Changes in the content of endogenous auxins in apical buds ofchenopodium rubrum L. Induced with respect to the endogenous rhythm in capacity to flower

The content of endogenous auxins was examined in apical buds ofChenopodium rubrum plants induced by a photoperiodic cycle of 16h darkness and 8h light followed by a dark period of various duration so as to correspond with either maximal or minimal flowering response in the endogenous rhythm in capacity to flower initiated by the photoperiodic treatment. Apical buds of potentially generative plants contained less auxins than apical buds of plants which remained in the vegetative state. Apical buds from plants treated with kinetin (1. 10^-3 M) and therefore remaining in the vegetative state showed an auxin level comparable to that of untreated plants exhibiting minimal flowering response irrespective of the duration of the second dark period. Plants cultivated on a sucrose solution (0.6 M) during the second dark period became generative even at the normal minimum of flowering. The auxin content of the apical buds was low, similarly as in untreated plants induced for a period leading to maximal flowering response. On the other hand, apical buds from plants grown on sucrose solution during a dark period leading to the manifestation of maximal flowering response showed a relatively high auxin content comparable to that found in untreated plants which had obtained a more extended induction by three photoperiodic cycles. The results are discussed with respect to the possible role of endogenous auxins in the regulation of the changes in growth correlations occurring in the shoot apex during photoperiodic induction and in the expression of the competence to flower.     

An investigation of circadian rhythm in Escherichia coli

Background: Persuasive evidence for circadian programs in non-photosynthetic bacteria other than cyanobacteria is still lacking, we aimed to investigate the circadian rhythm of specific growth rate in Escherichia coli ATCC 25922, one of the important prokaryotes. Methods: To grow E. coli under different light and dark conditions. When the growth entered into the stationary phase, we stopped the culture and obtained the viable counts by MTT assay every 3 h. The specific growth rates (SGRs) were calculated and analyzed with cosinor method for potential rhythms. Results: Single cosinor method revealed that the SGR of E. coli displayed rhythmic variations with a period of around 24 h both under light/dark cycles and under constant darkness. The best-fitting periods and best-fitting cosine curves were acquired. Conclusions: The SGR of E. coli (ATCC 25922) in a culture medium with limiting substrates in the stationary and death phases displayed rhythmic variations with a period of around 24 h under light/dark cycles and constant darkness conditions.     

Light level does not alter ethylene sensitivity in radish or pea

Ethylene accumulation occurs in many plant growth environments. In some instances, low photosynthetic photon flux (PPF) is also a stress factor. Ethylene helps regulate the shade-avoidance mechanism and synthesis rates can be altered by light. We thus hypothesized that ethylene sensitivity in whole plants may be altered in low light. Radish (Raphanus sativus) and pea (Pisum sativum) plants were selected as models due to their rapid growth, use in previous studies and difference in growth habit. We first characterized radish and pea sensitivity to ethylene. Radish vegetation was less sensitive to ethylene than pea vegetation. Pea reproductive yield was highly sensitive. Plants grown under low light levels are typically etiolated and less robust than plants grown under higher light. In a second series of studies we examined the interaction of ethylene (50 ppb pea, 200 ppb radish) with PPFs from 50 to 400 μmol m^?2 s^?1. There was no statistically significant interaction between ethylene sensitivity and PPF, indicating that high PPF does not mitigate the detrimental effects of chronic low-level ethylene exposure. This also suggests there is no crosstalk between the shade avoidance pathway and the primary ethylene signaling pathway.     

Circadian expression of DNA methylation and demethylation genes in zebrafish gonads

This research aimed at investigating the light synchronization and endogenous origin of daily expression rhythms of eight key genes involved in epigenetic mechanisms (DNA methylation and demethylation) in zebrafish gonads. To this end, 84 zebrafish were distributed into six tanks, each one containing 14 fish (7 males and 7 females). Animals were subjected to 12 h light:12 h dark cycles (LD, lights on at ZT0 h) and fed randomly three times a day during the light phase. Locomotor activity rhythms were recorded in each tank for 20 days to test their synchronization to light. Then, zebrafish were fasted for one day and gonad samples were collected every 4 h during a 24 h cycle (ZT2, 6, 10, 14, 18, and 22 h). The results revealed that most of the epigenetic genes investigated exhibited a significant daily rhythm. DNA methylation genes (dnmt4, dnmt5, dnmt7) exhibited a daily rhythm of expression with a nocturnal acrophase (ZT14:01~ZT22:17 h), except for dnmt7 in males (ZT2:25 h). Similarly, all DNA demethylation genes (tet2, tdg, mb4, gadd45aa, and apobec2b) revealed the existence of statistically significant daily rhythms, except for gadd45aa in females. In females, tdg, mb4, and apobec2b presented a nocturnal peak (ZT14:20 ~ ZT22:04 h), whereas the tet2 acrophase was diurnal (ZT4:02 h). In males, tet2, tdg, and gadd45aa had nocturnal acrophases (ZT18:26~ZT21:31 h), whereas mb4 and apobec2b displayed diurnal acrophases (ZT5:28 and ZT4:02 h, respectively). To determine the endogenous nature of gene expression rhythms, another experiment was performed: 12 groups of 14 fish (7 males and 7 females) were kept in complete darkness (DD) and sampled every 4 h during a 48 h cycle (CT2, 6, 10, 14, 18, 22, 26, 30, 34, 38, 42, and 46 h). Under DD, most of the genes (7 out of 8) presented circadian rhythmicity with different endogenous periodicities (tau), suggesting that the epigenetic mechanisms of DNA methylation and demethylation in the gonads follow an internal control, functioning as part of the translation network linking the environment into somatic signals in fish reproduction.     

The circadian rhythms of photosynthesis,ATP content and cell division in Microcystis aeruginosa PCC7820

Microcystis aeruginosa is one of the most common blue-green algae species that forms harmful water bloom, which frequently causes serious ecological pollution and poses a health hazard to animals and humans. To understand the progression of algal blooms and to provide a theoretical basis for predicting and preventing the occurrence of algal blooms and reducing the harm of algal bloom to environment, we investigated the diurnal variation of photosynthesis, ATP content and cell division in M. aeruginosa PCC7820. The results showed that the photosynthesis and ATP content of M. aeruginosa PCC7820 exhibited clear circadian rhythm with a period of approximately 24 h and that the periodic rhythms continued for at least three cycles under continuous light conditions. Furthermore, the period length showed that a temperature compensation effect and changes in light cycle or temperature could reset the phase of circadian rhythm. These results indicate that the circadian rhythms of physiological process in M. aeruginosa PCC7820 are controlled by the endogenous circadian clock. Examinations of the number, size and cytokinin content of cells also reveal that the cell division of M. aeruginosa PCC7820 with the generation time of 38.4 h exhibits robust circadian rhythms with a period close to 24 h. The circadian rhythms of cell division may be generated by a biological clock through regulation of the cell division phase of M. aeruginosa PCC7820 via a gating mechanism. The phases in which cell division slows or stop recur with a circadian periodicity of about 24 h.     

Rhythmic changes in ribonuclease activity in relation to nucleic acid synthesis in cotyledons ofChenopodium rubrum L. and to floral induction of the plants

Ribonuclease (RNAse) activity was investigated in cotyledons ofChenopodium rubrum plants subjected to various conditions of illumination (photoperiodic induction, continuous light, induction cancelled by interrupting the dark period by a light-break). At the end of the dark period of the single inductive cycles RNAse activity of induced plants was inferior to that of plants grown in continuous light. At the end of the first two cycles the activity was lowest after the interruption of the dark period by light. The investigation of the enzyme in 6h intervals showed rhythmic changes in activity to occur in induced plants. Enzyme activity followed a pattern opposed to this of nucleic acid (NA) synthesis in the cotyledons. In plants from continuous light the enzyme activity did not show any rhythm and in plants having obtained a light-break during the inductive period the rhythm was less distinct than in the induced ones. The period length of the endogenous rhythm of NA synthesis in the cotyledons is about half as long as this of flowering and the peaks of flowering coincide with the throughs of NA synthesis.     

Cycling of clock genes entrained to the solar rhythm enables plants to tell time: data from arabidopsis

Background and Aims An endogenous rhythm synchronized to dawn cannot time photosynthesis-linked genes to peak consistently at noon since the interval between sunrise and noon changes seasonally. In this study, a solar clock model that circumvents this limitation is proposed using two daily timing references synchronized to noon and midnight. Other rhythmic genes that are not directly linked to photosynthesis, and which peak at other times, also find an adaptive advantage in entrainment to the solar rhythm.Methods Fourteen datasets extracted from three published papers were used in a meta-analysis to examine the cyclic behaviour of the Arabidopsis thaliana photosynthesis-related gene CAB2 and the clock oscillator genes TOC1 and LHY in T cycles and N–H cycles.Key Results Changes in the rhythms of CAB2, TOC1 and LHY in plants subjected to non-24-h light:dark cycles matched the hypothesized changes in their behaviour as predicted by the solar clock model, thus validating it. The analysis further showed that TOC1 expression peaked ∼5·5 h after mid-day, CAB2 peaked close to noon, while LHY peaked ∼7·5 h after midnight, regardless of the cycle period, the photoperiod or the light:dark period ratio. The solar clock model correctly predicted the zeitgeber timing of these genes under 11 different lighting regimes comprising combinations of seven light periods, nine dark periods, four cycle periods and four light:dark period ratios. In short cycles that terminated before LHY could be expressed, the solar clock correctly predicted zeitgeber timing of its expression in the following cycle.Conclusions Regulation of gene phases by the solar clock enables the plant to tell the time, by which means a large number of genes are regulated. This facilitates the initiation of gene expression even before the arrival of sunrise, sunset or noon, thus allowing the plant to ‘anticipate’ dawn, dusk or mid-day respectively, independently of the photoperiod.     

Suppression of telomere-binding protein gene expression represses seed and fruit development in tomato

Tomato (Solanum lycopersicum L.) plants were transformed with an antisense construct of a cDNA encoding tomato telomere-binding protein (LeTBP1) to describe the role of a telomere-binding protein at the whole plant level. Fruit size decreased corresponding to the degree of suppression of LeTBP1 expression. This inhibition of fruit development was likely due to a decrease in the number of seeds in the LeTBP1 antisense plants. Pollen fertility and pollen germination rate decreased in accordance with the degree of suppression of LeTBP1 expression. Ovule viability was also reduced in the LeTBP1 antisense plants. Although plant height was somewhat reduced in the antisense plants compared to the control plants, the number and weight of leaves were unaffected by LeTBP1 suppression. The number and morphology of flowers were also normal in the antisense plants. These indicate that reduced fertility in the antisense plants is not an indirect effect of altered vegetative growth. LeTBP1 expression was sensitive to temperature stress in wild-type plants. We conclude that LeTBP1 plays a critical role in seed and fruit development rather than vegetative growth and flower formation.     

Spontaneous action potentials and circumnutation in <Emphasis Type="Italic">Helianthus annuus</Emphasis>

Action potentials (APs) in plants are involved in fast leaf or trap closure as well as elongation, respiration, photosynthesis, and fertilization regulation. Here, spontaneous APs (SAPs) in relation to endogenous stem movement named circumnutation (CN) have been investigated in Helianthus annuus in different light conditions in freely circumnutating and immobilized plants. Extracellular electrical measurements and time-lapse photography were carried out simultaneously. The parameters of CN (trajectory length, period, and direction) and the number and transmission direction of SAPs were analysed. In continuous light (25–40 μmol m^?2 s^?1), all plants circumnutating vigorously in a regular elliptical manner and no SAPs were observed. In light/dark conditions, the plants circumnutated in a daily pattern, most SAPs were observed in the dark and freely circumnutated sunflowers had two times more SAPs (10 SAPs/24 h/plant) than the immobilized plants (5 SAPs/24 h/plant). In continuous very low light (5 μmol m^?2 s^?1), the plants circumnutated weakly and irregularly and SAPs appeared without the circadian pattern. 3–5 SAPs/24/plant occurred in the freely circumnutating and immobilized plants. In light/dark and continuous very low light conditions, an ultradian rhythm of SAPs was observed and the mean spacing between SAPs was approx. 121–271 min in the freely circumnutating and immobilized plants. Under all light conditions, more SAPs were transmitted basipetally than acropetally. One-hour lasting series of 3–4 min spaced SAPs locally propagated were observed as well in very low light. Basipetal and acropetal SAPs passing along the stem motor region accompany irregularity, changes in the CN trajectory direction, and stem torsion. These results demonstrate that APs and CN changes play a role in plant adaptation to light conditions and that there is an ultradian rhythm of SAPs beside ultradian CN rhythm.     

Bioethanol production from Scenedesmus obliquus sugars: the influence of photobioreactors and culture conditions on biomass production

A closed-loop vertical tubular photobioreactor (PBR), specially designed to operate under conditions of scarce flat land availability and irregular solar irradiance conditions, was used to study the potential of Scenedesmus obliquus biomass/sugar production. The results obtained were compared to those from an open-raceway pond and a closed-bubble column. The influence of the type of light source and the regime (natural vs artificial and continuous vs light/dark cycles) on the growth of the microalga and the extent of the sugar accumulation was studied in both PBRs. The best type of reactor studied was a closed-loop PBR illuminated with natural light/dark cycles. In all the cases, the relationship between the nitrate depletion and the sugar accumulation was observed. The microalga Scenedesmus was cultivated for 53 days in a raceway pond (4,500 L) and accumulated a maximum sugar content of 29 % g/g. It was pre-treated for carrying out ethanol fermentation assays, and the highest ethanol concentration obtained in the hydrolysate fermented by Kluyveromyces marxianus was 11.7 g/L.     

Dim Nighttime Illumination Interacts With Parametric Effects of Bright Light to Increase the Stability of Circadian Rhythm Bifurcation in Hamsters

The endogenous circadian pacemaker of mammals is synchronized to the environmental day by the ambient cycle of relative light and dark. The present studies assessed the actions of light in a novel circadian entrainment paradigm where activity rhythms are bifurcated following exposure to a 24-h light:dark:light:dark (LDLD) cycle. Bifurcated entrainment under LDLD reflects the temporal dissociation of component oscillators that comprise the circadian system and is facilitated when daily scotophases are dimly lit rather than completely dark. Although bifurcation can be stably maintained in LDLD, it is quickly reversed under constant conditions. Here the authors examine whether dim scotophase illumination acts to maintain bifurcated entrainment under LDLD through potential interactions with the parametric actions of bright light during the two daily photophases. In three experiments, wheel-running rhythms of Syrian hamsters were bifurcated under LDLD with dimly lit scotophases, and after several weeks, dim scotophase illumination was either retained or extinguished. Additionally, “full” and “skeleton” photophases were employed under LDLD cycles with dimly lit or completely dark scotophases to distinguish parametric from nonparametric effects of bright light. Rhythm bifurcation was more stable in full versus skeleton LDLD cycles. Dim light facilitated the maintenance of bifurcated entrainment under full LDLD cycles but did not prevent the loss of rhythm bifurcation in skeleton LDLD cycles. These studies indicate that parametric actions of bright light maintain the bifurcated entrainment state; that dim scotophase illumination increases the stability of the bifurcated state; and that dim light interacts with the parametric effects of bright light to increase the stability of rhythm bifurcation under full LDLD cycles. A further understanding of the novel actions of dim light may lead to new strategies for understanding, preventing, and treating chronobiological disturbances. (Author correspondence: jevans@msm.edu)     

A circadian and an ultradian rhythm are both evident in root growth of rice

This paper presents evidence for the existence of both a circadian and an ultradian rhythm in the elongation growth of rice roots. Root elongation of rice (Oryza sativa) was recorded under dim green light by using a CCD camera connected to a computer. Four treatment conditions were set-up to investigate the existence of endogenous rhythms: 28 °C constant temperature and continuous dark (28 DD); 28 °C constant temperature and alternating light and dark (28 LD); 33 °C constant temperature and continuous dark (33 DD); and diurnal temperature change and alternating light and dark (DT-LD). The resulting spectral densities suggested the existence of periodicities of 20.4-25.2 h (circadian cycles) and 2.0-6.0 h (ultradian cycles) in each of the 4 treatments. The shorter ultradian cycles can be attributed to circumnutational growth of roots and/or to mucilage exudation. The average values across all the replicate data showed that the highest power spectral densities (PSDs) corresponded to root growth rhythms with periods of 22.9, 23.7, and 2.1 h for the 28 DD, 28 LD, and 33 DD treatments, respectively. Accumulation of PSD for each data set indicated that the periodicity was similar in both the 28 DD and 33 DD treatments. We conclude that a 23-h circadian and a 2-h ultradian rhythmicity exist in rice root elongation. Moreover, root elongation rates during the day were 1.08 and 1.44 times faster than those during the night for the 28 LD and DT-LD treatments, respectively.     

Endogenous rhythms of locomotion in the American horseshoe crab, Limulus polyphemus

American horseshoe crabs (Limulus polyphemus) exhibit clear circadian rhythms of visual sensitivity in the laboratory and in the field they exhibit seasonal patterns of mating behavior that are closely associated with the tides. Recent reports suggest that Limulus locomotor activity may be controlled by endogenous circadian and/or circatidal clocks and that light:dark (LD) cycles may affect the rhythmic output of both of these clocks. In this study, we examined locomotor behavior in the laboratory to determine the extent of this endogenous activity and to examine the influence of LD cycles on these rhythms. Thirty-three L. polyphemus were captured during the breeding season and their activity was monitored with activity boxes and “running wheels” in seawater kept at constant temperature and salinity. Activity patterns were analyzed using visual inspection of actograms and Chi-square and Lomb-Scargle periodograms. Overall, 36% of the animals was significantly more active during L, while only 12% was more active during D (52% showed no preference). Circatidal rhythms were observed in LD in 67% of the horseshoe crabs. Surprisingly, LD cycles appeared to synchronize these rhythms at times. In DD, the majority of animals tested (63%) exhibited circatidal rhythms that persisted for at least seven days. Overall, the results demonstrate that an endogenously controlled tidal rhythm of locomotion operates during, and significantly after, the breeding season in this species. In addition, the present results are consistent with the presence of circalunidian oscillators controlling these rhythms.     

Effects of Exogenously Applied Gibberellins and Thidiazuron on Phytohormone Profiles of Long-Shoot Buds and Cone Gender Determination in Lodgepole Pine

In long-shoot buds of lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm), cone-bud initiation and gender differentiation occur in a site-specific manner: female cone buds are normally restricted to the distal portion, whereas male cone buds are located in the proximal portion. Application of a paste containing two plant growth regulators gibberellins A₄ + A₇ (GA_4/7) combined with thidiazuron (TDZ) to long-shoot buds prior to cone-bud gender determination altered endogenous phytohormone profiles and induced female cone-bud formation in the proximal portion of the long-shoot bud, where male cone buds normally occur. Induced cone clusters observed in the following spring were either entirely female or a mixture of both female and male cones. Endogenous phytohormones in the long-shoot bud tissues were quantified by the stable isotope dilution method using high performance liquid chromatography-electrospray ionization tandem mass spectrometry in multiple-reaction monitoring mode. Applied GA_4/7 + TDZ led to increased concentrations of endogenous zeatin-type cytokinins, that is, trans-zeatin riboside and dihydrozeatin riboside, whereas concentrations of abscisic acid (ABA) and its catabolite, ABA glucose ester, were decreased, all relative to control, in untreated long-shoot bud tissue. Concentrations of extractable GA₄ and GA₇ declined in long-shoot bud tissues over 4 weeks following treatment with exogenous GA_4/7. This study demonstrates that high levels of endogenous zeatin-type cytokinins, together with reduced levels of ABA, both induced by applied GA_4/7 + TDZ, are positively associated with an increased female cone-bud formation in long-shoot buds.     

Isolation and Functional Analysis of the Poplar RbcS Gene Promoter

A tissue-specific promoter, Pt-RbcS, from Populus was isolated and cloned based on alignment of AtRBCS-2B cDNA with genomic Populus sequences. Sequence analysis of Pt-RbcS revealed cis-acting regulatory elements in the promoter region, including an ATCT-motif, BoxI, GAG-motif, I-box, G-box, BoxII, GATA-motif, and TCT-motif, which are involved in light responses. In transgenic tobacco lines carrying the β-glucuronidase (GUS) gene driven by the Pt-RbcS promoter, GUS expression was detected in leaves and stems, but not in roots. Transgenic poplar lines harboring constructs carrying the GUS gene driven by truncated Pt-RbcS promoters revealed distinctive expression patterns for five different promoter constructs. The Pt-RbcS promoter was expressed preferentially in photosynthetic tissues such as leaves and stems. Moreover, deletion analysis of the 1,547 bp Pt-RbcS promoter region revealed that a 927-bp DNA segment is critical for expression of Pt-RbcS in green tissues. Overall, our study suggests that the Pt-RbcS promoter from Populus could be applied to genetically improve the photosynthetic efficiency of woody plants.