The constitutively active missense allele of Arabidopsis phytochrome B, or , encodes a polypeptide that adopts a light-insensitive, physiologically active conformation capable of sustaining photomorphogenesis in darkness. Here, we show that the orthologous allele of rice phytochrome B ( ) also encodes a dominant "constitutively active" photoreceptor through comparative phenotypic analyses of and transgenic lines of four eudicot species, , (tobacco) and cv. MicroTom (tomato), and of two monocot species, ssp. japonica and . Reciprocal transformation experiments show that the gain-of-function nstitutive hotomorphogenic () phenotypes by expression are stronger in host plants within the same class than across classes. Our studies also reveal additional -dependent traits in adult plants, which include extreme shade tolerance, both early and late flowering behaviors, delayed leaf senescence, reduced tillering, and even viviparous seed germination. However, the strength of these gain-of-function phenotypes depends on the specific combination of allele and species/cultivar transformed. Flowering and tillering of - and -expressing lines of rice Nipponbare and Kitaake cultivars were compared, also revealing differences in allele versus genotype interaction on the phenotypic behavior of the two rice cultivars. In view of recent evidence that the regulatory activity of YHB is not only light insensitive but also temperature insensitive, selective expression is expected to yield improved agronomic performance of both dicot and monocot crop plant species not possible with wild-type alleles.