Two distinct blue-light responses regulate the levels of transcripts of specific nuclear-coded genes in pea

Fluence-response characteristics for blue-light(BL)-mediated changes in the steady-state levels of Cab-, pEA215- and pEA207-RNA in red-light(RL)-grown pea (Pisum sativum L. cv. Alaska) seedlings indicate the existence of two BL responses: a blue-low-fluence (BLF) response, causing an increase in Cab- and pEA215-RNA, and a blue-high-fluence (BHF) response, causing a return to control levels for Cab- and pEA215-RNA and a decrease in pEA207-RNA levels (Warpeha and Kaufman, 1989, Plant Physiol., 91, 1030—1035). We now show that under dark growth conditions, only the BLF response is apparent; Cab- and pEA215-RNA increase at all fluences tested, whereas pEA207-RNA levels are unaltered over the range of BL fluence tested. The treatment of dark-grown seedlings with RL immediately prior to BHF irradiation does not elicit the BHF response for Cab-, pEA215- and pEA207-RNA, indicating that the role of growth in RL is to enable the seedling to reach a particular developmental state, rather than ensuring the presence of active phytochrome at the time of BL-irradiation. The apical bud of RL-grown seedlings has only the BLF response; Cab-RNA levels increase while pEA207-RNA exhibits no change at any of the fluences tested. The developing leaves of the fourth node show the BHF response; both Cab- and pEA207-RNA decrease following treatment with high-fluence BL. These data also indicate the necessity for reaching a specific developmental state before the BHF response can be activated.