Discussion

The 139-kbp maize plastid chromosome is comprised of two large inverted repeats of 22 kbp each plus two unique regions.Thus，the chromosome contains 117 kbp of unique sequences［19］.Photogenes occupy about 19%of the unique sequences and are distributed among at least six different transcription units（Muskavitch K M T，Krebbers E T，Bogorad L，unpublished data）［25］.These transcription units could not be distinguished by the kinetics of increase in complementary RNAs upon illumination of dark-grown plants but showed three distinctive patterns of change on prolonged continuous illumination［25］.In this investigation，we have focused on the question of whether or not phytochrome is involved in regulating the initial increases of RNA levels for these maize photogenes upon illumination of dark-grown seedlings.

Many photoregulated developmental phenomena in plants are mediated by phytochrome.Although it is not clear yet exactly how and at which level it acts，Mohr［21］proposed that phytochrome might regulate“differential gene activation and gene repression.”This hypothesis has been supported by the observations that the levels of RNA transcripts of rcS，cab（in barley，Lemna，pea and bean），ps2B（in mustard，pea and bean），and rcL（in pea and bean）are affected by phytochrome［2，16，20，26，28，29］.We have used somewhat different illumination procedures and come to the same conclusions for etiolated maize seedlings with regard to these as well as other genes.

We have shown here，by dot blot and Northern hybridization procedures，that increases in the levels of transcripts complementary to maize photogenes，ps2B，rcL，pslAl，cflA，cfOⅢ，and cf1BE as well as the photogenes UORF 443，ORF 353，and ORF 473 observed after illumination of dark-grown seedlings（Muskavitch K M T，Krebbers E T，Bogorad L，unpublished data）are regulated via phytochrome.The increases are observed after brief illumination with R but the effect of illumination by R is completely or largely reversed by FR，as is characteristic of phytochrome-mediated photomorphogenesis.However，the possibility of involvementof photoreceptors other than phytochrome cannot be ruled out；the synthesis of some plant proteins is responsive to blue light［24］.Rodermel and Bogorad［25］have shown that，in continuously illuminated dark-grown seedlings，the levels of transcripts of several maize photogenes drop after reaching maxima after 10 to 20 h in the light.The results described in this paper do not address the mechanism responsible for these drops.(https://www.daowen.com)

Many developmental and metabolic changes are triggered by illumination of etiolated seedlings.For example，some limited photosynthetic activity begins within a few minutes after commencement of illumination.The optimal experimental arrangement is to induce a change large enough to measure with confidence in the parameter under study while eliciting no，or only the smallest possible，changes in other，possibly interacting，systems.We have chosen experimental conditions close to the minimum needed to elicit clearly measurable changes in RNA pool sizes in order to minimize possible effects of secondary changes on RNA metabolism.Earlier work showed that illumination of etiolated maize seedlings with white light for 1 min initiated plastid maturation effects that were completed over a period of about 3 h in darkness，and could be reinforced by an additional brief exposure to light［14］.

In the present experiments，we have measured only RNA pool sizes because of technical limitations that do not permit us to determine rates of transcription in vivo.The size of each pool reflects the sum of synthesis and destruction.It seems unlikely that changes in pool sizes observed here result from constant rates of synthesis of photogene RNAs coupled with photoinduced changes in RNA lifetimes without postulating rises and falls in the activities of a number of ribonucleases that act on specific transcnpts.First，RNA pools for most maize plastid genes do not change significantly during light-induced development of etioplasts to chloroplasts［25］.Second，transcripts of at least one region of the maize plastid chromosome decrease，rather than increase，in amount after illumination（Lukens J，Bogorad L，unpublished data）.Thus，it seems most reasonable to conclude that phytochrome，acting somewhere in the cell，regulates one or more events leading to increases in the rates of transcription of maize plastid photogenes.This suggestion is also supported by Gallagher and Ellis's finding，using isolated nuclei from dark and light-grown pea leaves，that light increases the transcription of the genes cab and res［15］.