Brassinosteroid (BR), an endogenous steroid growth regulator of higher plants, enhances expansion and division of the cell in a number of plant species. It has been recently reported that a shared auxin–BR signalling pathway is involved in the seedling growth in Arabidopsis. Here, we show that BR specifically enhanced the expression of AtACS4, which encodes an auxin‐responsive ACC synthase 4, by a distinct temporal induction mechanism compared with that of IAA in etiolated Arabidopsis seedlings. This BR induction of AtACS4 was undetectable in the light‐grown seedlings. In addition, BR failed to activate the AtACS4 gene in auxin‐resistant1 (axr1‐3) and auxin‐resistant2 (axr2‐1), both of which are auxin‐resistant mutants. Thus, it appears that there is a possible regulatory link between light, auxin and BR to control ethylene synthesis in Arabidopsis young seedlings. Analysis of transgenic Arabidopsis plants harbouring AtACS4::GUS fusion revealed the AtACS4 promoter‐driven GUS activity in the highly elongating zone of the hypocotyls in response to BR treatment. Furthermore, Arabidopsis plants homozygous for the T‐DNA insertion in the AtACS4 gene exhibited longer hypocotyls and roots than those of control seedlings. Taken together, these results suggest that the BR‐induced ethylene production may participate in the elongation growth response in early seedling development of Arabidopsis.
Indole-3-acetic acid (IAA) markedly increased ethylene production by inducing the expression of three 1aminocyclopropane-1-carboxylate (ACC) synthase cDNAs (pVR-ACS1, pVR-ACS6 and pVR-ACS7) in mung bean hypocotyls. Results from nuclear run-on transcription assay and RNA gel blot studies revealed that all three genes were transcriptionally active displaying unique patterns of induction by IAA and various hormones in etiolated hypocotyls. Particularly, 24-epibrassinolide (BR), an active brassinosteroid, specifically enhanced the expression of VR-ACS7 by a distinct temporal induction mechanism compared to that of IAA. In addition, BR synergistically increased the IAA-induced VR-ACS6 and VR-ACS7 transcript levels, while it effectively abolished both the IAA- and kinetin-induced accumulation of VR-ACS1 mRNA. In light-grown plants, VR-ACS1 was induced by IAA in roots, and VR-ACS6 in epicotyls. IAA- and BR-treatments were not able to increase the VR-ACS7 transcript in the light-grown tissues. These results indicate that the expression of ACC synthase multigene family is regulated by complex hormonal and developmental networks in a gene- and tissue-specific manner in mung bean plants. The VR-ACS7 gene was isolated, and chimeric fusion between the 2.4 kb 5'-upstream region and the beta-glucuronidase (GUS) reporter gene was constructed and introduced into Nicotiana tabacum. Analysis of transgenic tobacco plants revealed the VR-ACS7 promoter-driven GUS activity at a highly localized region of the hypocotyl-root junction of control seedlings, while a marked induction of GUS activity was detected only in the hypocotyl region of the IAA-treated transgenic seedlings where rapid cell elongation occurs. Although there was a modest synergistic effect of BR on the IAA-induced GUS activity, BR alone failed to increase the GUS activity, suggesting that induction of VR-ACS7 occurs via separate signaling pathways in response to IAA and BR. A scheme of the multiple regulatory pathways for the expression of ACC synthase multigene family by auxin and BR is presented.
Capsicum annuum tobacco mosaic virus (TMV)-induced clone 1 (CaTin1) gene was expressed early during incompatible interaction of hot pepper (Caspsicum annuum) plants with TMV and Xanthomonas campestris. RNA-blot analysis showed that CaTin1 gene was expressed only in roots in untreated plants and induced mainly in leaf in response to ethylene, NaCl, and methyl viologen but not by salicylic acid and methyl jasmonate. The ethylene dependence of CaTin1 induction upon TMV inoculation was demonstrated by the decrease of CaTin1 expression in response to several inhibitors of ethylene biosynthesis or its action. Transgenic tobacco (Nicotiana tabacum) plants expressing CaTin1 gene in sense-or antisense-orientation showed interesting characteristics such as the accelerated growth and the enhanced resistance to biotic as well as abiotic stresses. Such characteristics appear to be caused by the elevated level of ethylene and H 2 O 2 . Moreover, in transgenic plants expressing antisense CaTin1 gene, the expression of some pathogenesis-related genes was enhanced constitutively, which may be mainly due to the increased ethylene level. The promoter of CaTin1 has four GCC-boxes, two AT-rich regions, and an elicitor-inducible W-box. The induction of the promoter activity by ethylene depends on GCC-boxes and by TMV on W-box. Taken together, we propose that the CaTin1 up-regulation or down-regulation interferes with the redox balance of plants leading to the altered response to ethylene and biotic as well as abiotic stresses.Multiple preformed antimicrobial compounds contribute to the constitutive defense machinery of plants against pathogenic organisms. In addition, plants can trigger the inducible defense programs upon the perception of invaders. The early signaling molecules leading to R-gene-mediated resistance are ion fluxes, GTP-binding proteins, protein kinases, phosphatases, and phospholipases (Hammond-Kosack and Jones, 1996). Plants also respond to external stimuli such as microbial elicitors of cell death and/or defense responses by changing the calcium influx of the cell (Ebel and Scheel, 1997;Higgins et al., 1998). The increase of cytosolic calcium leads to cell death, which seems necessary for hypersensitive cell death triggered by rust fungi (Xu and Heath, 1998). The calcium channel blocker La 31 , on the other hand, prevents the bacteriainduced hypersensitive response (HR) in soybean (Glycine max) leaves (Levine et al., 1996). The burst in the oxidative metabolism generated within minutes of infection leads to the accumulation of the reactive oxygen intermediates (ROI) such as H 2 O 2 and superoxide anion (O 2 d 2 ), involving NADPH-dependent oxidase. ROI serve as signaling molecules, for example in the recognition of the attack by fungal pathogens (Lamb and Dixon, 1997).Ethylene is widely known to modulate the organ senescence induced by various stress factors such as plant pathogens (Moore et al., 2000), O 3 2 (Pell et al., 1997), and hypoxia (He et al., 1996). O 3 2 induces genes involved in the ethylene biosynthes...
Auxin induces the expression of the two ethylene-biosynthetic genes VR-ACS6 and VR-ACS7 in etiolated mung bean hypocotyls. However, while it also enhances VR-ACS6 expression in light-grown tissues, it does not up-regulate VR-ACS7 expression in these tissues. Here we show that transfer of 3-day-old etiolated seedlings into light quickly reduced the auxin-induced expression of both genes. However, while auxin-induced VR-ACS6 expression recovered after 24 h of light, VR-ACS7 transcription continued to reduce and was almost completely absent at 36 h. Thus, light differentially modulates the expression of the auxin-inducible VR-ACS genes. In hormone-treated etiolated seedlings, VR-ACS7 was primarily induced in the rapidly elongating zones of hypocotyl and epicotyl tissues, while auxin-induced VR-ACS6 mRNA was evenly distributed throughout the whole seedling. VR-ACS7 promoter-driven beta-glucuronidase (GUS) activity in auxin-treated etiolated transgenic Arabidopsis seedlings was observed in the highly elongating zones of the hypocotyl. During de-etiolation, the GUS activity gradually declined to become confined to the uppermost region of hypocotyls. In situ mRNA localization studies showed that in etiolated mung bean hypocotyls, the auxin-dependent VR-ACS7 transcript was predominantly present in the epidermis, which is the driving site for auxin-mediated elongation. Thus, it appears that the modulation by light of auxin-induced VR-ACS7 expression may correlate closely with the elongation growth response in early seedling development.
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