytochrome signaling mechanism, I searched Arabidopsis cDNA library and found new phytochrome interacting proteins. One is NPH3-L protein. And the nph3-l mutant show longer hypocotyl length suggesting their positive role for phytochrome-mediated inhibition of hypocotyl length.
Another interacting protein is TZP which is known as a transcriptional regulator localized in the nucleus. The TZP bound both phytochrome A and phytochrome B. The overexpression lines of TZP had longer hypocotyl length than that of wild type under red light condition, which means TZP act as a negative regulator in phytochrome B-mediated signaling.
In addition to the identification of phytochrome interacting factor, PIF3-signaling related protein was found by microarray analysis. RHA3B which is RING finger protein is likely to be regulated by PIF3. Through the results of mutant analysis, RHA3B was thought as a positive component in PIF3 signaling.