Spectrophotometric phytochrome measurements in light-grown Avena sativa L

Thermal Reversion of Plant Phytochromes

Phytochromes are red/far-red reversible photoreceptors essential for plant growth and development. Phytochrome signaling is mediated by the physiologically active far-red-absorbing Pfr form that can be inactivated to the red-absorbing Pr ground state by light-dependent photoconversion or by light-independent thermal reversion, also termed dark reversion. Although the term "dark reversion" is justified by historical reasons and frequently used in the literature, "thermal reversion" more appropriately describes the process of light-independent but temperature-regulated Pfr relaxation that not only occurs in darkness but also in light and is used throughout the review. Thermal reversion is a critical parameter for the light sensitivity of phytochrome-mediated responses and has been studied for decades, often resulting in contradictory findings. Thermal reversion is an intrinsic property of the phytochrome molecules but can be modulated by intra- and intermolecular interactions, as well as biochemical modifications, such as phosphorylation. In this review, we outline the research history of phytochrome thermal reversion, highlighting important predictions that have been made before knowing the molecular basis. We further summarize and discuss recent findings about the molecular mechanisms regulating phytochrome thermal reversion and its functional roles in light and temperature sensing in plants.

Perception of shade

Plants perceive shade by responding to both the fluence rate and to the spectral quality of the natural radiation environment. Changes in fluence rate are perceived by separate photoreceptors absorbing in both the blue and the red wavebands. The identity of the photoreceptor (or photoreceptors) responding to changes in the fluence rate of blue light is unknown (see Briggs, this volume). Physiological responses to changes in the fluence rate in the red waveband appear to be mediated through phytochrome. The relative roles played by the blue-light-absorbing photoreceptor and phytochrome in determining the response to changes in fluence rate varies between species and organs and is also dependent on the physiological age of the plant. Evidence is also presented that supports the concept that phytochrome functions to perceive the specific form of shade caused by surrounding competitive vegetation.

Phytochrome: molecular properties and biogenesis

Native Avena phytochrome, recently shown to have a monomeric molecular mass of 124 kDa, has molecular properties that differ significantly from those of the extensively characterized ‘ 120’ kDa or ‘large’ phytochrome preparations now known to contain a mixture of proteolytically degraded 118 and 114 kDa polypeptides. For example, 124 kDa phytochrome has a blocked N-terminus, a P fr λ max of 730 nm, a higher photostationary state in red light (86% P fr ), exhibits no dark reversion and shows no differential reactivity of P r and P fr toward a chemical probe of hydrophobic domains. The data indicate that the proteolytically removed 6-10 kDa polypeptide segment (s) is critical to the spectral and structural integrity of the photoreceptor; that at least part of the cleaved domain is located at the N-terminus of the molecule; that this domain influences the chemical reactivity of the chromophore with the external medium; and that a current hypothesis that P r —P fr photoconversion results in the exposure of a hydrophobic domain on the molecule is inconsistent with the properties of native phytochrome. Phytochrome has been found to exert rapid negative feedback control over the level of its own translatable mRNA. P fr formation in etiolated tissue causes a decline in translatable phytochrome mRNA that is detectable within 15—30 min and that results in more than a 95 % reduction within 2 h. Less than 1 % P fr is sufficient to induce 60 % of the maximum response, which is saturated at 20 % P fr or less. The rapidity of this autoregulatory control makes phytochrome itself an attractive system for investigating phytochrome-regulated gene expression. A project to clone phytochrome complementary DNA (cDNA) has been initiated. A major obstacle in this work has been the unexpectedly low abundance of phytochrome mRNA, less than 0.005 % of the poly (A) RNA in etiolated tissue. cDNA made from poly (A) RNA enriched ca. 200-fold in phytochrome mRNA has been cloned and bacterial colonies have been screened with a synthetic oligodeoxynucleotide hybridization probe. The sequence of this probe was derived from a known partial amino acid sequence of the phytochrome protein. Difficulties encountered with this approach are discussed.

Temporal and light regulation of the expression of three phytochromes in germinating seeds and young seedlings of Avena sativa L

An oat (Avena sativa L.) plant contains at least three phytochromes, which have monomeric masses of 125, 124, and 123 kilodaltons (kDa) (Wang et al., 1991, Planta 184, 96-104). The 124-kDa phytochrome is most abundant in dark-grown seedlings, while the other two phytochromes predominate in light-grown seedlings. Using three monoclonal antibodies, each specific to one of the three phytochromes, we have monitored by immunoblot assay the expression of these three phytochromes in the 5 d following onset of imbibition of seeds. On a per-organism basis, each of these three phytochromes increased in abundance for the first 3 d in the light, or for the first 4 d in darkness, after which they each began to decrease in quantity. When 3-d-old dark-grown seedlings were transferred to the light, the abundance of each of these three phytochromes decreased both in absolute amount and relative to the phytochrome levels in control seedlings kept in darkness. In contrast, when 3-d-old light-grown seedlings were transferred to darkness, the abundance of the 124-kDa and 125-kDa phytochromes increased while that of 123-kDa phytochrome remained unchanged. In each case, the level of phytochrome was greater than that of control seedlings maintained in the light. Thus, in addition to temporal regulation, all three phytochromes exhibit photoregulated expression at the protein level, although the magnitude of this photoregulation varies substantially.

phyB is evolutionarily conserved and constitutively expressed in rice seedling shoots

Southern blot analysis indicates that the rice genome contains single copies of genes encoding type A (phyA) and type B (phyB) phytochromes. We have isolated overlapping cDNA and genomic clones encoding the entire phyB polypeptide. This monocot sequence is more closely related to phyB from the dicot, Arabidopsis (73% amino acid sequence identity), than it is to the phyA gene in the rice genome (50% identity). These data support the proposal that phyA and phyB subfamilies diverged early in plant evolution and that subsequent divergence accompanied the evolution of monocots and dicots. Moreover, since rice and Arabidopsis phyB polypeptides are more closely related to one another (73% identity) than are monocot and dicot phyA sequences (63-65% identity), it appears that phyB has evolved more slowly than phyA. Sequence conservation between phyA and phyB is greatest in a central core region surrounding the chromophore attachment site, and least toward the amino-terminal and carboxy-terminal ends of the polypeptides, although hydropathy analysis suggests that the overall structure of the two phytochromes has been conserved. Gene-specific Northern blot analysis indicates that, whereas phyA is negatively regulated by phytochrome in rice seedling shoots in the manner typical of monocots, phyB is constitutively expressed irrespective of light treatment. In consequence, phyA and phyB transcripts are equally abundant in fully green tissue. Since Arabidopsis phyB mRNA levels are also unaffected by light, the present results suggest that this mode of regulation is evolutionarily conserved among phyB genes, perhaps reflecting differences in the functional roles of the different phytochrome subfamilies.

Phytochrome in green tissue: Spectral and immunochemical evidence for two distinct molecular species of phytochrome in light-grown Avena sativa L

A method is described for the extraction of phytochrome from chlorophyllous shoots of Avena sativa L. Poly(ethyleneimine) and salt fractionation are used to reduce chlorophyll and to increase the phytochrome concentration sufficiently to permit spectral and immunochemical analyses. The phototransformation difference spectrum of this phytochrome is distinct from that of phytochrome from etiolated shoots in that the maximum in the red region of the difference spectrum is shifted about 15 nm to a shorter wavelength. Immunochemical probing of electroblotted proteins (Western blotting), using a method sensitive to 50 pg, demonstrates the presence of two polypeptides in green tissue that bind antiphytochrome antibodies: a predominant species with a relative molecular mass (Mr) of 118000 and a lesser-abundant 124000-Mr polypeptide. Under nondenaturing conditions all of the 124000-Mr species is immunoprecipitable, but the 118000-Mr species remains in the supernatant. Peptide mapping and immunochemical analysis with monoclonal antibodies show that the 118000-Mr species has structural features that differ from etiolated-oat phytochrome. Mixing experiments show that these structural differences are intrinsic to the molecular species from these two tissues rather than being the result of post-homogenization modifications or interfering substances in the green-tissue extracts. Together the data indicate that the phytochrome that predominates in green-tissue has a polypeptide distinct from the well-characterized molecule from etiolated tissue.

Immunochemical detection with rabbit polyclonal and mouse monoclonal antibodies of different pools of phytochrome from etiolated and green Avena shoots

While two monoclonal antibodies directed to phytochrome from etiolated oat (Avena sativa L.) shoots can precipitate up to about 30% of the photoreversible phytochrome isolated from green oat shoots, most precipitate little or none at all. These results are consistent with a report by J.G. Tokuhisa and P.H. Quail (1983, Plant Physiol. 72, Suppl., 85), according to which polyclonal rabbit antibodies directed to phytochrome from etiolated oat shoots bind only a small fraction of the phytochrome obtained from green oat shoots. The immunoprecipitation data reported here indicate that essentially all phytochrome isolated from green oat shoots is distinct from that obtained from etiolated oat shoots. The data indicate further that phytochrome from green oat shoots might itself be composed of two or more immunochemically distinct populations, each of which is distinct from phytochrome from etiolated shoots. Phytochrome isolated from light-grown, but norflurazon-bleached oat shoots is like that isolated from green oat shoots. When light-grown, green oat seedlings are kept in darkness for 48 h, however, much, if not all, of the phytochrome that reaccumulates is like that from etiolated oat shoots. Neither modification during purification from green oat shoots of phytochrome like that from etiolated oat shoots, nor non-specific interference by substances in extracts of green oat shoots, can explain the inability of antibodies to recognize phytochrome isolated from green oat shoots. Immunopurified polyclonal rabbit antibodies to phytochrome from etiolated pea (Pisum sativum L.). shoots precipitate more than 95% of the photoreversible phytochrome obtained from etiolated pea shoots, while no more than 75% of the pigment is precipitated when phytochrome is isolated from green pea shoots. These data indicate in preliminary fashion that an immunochemically unique pool of phytochrome might also be present in extracts of green pea shoots.

Phytochrome quantitation in crude extracts of Avena by enzyme-linked immunosorbent assay with monoclonal antibodies

An enzyme-linked immunosorbent assay (ELISA), which uses both rabbit polyclonal and mouse monoclonal antibodies to phytochrome, has been adapted for quantitation of phytochrome in crude plant extracts. The assay has a detection limit of about 100 pg phytochrome (<1 fmol of monomer) and can be completed within 10 h. Nonspecific interference by crude plant extracts was detected and corrected for. Quantitation of phytochrome in crude extracts of etiolated oat (Avena sativa L.) seedlings by ELISA gave values that agreed well with those obtained by spectrophotometric assay. When etiolated oat seedlings were irradiated continuously for 24 h, the amount of phytochrome detected by ELISA and by spectrophotometric assay in crude extracts of these seedlings decreased by more than 1000-fold and about 100-fold, respectively. This discrepancy indicates that phytochrome in light-treated plants may be antigenically distinct from that found in fully etiolated plants. Both a decrease in the light and an increase in the dark of phytochrome content was observed in crude extracts of light-grown oat shoots, both green and Norflurazon-bleached, in response to a 12:12-h light-dark cycle. When these light-grown oat seedlings were kept in darkness for 48 h, phytochrome content detected by ELISA increased by 50-fold in crude extracts of green oat shoots, but only about 12-fold in extracts of herbicide-treated oat shoots. Phytochrome reaccumulation in green oat shoots was initially more rapid in the more mature cells of the primary leaf tip than near the basal part of the shoot. The inhibitory effect of Norflurazon on phytochrome accumulation was much more evident near the leaf tip than the shoot base. A 5-min red irradiation of oat seedlings at the end of a 48-h dark period resulted in a subsequent, massive decrease in phytochrome content in crude extracts from both green and Norflurazon-bleached oat shoots. These observations eliminate the possibility that substantial accumulation of chromophore-free phytochrome was being detected and indicate that Norflurazon has a substantial effect on phytochrome accumulation during a prolonged dark period.

Analysis of phytochrome kinetics in light-grown Avena sativa L. seedlings

The phytochrome content, the rate of phytochrome accumulation after a light/dark transition and the rate of phytochrome destruction after a 1.5 d reaccumulation period in darkness were measured in light grown Avena sativa L. seedlings. The results using spectrophotometrical methods (Norflurazon treated seedlings) and the radio-immunoassay (RIA) (green seedlings) were almost identical. The rate of phytochrome synthesis was analysed by measuring the activity of poly(A(+))-RNA coding for the phytochrome apoprotein. It was demonstrated that the rate of phytochrome synthesis is different in light and in dark. These results were confirmed by measuring the incorporation of radioactive label in vivo. Five minutes red (and 5 min far-red) light strongly reduces the rate of phytochrome synthesis. Even after prolonged dark periods only 50% of the initial rate of phytochrome synthesis is recovered for light and dark grown seedlings which received one red light pulse.

Integral association of phytochrome with a membranous fraction fromAvena shoots: in vivo characterization and physiological significance

Phytochrome in the far-red light absorbing form (Pfr) was observed to disappear in vivo more rapidly from the non-cation-requiring pelletable phytochrome population than from the supernantant phytochrome population of oat seedlings given an increasing dark incubation after red irradiation. The amount of pelletable phytochrome in the red light absorbing form (Pr) remained relatively stable while supernatant Pr was lost. These observations indicated that supernant Pfr was subject to loss during the incubation, while pelletable Pfr was subject to both dark reversion and loss.During the incubation, the ability of far-red irradiation to reverse the red-induced increase in phytochrome pelletability was lost, with kinetics similar to those of the loss of pelletable Pfr.Far-red reversibility of the red-induced increase in coleoptile elongation correlated with the change intotal Pfr in both supernatant and pelletable phytochrome populations, but with the change in the ratio of Pfr to total phytochrome only in the pelletable phytochrome population.The possible significance of these results is discussed with reference to the action of phytochrome in the photocontrol of physiological growth responses.

The influence of light quality on the phytochrome content of light grown Sinapis alba L. and Phaseolus aureus Roxb

The effect on the phytochrome system of light regimes establishing a range of photoequilibria was studied in two light grown dicotyledonous plants, both of which were treated with the herbicide SAN 9789 to prevent chlorophyll accumulation. In Sinapis alba L. cotyledons the results are comparable with phytochrome behaviour in etiolated mustard seedlings; the level of Pfr becomes independent of wave-length whereas the total phytochrome level is wave-length dependent. Contrasting properties are exhibited in Phaseolus aureus Roxb. leaves in which total phytochrome is unaffected by light quality; consequently the Pfr level is dependent on wavelength. Nevertheless, the amount of phytochrome in mung leaves increased after transfer to darkness suggesting that light still has a profound influence on the phytochrome system, even though light quality during the light period and prior to darkness does not.

The phytochrome system in light-grown Zea mays L

The phytochrome system is analyzed in light-grown maize (Zea mays L.) plants, which were prevented from greening by application of the herbicide SAN 9789. The dark kinetics of phytochrome are not different in the first, second or third leaf. It is concluded that in light-grown maize plants phytochrome levels are regulated by Pr formation and Pfr and Pr destruction, rather than by Pfr→Pr dark reversion. Pr undergoes destruction after it has been cycled through Pfr. The consequences of this Pr destruction on the phytochrome system are discussed.

Inhibition of carotenoid biosynthesis by the herbicide SAN 9789 and its consequences for the action of phytochrome on plastogenesis

Treatment of the mustard (Sinapis alba L.) seedling with the herbicide SAN 9789 inhibits synthesis of colored carotenoids and interferes with the formation of plastid membrane lipids without affecting growth and morphogenesis significantly. In farred light, which is hardly absorbed by chlorophyll, development of plastid ultrastructure, synthesis of ribulosebisphosphate carboxylase and synthesis of chlorophyll are not affected by SAN 9789. It is concluded that normal phytochrome actions on plastid structural development, protein and chlorophyll syntheses are not affected by the absence of carotenoids provided that there is no significant light absorption in chlorophyll. The findings show that the inhibition of synthesis of one set of plastid membrane components (the carotenoids) does not stop synthesis of other components such as chlorophyll and does not halt membrane assembly. Supplementary experiments with the closely related compound SAN 9785, which affects the amount and composition of plastid lipids but not carotenoid and chlorophyll syntheses, suggest that the effect of the herbicide SAN 9789 is due exclusively to its inhibition of synthesis of colored carotenoids. In the presence of SAN 9789 white or red light at high fluence rate causes photodestruction of chlorophyll and ribulosebisphosphate carboxylase and photodecomposition of thylakoids. These effects are interpreted as resulting exclusively from the self-photooxidation and photosensitizing action of chlorophyll once the protection by carotenoids of chlorophyll against self- and sensitized photooxidation is lost.