Individual phytochrome genes share regions of high homology which have facilitated the isolation of many phytochrome sequences from both higher and lower plants. Transgenes have been used to study the transcriptional control of phytochrome gene expression, and to create phytochrome-deficient lines in polyploid species. Transgenically produced phytochrome apoproteins assemble with endogenous chromophore to become photoactive, and in most cases confer predictable light responses on the transgenic plants. The biological activity of transgenically produced phytochromes has been used to investigate the functions of domains within the phytochrome molecule. Physiological analysis of plants expressing phytochrome transgenes has revealed the potential of transgenic approaches to modifying the architecture of crop plants. Tobacco plants expressing to moderate levels an oat PHYA transgene exhibit proximity-conditional dwarfing which leads to up to 20% improvement in harvest index in the field. These results provide prospects for a new genetic engineering approach to crop plant improvement.