Phytochrome control of specific mRNA levels in developing pea buds : the presence of both very low fluence and low fluence responses.

We have examined phytochrome regulated changes in transcript abundance for 11 different light regulated mRNAs in developing pea buds. Fluence-response curves were measured for changes in transcript abundance in response to red light pulses in both the low and very low fluence ranges. Most transcripts show only low fluence responses, with a threshold of approximately 10 micromoles per square meter. All of the low fluence responses are reversible by far red light. One transcript shows a very low fluence response, with a threshold of approximately 10(-4) micromoles per square meter. As expected, the very low fluence response is not far red reversible and in fact can be induced by far red light.Various fluences of red light were also used as pretreatments before transferring seedlings to continuous white light. One transcript responds to pretreatments in the very low fluence range, several respond to pretreatments in the low fluence range (including chlorophyll a/b binding protein RNA and ribulose-1,5-bisphosphate carboxylase RNA), and several show no response to the red light under these conditions. The threshold of these low fluence responses is approximately 10(2) micromoles per square meter, one order of magnitude greater than the threshold of the low fluence responses to red light alone.The transcripts may also be grouped by their responses to white light treatment alone. Three of the clones correspond to transcripts whose abundance decreases after a 24 hour white light treatment. The remainder of the mRNAs increase between 2- and 10-fold in response to the 24 hour white light.