Some of phytochrome (phy)-induced morphogenesis in plants are amplified by a signal produced by red light (R) pre-irradiation other than far-red light (FR)-absorbing form of phy (Pfr). With anthocyanin synthesis in broom sorghum (Sorghm bicolor Moench) the signal named cryptic red-light signal (CRS) has been characterized in detail concerning production, action and other physiological properties. The idea of CRS provides an adequate explanation to not only anthocyanin synthesis, but also some other low-fluence responses of phy that have not fully been explained by the classical Pr/Pfr photoconversion mechanism. This article summarizes the findings about CRS, reviewing the reports on R pre-irradiation effects on phy actions as well as rapid escape from FR reversion and rapid dark reversion of Pfr.