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 response 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 determined with a 200-s light pulse of wavelengths from 347 to 693 nm had peaks at 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 the 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 fluence rate regardless of the pulse length, but the level of this plateau depended on the pulse length. (iv) The effect of R increased with increasing pulse length when compared at the same fluence, whether saturating or unsaturating; thus, no reciprocity law holds. These results indicate that the photoreceptor for CRS production is a phy, Pfr being active, which presumably shows very fast dark reversion to the R-absorbing form without absorbing FR. The possible CRS-production mechanism of the phy and its significance in the so-called R high-irradiance response of phy are discussed.