Temporal and light regulation of the expression of three phytochromes in germinating seeds and young seedlings of Avena sativa L.

An oat (Avena sativa L.) plant contains at least three phytochromes, which have monomeric masses of 125, 124, and 123 kilodaltons (kDa) (Wang et al., 1991, Planta 184, 96-104). The 124-kDa phytochrome is most abundant in dark-grown seedlings, while the other two phytochromes predominate in light-grown seedlings. Using three monoclonal antibodies, each specific to one of the three phytochromes, we have monitored by immunoblot assay the expression of these three phytochromes in the 5 d following onset of imbibition of seeds. On a per-organism basis, each of these three phytochromes increased in abundance for the first 3 d in the light, or for the first 4 d in darkness, after which they each began to decrease in quantity. When 3-d-old dark-grown seedlings were transferred to the light, the abundance of each of these three phytochromes decreased both in absolute amount and relative to the phytochrome levels in control seedlings kept in darkness. In contrast, when 3-d-old light-grown seedlings were transferred to darkness, the abundance of the 124-kDa and 125-kDa phytochromes increased while that of 123-kDa phytochrome remained unchanged. In each case, the level of phytochrome was greater than that of control seedlings maintained in the light. Thus, in addition to temporal regulation, all three phytochromes exhibit photoregulated expression at the protein level, although the magnitude of this photoregulation varies substantially.