Phytochrome regulation of greening in barley : effects on mRNA abundance and on transcriptional activity of isolated nuclei.

Red light treatment of etiolated barley (Hordeum vulgare L.) seedlings causes an increase in the relative abundance of the mRNA for light-harvesting, chlorophyll a/b-binding polypeptides, and a decrease in the relative abundance of the mRNA for the NADPH:protochlorophyllide oxidoreductase. It also increases transcriptional activity of subsequently isolated nuclei for the mRNA for the chlorophyll-binding polypeptides and reduces it for the reductase. These results confirm those published previously. Fluence-response and kinetic studies support the hypothesis that the abundance of mRNA for the chlorophyll-binding polypeptides may be transcriptionally limited. They do not support the same hypothesis for the reductase. Red light treatments in the very low fluence range significantly decrease transcriptional activity in isolated nuclei for RNAs hybridizable by the reductase probe, but have little effect on mRNA abundance. By contrast, red light treatments in the low fluence range bring about a sharp decrease in reductase mRNA abundance with little further effect on transcription, suggesting light regulation at the level of mRNA stability rather than transcription. The fluence-response relationships for increase in abundance of mRNA for the chlorophyll-binding polypeptides is similar to that published elsewhere for elimination of the lag period of chlorophyll accumulation in barley. However, kinetic studies published elsewhere show that for elimination of the lag in chlorophyll accumulation, the dependence on the far red-absorbing form of phytochrome concentration is significantly different from the dependence of the transcriptional changes, suggesting that although transcription might be what limits mRNA abundance for the chlorophyll-binding proteins, the mRNA abundance can not be what limits chlorophyll accumulation per se.