Low Fluence Response Research Articles

Chloroplast-avoidance response induced by high-fluence blue light in prothallial cells of the fern adiantum capillus-veneris as analyzed by microbeam irradiation

Chloroplast movement was induced by partial cell illumination using a high-fluence blue microbeam in light-grown and dark-adapted prothallial cells of the fern Adiantum capillus-veneris. Chloroplasts inside the illuminated area moved out (high-fluence response [HFR]), whereas those outside moved toward the irradiated area (low-fluence response [LFR]), although they stopped moving when they reached the border. These results indicate that both HFR and LFR signals are generated by high-fluence blue light of the same area, and that an LFR signal can be transferred long-distance from the beam spot, although an HFR signal cannot. The lifetime of the HFR signal was calculated from the traces of chloroplast movement induced by a brief pulse from a high-fluence blue microbeam to be about 6 min. This is very short compared with that of the LFR (30-40 min; T. Kagawa, M. Wada [1994] J Plant Res 107: 389-398). These data indicate that the signal transduction pathways of the HFR and the LFR must be distinct.

In Vivo Characterization of Chimeric Phytochromes in Yeast

Phytochromes are plant photoreceptors that play a major role in photomorphogenesis. Two members of the phytochrome family have been characterized in some detail. Phytochrome A, which controls very low fluence and high irradiance responses, is rapidly degraded in the light, forms sequestered areas of phytochrome (SAPs), and does not exhibit dark reversion in monocotyledonous seedlings. Phytochrome B mediates red/far-red reversible responses, is stable in the light, and does not form SAPs. We report on the behavior in yeast of the phytochrome apoproteins of rice PHYA, tobacco PHYB, and chimeric PHYAB and PHYBA and on the behavior of the respective holoprotein adducts after assembly with phycocyanobilin chromophore (PHY*). SAP-like formation in yeast was not observed for PHYB, but was detectable for PHYA, PHYAB, and PHYBA. Rice PHYA* did not undergo dark reversion in yeast. Surprisingly, all other tested phytochrome constructs did exhibit dark reversion, including chimeric phytochromes with a short N-terminal part of tobacco PHYB or parsley PHYA fused to rice PHYA. Furthermore, the proportion of phytochrome undergoing dark reversion and the rate of reversion were increased for both the N terminus-swapped constructs and PHYBA*. These results are discussed with respect to structure/function analysis of phytochromes A and B.

Different Regulation of β-amylase and Starch Phosphorylase by Light in Dormant and Non-Dormant Turions of Spirodela polyrhiza

Summary The influence of single red light pulses and of continuous red light on the activity of two enzymes, i.e. β-amylase (EC 3.2.1.2) and starch phosphorylase (EC 2.4.1.1), was investigated in dormant and non-dormant turions of Spirodela polyrhiza. The activity of β-amylase was induced by continuous red light but not by red light pulses in dormant turions. In non-dormant turions, however, the activity of β-amylase following a single red light pulse was two times higher than after continuous red light. This high effectiveness of a single red light pulse, as compared with continuous red, represents a novel mode of regulation of enzyme activity by light. It is suggested that a low fluence response of phytochrome is abolished by a high irradiance response elicited by continuous red light. The light-induced increase in the activity of β-amylase is due to de novo synthesis of the enzyme as shown by Western blot analysis using polyclonal antibodies raised against β-amylase from Spirodela polyrhiza. The presence of light-activable precursor was excluded by showing that cysteine added to the extraction medium did not influence the activity of β-amylase. In contrast, the activity of starch phosphorylase was induced by continuous red light in dormant turions but decreased in non-dormant turions. During the cold after-ripening treatment turions qualitatively changed their response to light, becoming light-insensitive when partly after-ripened (21 days). In all instances, a red light pulse was ineffective in inducing starch phosphorylase. Isoforms of β-amylase and starch phosphorylase were separated by non-denaturating PAGE. No special induction by light of any specific isoform of β-amylase and starch phosphorylase was found in turions. Since both enzymatic activities and their regulation by light do not correlate with the breakdown of starch, we conclude that they are not directly involved in the regulation of starch degradation in turions.

Phytochrome‐mediated germination control of Datura stramonium L. seeds after seed burial

A series of experiments investigated the light sensitivity of buried weed seeds both within and outside the soil gas microenvironment. Light sensitivity of Datura stramonium L. seeds was found to be increased markedly by a period of soil burial; seeds showed a pronounced germination response even in far‐red light (724 nm). Despite this elevated light sensitivity (even to less than 0.01% of incident light), soil overlying buried seeds was sufficient to neutralize the germination trigger. Furthermore, in situ irradiation of buried seeds was found to be virtually ineffective as a germination trigger, showing that the gaseous environment surrounding buried seeds may represent the main obstacle to germination. It is suggested in this study that the soil environment restricts the removal of the germination‐inhibiting products of fermentation metabolism. This suggestion is supported by the finding that gas exchange within the seed:soil complex, achieved by flushing with nitrogen, partially restores seed light sensitivity. In addition, it was found that with increasing sowing depth, the germination response was attenuated and also became less dependent on seed phytochrome photoequilibrium conditions. This demonstrates that the phenomenon of germination photoinduction can be reversed by other environmental factors that reduce or eliminate phytochrome physiological activity. Finally, it was shown that seed very low fluence response (VLFR) is triggered only when gas exchange around buried seeds co‐occurs with light exposure.

Modes of action of phytochromes

through largely unknown pathways into molecular and physiological changes that modulate growth and developPhytochrome is a family of photoreceptors whose ment. Phytochromes posses two photo-interconvertible apoproteins are encoded by divergent genes. Three forms, the R-absorbing form (Pr) and the FR-absorbing modes of action of phytochromes, very-low-fluence form (Pfr) and are synthesised in the Pr form. In recent responses (VLFR), low-fluence responses (LFR) and years it has been established that phytochromes are a high-irradiance responses (HIR), have been considered small family of photoreceptors whose apoproteins are in the literature to define the quantitative relationship encoded by diVerent genes (Clack et al., 1994; Hauser between response and predicted levels of the far-red et al., 1995). Traditionally, three ‘modes of action’ of light absorbing form of phytochrome. The results of phytochromes have been described: very-low-fluence recent experiments with phytochrome mutant and responses ( VLFR); low-fluence responses (LFR) and transgenic seedlings are consistent with the notion high-irradiance responses (HIR). Assigning particular that phytochrome B, and in some cases other stable responses to particular molecular species of phytochromes phytochromes, mediate LFR whereas phytochrome A is one of the important areas of research in the field mediates both VLFR and HIR. The mechanisms of photomorphogenesis. This review seeks to bring involved in phytochrome A inductive responses (VLFR) together data on the modes of action of phytochrome versus HIR appear to be different in some ways. with information on diVerent phytochrome molecular However, VLFR, LFR and HIR can converge to control species. the transcriptional activity of a single gene promoter. The analysis of phytochrome transduction chains is The implications of these findings for the under- currently concentrating the eVorts of many laboratories. standing of phytochrome phototransduction are Experimental approaches include the characterization of discussed. the photoreceptor molecule, in particular the biological significance of diVerent domains (Cherry et al., 1993;

Transcription of Arabidopsis and wheat Cab genes in single tobacco transgenic seedlings exhibits independent rhythms in a developmentally regulated fashion.

Transcription of Cab genes has been previously shown to be regulated by a circadian oscillator coupled to the red light-absorbing plant photoreceptor phytochrome in various plant species. In addition, it has recently been suggested that rhythmic expression of the Cab genes could also be affected by a phytochrome-independent circadian oscillator in a developmentally regulated fashion. This study has shown that a red light-insensitive oscillator and a phytochrome-coupled circadian clock indeed coregulate the oscillating expression of individual Cab genes at the level of transcription at an early developmental stage. The study involved analysing the expression patterns of transgenes, containing short fragments of the Arabidopsis thaliana Cab2 or the wheat Cab-1 promoter fused to the firefly luciferase reporter gene, by a video-imaging system in single, etiolated tobacco seedlings. Germination and red/far-red light treatments applied between 12 and 36 h after sowing lead to the appearance of two independent circadian rhythms. These rhythms coexist, both exhibiting period lengths close to 25 h but phased differently. However, repeated red-light treatments given 60 h or later after sowing synchronize these free-running rhythms and induce a single new circadian oscillation. These data indicate that both oscillators regulate the expression of the Cab genes studied at the level of transcription and that the cis-acting element(s) of the wheat Cab-1 and A. thaliana Cab2 genes mediating these responses are located on short, 250 bp promoter regions. Furthermore, these red-light induced rhythms are also inducible by far-red light treatments alone. Therefore, in tobacco, the phytochrome-coupled oscillator is regulated, at least partially, by the very low fluence response of phytochrome A.

Influence of phytochrome on leaf monoterpene variation in Satureja douglasii

Abstract Phytochrome influenced monoterpene variation in mature leaves of Satureja douglasii in a chemotype-dependent manner. Phytochrome significantly increased the concentrations (mg g -1 leaf dry weight) of camphor, pulegone and carvone and the yield (total amount mg g -1 leaf dry weight) in the pulegone and carvone chemotypes. However, phytochrome did not influence monoterpene concentrations nor yield in the bicyclic or isomenthone chemotypes. The phytochrome action in the pulegone type is a low fluence response, whereas in the carvone type it is both a low and a very low fluence response. However, statistically, the composition (percent of total monoterpenes) that defines the chemotypes was not significantly influenced by phytochrome and supports other studies suggesting genetic control of the chemotype composition. The chemotypes tend to occur in a particular light environment, although one occurs across a wide range of them. The chemotype-dependent phytochrome response for monoterpene variation may result from adaptation to the light environments where the chemotypes usually occur.

The VLF loci, polymorphic between ecotypes Landsberg erecta and Columbia, dissect two branches of phytochrome A signal transduction that correspond to very‐low‐fluence and high‐irradiance responses

Phytochromes play a key role in the perception of light signals by plants. In this study, the three classical phytochrome action modes, i.e. very‐low‐fluence responses (VLFR), low‐fluence responses (LFR) and high‐irradiance responses (HIR), were genetically dissected using phyA and phyB mutants of Arabidopsis thaliana (respectively lacking phytochrome A or phytochrome B) and a polymorphism between ecotypes Landsberg erecta and Columbia. Seed germination and potentiation of greening, hypocotyl growth inhibition and cotyledon unfolding in etiolated seedlings of the ecotype Landsberg erecta showed biphasic responses to the calculated proportion of active phytochrome established by one light pulse or repeated light pulses. The first phase, i.e. the VLFR, was absent in the phyA mutant, normal in the phyB mutant (both in the Landsberg erecta background) and severely deficient in Columbia. The second phase, i.e. the LFR, was present in the phyA mutant, deficient in the phyB mutant and normal in Columbia. Under continuous far‐red light, HIR of etiolated seedlings were absent in phyA and normal in phyB and Columbia. The segregation of VLFR in recombinant inbred lines derived from a cross between Landsberg erecta and Columbia was analysed by MAPMAKER/QTL. Two quantitative trait loci, one on chromosome 2 (VLF1) and another on chromosome 5 (VLF2), were identified as responsible for the polymorphism. Phytochrome A is proposed to initiate two transduction pathways, VLFR and HIR, involving different cells and/or different molecular steps. This is the first application of the analysis of quantitative trait loci polymorphic between ecotypes to dissect transduction chains of environmental signals.

The VLF loci, polymorphic between ecotypes Landsberg erecta and Columbia, dissect two branches of phytochrome A signal transduction that correspond to very-low-fluence and high-irradiance responses.

Phytochromes play a key role in the perception of light signals by plants. In this study, the three classical phytochrome action modes, i.e. very-low-fluence responses (VLFR), low-fluence responses (LFR) and high-irradiance responses (HIR), were genetically dissected using phyA and phyB mutants of Arabidopsis thaliana (respectively lacking phytochrome A or phytochrome B) and a polymorphism between ecotypes Landsberg erecta and Columbia. Seed germination and potentiation of greening, hypocotyl growth inhibition and cotyledon unfolding in etiolated seedlings of the ecotype Landsberg erecta showed biphasic responses to the calculated proportion of active phytochrome established by one light pulse or repeated light pulses. The first phase, i.e. the VLFR, was absent in the phyA mutant, normal in the phyB mutant (both in the Landsberg erecta background) and severely deficient in Columbia. The second phase, i.e. the LFR, was present in the phyA mutant, deficient in the phyB mutant and normal in Columbia. Under continuous far-red light, HIR of etiolated seedlings were absent in phyA and normal in phyB and Columbia. The segregation of VLFR in recombinant inbred lines derived from a cross between Landsberg erecta and Columbia was analysed by MAPMAKER/QTL. Two quantitative trait loci, one on chromosome 2 (VLF1) and another on chromosome 5 (VLF2), were identified as responsible for the polymorphism. Phytochrome A is proposed to initiate two transduction pathways, VLFR and HIR, involving different cells and/or different molecular steps. This is the first application of the analysis of quantitative trait loci polymorphic between ecotypes to dissect transduction chains of environmental signals.

Light environment, phytochrome and germination of Datura stramonium L. seeds

The role in germination of the maternal pre-existing far-red absorbing form of phytochrome (Pfr) was studied in seeds harvested at different developmental stages. Datura stramonium seeds acquired marked dormancy in the later ripening stages, independently of phytochrome phototransformation. However, the balance between stimulators, such as Pfr, and inhibitors (probably abscissic acid and/or phenolic substances) in the stage prior to complete dehydration appeared to play a crucial role in the degree of progeny dormancy. Enrichment of the far-red (FR) wavelength (simulated canopy light) induced light-dependent dormancy if stage IV progeny were incubated in darkness in suboptimal conditions. In contrast, natural storage of seeds in soil led to a uniform degree of dormancy. An increase in photosensitivity owing to storage in soil appeared to be linked to simultaneous loss of inhibitors. It should be noted, however, that this extreme sensitivity to very low Pfr levels (724 nm monochromatic light pulse) was acquired only by seeds stored deep in the soil (typical very low fluence response). It is postulated that these reactions are induced by an elevated quantity of phytochrome typical of etiolated tissue, in conditions of near-absence of inhibitors.

Phytochrome Control of Anthocyanin Biosynthesis in Tomato Seedlings: Analysis Using Photomorphogenic Mutants

AbstractAnthocyanin biosynthesis has been studied in hypocotyls and whole seedlings of tomato (Lycoperskon esculentum Mill.) wild types (WTs) and photomorphogenic mutants. In white light (WL)/dark (D) cycles the fri1 mutant, deficient in phytochrome A (phyA), shows an enhancement of anthocyanin accumulation, whereas the tri1 mutant, deficient in phytochrome Bl (phyBl) has a WT level of anthocyanin. Under pulses of red light (R) or R followed by far‐red light (FR) given every 4 h, phyA is responsible for the non‐R/FR reversible response, whereas phyBl is partially responsible for the R/FR reversible response. From R and blue light (B) pretreatment studies, B is most effective in increasing phytochrome responsiveness, whereas under R itself it appears to be dependent on the presence of phyBl. Anthocyanin biosynthesis during a 24 h period of monochromatic irradiation at different flu‐ence rates of 4 day‐old D‐grown seedlings has been studied. At 660 nm the fluence rate‐response relationships for induction of anthocyanin in the WT are similar, yet complex, showing a low fluence rate response (LFRR) and a fluence rate‐dependent high irradiance response (HIR). The high‐pigment‐1 (hp‐1) mutant exhibits a strong amplification of both the LFRR and HIR. The fri1 mutant lacks the LFRR while retaining a normal HIR. In contrast, a transgenic tomato line overexpressing the oat PHYA3 gene shows a dramatic amplification of the LFRR. The tri1 mutant, retains the LFRR but lacks the HIR, whereas the fri1, tri1 double mutant lacks both components. Only an LFRR is seen at 729 nm in WT; however, an appreciable HIR is observed at 704 nm, which is retained in the tri1 mutant and is absent in the fri1 mutant, indicating the labile phyA pool regulates this response component.

Dual effect of phytochrome A on hypocotyl growth under continuous red light

ABSTRACTThe role of phytochrome A in the control of hypocotyl growth under continuous red light (Rc) was investigated using phyA and phyB mutants of Arabidopsis thaliana, which lack phytochrome A (phyA) or phytochrome B (phyB), respectively, and transgenic seedlings of Nicotiana tabacum overexpressing Avena phyA, compared to the corresponding wild type (WT). In WT seedlings of A. thaliana, hypocotyl growth inhibition showed a biphasic response to the fluence rate of Rc, with a brake at 10−2μmol m−2 s−1. At equal total fluence rate, hourly pulses of red light caused slightly more inhibition than Rc. The response to very low fluences of continuous or pulsed red light was absent in the phyA and phyA phyB mutants and present in the phyB mutant. The second part of the response was steeper in the phyA mutant than in the WT but was absent in the phyB mutant. In WT tobacco the response to Rc was biphasic. Overexpression of Avena phyA enhanced the response only at very low fluence rates of Rc (< 10−2μmol m−2 s−1). In both species, the effect of hourly pulses of far‐red light was similar to the maximum inhibition observed in the first phase of the response to Rc. Using reciprocity failure (i.e. higher inhibition under continuous than pulsed light) as the operational criterion, a ‘true’ high‐irradiance reaction occurred under continuous far‐red light but not under Rc or red plus far‐red light mixtures. Native and overexpressed phyA are proposed to mediate very low fluence responses under Rc. In WT A. thaliana, this effect is counteracted by a negative action of phyA on phyB‐mediated low‐fluence responses.

A 146 bp fragment of the tobacco Lhcb1*2 promoter confers very-low-fluence, low-fluence and high-irradiance responses of phytochrome to a minimal CaMV 35S promoter.

The occurrence of very-low-fluence responses (VLFR), low-fluence responses (LFR) and high-irradiance responses (HIR) of phytochrome was investigated for the expression of the gene of beta-glucuronidase (gusA) under the control of the tobacco Lhcb1*2 promoter, in etiolated transgenic tobacco seedlings. The activity of beta-glucuronidase (GUS) showed biphasic responses to the calculated proportion of Pfr provided by light pulses. The first phase (i.e. the VLFR) showed a maximum for Pfr levels characteristic of far-red light. The second phase (i.e. the LFR) was observed at higher Pfr levels and was reversible by far-red light pulses. The strong effect of continuous far-red light (i.e. HIR) was fluence-rate-dependent and could not be replaced either by hourly pulses of the same spectral composition and total fluence or by very low fluences of red light. Deletion of the Lhcb1*2 promoter to -453 caused little loss of GUS activity. The -453 to -31, -270 to -31 and -176 to -31 fragments of the Lhcb1*2 promoter conferred proportionally normal VLFR, LFR and HIR to a truncated (-46 to +8) CaMV 35S minimal promoter. This is the first demonstration of the presence of three phytochrome action modes in the control of the transcriptional activity of a single gene. The cis-acting regulatory elements necessary for VLFR, LFR and HIR are present in a 146 bp fragment of the tobacco Lhcb1*2 promoter.

Physiological characterization of exaggerated-photoresponse mutants of tomato

Summary Four monogenic mutants in tomato ( Lycopersicon esculentum Mill.), i.e. three recessive mutations, high-pigment-1 ( hp-1 ), high-pigment-2 ( hp-2 ), and atroviolacea ( atv ), and one dominant mutation, Intense pigmentation ( Ip ), were used in this study. These mutants all show exaggerated photoresponses during deetiolation and seedlings having shorter hypocotyls and higher anthocyanin levels. The hp-1 and hp-2 have higher chlorophyll levels in immature fruit, giving them a dark green colour. Spectrophotometrical and immunological analyses of phytochrome A and phytochrome B revealed no differences between the mutants and the wild types (WTs), suggesting that the mutants are not photoreceptor mutants. Both hp-1 and hp-2 accumulate high levels of anthocyanin in continuous blue (B) and red (R) broad-band light. In contrast, atv has a WT level of anthocyanin in B and an exaggerated response in R. The Ip mutant has the opposite response: a WT level of anthocyanin in R and an exaggerated response in B. In B and R pretreat-ment studies, all mutants show an enhanced R/far-red light (FR)-reversible response compared with WT, but the Ip mutant shows a preferentially enhanced response in B. The hp-1 mutant exhibits a strong amplification of both the low fluence rate response and high irradiance response components of anthocyanin biosynthesis in red light. The atv mutant shows the strongest amplification of the HIR component. The Ip mutant exhibits an exaggerated anthocyanin response in B. All four mutants exhibit a normal elongation response to supplementary FR during the daily photoperiod.

Very low fluence and low fluence response in the induction and inhibition of seed germination in Celosia argentea

AbstractSeeds of Celosia argentea L. displayed an absolute requirement for light for germination. Germination could be induced by far-red light as well as red light, and therefore, the effect of red light was not completely reversible by far-red light. A considerable proportion of the seed population was sensitive to red light within the range 0.01–0.1 μmol m−2, which suggested that phytochrome was operating in the very low fluence response mode. Seeds showed a gradual increase in germination in response to red light with an increasing duration of previous dark imbibition. Interruption of the dark imbibition period with very low fluence (10−4−10–1 μmol m−2) or low fluence (1–103 μmol m−2) red light prevented germination subsequent to a terminal, saturating red light irradiation. Increasing responsiveness to red light with increase in imbibition temperature and inhibition of development of photosensitivity by cycloheximide suggested that phytochrome synthesis occurred during the dark imbibition period. An absolute requirement for light for seed germination, inhibition of seed germination on interrupting the dark imbibition period with very low fluence or low fluence red light, and the recovery from photoinhibition of seeds in darkness are consistent with the hypothesis that PhyA is the principal phytochrome involved in the photoregulation of seed germination in C. argentea.

Phytochrome A Mediates the Promotion of Seed Germination by Very Low Fluences of Light and Canopy Shade Light in Arabidopsis.

Seeds of the wild type (WT) and of the phyA and phyB mutants of Arabidopsis thaliana were exposed to single red light (R)/far-red light (FR) pulses predicted to establish a series of calculated phytochrome photoequilibria (Pfr/P). WT and phyB seeds showed biphasic responses to Pfr/P. The first phase, i.e. the very-low-fluence response (VLFR), occurred below Pfr/P = 10-1%. The second phase, i.e. the low-fluence response, occurred above Pfr/P = 3%. The VLFR was similarly induced by either a FR pulse saturating photoconversion or a subsaturating R pulse predicted to establish the same Pfr/P. The VLFR was absent in phyA seeds, which showed a strong low-fluence response. In the field, even brief exposures to the very low fluences of canopy shade light (R/FR ratio < 0.05) promoted germination above dark controls in WT and phyB seeds but not in the phyA mutant. Seeds of the phyA mutant germinated normally under canopies providing higher R/FR ratios or under deep canopy shade light supplemented with R from light-emitting diodes. We propose that phytochrome A mediates VLFR of A. thaliana seeds.

Role of Phytochrome B in the Induction of Seed Germination by Light in Arabidopsis thaliana

Summary The role played by phytochrome B in seed germination responses to light pulses was investigated by comparing wild-type (WT) and phyB-1 -mutant seeds of Arabidopsis thaliana (L.) Heynh. Similar dark germination rates were observed in WT and phyB-1 seeds stored dry for at least one month. The response of WT seeds to light pulses that established a series of calculated phytochrome photoequilibria was biphasic. In the first phase, i.e. the very-low-fluence response, similar slopes were observed in WT and phyB-1 -mutant seeds. In the second phase, i.e. the low-fluence response, the slope of the WT was 30 to 930% higher (depending on the batch, duration of dry storage, and temperature pretratements) than the slope of the phyB-1 mutant. Both hourly pulses of FR and continuous FR promoted germination above dark controls in both genotypes. Phytochrome B is proposed to make a higher contribution to low-fluence than very-low-fluence germination responses of Arabidopsis thaliana seeds.

New lv Mutants of Pea Are Deficient in Phytochrome B.

The lv-1 mutant of pea (Pisum sativum L.) is deficient in responses regulated by phytochrome B (phyB) in other species but has normal levels of spectrally active phyB. We have characterized three further lv mutants (lv-2, lv-3, and lv-4), which are all elongated under red (R) and white light but are indistinguishable from wild type under far-red light. The phyB apoprotein present in the lv-1 mutant was undetectable in all three new lv mutants. The identification of allelic mutants with and without phyB apoprotein suggests that Lv may be a structural gene for a B-type phytochrome. Furthermore, it indicates that the lv-1 mutation results specifically in the loss of normal biological activity of this phytochrome. Red-light-pulse and fluence-rate-response experiments suggest that lv plants are deficient in the low-fluence response (LFR) but retain a normal very-low-fluence-rate-dependent response for leaflet expansion and inhibition of stem elongation. Comparison of lv alleles of differing severity indicates that the LFR for stem elongation can be mediated by a lower level of phyB than the LFR for leaflet expansion. The retention of a strong response to continuous low-fluence-rate R in all four lv mutants suggests that there may be an additional phytochrome controlling responses to R in pea. The kinetics of phytochrome destruction and reaccumulation in the lv mutant indicate that phyB may be involved in the light regulation of phyA levels.

Phytochrome-mediated germination control of Hygrophila auriculata seeds following dry storage augmented by temperature pulse, hormones, anaerobiosis or osmoticum imbibition

A pulse of red light ( R) stimulates, and far-red light ( FR) inhibits germination of Hygrophila auriculata seeds with primary dormancy. These seeds lose their light requirement for germination after 4 months of dry storage in air. A secondary dormancy can then be induced by 5-day dark imbition in osmoticum in air (21% O 2). Red light will break this dormancy. Seeds stored in nitrogen gas (0% O 2) neither lose the light requirement, nor is a secondary dormancy imposed. However, FR stimulated some germination in osmoticum-induced dormant seeds is evident when such seeds are given a 1–6 hr 40°C pulse, gibberellin A 3 (GA 3), or ethylene treatments. These results suggest that changes of photosensitivity from a low fluence response to a very low fluence response may occur concurrently with the status of dormancy in seeds. The processes of dry after-ripening, the induction of secondary dormancy or of photosensitivity all require oxygen.

Individual Members of the Cab Gene Family Differ Widely in Fluence Response.

Chlorophyll a/b-binding protein genes (Cab genes) can be extremely sensitive to light. Transcript accumulation following a red light pulse increases with fluence over 8 orders of magnitude (L.S. Kaufman, W.F. Thompson, W.R. Briggs [1984] Science 226: 1447-1449). We have constructed fluence-response curves for individual Cab genes. At least two Cab genes (Cab-8 and AB96) show a very low fluence response to a single red light pulse. In contrast, two other Cab genes (AB80 and AB66) fail to produce detectable transcript following a single pulse of either red or blue light but are expressed in continuous red light. Thus, very low fluence responses and high irradiance responses occur in the same gene family.