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

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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. ( ii ) 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 P fr , 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 anthocyanin-producing mechanism to P fr . P fr 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 P fr is to determine the responsiveness to the effector P fr 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 P fr remains cryptic. The effect of a RL pretreatment and the effect of a UV-A pretreatment on responsiveness towards P fr (or, effectiveness of P fr ) were found to be totally independent of each other, even though it is the UV-A that permits operation of P fr .