Photocontrol of anthocyanin biosynthesis in tomato

Juvenile anthocyanin biosynthesis has been studied in dark-grown seedlings of tomato (Lycopersicon esculentum Mill.) wild types (WTs) and photomorphogenic mutants. During a subsequent 24-hr period of monochromatic irradiation at different fluence rates of red light (R) the fluence-rate response relationships for induction of anthocyanin in all the WTs are similar, yet complex, showing a response at low fluence rates (LFRR) followed by a fluence rate-dependent high irradiance response (HIR). In the hypocotyl this response is restricted to the sub-epidermal layer of cells. The high-pigment-1 (hp-1) mutant exhibits a strong amplification of both response components. Theatroviolacea (atv) mutant shows strongest amplification of the HIR component. In contrast, a transgenic line overexpressing an oat phytochrome A gene (PHYA3 ⁺) shows a most dramatic amplification of the LFRR component. The far-red light (FR)-insensitive (fri) mutant, deficient in phytochrome A (phyA), lacks the LFRR component whilst retaining a normal HIR. The temporarily R-insensitive (tri) mutant, deficient in phytochrome B1 (phyB1) retains the LFRR, but lacks the HIR. Thehp-1,fri andhp-1,tri double mutant, exhibit amplified, yet qualitatively similar responses to the monogenicfri andtri mutants. Thefri,tri double mutant lacks both response components in R, but a residual response to blue light (B) remains. Similarly, theaurea (au) mutant deficient in phytochrome chromophore biosynthesis and presumably all phytochromes, lacks both response components in the R and FR regions of the spectrum. Experiments at other wavelengths demonstrate that while there is only a small response in the FR spectral region (729 nm) in tomato, there is an appreciable HIR response in the near FR at 704 nm, which is retained in thetri mutant. This suggests that the labile phyA pool participates in the HIR at this wavelength. The intense pigmentation (Ip) mutant appears to be specifically deficient in the B1 induced anthocyanin biosynthesis. Adult plants, grown under fluorescent light/dark cycles, show a reduction of anthocyanin content of young developing leaves upon application of supplemtary or end-of-day FR. The involvement of different phytochrome species in anthocyanin biosynthesis based on micro-injection studies into theau mutant and studies using type specific phytochrome mutants is discussed.     

Effect of Light on Root Hair Formation in Arabidopsis thaliana Phytochrome-Deficient Mutants

Arabidopsis thaliana lacking phytochrome A, phytochrome B or both (double mutant) were analyzed by comparing their photoresponse with that of the wild type. Results indicate that root hair formation in Arabidopsis was strongly stimulated by light irradiation. Both phytochrome A and phytochrome B are responsible for photoinduction by continuous red light irradiation, while only phytochrome A mediates the response under continuous far-red light. The fluence response relationships to a red light pulse in the wild type displayed a biphasic trend similar to that previously observed in lettuce seedlings, with the first phase showing a sharp maximum at 78.3 Jm⁻², and the second one operating over a wider fluence range (3,100–9,400 Jm⁻²) two orders of magnitude higher than the first one. Analysis of the fluence response curves for red light induction in the phytochrome mutants revealed that phytochrome A is responsible for the first phase in the wild type, while the second is the result of the combined action of both phytochrome A and phytochrome B. Received 13 August 1999/ Accepted in revised form 22 December 1999     

Phytochrome regulation of greening in wild type and long-hypocotyl mutants ofArabidopsis thaliana

A brief pulse of red light (R) given to darkgrown seedlings ofArabidopsis thaliana (L.) Heyn. potentiates rapid synthesis of chlorophyll upon transfer to continuous white light. The time course for potentiation of rapid greening shows that a R pulse in the LF (low fluence) range has maximal effect within a few hours, and that there is a small VLF (very low fluence) component as well. Partial reversal of the effect of R by far-red light (FR) indicates that the pulse acts through phytochrome. As it does in the wild-type (WT), a pulse of R accelerates greening of long-hypocotyl (hy) mutants. The extent of induction by the R pulse was about the same in the WT and in allhy mutants studied. Reversibility by FR was greatly decreased in thehy-1 andhy-2 strains. It is possible that these mutants contain a species of phytochrome with defective phototransformation kinetics. If there is such a defective phytochrome species, it nevertheless appears to be active in the potentiation of rapid greening. Dedicated to Professor Hans Mohr on the occasion of his 60th birthday     

Co-action between phytochrome B and HY4 in Arabidopsis thaliana

A combination of physiological and genetic approaches was used to investigate whether phytochromes and blue light (BL) photoreceptors act in a fully independent manner during photomorphogenesis of Arabidopsis thaliana (L.) Heynh. Wild-type seedlings and phyA, phyBand hy4 mutants were daily exposed to 3 h BL terminated with either a red light (R) or a far-red light (FR) pulse. In wild-type and phyA-mutant seedlings, BL followed by an R pulse inhibited hypocotyl growth and promoted cotyledon unfolding. The effects of BL were reduced if exposure to BL was followed by an FR pulse driving phytochrome to the R-absorbing form (Pr). In the wild type, the effects of R versus FR pulses were small in seedlings not exposed to BL. Thus, maximal responses depended on the presence of both BL and the FR-absorbing form of phytochrome (Pfr) in the subsequent dark period. Impaired responses to BL and to R versus FR pulses were observed in phyB and hy4 mutants. Simultaneous irradiation with orange light indicated that BL, perceived by specific BL photoreceptors (i.e. not by phytochromes), required phytochrome B to display a full effect. These results indicate interdependent co-action between phytochrome B and BL photoreceptors, particularly the HY4 gene product. No synergism between phytochrome A (activated by continuous or pulsed FR) and BL photoreceptors was observed.Abbreviations BL blue light - D darkness - FR far-redlight - FRc continuous FR - Pfr FR-absorbing form of phytochrome - Pfr/P proportion of phytochrome as Pfr - phyA phytochrome A - phyB phytochrome B - R red light - WT wild type We thank Professors R.E. Kendrick and M. Koornneef (Wageningen Agricultural University, The Netherlands), Professor J. Chory (Salk Institute, Calif., USA) and the Arabidopsis Biological Resource Center (Ohio State University, Ohio, USA) for their kind provision of the original seed batches. This work was financially supported by CONICET, Universidad de Buenos Aires (AG 040) and Fundación Antorchas (A-12830/1 0000/9)     

The Expression of Light-Regulated Genes in the High-Pigment-1 Mutant of Tomato

Three light-regulated genes, chlorophyll a/b-binding protein (CAB), ribulose-1,5-bisphosphate carboxylase/oxygenase small subunit, and chalcone synthase (CHS), are demonstrated to be up-regulated in the high-pigment-1 (hp-1) mutant of tomato (Lycopersicon esculentum Mill.) compared with wild type (WT). However, the pattern of up-regulation of the three genes depends on the light conditions, stage of development, and tissue studied. Compared with WT, the hp-1 mutant showed higher CAB gene expression in the dark after a single red-light pulse and in the pericarp of immature fruits. However, in vegetative tissues of light-grown seedlings and adult plants, CAB mRNA accumulation did not differ between WT and the hp-1 mutant. The ribulose-1,5-bisphosphate carboxylase/oxygenase small subunit mRNA accumulated to a higher level in the hp-1 mutant than WT under all light conditions and tissues studied, whereas CHS gene expression was up-regulated in de-etiolated vegetative hp-1-mutant tissues only. The CAB and CHS genes were shown to be phytochrome regulated and both phytochrome A and B1 play a role in CAB gene expression. These observations support the hypothesis that the HP-1 protein plays a general repressive role in phytochrome signal transduction.     

Identification of two loci involved in phytochrome expression in Nicotiana plumbaginifolia and lethality of the corresponding double mutant

Four Nicotiana plumbaginifolia mutants exhibiting long hypocotyls and chlorotic cotyledons under white light, have been isolated from M2 seeds following mutagenesis with ethyl methane sulphonate. In each of these mutants, this partly etiolated in white light (pew) phenotype is due to a recessive nuclear mutation at a single locus. Complementation analysis indicates that three mutants, dap5, ems28 and ems3-6-34, belong to a single complementation group called pew1, while dap1 defines the pew2 locus. The mutants at pew1 contain normal levels of immunochemically detectable apoprotein of the phytochrome that is relatively abundant in etiolated seedlings, but are deficient in spectrophotometrically detectable phytochrome, whether seedlings are grown in darkness or light. Moreover, biliverdin, a precursor of the phytochrome chromophore, restores light-regulated responses in pew1 mutants and increases their level of photoreversible phytochrome when grown in darkness. These results indicate that the pew1 locus may be involved in chromophore biosynthesis. The mutant at the pew2 locus displays no photoreversible phytochrome in etiolated seedlings, but does contain normal levels of photoreversible phytochrome when grown in the light. Biliverdin had little effect on light-regulated responses in this mutant. In addition, biliverdin did not alter the level of phytochrome in etiolated seedlings. These observations lead us to propose that this mutant could be affected in the phyA gene itself. We have also obtained the homozygous double mutant at the pew1 and pew2 loci. This double mutant is lethal at an early stage of development, consistent with a critical role for phytochrome in early development of higher plants.     

Phytochrome A-mediated inhibition of seed germination in tomato

Far-red light (FR) inhibition of seed germination of tomato (Solanum lycopersicum L.) was studied with the phytochrome (phy)-hypersensitive mutants, hp-1w, hp-1w,fri1, a phyA-deficient double mutant, and hp-1w,tri1, a phyB1-deficient double mutant. Seeds of all mutants germinated readily in the dark at 25 degrees C, and the germination was retarded by a single 100-s FR pulse given 1-3 h after sowing. The effect of an FR pulse was red-light reversible in all mutants used. After 24 h where a single FR pulse was no longer effective, prolonged FR exposure or hourly FR pulses suppressed germination in hp-1w and hp-1w,tri1, whereas in hp-1w,fri1 the suppressive effect of FR was almost absent. The effect of the prolonged FR was greater than that of the hourly 3-min FR pulses having equal photon fluence, and was fluencerate dependent. Thus we conclude that the germination inhibition by FR in tomato seed consists of a low-fluence response and a high irradiance response (HIR); the latter is controlled by phyA, but not phyB1. This is the first indication of phyA being involved in the HIR of seed germination inhibition.     

Light exaggerates apical hook curvature through phytochrome actions in tomato seedlings

Contrary to the established notion that the apical hook of dark-grown dicotyledonous seedlings opens in response to light, we found in tomato (Solanum lycopersicum L.) that the apical hook curvature is exaggerated by light. Experiments with several tomato cultivars and phytochrome mutants, irradiated with red and far-red light either as a brief pulse (Rp, FRp) or continuously (Rc, FRc), revealed: the hook-exaggeration response is maximal at the emergence of the hypocotyl from the seed; the effect of Rp is FRp-reversible; fluence–response curves to a single Rp or FRp show an involvement of low and very low fluence responses (LFR, VLFR); the effect of Rc is fluence-rate dependent, but that of FRc is not; the phyA mutant (phyA hp-1) failed to respond to an Rp of less than 10⁻² μmol m⁻² and to an FRp of all fluences tested as well as to FRc, thus indicating that the hook-exaggeration response involves phyA-mediated VLFR. The Rp fluence–response curve with the same mutant also confirmed the presence of an LFR mediated by phytochrome(s) other than phyA, although the phyB1 mutant (phyB1 hp-1) still showed full response probably due to other redundant phytochrome species (e.g., phyB2). Simulation experiments led to the possible significance of hook exaggeration in the field that the photoresponse may facilitate the release of seed coat when seeds germinate at some range of depth in soil. It was also observed that seed coat and/or endosperm are essential to the hook exaggeration.     

Overexpression of rice phytochrome A partially complements phytochrome B deficiency in Arabidopsis

The red/far-red reversible phytochromes play a central role in regulating the development of plants in relation to their light environment. Studies on the roles of different members of the phytochrome family have mainly focused on light-labile, phytochrome A and light-stable, phytochrome B. Although these two phytochromes often regulate identical responses, they appear to have discrete photosensory functions. Thus, phytochrome A predominantly mediates responses to prolonged far-red light, as well as acting in a non-red/far-red-reversible manner in controlling responses to light pulses. In contrast, phytochrome B mediates responses to prolonged red light and acts photoreversibly under light-pulse conditions. However, it has been reported that rice (Oryza sativa L.) phytochrome A operates in a classical red/far-red reversible fashion following its expression in transgenic tobacco plants. Thus, it was of interest to determine whether transgenic rice phytochrome A could substitute for loss of phytochrome B in phyB mutants of Arabidopsis thaliana (L.) Heynh. We have observed that ectopic expression of rice phytochrome A can correct the reduced sensitivity of phyB hypocotyls to red light and restore their response to end-of-day far-red treatments. The latter is widely regarded as a hallmark of phytochrome B action. However, although transgenic rice phytochrome A can correct other aspects of elongation growth in the phyB mutant it does not restore other responses to end-of-day far-red treatments nor does it restore responses to low red:far-red ratio. Furthermore, transgenic rice phytochrome A does not correct the early-flowering phenotype of phyB seedlings. Received: 12 July 1998 / Accepted: 13 August 1998     

Altered Phytochrome Regulation of Greening in an aurea Mutant of Tomato

A brief pulse of red light accelerates chlorophyll accumulation upon subsequent transfer of dark-grown tomato (Lycopersicon esculentum) seedlings to continuous white light. Such potentiation of greening was compared in wild type and an aurea mutant W616. This mutant has been the subject of recent studies of phytochrome phototransduction; its dark-grown seedlings are deficient in phytochrome, and light-grown plants have yellow-green leaves. The rate of greening was slower in the mutant, but the extent (relative to the dark control) of potentiation by the red pulse was similar to that in the wild type. In the wild type, the fluence-response curve for potentiation of greening indicates substantial components in the VLF (very low fluence) and LF (low fluence) ranges. Far-red light could only partially reverse the effect of red. In the aurea mutant, only red light in the LF range was effective, and the effect of red was completely reversed by far-red light. When grown in total darkness, aurea seedlings are also deficient in photoconvertible PChl(ide). Upon transfer to white light, the aurea mutant was defective in both the abundance and light regulation of the light-harvesting chlorophyll a/b binding polypeptide(s) [LHC(II)]. The results are consistent with the VLF response in greening being mediated by phytochrome. Furthermore, the data support the hypothesis that light modulates LHC(II) levels through its control of the synthesis of both chlorophyll and its LHC(II) apoproteins. Some, but not all, aspects of the aurea phenotype can be accounted for by the deficiency in photoreception by phytochrome.     

Coupling of phytochrome B to the control of hypocotyl growth in Arabidopsis

Etiolated seedlings of the wild-type (WT) and of the phyB-1 mutant of Arabidopsis thaliana (L.) Heynh. were exposed to red-light (R) and far-red light (FR) treatments to characterize the action of phytochrome B on hypocotyl extension growth. A single R or FR pulse had no detectable effects on hypocotyl growth. After 24-h pre-treatment with continuous FR (FRc) a single R, compared to FR pulse inhibited (more than 70%) subsequent hypocotyl growth in the WT but not in the phyB-1 mutant. This effect of FRc was fluence-rate dependent and more efficient than continuous R (Rc) or hourly FR pulses of equal total fluence. Hypocotyl growth inhibition by Rc was larger in WT than phyB-1 seedlings when chlorophyll screening was reduced either by using broadband Rc (maximum emission 610 nm) or by using narrow-band Rc (658 nm) over short periods (24 h) or with seedlings bleached with Norflurazon. Hourly R or R + FR pulses had similar effects in WT and phyB-1 mutant etiolated seedlings. It is concluded that phytochrome B is not the only photoreceptor of Rc and that the action of phytochrome B is enhanced by a FRc high-irradiance reaction. Complementary experiments with the phyA-201 mutant indicate that this promotion of a phytochrome B-mediated response occurs via co-action with phytochrome A.Abbreviations D darkness - FR far-red light - FRc continuous FR - Pfr FR-absorbing form of phytochrome - HIR high-irradiance reaction - Pfr/P proportion of phytochrome as Pfr - phyA phytochrome A - phyB phytochrome B - R red light - Rc continuous R - WT wild-type I thank Professors R.E. Kendrick and M. Koornneef (Wageningen Agricultural University, The Netherlands) and Professor J. Chory (Salk Institute, Calif., USA) for their kind provision of the original WT and phyB-1 and phyA-201 seed, respectively. This work was financially supported by grants PID and PID-BID from CONICET, AG 040 from Universidad de Buenos Aires and A 12830/1-000019 from Fundación Antorchas.     

Genetic and transgenic evidence that phytochromes A and B act to modulate the gravitropic orientation of Arabidopsis thaliana hypocotyls

Hypocotyls of Arabidopsis thaliana exhibit negative gravitropism in the dark, growing against the gravity vector. The direction of growth is randomized in red light (R). In single mutants lacking either phytochrome A or B randomization of hypocotyl orientation in R is retained. However, a double mutant lacks this response, indicating that either phytochrome A or B is capable of inducing randomization and phytochrome A and B are the only phytochromes involved in this process. The induction of randomization was confirmed using lines that express to different levels PHYA and PHYB cDNAs. Overexpression of PHYA cDNAs induced randomization of hypocotyl orientation in the dark. Dark randomization was also seen in the phyB-1 mutant but not in two other phyB alleles, suggesting that dark randomization in the phyB-1 line may be due to a second mutation. When germination was induced by gibberellin, rather than exposure to brief white light, randomization in the dark associated with phytochrome A overproduction was not observed but was retained in the phyB-1 mutant. Overexpression of PHYB cDNAs induced a light-dependent randomization of hypocotyl orientation that responded to R:far-red light ratio. We conclude that the default situation in Arabidopsis hypocotyls is, therefore, negative gravitropism, and either phytochrome A or phytochrome B can mediate randomization.     

The mapping of phytochrome genes and photomorphogenic mutants of tomato

The map positions of five previously described phytochrome genes have been determined in tomato (Lycopersicon esculentum Mill.) The position of the yg-2 gene on chromosome 12 has been confirmed and the classical map revised. The position of the phytochrome A (phy A)-deficient fri mutants has been refined by revising the classical map of chromosome 10. The position of the PhyA gene is indistinguishable from that of the fri locus. The putative phyB1-deficient tri mutants were mapped by classical and RFLP analysis to chromosome 1. The PhyB1 gene, as predicted, was located at the same position. Several mutants with the high pigment (hp) phenotype, which exaggerates phytochrome responses, have been reported. Allelism tests confirmed that the hp-2 mutant is not allelic to other previously described hp (proposed here to be called hp-1) mutants and a second stronger hp-2 allele (hp-2 ^j ) was identified. The hp-2 gene was mapped to the classical, as well as the RFLP, map of chromosome 1. Received: 24 May 1996 / Accepted: 14 June 1996     

Far-red light-insensitive, phytochrome A-deficient mutants of tomato

We have selected two recessive mutants of tomato with slightly longer hypocotyls than the wild type, one under low fluence rate (3 mol/m²/s) red light (R) and the other under low fluence rate blue light. These two mutants were shown to be allelic and further analysis revealed that hypocotyl growth was totally insensitive to far-red light (FR). We propose the gene symbol fri (far-red light insensitive) for this locus and have mapped it on chromosome 10. Immunochemically detectable phytochrome A polypeptide is essentially absent in the fri mutants as is the bulk spectrophotometrically detectable labile phytochrome pool in etiolated seedlings. A phytochrome B-like polypeptide is present in normal amounts and a small stable phytochrome pool can be readily detected by spectrophotometry in the fri mutants. Inhibition of hypocotyl growth by a R pulse given every 4 h is quantitatively similar in the fri mutants and wild type and the effect is to a large extent reversible if R pulses are followed immediately by a FR pulse. After 7 days in darkness, both fri mutants and the wild type become green on transfer to white light, but after 7 days in FR, the wild-type seedlings that have expanded their cotyledons lose their capacity to green in white light, while the fri mutants de-etiolate. Adult plants of the fri mutants show retarded growth and are prone to wilting, but exhibit a normal elongation response to FR given at the end of the daily photoperiod. The inhibition of seed germination by continuous FR exhibited by the wild type is normal in the fri mutants. It is proposed that these fri mutants are putative phytochrome A mutants which have normal pools of other phytochromes.     

Phytochrome A and phytochrome B1 control the acquisition of competence for shoot regeneration in tomato hypocotyl

 We analysed the light-dependent acquisition of competence for adventitious shoot formation in hypocotyls of phytochrome A (fri) and phytochrome B1 (tri) mutants of tomato and their wild type by pre-growing the seedlings under different light quality. The regenerative response in vitro of explants from etiolated seedlings was reduced in comparison to that displayed by light-grown ones. Our results indicate that the light-dependent acquisition of competence for shoot regeneration in the tomato hypocotyl is regulated by phytochrome and antagonistically by a blue-light receptor. By using phytochrome mutants and narrow wave band light we showed that it is mediated at least by two distinct phytochrome species: phytochrome B1 and phytochrome A. The action of phytochrome B1 during seedling growth was sufficient to induce the full capacity of the subsequent regenerative response in vitro in explants from all positions along the hypocotyls. In contrast far-red light acting through phytochrome A did not induce the full capability of shoot regeneration from middle and basal segments of the hypocotyl when phytochrome B1 was absent (tri mutant). A few middle and basal hypocotyl explants pre-grown in blue light regenerated shoots. Received: 12 April 1999 / Revision received: 5 July 1999 · Accepted: 6 August 1999     

Temperature-dependent internode elongation in vegetative plants of Arabidopsis thaliana lacking phytochrome B and cryptochrome 1

 Vegetative plants of Arabidopsis thaliana (L.) Heynh. form a compact rosette of leaves in which internode growth is virtually arrested. Rapid extension of the internodes occurs after flower buds are present in the reproductive apex. Under natural radiation, continuous light from fluorescent lamps, or short photoperiods of light from fluorescent lamps, plants of the phyB cry1 double mutant (lacking both phytochrome B and cryptochrome 1) did not form normal rosettes because all the internodes showed some degree of elongation. Internode elongation was weak in the phyB single mutant and absent in the cry1 mutant, indicating redundancy between phytochrome B and cryptochrome 1. The absence of phytochrome A caused no effects. The failure to form normal rosettes was conditional because internode elongation was arrested at low temperatures in all the mutant combinations. In contrast, the temperature dependence of phytochrome B and cryptochrome 1 effects on hypocotyl growth was weak. The elongation of the internodes in phyB cry1 was not accompanied by early flowering as showed by the lack of effects on the final number of leaves. Apex dissection indicated that in phyB cry1 double mutants internode elongation anticipated the transition from the vegetative to the reproductive stage. Thus, stem growth in Arabidopsis thaliana is not fully dependent on the program of reproductive development. Received: 2 June 1999 / Accepted: 13 August 1999     

Phenotype of the tomato high pigment-2 mutant is caused by a mutation in the tomato homolog of DEETIOLATED1.

Tomato high pigment (hp) mutants are characterized by their exaggerated photoresponsiveness. Light-grown hp mutants display elevated levels of anthocyanins, are shorter and darker than wild-type plants, and have dark green immature fruits due to the overproduction of chlorophyll pigments. It has been proposed that HP genes encode negative regulators of phytochrome signal transduction. We have cloned the HP-2 gene and found that it encodes the tomato homolog of the nuclear protein DEETIOLATED1 (DET1) from Arabidopsis. Mutations in DET1 are known to result in constitutive deetiolation in darkness. In contrast to det1 mutants, tomato hp-2 mutants do not display any visible phenotypes in the dark but only very weak phenotypes, such as partial chloroplast development. Furthermore, whereas det1 mutations are epistatic to mutations in phytochrome genes, analysis of similar double mutants in tomato showed that manifestation of the phenotype of the hp-2 mutant is strictly dependent upon the presence of active phytochrome. Because only one DET1 gene is likely to be present in each of the two species, our data suggest that the phytochrome signaling pathways in which the corresponding proteins function are regulated differently in Arabidopsis and tomato.     

Growth analysis of wild-type and photomorphogenic-mutant tomato plants

A custom designed growth-measuring apparatus, controlled by a microcomputer has been used to study extension growth kinetics of wild-type and photomorphogenic-mutant tomato ( Lycopersicon esculentum Mill.) plants with and without end-of-day farred light (EODFR). The following photomorphogenic mutants were used. Far-red insensitive ( fri ^.1): deficient in phytochrome A (phyA); temporarily red light-insensitive ( tri ^.3): deficient in phytochrome Bl (phyB1), and their isogenic wild type (WT) cv. MoneyMaker. aurea (au) : deficient in phytochrome chromophore biosynthesis; high-pigment-1 ( hp-1 ): exhibiting exaggerated phytochrome responses, and their isogenic WT cv. Ailsa Craig. The stem elongation rate (SER) during a 24-h period of all the genotypes studied shows a similar pattern, having two dramatic transients, one shortly after the onset of the light period (a sharp decline in SER) and another shortly after the start of the dark period (a sharp increase in SER). These transients are probably associated with water relations as a consequence of opening and closure of the stomata. The fastest SER occurs during the dramatic oscillations early in the dark period. Between the genotypes there are large quantitative differences in SER. All the genotypes tested exhibited a strong EODFR response, resulting in a relative promotion of SER during the first period after the start of EODFR and in the subsequent light and dark periods. These results indicate that phyA, absent in the fri ^.1 mutant, does not play a major role in SER of light-grown tomato plants, whereas phyB 1, absent in the tri ^.3 mutant, is partly responsible for the compact stature of WT plants. An additional phytochrome other than phy A and phy B1 must therefore be capable of eliciting the EODFR response.     

Phytochrome A, phytochrome B and other phytochrome(s) regulate ATHB-2 gene expression in etiolated and green Arabidopsis plants

The expression of the Arabidopsis ATHB-2 gene is light-regulated both in seedlings and in adult plants. The gene is expressed at high levels in rapidly elongating etiolated seedlings and is down-regulated by a pulse of red light (R) through the action of a phytochrome other than phytochrome A or B, or by a pulse of far-red light (FR) through the action of phytochrome A. In green plants, the expression of the ATHB-2 gene is rapidly and strongly enhanced by lowering the R:FR ratio perceived by a phytochrome other than A or B. Returning the plant to a high R:FR ratio results in an equally rapid decrease of the ATHB-2 mRNA. Consistently, plants overproducing ATHB-2 show developmental phenotypes characteristic of plants grown in low R:FR: elongated petioles, reduced leaf area, early flowering, and reduced number of rosette leaves. Taken together, the data strongly suggest a direct involvement of ATHB-2 in light-regulated growth phenomena throughout Arabidopsis development.     

Impaired phytochrome-mediated shade-avoidance responses in the aurea mutant of tomato

Internode extension-growth responses to neighbouring plants and to red to far-red ratios (R:FR) were investigated in wild-type (WT) and aurea (au)-mutant seedlings of tomato grown under natural radiation. The genomic location of the au mutant is not known, but one of its consequences is the reduced phytochrome level. In WT seedlings, internode growth was promoted by the presence of non-shading neighbours reflecting far-red light (FR), the shade of a tall canopy, FR provided as a supplement during the photoperiod, and FR pulses either provided at the end of the day or delayed into the dark period. Supplementary FR during the photoperiod also promoted growth in herbicide-treated partially bleached WT seedlings. The au mutant showed higher background extension-growth rates, but only responded to the most severe treatments: deep shade light and very low R:FR at the end of the day, i.e. au-mutant seedlings were less sensitive than WT seedlings to R:FR signals. Wild-type seedlings were transferred from the glasshouse to a growth room and exposed to white light with two levels of phytochrome-absorbable radiation but similar phytochrome photoequilibria and radiation for photosynthesis. The plants exposed to the lowest level showed a transient increase of internode extension growth rate and a simultaneous reduction of response to FR pulses, i.e. reproduced some of the features of au-mutant seedlings. Phytochrome itself could set the degree of response to Pfr during neighbour detection.