The 5''-proximal region of the wheat Cab-1 gene contains a 268-bp enhancer-like sequence for phytochrome response.

We have previously reported that the expression of the wheat Cab-1 gene is subject to phytochrome regulation and a 1.8-kb 5' upstream sequence of this gene is sufficient for the regulated expression. To delineate sequences for the phytochrome response we analyzed a series of 5' deletion mutants as well as chimeric gene constructs comprising different sequences of the Cab-1 upstream region in transgenic tobacco seedlings. We found that a deletion mutant containing a 357-bp 5' upstream sequence still exhibits maximal levels of phytochrome-regulated expression. A 268-bp enhancer-like element, located between -89 and -357, is responsible for the phytochrome response of the Cab-1 gene; sequences upstream from -357 to -843 and downstream from -124 to +1100 are probably not involved. Finally, we show that the Cab-1 mRNA stability is not regulated by phytochrome.     

Differences in Photoresponse and Phytochrome Spectrophotometry Between Etiolated and De-etiolated Pea Stem Tissue

Morphologically similar pea plants having a 4-fold difference in spectrophoto-metrically detectable phytochrome can be produced by pretreatment of etiolated plants with red light (R) or with red and far-red light combined (RF). A search for response differences which could be ascribed to differences in phytochrome content has resulted only in the establishment of differences due to de-etiolation. Segments of etiolated plants differ from those of plants de-etiolated by R and RF pretreatments in 2 ways. Segments from etiolated plants appear to respond rapidly to the far-red absorbing form of phytochrome (P_FR), while segments from de-etiolated plants do not respond rapidly to P_FR. This statement is based upon 2 observations: (i) the red light induced growth inhibition in segments from etiolated plants rapidly escapes reversibility by far-red light, while with segments from R or RF pretreated plants, the red light effect is fully reversed by subsequent far-red light for up to 2 hr; and (ii) segments from etiolated plants were inhibited to a greater degree than were segments from RF pretreated plants when various photostationary state levels of P_FR were maintained for 30 or 90 min and then removed by photoconversion to P_R. The in vivo nonphotochemical transformation curves of the phytochrome of etiolated and RF pretreated plants appear to differ in 2 related respects: (i) the amount of phytochrome destroyed in de-etiolated tissue is greater than that in etiolated tissue, perhaps as a result of the fact that (ii) the rate and extent of apparent reversion of P_FR to P_R in etiolated tissue is about twice that in de-etiolated tissue.     

G-proteins in etiolated Avena seedlings. Possible phytochrome regulation

The molecular mechanism of light signal transduction in plants mediated by the photosensor phytochrome is not well understood. The possibility that phytochrome initiates the signal transduction chain by modulating a G-protein-like receptor is examined in the present work. Etiolated Avena seedlings contain G-proteins as examined in terms of the binding of GTP as well as by cross-reaction with mammalian G-protein antibodies. The binding of GTP was regulated in vivo by red/far-red light. The possible involvement of G-proteins in the phytochrome-mediated signal transduction in etiolated Avena seedlings has been implicated from the study of the light regulated expression of the Cab and phy genes.     

Light-grown plants of transgenic tobacco expressing an introduced oat phytochrome A gene under the control of a constitutive viral promoter exhibit persistent growth inhibition by far-red light

A comparison of the photoregulation of development has been made for etiolated and light-grown plants of wild-type (WT) tobacco (Nicotiana tabacun L.) and an isogenic transgenic line which expresses an introduced oat phytochrome gene (phyA) under the control of a constitutive viral promoter. Etiolated seedlings of both the WT and transgenic line showed irradiance-dependent inhibition of hypocotyl growth under continuous far-red (FR) light; transgenic seedlings showed a greater level of inhibition under a given fluence rate and this is considered to be the result of the heterologous phytochrome protein (PhyA) functioning in a compatible manner with the native etiolated phytochrome. Deetiolation of WT seedlings resulted in a loss of responsiveness to prolonged FR. Light-grown transgenic seedlings, however, continued to respond in an irradiance-dependent manner to prolonged FR and it is proposed that this is a specific function of the constitutive PhyA. Mature green plants of the WT and transgenic lines showed a qualitatively similar growth promotion to a brief end-of-day FR-treatment but this response was abolished in the transgenic plants under prolonged irradiation by this same FR source. Growth inhibition (McCormac et al. 1991, Planta 185, 162–170) and enhanced levels of nitrate-reductase activity under irradiance of low red:far-red ratio, as achieved by the FR-supplementation of white light, emphasised that the introduced PhyA was eliciting an aberrant mode of photoresponse compared with the normal phytochrome population of light-grown plants. Total levels of the oat-encoded phytochrome in the etiolated transgenic tobacco were shown to be influenced by the wavelength of continuous irradiation in a manner which was qualitatively similar to that seen for the native, etiolated tobacco phytochrome, and distinct from that seen in etiolated oat tissues. These results are discussed in terms of the proposal that the constitutive oat-PhyA pool in the transgenic plants leads to a persistence of a mode of response normally restricted to the situation in etiolated plants.Abbreviations FR far-red light - R red light - WL white light - WL + FR white light supplemented with FR - HIR high-irradiance response - PAR photosynthetically active radiation - Pr, Pfr R- and FR-absorbing forms of phytochrome - Ptot total phytochrome - phyA (PhyA) gene (encoded protein) for phytochrome - WT wild type This work was supported by an Agricultural and Food Research Council research grant to H.S. and A.M.; J.R. Cherry and R.D. Vierstra, (Department of Horticulture, University of Wisconsin-Madison, USA) are thanked for the provision of the transgenic tobacco line.     

Phytochrome is not involved in the red-light-enhancement of the stomatal blue-light-response in wheat seedlings

The stomatal response to blue light (BL) in wheat seedlings ( Triticum aestivum L. cv. Starke II, Weibull) was enhanced by background red light (R). This enhancement was only slightly affected by the addition of background far-red light (FR). Under similar light treatments, the addition of FR induced a 43% transformation from the far-red-absorbing form towards the red-absorbing form of phytochrome from etiolated oat ( Avena sativa L. cv. Sol II), immobilized on phenyl-sepharose. Furthermore, the enhancement of the stomatal BL-response by 15 min R was not reversed by a subsequent irradiation with 5 min FR. It is concluded that the red-light-enhancement of the stomatal blue-light-response in wheat seedlings does not involve a change in the photostationary state of phytochrome.     

Origins of phytochrome-modulated Lhcb mRNA expression in seed plants

The levels of Lhcb mRNA in higher plants are regulated by phytochrome, cryptochrome, and an endogenous circadian oscillator. To determine whether similar regulatory mechanisms operate in the ancient gymnosperm Ginkgo biloba, we measured Lhcb mRNA levels in seedlings in response to different light conditions. Removal of a diurnally oscillating light stimulus caused dampening of maximal Lhcb mRNA accumulation levels, with little change in periodicity. Although low fluence pulses of both red and blue light given to etiolated seedlings caused maximal accumulation of Lhcb mRNAs characteristic of the phasic/circadian response seen in flowering plants, the additional initial acute response seen in flowering plants was absent. The induction of Lhcb gene expression in both cases was at least partially reversible by far-red light, and appeared biphasic over a range of red fluences. Together, these data indicate that Lhcb genes in G. biloba appear to be regulated in a manner similar to that of flowering plants, whereas signaling and attenuation of mRNA levels through the photoreceptor systems and circadian clock show features distinct from those characterized to date. The implications for these findings are discussed in light of the evolution of circadian clock input signaling.