Phytochrome elicits the cryptic red-light signal which results in amplification of anthocyanin biosynthesis in sorghum

Anthocyanin synthesis in Sorghum bicolor Moench induced by a low-fluence re sponse of phytochrome (phy) is multiplicatively amplified by a cryptic red- light signal (CRS) produced by red light (R). The photoreceptor for CRS and its features in CRS production were studied. (i) An action spectrum determ ined with a 200-s light pulse of wavelengths from 347 to 693 nm had peaks a t 657 and 378 nm. (ii) The CRS-producing effect of R, even as short a pulse as 20 s, was neither suppressed by an immediately subsequent far-red light (FR) pulse nor increased by placing a dark interval of 180 s between R and FR; simultaneous FR, however, suppressed the R action in accordance with t he resulting ratios of the FR-absorbing form (Pfr) to total phy. (iii) The effect of R increased with increasing fluence rate to plateau at the same f luence rate regardless of the pulse length, but the level of this plateau d epended on the pulse length. (iv) The effect of R increased with increasing pulse length when compared at the same fluence, whether saturating or unsa turating; thus, no reciprocity law holds. These results indicate that the p hotoreceptor for CRS production is a phy, Pfr being active, which presumabl y shows very fast dark reversion to the R-absorbing form without absorbing FR. The possible CRS-production mechanism of the phy and its significance i n the so-called R high-irradiance response of phy are discussed.