UV-B stress-induced ABA production in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> mutants defective in ethylene signal transduction pathway

In this study, we examined the influence of UV-B radiation (280–320 nm) on ABA accumulation in 14-day-old Arabidopsis thaliana (L.) Heynh plants of wild type (WT), ethylene receptor mutant (etr1-1), and mutant with a constitutively active ethylene signal transduction pathway (ctr1-1). ABA content in nonirradiated WT plants was twice higher than in each mutant. UV-B irradiation caused dose-dependent ABA accumulation in WT plants. In the etr1-1 mutant, the amount of accumulated ABA was significantly less. In the ctr1-1 mutant, ABA content didn’t increase after UV-B irradiation. These data suggest that start of stress-induced ABA formation requires the adjustable ethylene signal pathway. In the ctr1-1 mutant, a constitutively active (nonadjustable) ethylene signal pathway blocks stress-induced ABA accumulation.     

Light regulation of succinate dehydrogenase expression in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> leaves

A potential mechanism of light regulation of the succinate dehydrogenase (SDH) expression in Arabidopsis thaliana leaves was studied. As was shown by dot-hybridization and polymerase chain reaction in real time (RT-PCR), the SDH mRNA level in wild-type Arabidopsis thaliana plants changed depending on light conditions. The level of SDH mRNA in darkness was higher than in the light. The analysis of Arabidopsis thaliana plants carrying the mutant genes of phytochromes A and B showed that phytochrome A was involved in the regulation of the SDH enzyme activity. The active form of phytochrome A suppressed the SDHI-2 gene expression, and that resulted in decreasing activity of SDH.     

Interaction of jasmonic acid and blue light in the regulation of arabidopsis morphogenesis

The effects of blue light (BL) and jasmonic acid (JA) on morphogenesis of Arabidopsis thaliana (L.) Heynh seedlings of genotypes Col and Ler and their mutants, namely, axr1-3 and jar1-1 mutants resistant to IAA and JA, respectively, and a CRY1 photoreceptor-deficient mutant hy4 were studied. Both 1 μM JA and BL exposure retarded hypocotyl growth of Ler, Col, and jar1-1 seedlings, whereas JA had no effect on hypocotyl growth of axr1-3, but the suppression of hypocotyl growth of this mutant by BL was even more noticeable than that of Ler, Col, and jar1-1. JA and BL applied simultaneously inhibited hypocotyl growth of axr1-3 and especially of Ler, Col, and jar1-1 more than either of factors applied separately. The hy4 mutant did not respond to BL, whereas JA stimulated its hypocotyl growth. JA did not change the cotyledon size of Col, axr1-3, and jar1-1 and reduced the cotyledon size of Ler and hy4. BL enhanced the cotyledon growth of all wild-type and mutant plants used in the study. The cotyledon sizes of all plants except Ler were also increased when JA and BL were applied together. Some of the growth responses correlated with the endogenous IAA and ABA contents. Thus, for example, the hypocotyl and cotyledon growth retardation of Ler seedlings in the presence of JA correlated with a reduced level of free IAA and a considerable increase in the free ABA level in plants grown both in darkness and in BL. Under other growth conditions, no correlation between the endogenous IAA and ABA levels and A. thaliana seedling growth was noted. The interaction between the signal transduction pathways triggered by BL and JA at the early stages of arabidopsis morphogenesis is discussed on the basis of Col, Ler, axr1-3, and jar1-1 hypocotyl growth responses.     

Effect of gibberellin on <Emphasis Type="Italic">Arabidopsis</Emphasis> growth,development, and hormonal balance under green and blue light

The role of gibberellins in plant morphology under selective light was studied. A comparison of the effects of green and blue light on growth, development, and hormonal balance was performed for two Arabidopsis thaliana (L.) Heynh. (ecotype Landsberg erecta lines: wild type Ler and its ga4-1 mutant with suppressed GA_4/1 synthesis. The absence of active GA_4/1 from ga4-1 mutant determined its retarded growth, slowed passing through developmental phases, suppressed apical dominance, and reduced seed productivity. The retarded growth and development of the mutant was related to changed hormonal balance in them as compared to wild-type line: IAA content and the IAA/ABA ratio were declined, zeatin riboside and ABA accumulated. Green light retarded stem elongation and branching, reduced leaf specific surface density and plant seed productivity, and retarded plant transition to reproduction to a greater degree at GA_4/1 deficit (ga4-1) than at its normal content (Ler).     

Interaction between ethylene and ABA in the regulation of polyamine level in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> during UV-B stress

The effects of treatment with ethylene (0.01–100 μl/l) on ABA and polyamine contents and treatment with ABA on ethylene synthesis, polyamines content, and the resistance to UV-B radiation of two-week-old Arabidopsis thaliana (L.) Heynh, Columbia ecotype plants grown u?er sterile conditions were studied. Ethylene stimulated the accumulation of polyamines only at concentrations of 0.1–10 μl/l, which could activate ABA synthesis. Treatment with ABA (50–5000 μM, 1 μl per plant) decreased the UV-B-induced ethylene synthesis and a spermine and spermidine loss, increasing the content of putrescine, the precursor of these polyamines. ABA inhibited fresh weight accumulation in irradiated and nonirradiated plants but prevented them from severe damage and death at the high (18 kJ/m²) and lethal (27 kJ/m²) UV-B dose, respectively. The data obtained demonstrated a mutual regulation of ethylene and ABA syntheses and the participation of these hormones in the control of the polyamine level during adaptation of A. thaliana to UV-B stress.     

An improved method to identify the T-DNA insertion site in transgenic <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> genome

An improved method to identify the T-DNA insertion site in transgenic Arabidopsis thaliana (Columbia ecotype) genome was presented. Firstly, the pre-adaptor was amplified by PCR from the plasmid pLASC11.12.8 and digested by HindIII to produce the adaptor. After treated with calf intestine alkaline phosphatase, the adaptor was ligated to the genomic restriction digested fragment with the same restriction endonucleases. Then two rounds of PCR (nested-PCR) were carried out and an unknown sequence between the T-DNA and the adaptor was amplified. Further analysis would reveal the accurate site of T-DNA insertion into transgenic A. thaliana genome. This text was submitted by the authors in English.     

Cumulative effect of low and high atrazine concentrations on <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> plants

Atrazine belongs to the widely used herbicides blocking the electron transport chain in chloroplasts, thus resulting in the generation of active oxygen species. In the present work, we demonstrated that, at low concentrations mimicking residual amounts, atrazine enhanced the susceptibility of Arabidopsis plants to further treatments with the same herbicide applied at the recommended field rate. Arabidopsis thaliana plants were treated three times (at five-day intervals) with 1 µM atrazine. Five days after the last treatment, the plants were sprayed with 5 mM atrazine. Atrazine increased the levels of lipid peroxidation products, hydrogen peroxide, and ion leakage, and caused changes in the activities of antioxidant enzymes, such as superoxide dismutase, guaiacol peroxidase, and catalase.From Fiziologiya Rastenii, Vol. 52, No. 2, 2005, pp. 243–249.Original English Text Copyright © 2005 by Ivanov, Alexieva, Karanov.This article was submitted by the authors in English.This revised version was published online in April 2005 with a corrected cover date.     

A new <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> deletion mutant <Emphasis Type="Italic">apetala1-20</Emphasis>

A new deletion allele of the APETALA1 (AP1) gene encoding a type II MADS-box protein with the key role in the initiation of flowering and development of perianth organs has been identified in A. thaliana. The deletion of seven amino acids in the conserved region of the K domain in the ap1-20 mutant considerably delayed flowering and led to a less pronounced abnormality in the corolla development compared to the weak ap1-3 and intermediate ap1-6 alleles. At the same time, a considerable stamen reduction has been revealed in ap1-20 as distinct from ap1-3 and ap1-6 alleles. These data indicate that the K domain of AP1 can be crucial for the initiation of flowering and expression regulation of B-class genes controlling stamen development.     

Ethylene evolution and ABA and polyamine contents in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> during UV-B stress

Changes in plant growth, membrane integrity, ethylene evolution, ABA content, and the content of free polyamines were examined in 14-day-old Arabidopsis thaliana (L.) Heynh., strain Columbia (Col-0) plants after a single UV-B irradiation with low (3 kJ/m²), moderate (6–9 kJ/m²), high (18 kJ/m²), and lethal (27 kJ/m²) doses. The UV-B treatment caused dose-dependent suppression of plant growth. One hour after irradiation, the membrane damage was evident from the increased leakage of electrolytes. The low-dose and moderate-dose irradiation caused a transient increase in evolution of ethylene and in the content of putrescine (spermidine and spermine precursor) with the peaks of these parameters attained at 5 and 24 h, respectively. The high-and lethaldose irradiation induced a smaller rise in ethylene evolution, with a slight trend to its decrease, especially, after the exposure to the lethal dose. The high and lethal doses of UV-B suppressed putrescine accumulation, depleted spermidine and spermine pools, and caused severe injuries and plant death. During the first day after irradiation, the ABA content increased in proportion to the irradiation dose. On the second day, the accumulation of ABA was observed in plants irradiated with moderate doses. The accumulation was arrested after a high-dose irradiation and was diminished by 45% after a lethal dose treatment. The results provide evidence for the involvement of ethylene, ABA, and polyamines in plant responses induced by UV-B irradiation.     

Effect of salicylic acid on the development of induced Thermotolerance and induction of heat shock protein synthesis in the <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> cell culture

Salicylic acid (SA) could be involved in the development of tolerance to abiotic stresses, to heat shock in particular. Under normal conditions (26°C), treatment with SA improved the tolerance of heterotrophic Arabidopsis thaliana (L.) Heynh culture to severe heat shock (50°C). Under mild heat shock (37°C) inducing the development of thermotolerance, the presence of SA, in contrast, reduced the capability of arabidopsis cells to tolerate high temperature (50°C) and simultaneously suppressed induction of HSP synthesis (Hsp101 and Hsp17.6) important for the development of induced thermotolerance. Since SA suppressed cell respiration and activated the alternative pathway of electron transport, SA is supposed, by modulating mitochondria functions, to be an endogenous regulator of plant stress gene expression.     

<Emphasis Type="Italic">Anthocyaninless1</Emphasis> gene of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> encodes a UDP-glucose:flavonoid-3-<Emphasis Type="Italic">O</Emphasis>-glucosyltransferase

We isolated several mutants of Arabidopsis thaliana (L.) Heynh. that accumulated less anthocyanin in the plant tissues, but had seeds with a brown color similar to the wild-type. These mutants were allelic with the anthocyaninless1 (anl1) mutant that has been mapped at 15.0 cM of chromosome 5. We performed fine mapping of the anl1 locus and determined that ANL1 is located between the nga106 marker and a marker corresponding to the MKP11 clone. About 70 genes are located between these two markers, including three UDP-glucose:flavonoid-3-O-glucosyltransferase-like genes and a glutathione transferase gene (TT19). A mutant of one of the glucosyltransferase genes (At5g17050) was unable to complement the anl1 phenotype, showing that the ANL1 gene encodes UDP-glucose:flavonoid-3-O-glucosyltransferase. ANL1 was expressed in all tissues examined, including rosette leaves, stems, flower buds and roots. ANL1 was not regulated by TTG1.     

A meiotic time-course for<Emphasis Type="Italic"> Arabidopsis thaliana</Emphasis>

A meiotic time-course for Arabidopsis pollen mother cells has been established based on BrdU pulse-labelling of nuclear DNA in the meiotic S-phase. Labelled flower buds were sampled at intervals and the progress of labelled cells through meiosis assessed by anti-BrdU antibody detection. The overall duration of meiosis from the end of meiotic S-phase to the tetrad stage, at 18.5°C, was 33 h, which is about three times longer than the mitotic cell cycle in seedlings. The onset of leptotene was defined by reference to the loading of the axis-associated protein Asy1, and this permitted the detection of a definite G2 stage, having a maximum duration of 9 h. It is likely, from two independent sources of evidence, that the meiotic S-phase has a duration similar to that of G2. The durations of leptotene and zygotene/pachytene are 6 h and 15.3 h, respectively, but the remaining meiotic division stages are completed very rapidly, within 3 h. The establishment of a meiotic time-course provides a framework for determining the relative timing and durations of key molecular events of meiosis in Arabidopsis in relation to cytologically defined landmarks. In addition, it will be important in a broader developmental context for determining the timing of epigenetic mechanisms that are known or suspected to occur during meiosis.     

<Emphasis Type="Italic">pgd1</Emphasis>, an <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> deletion mutant,is defective in pollen germination

An Arabidopsis deletion mutant was fortuitously identified from the alpha population of T-DNA insertional mutants generated at the University of Wisconsin Arabidopsis Knockout Facility. Segregation and reciprocal crosses indicated that the mutant was a gametophytic pollen sterile mutant. Pollen carrying the mutation has the unusual phenotype that it is viable, but cannot germinate. Thus, the mutant was named pollen germination defective mutant 1 (pgd1), based on the pollen phenotype. Flanking sequences of the T-DNA insertion in the pgd1 mutant were identified by thermal asymmetric interlaced (TAIL) PCR. Sequencing of bands from TAIL PCR revealed that the T-DNA was linked to the gene XLG1, At2g23460, at its downstream end, while directly upstream of the T-DNA was a region between At2g22830 and At2g22840, which was 65 genes upstream of XLG1. Southern blotting and genomic PCR confirmed that the 65 genes plus part of XLG1 were deleted in the pgd1 mutant. A 9,177 bp genomic sequence containing the XLG1 gene and upstream and downstream intergenic regions could not rescue the pgd1 pollen phenotype. One or more genes from the deleted region were presumably responsible for the pollen germination defect observed in the pgd1 mutant. Because relatively few mutations have been identified that affect pollen germination independent of any effect on pollen viability, this mutant line provides a new tool for identification of genes specifically involved in this phase of the reproductive cycle.     

<Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> is a susceptible host plant for the holoparasite <Emphasis Type="Italic">Cuscuta </Emphasis>spec

Arabidopsis thaliana and Cuscuta spec. represent a compatible host–parasite combination. Cuscuta produces a haustorium that penetrates the host tissue. In early stages of development the searching hyphae on the tip of the haustorial cone are connected to the host tissue by interspecific plasmodesmata. Ten days after infection, translocation of the fluorescent dyes, Texas Red (TR) and 5,6-carboxyfluorescein (CF), demonstrates the existence of a continuous connection between xylem and phloem of the host and parasite. Cuscuta becomes the dominant sink in this host–parasite system. Transgenic Arabidopsis plants expressing genes encoding the green fluorescent protein (GFP; 27 kDa) or a GFP–ubiquitin fusion (36 kDa), respectively, under the companion cell (CC)-specific AtSUC2 promoter were used to monitor the transfer of these proteins from the host sieve elements to those of Cuscuta. Although GFP is transferred unimpedly to the parasite, the GFP–ubiquitin fusion could not be detected in Cuscuta. A translocation of the GFP–ubiquitin fusion protein was found to be restricted to the phloem of the host, although a functional symplastic pathway exists between the host and parasite, as demonstrated by the transport of CF. These results indicate a peripheral size exclusion limit (SEL) between 27 and 36 kDa for the symplastic connections between host and Cuscuta sieve elements. Forty-six accessions of A. thaliana covering the entire range of its genetic diversity, as well as Arabidopsis halleri, were found to be susceptible towards Cuscuta reflexa.     

<Emphasis Type="Italic">CKB1</Emphasis> is involved in abscisic acid and gibberellic acid signaling to regulate stress responses in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

Casein kinase II (CK2), an evolutionarily well-conserved Ser/Thr kinase, plays critical roles in all higher organisms including plants. CKB1 is a regulatory subunit beta of CK2. In this study, homozygous T-DNA mutants (ckb1-1 and ckb1-2) and over-expression plants (35S:CKB1-1, 35S:CKB1-2) of Arabidopsis thaliana were studied to understand the role of CKB1 in abiotic stress and gibberellic acid (GA) signaling. Histochemical staining showed that although CKB1 was expressed in all organs, it had a relatively higher expression in conducting tissues. The ckb1 mutants showed reduced sensitivity to abscisic acid (ABA) during seed germination and seedling growth. The increased stomatal aperture, leaf water loss and proline accumulation were observed in ckb1 mutants. In contrast, the ckb1 mutant had increased sensitivity to polyaluminum chloride during seed germination and hypocotyl elongation. We obtained opposite results in over-expression plants. The expression levels of a number of genes in the ABA and GA regulatory network had changed. This study demonstrates that CKB1 is an ABA signaling-related gene, which subsequently influences GA metabolism, and may play a positive role in ABA signaling.