Mode of coaction between blue/UV light and light absorbed by phytochrome in light-mediated anthocyanin formation in the milo ( Sorghum vulgare Pers.) seedling

Abstract

Anthocyanin formation in milo (Sorghum vulgare Pers.) seedlings (coleoptile, mesocotyl, taproot) occurs only in white light and blue/UV light (BL/UV), while red light (RL) and far-RL are totally ineffective. However, after a BL/UV pretreatment, the participation of phytochrome can be demonstrated. With a short-wavelength light source [peak emission in longwave UV (UV-A)], the mode of coaction between BL/UV and light absorbed by phytochrome (RL) was studied with the following principal results. (i) As soon as the seedling becomes competent to respond to UV-A (with regard to anthocyanin formation), the involvement of phytochrome can be detected. (it) A 5-min pulse of UV-A has a strong effect on the anthocyanin synthesis in the milo mesocotyl. This effect is fully reversible if a long-wavelength far-RL pulse (RG9 light) is given immediately after the UV-A light pulse. (iii) When seedlings treated with 5 min of UV-A and 5 min of RG9 light are kept in darkness for 3 hr and then transferred to RL, anthocyanin appears. (iv) In continuous UV-A treatment, anthocyanin accumulation starts after a lag phase of 3.5 hr (25°C). A RL pretreatment prior to the onset of UV-A treatment strongly increases anthocyanin accumulation in UV-A, though the lag phase is not affected. Moreover, a RL pretreatment does not affect the time course for escape from reversibility in UV-A. It is concluded from these data that BL/UV cannot mediate induction of anthocyanin synthesis in the absence of Pf,, the active form of phytochrome that absorbs maximally in the far-red. Rather, the action of BL/UV must be considered to establish responsiveness of the anthocyaninproducing mechanism to Pfr. Pfr operates in this system via two different channels. As the effector of the terminal response, it sets in motion the signal-response chain that eventually leads to the appearance of anthocyanin. This is a slow process with a lag phase of the order of 3.5 hr. The second function of Pfr is to determine the responsiveness to the effector Pfr in mediating anthocyanin synthesis. This is a very fast and highly sensitive phytochrome action that can be detected readily within 1 min. However, as long as the plant has not received BL/UV, the strong effect of RL on the effectiveness of Pfr remains cryptic. The effect of a RL pretreatment and the effect of a UV-A pretreatment on responsiveness towards Pfr (or, effectiveness of Pfr) were found to be totally independent ofeach other, even though it is the UV-A that permits operation of Pfr.

Cryptochrome, Phytochrome, and Anthocyanin Production

“…These results are consistent with previous findings (3-5, 10, 11, 19-22). Blue pretreatments are more effective than R and FR ones in increasing AR (Table III), another observation consistent with previous findings (3)(4)(5)(19)(20)(21)(22). The effects of BL pretreatments on AR are enhanced by adding R and reduced by adding FR during the pretreatments (Table III).…”

“…An interaction between the two photoreceptors under the prolonged exposures required for a high level of anthocyanin production has been inferred (10, 1 1) from data showing a specific cryptochrome-mediated effect of UV-A/BL pretreat-ments on the R-FR reversibility of anthocyanin production (sequential interaction [3,4,[19][20][21][22] .JU. (23), was used for the in vivo assays.…”

Comparison of Photomorphogenic Responses to UV Light in Red and White Cabbage (Brassica oleracea L.)

“…In summary, our results are not consistent with the theory (6,14) that light absorbed by UV photoreceptors can lead to an increase in responsiveness toward Pfr in a subsequent dark period.…”

Regulation of Early Light-Inducible Protein Gene Expression by Blue and Red Light in Etiolated Seedlings Involves Nuclear and Plastid Factors

“…A requirement of both phytochrome and cryptochrome has been reported for the induction of genes for enzymes of anthocyanin synthesis (Oelmiiller and Mohr, 1985) and induction of cab and rbcS transcript levels (Fluhr and Chua, 1986). Also, the accumulation of chloroplast-encoded transcript psbD-psbC was shown to be regulated by both receptors (Gamble and Mullet, 1989).…”