A stable phytochrome pool regulates the expression of the phytochrome I gene in pea seedlings.

Two different molecular species of phytochrome apoprotein in terms of amino-acid sequences, phytochrome I (PI) and phytochrome II (PII), are found in pea tissues (H. Abe et al., 1989, Plant Cell Physiol. 30, 1089-1097). The single-copy pea PI gene produces at least three distinct transcripts (mRNAs 1, 2 and 3), differing in the length of the 5' non-coding sequence. The abundance of PI mRNA1 and mRNA2 in pea (Pisum sativum L. cv. Alaska) seedlings was determined using a primer extension assay. When dark-grown seedlings pretreated with a red light pulse or white-light-grown seedlings were briefly irradiated with far-red light (FR) 4h after transfer to darkness, the abundance of PI mRNA1 and mRNA2 significantly increased within a few hours. This effect was also observed when the FR was given after a 16-h dark incubation period. This FR effect was reversed by a subsequent irradiation with red light. Therefore, this photoreversible effect, observed during the prolonged dark period, can be ascribed to a phytochrome pool that is stable in its FR-absorbing form (Pfr).