Continuous Far-red Irradiation Research Articles

Fluorescence and photochemical properties of phytochromes in wild‐type wheat and a transgenic line overexpressing an oat phytochrome A (PHYA) gene: functional implications

AbstractEtiolated seedlings of wild‐type wheat and a transgenic line overexpressing an oat PHYA gene were investigated by the use of in situ low‐temperature fluorescence spectroscopy. The red‐absorbing phytochrome form, Pr, was characterized by (1) fluorescence emission spectrum; (2) total phytochrome content, and (3) by the extent of the Pr → lumi‐R photoconversion at low temperature (γ1), and of the Pr → Pfr photoconversion at ambient temperature (γ2) as derived from emission data. All the characteristics were shown to be variable and to depend on (1) organ and tissue used; (2) seedling age; (3) transgenic wheat modification, and (4) continuous far‐red irradiation of seedlings during their growth. These variations were interpreted in terms of the existence in wheat seedlings of the two phenomenological Pr types: (a), Pr′– major longer wavelength (687/673 nm, emission/absorption maxima) variable and light‐labile with γ1 ≈ 0·5; and (b), Pr′′– minor, shorter wavelength (682/668 nm), relatively constant with its concentration not changing significantly with the increase of total phytochrome content in tissues and light‐stable with γ1 ≤ 0·05–0·1. Overexpression of oat phyA increases primarily the content of Pr′ suggesting that it is comprised of phyA (phyA′) whereas Pr′′ is believed to consist of the minor phyA fraction (phyA′′) and phyB. The transgenic wheat line has been demonstrated to have a modified phenotype – the appearance of the far‐red high irradiance reaction (FR‐HIR) (Shlumukov et al. Plant, Cell and Environment 24, 703–712). The increased content of phyA′ in the transgenic line, whereas the total [phyA′′ + phyB] remains the same as in the wild type, indicates that the phyA′ pool is primarily responsible for the observed modification of the phenotype and suggests that even in wild‐type plants the phyA′ component of the phyA pool may mediate the FR‐HIR.

Photoresponses of transgenic Arabidopsis seedlings expressing introduced phytochrome B‐encoding cDNAs: evidence that phytochrome A and phytochrome B have distinct photoregulatory functions

The photocontrol of hypocotyl elongation has been studied in two transgenic lines of Arabidopsis thaliana which contain elevated levels of phytochrome B encoded by either an introduced rice‐ or Arabidopsis‐derived cDNA driven by the 35S CaMV promoter. Inhibition of hypocotyl growth in etiolated seedlings of the phyB‐transformed lines was saturated at photon fluence rates of continuous red light (R) which were markedly lower than those required for inhibition of growth in seedlings of the isogenic wild‐type (WT). Inhibition of hypocotyl growth in etiolated seedlings of the phyB‐transgenic lines under continuous far‐red irradiation (FR), however, showed the same relationship with fluence rate as WT. Light‐grown seedlings of the phyB‐transgenic lines responded to end‐of‐day FR by an acceleration of growth, in a manner comparable with WT. This response was unaltered when the end‐of‐day FR was extended from a 15 min pulse to 14 h of continuous irradiation. The response of light‐grown, phyB‐transformed seedlings to decreasing R:FR ratio was also qualitatively similar to WT, i.e. increased elongation growth of the hypocotyl and petioles occurred under low R:FR quantum ratio. However, absolute elongation growth was markedly less in the transgenic seedlings at all R:FR ratios tested than in WT. Together, these data indicate that seedlings over‐expressing phytochrome B are more responsive to R than are WT, but are unaltered in their responsiveness to FR. By contrast, seedlings overexpressing phytochrome A are more responsive than WT to both R and FR; whereas the phytochrome B‐deficient mutant hy3 is unresponsive to R while retaining WT‐like responsiveness to FR. These data indicate that in WT etiolated seedlings phytochrome A mediates the effects of continuous FR, and phytochrome B the effects of continuous R. The evidence thus supports the conclusion that these two molecular species of the photoreceptor have differential regulatory roles in the plant.

Kinetic Study on the Development of the Capacity to Accumulate Chlorophyll <italic>A</italic> without a Lag Phase during Continuous Far-Red Irradiation in <italic>Pharbitis nil</italic> Seedlings

To analyze how the capacity to accumulate chlorophyll a (Chi a) without a lag phase develops by continuous far-red light (FR) irradiation, the time course of the capacity was investigated under various FR-dark conditions in Pharbitis seedlings. The capacity, which is negligible in darkness, rapidly developed on transfer to continuous FR irrespective of the age of the seedlings, the maximum level being attained about 18-24 h after the onset of.FR irradiation. The-capacity gradually decreased thereafter whether in darkness or under extended FR irradiation. When the seedlings pretreated by 6-12 h of FR followed by 66-60 h darkness (which have no capacity) were exposed to the second FR irradiation for up to 72 h, the level of the capacity acquired during the second FR irradiation was the lower the higher the level acquired by the first FR irradiation. A possible mechanism which determines the time course of the development of the capacity is discussed.

Development- and light-dependent regulation of the expression of two different chalcone synthase transcripts in mustard cotyledons

Two different chalcone synthase (CHS) transcripts show similar expression characteristics under different light regimes in cotyledons of mustard (Sinapis alba L.). Etiolated seedlings show an increase in dark-expression 36-42 h after sowing. Under continuous red or far-red irradiation both CHS transcripts start to accumulate to levels above those of the dark control at 24-27 h after sowing. This time point can therefore be considered as the starting (or competence) point for phytochrome control of CHS. Continuous far-red irradiation stimulates transcript accumulation more than red light, indicating the involvement of a high-irradiance response (HIR). Irradiation of etiolated seedlings with 5 min long-wavelength far-red light (RG9) at 6-21 h after sowing decreases CHS-mRNA levels below those of the dark control. It is concluded that CHS dark-expression in etiolated seedlings is controlled by a pool of stabletype phytochrome which is derived from seed tissue. By contrast, an RG9-light pulse given to etiolated seedlings 30 h after sowing causes accumulation of CHS-mRNA above the dark-control level. This response and the HIR are attributed to the action of labile phytochrome for which the seedling becomes competent at the starting point 24-27 h after sowing. The different starting points for CHS-mRNA expression in darkness and in light (36 h and 24 h, respectively, after sowing) also indicate that the tested CHS genes in mustard are under the photocontrol of two distinct phytochrome pools. Northern analysis shows that both CHS-mRNAs are expressed in primary leaves, epicotyls and young flower buds. In-situ hybridization with gene-specific CHS probes reveals similar expression patterns for both transcripts in cotyledons of seedlings grown under 42 h continuous far-red light.

Photomorphogenesis of the Gynophore, Pod and Embryo in Peanut, Arachis hypogaea L.

Darkened excized gynophores ceased to elongate after 8–10 days in vitro and started to form a pod. Gynophore elongation was inhibited to a greater extent in total darkness than under low irradiance, while pod and embryo growth was stimulated in darkness only. Intact gynophores, enclosed in transparent vials containing glass beads, continued to elongate in both light and darkness. In light the elongating gynophores thickened as they penetrated between the glass beads, forming a seedless pod at the bottom of the vials. In the dark the elongating gynophores produced small pods in which the seeds had started to grow. Excized gynophores elongated in vitro under continuous white light at a rate similar to that of intact exposed gynophores. The rate of elongation in vitro, was lower under continuous blue or red-enriched light, than under white light, and was further reduced under continuous far-red irradiation. Pods did not form during any of the continuous irradiation treatments but only after transfer to darkness, the largest pods forming after continuous far-red irradiation. As little as 10 min daily exposure to red or far-red irradiance had the same effect on gynophore elongation as continuous irradiation. Pods formed only when the daily periods of far-red irradiation were 30 min or less. Reducing the daily exposures to 2 min decreased the time to onset of pod formation from 30 to 16 days. Far-red following red irradiation was effective in inhibiting gynophore elongation stimulated by red irradiation. Pod formation in red/far-red irradiation was only 50 per cent of that observed in far-red irradiation. The involvement of light in continual gynophore elongation and in the concomitant inhibition of proembryo growth is discussed.

On the Role of Abscisic Acid in Phytochrome-mediated Photomorphogenesis )

Summary (±)- Abscisic acid completely prevents germination of mustard (Sinapis alba L.) seeds at concen trations higher than 2 μg · ml−1 in darkness as well as in continuous red or far-red light. However, growth of seedlings germinated on water for 36 h is only partly inhibited and the phytochrome-mediated (continuous far-red irradiation) photomorphogenesis is not impaired by 5 μg · ml−1 of the hormone. The development of 10 inducible enzyme activities which vary largely with respect to the extent of phytochrome effectiveness in the cotyledons of 36 h old mustard seedlings is either independent of or multiplicatively reduced by the hormone in darkness and continuous far-red light respectively. It is concluded that abscisic acid plays no essential role in photomorphogenesis of mustard seedlings and that the susceptibility (competence) to this hormone and to phytochrome during embryo development are separated in time.

Phytochrome regulation of phenylalanine ammonia-lyase (PAL) activity in radish cotyledons: A threshold mechanism

The time-course of phenylalanine ammonia-lyase (PAL) activity and of apparent phytochrome content has been determined under continuous farred irradiation. The kinetics of PAL (EC 4.3.1.5) show four phases: after a lagphase ( 1) which is detectable only in the early stages of the development, PAL activity increases ( 2), reaches a maximum ( 3) and then decreases ( 4). The development of PAL activity is controlled by the far-red absorbing form of the phytochrome (Pfr) through an all-or-none phenomenon. Whilst phytochrome content is above a critical value ( C m ) PAL activity increased. The lag-phase, the maximum and the decrease of PAL activity can be explained by the fact that the phytochrome content is below the C m .

Phytochrom-induzierte Regeneration von Adventivwurzeln beim Senfkeimling (Sinapis alba L.)

Isolated cotyledons and hypocotyl cuttings (“Restkeimlinge”) taken from young (36 h old) etiolated mustard seedlings at a stage of development at which nutrients are not yet limiting require active phytochrome (Pfr) to regenerate roots. The influence of photosynthesis on rooting (Lovell et al., 1969, 1972, 1973) appears to be limited to cotyledons depleted of storage materials. These conclusions are based on the following experimental results: 1. Continuous far-red irradiation (“Dauer-DR”) is as effective as continuous red irradiation (“Dauer-HR”) in stimulating rooting (Fig. 1). 2. Continuous far-red irradiation can be largely replaced by a series of 5 min red/12 h dark cycles. Five min far-red irradiations have no detectable effect if given alone, but fully reverse the action of red if given subsequently to the red irradiation (Fig. 3).

Über den einfluss von licht auf den umsatz von flavonolen und isoflavonen in Cicer arietinum

Continuous far-red irradiation of garbanzo seedlings ( Cicer arietinum L.) leads to repression of the synthesis of the isoflavones formononetin and biochanin A. The inhibition of isoflavone formation has been shown to occur both in the roots and the aerial parts. In contrast to the two isoflavons, the flavonols kaempferol, quercetin, and isorhamnetin do not significantly accumulate in dark grown seedlings. Tracer experiments, however, have shown that the very small yields of flavonols in etiolated plants are partly due to accelerated turnover. The differences between isoflavones and flavonols regarding their formation and turnover are discussed.

Distribution of phenylalanine ammonia-lyase in etiolated and far-red irradiated radish seedlings

In dark-grown Raphanus seedlings, most of the PAL activity is found in roots where it increases sigmoidally during organ development. In hypocotyls, the dark increase of enzyme activity is linear with time. In cotyledons and hooks, dark activity is very low and remains constant. After onset of continuous far-red irradiation, an activity increase is observed in all parts of the seedling. In cotyledons and hooks, the increase is followed by a decrease. This is comparable to light-induced PAL activity described in other materials. In roots and hypocotyls, the initial increase is not followed by a decrease. In dark-grown roots and hypocotyls PAL activity is correlated to fresh weight augmentation. In no part of the seedling could a correlation be found between light-induced PAL activity and anthocyanin formation.