An amino-terminal deletion of rice phytochrome A results in a dominant negative suppression of tobacco phytochrome A activity in transgenic tobacco seedlings

Overexpression of phytochrome A results in an increased inhibition of hypocotyl elongation under red and far-red light. We used this approach to assay for the function of N-terminal mutations of rice (Oryza sativa L.) phytochrome A. Transgenic tobacco seedlings that express the wild-type rice phytochrome A (RW), a rice phytochrome A lacking the first 80 amino acids (NTD) or a rice phytochrome A with a conversion of the first 10 serines into alanine residues (S/A) were compared with untransformed wild-type tobacco (Nicotiana tabacum L. cv. Xanthi) seedlings. Experiments under different fluence rates showed that RW and, even more strongly, S/A increased the response under both red and far-red light, whereas NTD decreased the response under far-red light but hardly altered the response under red light. These results indicate that NTD not only lacks residues essential for an increased response under red light but also distorts the wild-type response under far-red light. Wild-type rice phytochrome A and, even more so, S/A mediate an enhanced phytochrome A as well as phytochrome B function, whereas NTD interferes with the function of endogenous tobacco phytochrome A as well as that of rice phytochrome A when co-expressed in a single host. Experiments with seedlings of different ages and various times of irradiation under far-red light demonstrated that the effect of NTD is dependent on the stage of development. Our results suggest that the lack of the first 80 amino acids still allows a rice phytochrome A to interact with the phytochrome transduction pathway, albeit nonproductively in tobacco seedlings.Abbreviations HIR high-irradiance response - NTD N-terminal deletion mutant of rice phytochrome A - Pfr far-red-absorbing form of phytochrome - Pr red-absorbing form of phytochrome - RW rice wild-type phytochrome A - S/A serine-to-alanine mu-tant of rice phytochrome A - wNTD weakly expressing NTD line - XAN wild-type tobacco cv. Xanthi We thank Masaki Furuya (Adv. Research Laboratory, Hitachi, Saitama, Japan) and Akira Nagatani (RIKEN Institute, Saitama, Japan) for providing the monoclonal antibodies mAP5 and mAR14. The work was supported by a grant from the Human Frontier Science Program. K.E. was a recipient of a Landesgraduiertenförderung fellowship.