Identification of a negative regulator for the pathogenicity island of enterohemorrhagic<Emphasis Type="Italic">Escherichia coli</Emphasis> O157:H7

EnterohemorrhagicEscherichia coli (EHEC) forms histological lesions termed attaching and effacing lesions (A/E lesions) on infected large intestine tissue. The major virulence factors involved in A/E lesions reside on a locus of enterocyte effacement (LEE), a pathogenicity island. The LEE comprises 41 specific open reading frames, of which most are organized in 5 major operons,LEE1,LEE2,LEE3,LEE4, andtir (LEE5). The expression of LEE genes is regulated in a complicated manner by environmental factors such as temperature, osmolarity, and quorum sensing. Current knowledge is that regulation is hierarchical: a pivotal positive regulator,ler, is first stimulated, which in turn activates the expression of other operons. Herein, we report on the presence of a negative regulation protein located within the LEE.L0044 is 372 bp in length and is located outside of the 5 major operons. An isogenicL0044 deletion mutant displayed loss of the repression phenotype and increased synthesis of several LEE proteins when bacteria were cultured under repressive conditions that disfavor expression of LEE proteins. Reciprocally,trans expression ofL0044 suppressed the expression of the LEE. Furthermore, mRNA ofler increased as a result of deletingL0044, and disruptingler in aL0044-deleted background reversed the loss of the repression phenotype. Thus,L0044 plays a role in regulating the expression of virulence genes in EHEC by modulating the activation ofler.     

K252a-sensitive protein kinases but not okadaic acid-sensitive protein phosphatases regulate methyl jasmonate-induced cytosolic Ca2+ oscillation in guard cells of Arabidopsis thaliana

Methyl jasmonate (MeJA) induces stomatal closure similar to abscisic acid (ABA), and MeJA signaling in guard cells shares some signal components with ABA signaling. As part of this process, MeJA as well as ABA induce the elevation and oscillation of cytosolic free-calcium concentrations ([Ca²⁺]_cyt) in guard cells. While abscisic acid-induced [Ca²⁺]_cyt oscillation has been extensively studied, MeJA-induced [Ca²⁺]_cyt oscillation is less well understood. In this study, we investigated the effects of K252a (a broad-range protein kinase inhibitor) and okadaic acid (OA, a protein phosphatase 1 and 2A inhibitor) on MeJA-induced [Ca²⁺]_cyt oscillation in guard cells of Arabidopsis thaliana ecotype Columbia expressing the Ca²⁺ reporter yellow cameleon 3.6. The protein kinase inhibitor K252a abolished MeJA-induced stomatal closure and reduced MeJA-elicited [Ca²⁺]_cyt oscillation. The protein phosphatase inhibitor OA, on the other hand, did not inhibit these processes. These results suggest that MeJA signaling involves activation of K252a-sensitive protein kinases upstream of [Ca²⁺]_cyt oscillation but not activation of an OA-sensitive protein phosphatase in guard cells of A. thaliana ecotype Columbia.     

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).     

Effect of jasmonic acid on morphogenesis and photosynthetic pigment level in Arabidopsis seedlings grown under green light

The role of jasmonic acid (JA) in plant photomorhogenesis under green light (GL) was studied. The effect of GL of different intensity (8.1 and 18.1 W/m²) with or without 1 μM JA treatment on growth of plants and photosynthetic pigment level was compared for two types of Arabidopsis thaliana (L.) Heynh. Landsberg erecta ecotype plants: Ler, the wild type, and hy4, a mutant. A much more pronounced growth of hypocotyls and cotyledons of Ler plants in GL was observed compared to that of hy4 with suppressed cryptochrome 1 (cry1), a GL photoreceptor. Treatment with JA in GL caused retardation of hypocotyl and cotyledon growth of Ler plants; however, it stimulated their growth in hy4 plants. JA reduced the chlorophyll a and total carotenoids levels in cotyledons of Arabidopsis plants in GL. Blocked GL signal transduction due to the absence of cry1 in hy4, as well as the higher intensity of GL reduced the negative effect of exogenous JA on growth of cotyledons and photosynthetic pigments. The data obtained provide a basis for discussion of interaction between the JA and GL signals in the growth regulation controlled by cry1. Original Russian Text ? I.F. Golovatskaya, R.A. Karnachuk, 2008, published in Fiziologiya Rastenii, 2008, Vol. 55, No. 2, pp. 240–244.     

Interaction of gibberellic acid and 24-epibrassinolide in the regulation of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> seedling scotomorphogenesis

The effects of GA₃, 24-epibrassinolide (EBL), and their combination on morphogenesis of Arabidopsis thaliana (L.) Heynh seven-day-old seedlings were studied. Four plant lines were analyzed: wild type Ler and ga4-1 mutant, belonging to the Landsberg erecta ecotype and wild type Col and det2 mutant, both of the Columbia ecotype. In ga4-1 and det2, GA_4/1-and brassinosteroid-deficient mutants, the highest hypocotyl growth response to the lack of hormones was noted. The cotyledon shape and size were dependent on EBL, and the root length was both GA₃-and EBL-regulated, indicating organ specificities in the responses to these hormones. Simultaneous treatment of dark-grown plants with GA₃ and EBL exerted an additive stimulatory effect on the root growth of det2, reduced the inhibitory effect of EBL on hypocotyl elongation of ga4-1, and enhanced the effect of EBL on hypocotyl and cotyledon elongation of det2.     

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.     

The role of low intensity red luminescent radiation in the control of Arabidopsis thaliana morphogenesis and hormonal balance

The effects of low intensity red luminescent radiation emitted by the polyethylene light-correcting film due to the conversion of UV-A radiation on Arabidopsis thaliana (L.) Heynh. morphogenesis and hormonal balance were studied. Wild-type Ler plants and two mutants, hy3 and hy4, displaying disturbances in the synthesis of phytochrome B and cryptochrome 1, respectively, were compared. In wild-type and hy4 plants grown under the light-correcting film, growth and development were substantially accelerated, whereas, in hy3 plants, they were retarded. These changes were correlated with changes in the levels of endogenous hormones, both growth activators and inhibitors. We concluded that low intensity red luminescent radiation affected the plant hormonal balance. In its turn, the changes in the hormone ratios, growth stimulators and inhibitors, affected the rate of plant growth and their productivity.     

Endogenous Phytohormones and Regulation of Morphogenesis of Arabidopsis thalianaby Blue Light

We studied the effects of blue light (BL) on the levels of endogenous phytohormones (IAA, ABA, gibberellins, and cytokinins) and morphogenesis of the 7-day-old Arabidopsis thaliana(L.) Heynh seedlings of wild type (Ler) and its hy4mutant with a disturbed synthesis of cryptochrome 1 (CRY1), which is a receptor for BL. In darkness, the mutant contained considerably less free IAA and zeatin, but much more ABA as compared to the wild-type seedlings. BL retarded the hypocotyl growth in the wild-type seedlings but stimulated it in the mutant. Elongation of mutant hypocotyls was accompanied by accumulation of free IAA and a decrease in the content of free ABA; the level of cytokinins did not change. We believe that the response of the hy4hypocotyls to BL is mediated by a BL receptor distinct from cryptochrome 1. The conclusion is that light and hormonal signals interact in the control of the hypocotyl growth in A. thalianaseedlings.     

Enterohemorrhagic Escherichia coli senses microbiota-derived nicotinamide to increase its virulence and colonization in the large intestine

1. Download : Download high-res image (181KB)
2. Download : Download full-size image