Control by Phytochrome of Cytoplasmic and Plastid rRNA Accumulation in Cotyledons of Mustard Seedlings in the Absence of Photosynthesis

Abstract

Light stimulates the accumtulation of cytoplasmic and plastid rRNA in the cotyledons of mustard (Sinapis alba L.) seedlings. This effect is mediated by phytochrome. Continuous far red irradiation and brief red light pulses (the effect of which is reversible by brief far red light pulses) increase the amount of 1.3 X 106 + 0.7 X 106 and 1.1 X 101 + 0.56 X 106 molecular weight rRNA species. Large and small ribosomal subunit RNAs are maintained in a 1 :1 molar ratio in both cytoplasm and plastids, irrespective of seedling age and light treatment. Continuous white fluorescent light (which saturates chlorophyll synthesis) is less effective than contintuous far red light (essentially no chlorophyll synthesis) in producing the response, indicating that the accumulation of plastid rRNA is independent of the buildup of a functional photosynthetic apparatus. We conclude that the transcription of cytoplasmic and plastid rRNA cistrons is uinder phytochrome control.The involvement of phytochrome in the control of RNA metabolism of higher plants is well documented (2,10,20,27,32). So far there is no information regarding the species of RNA affected or the mechanism(s) by which the observed responses on the RNA level are produced. This situation is especially unsatisfactory because the only general hypothesis put forward until now to explain the phytochrome-mediated photomorphogenesis in molecular terms postulates specific derepression and repression of gene activities as a mechanism of phytochrome control. This "hypothesis of differential gene activation and gene repression" by Pfr (16,25)

Transcriptional regulation of a gene encoding the small subunit of ribulose-1,5-bisphosphate carboxylase in soybean tissue is linked to the phytochrome response.

“…This calculation is comparable to the data of Silverthorne and Tobin (25) and Gallagher and Ellis (10), who have estimated that 40 and 64%, respectively, of the total transcription in isolated light-grown nuclei is alphaamanitin resistant. We did not observe an increase in rRNA transcription in the light as has been seen in pea, L. gibba, and mustard plants (10,25,31,32).…”

“…We examined the effects of light on rRNA synthesis in soybean plants, since exposure to light increases rRNA synthesis above dark-grown levels in some plant systems (10,20,25,31,32). In nuclei from plants grown in both light and darkness, approximately 60 to 80%o of the transcription was resistant to an alpha-amanitin concentration of 1 ,ug/ml.…”

Control by Phytochrome of Cytoplasmic Precursor rRNA Synthesis in the Cotyledons of Mustard Seedlings

“…Seedlings of mustard (Sinapis alba L.; same batch of seeds) were grown under axenic conditions at 25°C and irradiated with continuous standard far-red light (3.5 wm-2) as described previously (23). No (1100 TBq mol-1; Amersham-Buchler, Braunschweig) in a 4-cmdiameter petri dish on the shaker.…”

“…For instance, since rRNA can be subject to turnover (15,26), the measured increase of accumulation could be due either to an enhancement of the formation or a slowing down of the degradation of rRNA. The occurrence of degradation of cytoplasmic rRNA is clearly shown in older mustard cotyledons (>60 h after sowing), but might be masked by rRNA synthesis before this time (23). Previous attempts to demonstrate a phytochrome-mediated stimulation of de novo synthesis of RNA have failed (6) or have been regarded as growth effects not directly related to phytochrome action (13).…”

“…In a previous paper (23), we have shown that light, via phytochrome, stimulates the accumulation of cytoplasmic and plastid rRNA in the cotyledons of mustard seedlings. The details of the molecular mechanism of these photomorphogenetic responses are still unclear.…”