Photoregulated expression of the PsPK3 and PsPK5 genes in pea seedlings.
The PsPK3 and PsPK5 genes of the garden pea encode protein-serine/threonine kinases whose catalytic domains are closely related to known signal transducing kinases from animals and fungi. The PsPK3 polypeptide is predicted to be located in the nucleus, whereas PsPK5 is a homologue of NPH1, the probable blue light receptor for phototropism from Arabidopsis. We found previously that when etiolated pea seedlings are illuminated with continuous white light, PsPK3 and PsPK5 transcript levels within apical buds decline substantially, reaching their minimum levels within one day of exposure to light. The role of light in regulating the expression of the PsPK3 and PsPK5 genes was investigated further. To gain insight into the rapidity with which expression changes, 6-day old, dark-grown pea seedlings were transferred to continuous white light, and PsPK3 and PsPK5 RNA levels monitored over the ensuing 24 h. While transcripts from the RbcS gene family increase, the PsPK3 and PsPK5 mRNAs decline rapidly to their minimum levels. PsPK5 mRNA declines 10-fold in ca. 2 h, whereas PsPK3 mRNA declines 4-fold in ca. 8 h. We used single pulses of light to elucidate which photoreceptor triggers the negative regulation of PsPK3 and PsPK5 gene expression. To assess phytochrome involvement, etiolated seedlings were treated with single pulses of red light, red followed by far-red light, or far-red light alone. RbcS induction by a red light pulse is reversible with a subsequent far-red light pulse, clearly showing that phytochrome mediates its induction. Likewise, RbcS expression is induced with a single pulse of blue light or a dichromatic pulse of red+blue light. However, none of these pulses trigger the PsPK3 and PsPK5 mRNA levels to decline. Given the lack of effectiveness of light pulses, etiolated seedlings were transferred to continuous light of three different qualities to determine the spectral sensitivity of PsPK3 and PsPK5 gene expression. Exposure to continuous red, continuous far-red, or continuous blue light causes the PsPK3 and PsPK5 mRNAs to decline and transcripts from the RbcS and Cab gene families to increase. One likely explanation is that phytochrome A mediates the responses of these genes to continuous far-red light. The effectiveness of continuous red light and blue light in triggering the reduction in PsPK3 and PsPK5 mRNA levels and the increase in RbcS and Cab mRNAs may imply the participation of additional phytochromes and/or cryptochromes. Thus, the PsPK3 and PsPK5 genes exhibit responsiveness to continuous light, but a lack of responsiveness to single light pulses that is unusual, and perhaps unique, among light-regulated genes.