Far red light control of elongation in light-grown bean hypocotyl: Effect on different age zones and interaction with indole-3-acetic acid

The effect of far red light on the light-grown bean hypocotyland its interaction with indole-3-acetic acid (IAA) were studied.Elongation of younger zones of the hypocotyl was inhibited butthat of older zones was promoted by far red light. This wascontrolled by phytochrome. Both the hook and shank portionscould receive far red light and its effect could be transmittedto either portions of the hypocotyl. When IAA was applied to the upper cut surface of the hypocotylunit, elongation of the shank portion was promoted even withoutfar red irradiation. IAA did not change the aspect of the growthcurves but amplified the elongation of each zone. When IAA wasapplied to each zone of the shank portion, elongation of zonesolder than the treated one was promoted but that of youngerzones was inhibited. This effect was emphasized by far red light.When IAA was applied to the older shank portion, elongationof the treated zone was synergistically promoted by IAA andfar red light, but when applied to the elbow or younger shankportion, far red light completely nullified the promoting effectof IAA. (Received October 1, 1979; )     

Auxin and red light in the control of hypocotyl hook opening in beans

Evidence is presented to support the suggestion that endogenous auxinlike substances participate in controlling the unbending of the hypocotyl hook of Phaseolus vulgaris L. (cv. Black Valentine). An acidic indole was detected in hook diffusates by fluorometry; triiodobenzoic acid, an inhibitor of auxin transport, prevented red light-induced unbending, and indoleacetic acid can be substituted for tissue just above the elbow region as an inhibitor of opening. Indoleacetic acid also stimulated growth of shank cells, and red light increased the sensitivity of this tissue to the hormone. A small red light-induced stimulation of auxin transport through the inside half of the hypocotyl shank was observed and may be related to light-induced unbending of the hook.     

The Role of Various Regions of the Bean Hypocotyl on Red Light-induced Hook Opening

Measurement of various zones on the concave half of etiolated Phaseolus vulgaris L. (cv. Black Valentine) hypocotyls has shown that growth at the basal portion of the elbow and the contiguous upper portion of the shank was stimulated earliest by red light. Growth of these two zones was unaffected by the tissue of the convex half but was inhibited by tissue distal to them. The inhibition was alleviated by the continuous presence of shank tissue below the growing zones. Based on cuts made halfway through the hypocotyl at positions above, below, or between the two zones of growth, it is suggested that cells at the inner portion of the upper shank control in some way the light-induced growth of the elbow cells directly above.     

Stimulation by Ethylene of Axis and Hypocotyl Growth in Bean and Cocklebur Seedlings

Effects of ethylene on the elongation of bean (Phaseolus vulgaris) embryonic axes and hypocotyls, and of cocklebur (Xanthium pennsylvanicum) hypocotyls were studied. In the bean axes, exogenous ethylene was promotive in stimulating longitudinal growth during the early germination period, but thereafter it turned inhibitive. This transition of the ethylene action is likely involved in the appearance of newly differentiated tissues in the hypocotyl, which are negatively sensitive to the gas. The ethylene stimulated elongation of the axes was hardly affected by light or by the presence of the cotyledons. In the bean hypocotyl segment unit, elongation was stimulated by ethylene in its limited zone, when the concentration of ethylene and the exposure times to ethylene were adequate (0.3 to 30 μl/l, 6 to 8 h): Elongation in the much younger region near to the elbow was inhibited by ethylene treatment, whereas the treatment of the upper region of the shank with ethylene finally resulted in significantly increased growth as compared to the untreated controls. In the continuing presence of ethylene over 3 days, the elongation of every region was retarded markedly and radial growth was induced in most regions of the shank from just below the elbow. These ethylene responses occurred independently of red light irradiation, but the ethylene promotion of elongation was lost by shortening the segment height, by removing the hook portion from the segment unit, or with its natural disappearance as a result of ageing. Fundamentally, similar effects of ethylene was observed in cocklebur hypocotyls.     

Developmental regulation of pectic epitopes during potato tuberisation

We show, by immunogold labelling, that potato (Solanum tuberosum L. cv Karnico) pectic epitopes are developmentally regulated within regions of the stolon, in addition to showing tissue-specific differences in abundance and localisation. The (1-->4)-beta-D-galactan and (1-->5)-alpha-arabinan epitopes demarcate two distinct zones within stolons; galactans are enriched in primary walls of elongating cells proximal to the stolon hook, whilst arabinans predominate in younger cells distal to the hook. Low-methoxyl homogalacturonan epitopes are concentrated in the middle lamella and show a proximo-distal gradient in stolons similar to that of galactans, whilst high-methoxyl homogalacturonan is uniformly abundant. Calcium pectate is restricted to the middle lamella at cell corners and pit fields. Calcium-binding sites are uniformly present in stolon cell walls, but their total density is reduced and they become localised to a few cell corners in mature tubers, as determined by image-electron energy loss spectroscopy. During the transition from elongation growth to isodiametric expansion during tuberisation of the stolon hook, there were no detectable changes in pectic epitope abundance or localisation. As tubers matured, all epitopes increased in abundance in parenchymal cell walls, except for calcium pectate. We conclude that potentially significant changes in pectic composition occur as young cells distal to the stolon hook move into the zone of cell elongation proximal to the hook.     

Regional growth patterns in the hypocotyls of etiolated and green cress seedlings in light and darkness

Abstract. A system is described whereby seedling development can be analysed in terms of growth rates of specific 1 mm regions of the hypocotyl. The technique involves time-lapse photography of marked hypocotyls in a specially designed chamber which accommodates seedlings in various orientations with respect to gravity, and under irradiation regimes differing in light quality, quantity and direction. The results of a preliminary study of the upward growth of etiolated or green cress seedlings in darkness or overhead while light are reported. Highest growth rates in etiolated seedlings were observed in zones in the upper one-third of ihe hypocotyl. In green seedlings, growth was more prominent within the subapical zones. Light further restricted growth of the median and basal zones in both types of seedling. However, in their immediate responses to the onset of irradiation, green and etiolated seedlings differed markedly. In etiolated seedlings, recovery of growth at the apex was accompanied by the development of inhibition in the median-basal regions; green seedlings showed a transient inhibition of growth in the apical zone together with a strong immediate inhibition in the median-basal regions.     

Oncospheral hook morphogenesis in the davaineid cestode Inermicapsifer madagascariensis (Davaine, 1870) Baer, 1956

The fine structural characteristics of oncospheral hook morphogenesis in the davaineid cestode Inermicapsifer madagascariensis are described. The primordia of the hooks appear in the oncoblasts of embryos in the advanced stage of development, in which there is a greatly reduced number of blastomeres that exhibit a bilaterally symmetrical pattern in their organization.

The hook primordium, adjacent to the invaginated part of the nuclear envelope, is surrounded by an abundance of free ribosomes, mitochondrial aggregations and extended Golgi regions. Simultaneously with its elongation and transformation into a blade, the hook primordium material becomes differentiated to form an electron dense cortex and a less dense, inner, crystal-like core. At the beginning of shank formation, the blade of the hook protrudes outside the oncoblast. The membrane-enclosed point of exit of the blade is surrounded by a cytoplasmic sheath which later forms a circular, septate desmosome.

With oncoblast degeneration, muscle fibres attach directly to the collar and the base of the hook.     

Electron microscopy and histochemistry of oncospheral hook formation by the cestode Catenotaenia pusilla

Swiderski Z. 1973. Electron microscopy and histochemistry of oncospheral hook formation by the cestode Catenotaenia pusilla. International Journal for Parasitology3: 27–33. Ultrastructure and histochemistry of oncospheral hook development in the cestode C. pusilla are described. The six anlagen of embryonal hooks appear in six specialized hook forming cells (oncoblasts) in the advanced phase of the preoncosphere.Electron microscopy shows a close connection of hook primordium with an abundance of free ribosomes, extended Golgi regions, and mitochondrial aggregations, evidently engaged in the hook morphogenesis. The shank completion occurs simultaneously with the progressive degeneration of hook forming cells, which completely disappears in the last phase of hook development. The mature oncospheral hook is a heterogeneous, bipartite structure composed of a dense outer sheath or cortex and a less dense inner core.Histochemistry shows evident changes in the reactivity for —SH, and —S—S— groups through the consecutive stages of hook development. The early hook anlage shows strongly positive reaction for sulfhydryl (—SH) groups (unconsolidated prekeratin), which remains in the subsequent stages mainly in the zone of keratinization, undergoing continuous displacement toward the base during hook maturation.The sulfhydryl groups of prokeratin through the oxidation process form bisulfite (—S—S—) links of mature hook keratin; reactivity for —SH groups completely disappears. The difference in electron density between the outer and inner part of the hook corresponds to a different reactivity for —S—S— links; the outer sheath shows evidently stronger reaction than the inner core.     

Effect of the hypocotyl hook on greening in etiolated cucumber cotyledons

Four-day-old etiolated cucumber cotyledons (Cucumis sativus, L.) were excised and allowed to green in white fluorescent light at 28 C. Cotyledons excised with a full hypocotyl hook exhibited a lag phase of 1 hour before entering the rapid greening phase, whereas cotyledons excised without any hypocotyl hook exhibited a lag phase of 6 hours. Cotyledons excised with varying lengths of hypocotyl hook accumulated chlorophyll roughly in proportion to the hook length. When cotyledons were excised with a full hook and were partially or totally shielded from light with aluminum foil, the samples with the hook covered accumulated more chlorophyll than the wholly exposed samples. The samples with the cotyledons covered showed no net accumulation of chlorophyll irrespective of hook's exposure to light. These data suggest the contribution of some factor or factors by the hypocotyl hook which reduce the lag phase during greening.     

Regulation of pea epicotyl elongation by blue light : fluence-response relationships and growth distribution

Irradiation with blue light causes a rapid decrease in stem elongation in Pisum sativum. Growing plants under continuous red light allowed us to study the fluence dependence and spatial distribution of blue-induced growth effects without interference from large changes in the ratio of the far-red absorbing form of phytochrome to total phytochrome. The magnitude of the inhibition generated by a 30-second pulse of blue light was linearly related to the log of the fluence applied over two orders of magnitude. Reciprocity held for irradiations with a pulse length shorter than the lag time for the response. The spatial distribution of inhibition was studied by marking the growing zone and photographing the stem at 10-minute intervals before, during, and after a 1-hour exposure to blue light. The region just below the hook does not undergo any perceptible change in growth rate while growth is nearly 100% inhibited in the base of the third internode.     

Low-fluence red light increases the transport and biosynthesis of auxin

In plants, light is an important environmental signal that induces photomorphogenesis and interacts with endogenous signals, including hormones. We found that light increased polar auxin transport in dark-grown Arabidopsis (Arabidopsis thaliana) and tomato (Solanum lycopersicum) hypocotyls. In tomato, this increase was induced by low-fluence red or blue light followed by 1 d of darkness. It was reduced in phyA, phyB1, and phyB2 tomato mutants and was reversed by far-red light applied immediately after the red or blue light exposure, suggesting that phytochrome is involved in this response. We further found that the free indole-3-acetic acid (IAA) level in hypocotyl regions below the hook was increased by red light, while the level of conjugated IAA was unchanged. Analysis of IAA synthesized from [13C]indole or [13C]tryptophan (Trp) revealed that both Trp-dependent and Trp-independent IAA biosynthesis were increased by low-fluence red light in the top section (meristem, cotyledons, and hook), and the Trp-independent pathway appears to become the primary route for IAA biosynthesis after red light exposure. IAA biosynthesis in tissues below the top section was not affected by red light, suggesting that the increase of free IAA in this region was due to increased transport of IAA from above. Our study provides a comprehensive view of light effects on the transport and biosynthesis of IAA, showing that red light increases both IAA biosynthesis in the top section and polar auxin transport in hypocotyls, leading to unchanged free IAA levels in the top section and increased free IAA levels in the lower hypocotyl regions.     

The role of the apex in normal and tropic growth of sunflower hypocotyls

Regional growth in vertical and horizontal etiolated sunflower hypocotyls from which the apical hook tissue had been either partly or wholly excised, was measured 24 h later, the regions having been demarcated with resin beads. Removal of the cotyledons (an excision which included the distal end of the shoot apex) had little effect on growth during this period but excision of the apical hook significantly reduced growth. In vertically orientated seedlings, removal of half of the hook severely reduced growth in all other growing regions and removal of the entire hook totally inhibited growth. This inhibition of growth was not a consequence of the removal of the region of growth but a consequence of the removal of a region on which growth was dependent. In horizontal seedlings, the situation was more complex inasmuch as a horizontal orientation itself induced growth in previously non-growing regions. This new growth was localised in its extent and was not as severely affected by progressive excision of the hook as was growth in vertical seedlings. The results are discussed in terms of overall growth co-ordination in the hypocotyl.     

Control of Hypocotyl Hook Angle in Phaseolus mungo L. Role of Growth Substances

Effects of growth hormones on the hook angle and light responseof Phaseolus mungo L. hypocotyl hooks are described and theresults are discussed with reference to the functions of otherparts of the seedling in controlling the growth and shape ofthe hook. Apically applied IAA (indolyl acetic acid) prevented hook openingin decapitated seedlings in the dark and in all the red-irradiatedseedlings. [¹⁴C]IAA experiments showed that only a small quantityof IAA (2–6 ?g per hook) was required to produce theseeffects, and that transport of IAA through the hook was negligibleand unaffected by red irradiation. ABA (abscisic acid) had little effect on the hooks or theirlight response. Applied ethylene and IAA-induced ethylene slightly closed thehooks, but only slightly reduced light-induced opening. IAAreduced the effect of ethylene in the dark, but after irradiationthe hooks appeared more sensitive to the ethylene in the presenceof IAA, resulting in light-induced hook closure. Basally applied kinetin (6-furfurylaminopurine) prevented decapitatedhooks from opening in the dark, especially when GA₃ (gibberellicacid) was also present. Some combinations of kinetin and GA₃(with high kinetin concentrations) also prevented light-inducedopening, but combinations with lower kinetin concentrationsallowed almost as much opening as was found in intact hooks. It is proposed that the terminal parts act by regulating thesupply of cytokinins and gibberellins from the basal parts,and that IAA does not mediate this funotion in this species. The results are compared with those reported for other species.     

Interactions of light and ethylene in hypocotyl hook maintenance in Arabidopsis thaliana seedlings

Etiolated seedlings frequently display a hypocotyl or epicotyl hook which opens on exposure to light. Ethylene has been shown to be necessary for maintenance of the hook in a number of plants in darkness. We investigated the interaction of ethylene and light in the regulation of hypocotyl hook opening in Arabidopsis thaliana . We found that hooks of Arabidopsis open in response to continuous red, far-red or blue light in the presence of up to 100 μl l⁻¹ ethylene. Thus a change in sensitivity to ethylene is likely to be responsible for hook opening in Arabidopsis, rather than a decrease in ethylene production in hook tissues. We used photomorphogenic mutants of Arabidopsis to demonstrate the involvement of both blue light and phytochrome photosensory systems in light-induced hook opening in the presence of ethylene. In addition we used ethylene mutants and inhibitors of ethylene action to investigate the role of ethylene in hook maintenance in seedlings grown in light and darkness.     

Ethylene and carbon dioxide as mediators in the response of the bean hypocotyl hook to light and auxins

Summary Ethylene inhibits hook opening in the bean hypocotyl and at high concentrations induces closure of the hook. Indoleacetic acid and 2,4-dichlorophenoxyacetic acid, whose inhibitory effect on hook opening resembles that of ethylene, stimulate ethylene production from the hook tissue, and this ethylene production is physiologically active in inhibiting hook opening. It is concluded that the inhibition of opening by auxin is due at least in a major part to auxin-induced ethylene production by the hook tissue.Carbon dioxide promotes hook opening, apparently by antagonizing the action of endogenous ethylene. The concentration of respiratory CO₂ in the internal gas space of the hook tissue is high enough to play a role in the regulation of hook opening.Red light causes a decrease in ethylene production and an increase in CO₂ evolution from the hook tissue. These effects are partially reversible by far-red light. It is concluded that both ethylene and CO₂ serve as natural growth regulators which mediate the hypocotyl hook-opening response to light in bean seedlings.     

Regulation of differential growth in the apical hook of Arabidopsis.

Arabidopsis seedlings develop a hook-like structure at the apical part of the hypocotyl when grown in darkness. Differential cell growth processes result in the curved hypocotyl hook. Time-dependent analyses of the hypocotyl showed that the apical hook is formed during an early phase of seedling growth and is maintained in a sequential phase by a distinct process. Based on developmental genetic analyses of hook-affected mutants, we show that the hookless mutants (hls1, cop2) are involved in an early aspect of hook development. From time-dependent analyses of ethylene-insensitive mutants, later steps in hook maintenance were found to be ethylene sensitive. Regulation of differential growth was further studied through examination of the spatial pattern of expression of two hormone-regulated genes: an ethylene biosynthetic enzyme and the ethylene receptor ETR1. Accumulation of mRNA for AtACO2, a novel ACC (1-aminocyclopropane-1-carboxylic acid) oxidase gene, occurred within cells predominantly located on the outer-side of the hook and was tightly correlated with ethylene-induced exaggeration in the curvature of the hook. ETR1 expression in the apical hook, however, was reduced by ethylene treatment. Based on the expression pattern of ETR1 and AtACO2 in the hook-affected mutants, a model for hook development and maintenance is proposed.     

Genome-wide association study of shank length and diameter at different developmental stages in chicken F2 resource population

In order to find SNPs and genes affecting shank traits, we performed a GWAS in a chicken F2 population of eight half-sib families from five hatches derived from reciprocal crosses between an Arian fast-growing line and an Urmia indigenous slow-growing chicken. A total of 308 birds were genotyped using a 60K chicken SNP chip. Shank traits including shank length and diameter were measured weekly from birth to 12 weeks of age. A generalized linear model and a compressed mixed linear model (CMLM) were applied to achieve the significant regions. The value of the average genomic inflation factor (λ statistic) of the CMLM model (0.99) indicated that the CMLM was more effective than the generalized linear model in controlling the population structure. The genes surrounding significant SNPs and their biological functions were identified from NCBI, Ensembl and UniProt databases. The results indicated that 12 SNPs at 12 different ages passed the LD-adjusted 5% Bonferroni significant threshold. Two SNPs were significant for shank length and nine SNPs were significant for shank diameter. The significant SNPs were located near to or inside 11 candidate genes. The results showed that a number of significant SNPs in the middle ages were higher than the rest. The MXRA8 gene was related to the significant SNP at week 1 that promotes proliferation of growth plate chondrocytes. A unique SNP of Gga_rs16689511 located on chicken Z chromosome within the LOC101747628 gene was related to shank length at three different ages of birds (weeks 8, 9 and 11). The significant SNPs for shank diameter were found at weeks 4 and 7 (four and five SNPs respectively). The identifications of SNPs and genes here could contribute to a better understanding of the genetic control of shank traits in chicken.     

Light responses,growth factors and phytochrome transformations of Cucumis seedling tissues

Summary The elongation of hypocotyl segments cut from etiolated Cucumis sativus seedlings is not affected by a single red light exposure at the start of the 20-hour growth period, but is inhibited by brief exposures repeated each hour or two even though these give no greater total energy. The inhibition is annulled if each red exposure is followed by far-red. The time course of phytochrome transformations in this tissue after a single red light exposure, followed spectrophotometrically, shows no anomalous characteristics that might correlate with the unusual pattern of responsiveness to red light. In intact seedlings, hypocotyl elongation responds similarly, but the opening of the hypocotyl hook is saturated by a single initial red light treatment. Excised hypocotyl segments on water alone appear insensitive to repeated red light treatment, but the growth increments caused by the addition of potassium ion, 2-propanol or cobaltous ion, or by leaving the cotyledons attached, are all inhibited roughly 40%. However, continuous white light inhibits the entire growth increment, reducing elongation to that of the water controls. Some implications of these results for current hypotheses and future investigations on the mechanisms of growth regulation by light are discussed.Research carried out in part at Brookhaven National Laboratory under the auspices of the U.S. Atomic Energy Commission and supported in part by Cancer Research Funds of the University of Califonia and by National Science Foundation Grant GB-3248 to WKP.     

Analysis of growth during phototropic curvature of cress hypocotyls

Abstract. Growth rates throughout an organ curving phototropically under continuous, unilateral while light were monitored by lime-lapse photography of cress hypocotyls marked into 1 mm sections by two rows of ion-exchange beads. Curvature resulted from an integrated sequence of changes in growth rate on each side of the organ, but the actual patterns of change and, therefore rales of curvature, differed within even this one species, depending upon the immediate pretreatment of the seedlings. Transference of seedlings from darkness to unilateral irradiation gave immediate growth inhibition on both sides of the organ. Curvature resulted from differential recovery of growth rate, being seen first on the shaded side, most prominently in the apical regions; only 2h after initial exposure to light did growth recover on the lit (lower) side. On the other hand, transfer of seedlings from omnilateral to unilateral irradiation of the same intensity resulted in simultaneous growth inhibition on the irradiated side and stimulated growth on the shaded side: this growth stimulation of the shaded side was greater than occurred in totally darkened control plants.     

Photocontrol of the hypocotyl hook opening of Sinapis alba seedlings. Involvement of phytochrome and a high irradiance response

Baumgartner, N. and Fondeville, J. C. 1989. Photocontrol of the hypocotyl hook opening of Sinapis alba seedlings. Involvement of phytochrome and a high irradiance response.
A statistical evaluation of the hypocotyl hook opening (hook opening index) was used for measurement of the hook angle in lots of etiolated Sinapis alba L. cv. Albatros seedlings. Studies of the kinetics for hook opening were carried out in continuous fluorescent white, blue and red light (6, 15 and 40 μmol m^-2s^-1) with 2-day-old dark-grown seedlings. At the beginning of the irradiation period the photoresponse in red light was the opposite to that in blue (low photon fluences). Blue rapidly induced the hook opening (in less than 20 min), while red produced hook tightening (photon fluences up to 70 mmol m^-2), which precedes the normal progressive hook opening. For low fluences, the data were consistent with the involvement of phytochrome and a specific blue light photoreceptor. A phytochrome effect was observed in the hook opening, dependent upon a high irradiance response (HIR). This HIR (like that for the inhibition of the hypocotyl elongation) was characterized by a wavelength response curve with maxima in the blue and far-red regions of the spectrum.