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

Photoperception and de-etiolation

In seedlings or sprouts of higher plants, photomorphogenesis is the strategy of development if and as long as abundant light is available, and scotomorphogenesis (etiolation) is the developmental strategy of choice as long as light is not yet, or no longer, available. The transition from scotomorphogenesis to photomorphogenesis (called de-etiolation) can be considered a process in which a single, well defined environmental factor causes a plant to change its pattern of gene expression. The present article focuses on the question: what is the photosensory system, including photoreception and signal transduction, through which a plant can detect those light conditions that justify the (gradual) shift from scotomorphogenesis to photomorphogenesis, i.e. de-etiolation, which implies a strong and partly irreversible investment of m atter and energy? The significance of phytochrome for signal reception, the mode of signal expression, and the time course of signal transduction in phytochromemediated responses are reviewed briefly. The emphasis is on amplification of the phytochrome signal by red, blue and ultraviolet light (measured as responsivity amplification) because these recent findings may lead to a better understanding of the responses of plants under natural light conditions.

Expression of AP 3, 4 and 5 isophytochromes in etiolated oat seedlings (Avena sativa L.)

Type 1 phytochrome from etiolated oat seedlings was digested with V-8 protease. Microsequencing of a 13 kDa fragment yielded a sequence of 31 amino acids. The fragment starting with the alanine residue at position 427 of the entire phytochrome amino acid sequence revealed a heterogeneity (threonine, alanine and asparagine) at position 10. This demonstrates that the phytochrome type A genes AP3, 4 and 5 are expressed as proteins.

Phytochrome and protein phosphorylation

The molecular mode of signal transduction triggered by phytochrome is unknown. One characteristic structural/topographic feature of the physiologically active form (Pfr) of phytochrome is that its tetrapyrrole chromophore becomes preferentially exposed in the Pfr form (compared to the Pr form). Phytochrome in its Pfr form appears to affect phosphorylation of cellular proteins. The literature on the phytochrome-mediated protein phosphorylation has been reviewed in an attempt to search for the role of the chromophore topography of phytochrome in the signal transduction process. In order to initiate a dephosphorylation-phosphorylation cascade as a possible step for the signal transduction, it may interact with a cellular protein kinase to inhibit its activity. This hypothesis has been reviewed with results from phosphorylation inhibition assays by the Pfr form of phytochrome and in light of the inhibition of protein kinase activity by tetrapyrroles in general.

Nucleotide sequence and expression of the phytochrome gene in Pisum sativum: Differential regulation by light of multiple transcripts

Complementary DNA and genomic DNA that code for phytochrome apoprotein in Pisum sativum cv. Alaska were cloned in λgt11 and λEMBL3, respectively, and sequenced. Southern and slot-blot analysis of pea nuclear DNA showed that there was only one copy of phytochrome gene (phy) per haploid genome. The phy gene, which consisted of five exons and four short introns, coded for a polypeptide comprising 1124 amino acid residues. Detailed analysis of the 5' end of phy transcripts by nuclease S1 mapping and primer extension demonstrated the presence of three distinct transcripts (RNAs 1,2, and 3) differing in the length of the 5' non-coding sequence (63, 285, and about 465 nucleotides long, respectively). A short upstream open reading frame was found in RNA 2, while RNA 3 contained an additional upstream open reading frame. The relative levels of RNAs 1, 2, and 3 per unit weight of poly(A)(+) RNA, which were semi-quantitatively estimated by nuclease S1 mapping, primer extension, and Northern hybridization, were 88, 11, and 1 in the dark, and 1, 6, and 6 in the light, respectively. Shifting of the plants from the dark to the light confirmed that the levels of the three transcripts were regulated by light in different ways, namely, elevated (RNA 3), weakly reduced (RNA 2), and strongly reduced (RNA 1).

Phytochrome action in light-grown mustard: kinetics, fluence-rate compensation and ecological significance

Internode extension in young, light-grown mustard plants was measured continuously to a high degree of resolution using linear voltage displacement transducers. Plants were grown in background white light (WL) and the first internode was irradiated with supplementary far-red (FR) from fibre-optic light guides, depressing the Pfr/P (ratio of FR-absorbing form of phytochrome to total spectrophotometrically assayable phytochrome) within the internode and causing an acceleration of extension rate. The internode was sensitive to periods of FR as brief as 1 min, with a sharp increase in extension rate occurring after the return to background WL only. The mean latent period of the response to FR was approx. 10 min. Periods of FR longer than approx. 35 min caused an apparently biphasic growth response, with an initial sharp acceleration in extension rate (Phase 1) being followed by a brief deceleration and a further acceleration to a more-or-less steady elevated rate, somewhat less than the first peak (Phase 2). With such longer-term FR, extension rate decelerated upon FR switch-off after a mean lag of approx. 6 min, achieving the prestimulation extension rate within 16 min. The magnitude of the FR-induced increase in extension rate, expressed as a percentage of the rate in WL alone, was an inverse, linear function of the phytochrome photoequilibrium (i.e. Pfr/P, measured in etiolated test material irradiated under the same geometry) over the range 0.17 to 0.63. This relationship was not significantly affected by variations in backround WL fluence rate over the range 50-150 μmol·m(-2)·s(-1) and was held both for Phase 1 and Phase 2 of the response. The data provide evidence for rapid coupling/uncoupling between phytochrome and its transduction chain in the light-grown plant and for fluence-rate compensation of the regulation of extension rate. The extensive linearity of the relationship between phytochrome photoequilibrium and proportional extension rate increment allows for fine tuning in shade avoidance. The results are discussed with respect to recent evidence on the nature of phytochrome in light-grown plants and in relation to the function of phytochrome in plants growing in the natural environment.

Phytochrome and the regulation of the expression of its genes

In attempting to understand the mechanism of phytochrome action we are studying structural properties of the photoreceptor molecule and the autoregulation of expression of phytochrome genes. Run-off transcription assays in isolated nuclei from Avena indicate that phytochrome decreases the transcription of its own genes threefold in less than 15 min form Pfr formation. The extent of this decrease is insufficient to account for the observed 10- to 50-fold decrease in mature phytochrome mRNA levels, suggesting that enhanced degradation may also play a significant role in determining the level of this mRNA. Structural analysis of native phytochrome from Avena indicates that the molecule is an elongated dimer of 124 kDa monomers, each consisting of a globular, 74 kDa, NH2-terminal domain bearing the single chromophore at Cys-321, and a more open COOH-terminal domain that bears the dimerization site. Controlled proteolysis and binding of monoclonal antibodies to mapped epitopes has identified two regions, one in the 6-10 kDa NH2-terminal segment and the other ca. 70 kDa from the NH2-terminus, that undergo photoconversion-induced conformational changes and are therefore candidates for involvement in the molecule's regulatory function. Comparison of the full-length amino acid sequences of Avena and Cucurbita phytochromes, derived from nucleotide sequence analysis, indicates overall homology of 65%. The most highly conserved regions are those immediately surrounding the chromophore attachment site, where 29 residues are invariant, and a hydrophobic region between residues 150 and 300, postulated to form a cavity containing the chromophore. In contrast, a strikingly lower level of homology exists at the COOH-terminus of the polypeptide between residues 800 and 1128, indicating a possible lack of involvement of this region in phytochrome function.

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.

Analysis of microsomal flavoproteins from Phycomyces sporangiophores: Candidates for the blue-light photoreceptor

To help identify components of the blue-light photoreceptor system for phototropism in Phycomyces blakesleeanus Bgff., proteins from a microsomal fraction obtained from synchronous sporangiophores were studied. By two-dimensional gel electrophoresis, two proteins (FP1, FP2) with covalently bound flavins were found. FP1 has a molecular weight of 71 000 and an isoelectric point of 6.6; FP2 has a molecular weight of 59 000 and an isoelectric point of 6.5. These flavoproteins were purified by column chromatography and gel filtration while assaying for flavins by fluorescence. The relative concentrations of FP1 and FP2 were affected by light applied during growth. These flavoproteins are likely components of the blue-light photoreceptor complex mediating phototropism in Phycomyces.

Phytochrome regulation of phytochrome mRNA abundance

Pure phytochrome RNA sequence synthesized in an SP6-derived in vitro transcription system has been used as a standard to quantitate phytochrome mRNA abundance in Avena seedlings using a filter hybridization assay. In 4-day-old etiolated Avena seedlings phytochrome mRNA represents ∼0.1% of the total poly(A)(+) RNA. Irradiation of such seedlings with a saturating red-light pulse or continuous white light induces a decline in this mRNA that is detectable within 30 min and results in a 50% reduction by ∼60 min and >90% reduction within 5 h. The effect of the red-light pulse is reversed, approximately to the level of the far-red control, by an immediately subsequent far-red pulse. In seedlings maintained in extended darkness after the red-light pulse, the initial rapid decline in phytochrome mRNA level is followed by a slower reaccumulation such that 50-60% of the initial abundance is reached by 48 h. White-light grown seedlings transferred to darkness exhibit a similar accumulation of phytochrome mRNA that is accelerated by removal of residual Pfr with a far-red light pulse at the start of the dark period. The data establish that previously reported phytochrome-regulated changes in translatable phytochrome mRNA levels result from changes in the physical abundance of this mRNA rather than from altered translatability.

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.