VR-ACS6 is an auxin-inducible 1-aminocyclopropane-1-carboxylate synthase gene in mungbean (Vigna radiata)

Role of ethylene and related gene expression in the interaction between strawberry plants and the plant growth-promoting bacterium Azospirillum brasilense

Induced systemic resistance (ISR) is one of the indirect mechanisms of growth promotion exerted by plant growth-promoting bacteria, and can be mediated by ethylene (ET). We assessed ET production and the expression of related genes in the Azospirillum-strawberry plant interaction. Ethylene production was evaluated by gas chromatography in plants inoculated or not with A. brasilense REC3. Also, plants were treated with AgNO₃ , an inhibitor of ET biosynthesis; with 1-aminocyclopropane-1-carboxylic acid (ACC), a precursor of ET biosynthesis; and with indole acetic acid (IAA). Plant dry biomass and the growth index were determined to assess the growth-promoting effect of A. brasilense REC3 in strawberry plants. Quantitative real time PCR (qRT-PCR) was performed to analyse relative expression of the genes Faetr1, Faers1 and Faein4, which encode ET receptors; Factr1 and Faein2, involved in the ET signalling pathway; Faacs1 encoding ACC synthase; Faaco1 encoding ACC oxidase; and Faaux1 and Faami1 for IAA synthesis enzymes. Results showed that ET acts as a rapid and transient signal in the first 12 h post-treatment. A. brasilense REC3-inoculated plants had a significantly higher growth index compared to control plants. Modulation of the genes Faetr1, Faers1, Faein4, Factr1, Faein2 and Faaco1 indicated activation of ET synthesis and signalling pathways. The up-regulation of Faaux1 and Faami1 involved in IAA synthesis suggested that inoculation with A. brasilense REC3 induces production of this auxin, modulating ET signalling. Ethylene production and up-regulation of genes associated with ET signalling in strawberry plants inoculated with A. brasilense REC3 support the priming activation characteristic of ISR. This type of resistance and the activation of systemic acquired resistance previously observed in this interaction indicate that both are present in strawberry plants, could act synergistically and increase protection against pathogens.

Molecular cloning and expression analysis of an 1-aminocyclopropane-1-carboxylate synthase gene from Oncidium Gower Ramsey

1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) is a rate-limiting enzyme in the biosynthesis of ethylene which regulates many aspects of the plant development and responses to biotic and abiotic stresses. In this study, a full-length cDNA of ACC synthase, OnACS2, was cloned from the senescing flower of Oncidium Gower Ramsey by RACE. The full-length cDNA of OnACS2 (GenBank accession no. JQ822087) was 1557 bp in length with an open reading frame (ORF) of 1308 bp encoding for a protein of 435 amino acid residues. The predicted OnACS2 protein had a molecular mass of 49.1 kDa with pI value of 7.51. Phylogenetic analysis indicated its evolutionary relationships with corresponding orthologous sequences in orchids, Hosta ventricosa and monocots. Real-time PCR assay demonstrated that OnACS2 was constitutively expressed in all tested organs with the highest transcript level in the gynandria. Differential expression pattern of OnACS2 gene correlated to the ethylene production and the subsequent occurrence of senescent symptoms in flower suggested that OnACS2 probably played an important role in the initiation of flower senescence.

Responses of rose RhACS1 and RhACS2 promoters to abiotic stresses in transgenic Arabidopsis thaliana

Promoter activities of RhACS1 and RhACS2 , two rose genes involved in ethylene biosynthesis, are highly sensitive to various abiotic stresses in an organ-specific manner. Our previous studies indicated that two rose (Rosa hybrida) 1-aminocyclopropane-1-carboxylic acid synthase genes, RhACS1 and RhACS2, play a role in dehydration-induced ethylene production and inhibition of cell expansion in rose petals. Here, both RhACS1 and RhACS2 promoters were analyzed using histochemical staining and glucuronidase synthase (GUS) gene reporter activity assays following their introduction into transgenic Arabidopsis thaliana plants. It was found that the promoter activities of both genes were strong throughout the course of development from young seedlings to mature flowering plants in various organs, including hypocotyls, cotyledons, leaves, roots and lateral roots. RhACS1 promoter activity was induced by drought in both rosette leaves and roots of transgenic A. thaliana lines, but was reduced following a re-hydration treatment. In contrast, RhACS2 promoter activity was decreased by drought in rosette leaves, while its response pattern was similar to that of RhACS1 in roots. A mannitol treatment induced the activity of both the RhACS1 and RhACS2 promoters, indicating that both genes are also regulated by osmotic stress. In addition, RhACS2 appeared to be abscisic acid (ABA)-inducible, while RhACS1 was less sensitive to ABA. Finally, four truncated sequences of the RhACS1 promoter were generated and GUS activity assays demonstrated that deleting a 327 bp region between bp 862 and -535 resulted in a substantial decrease of the promoter activity. Taken together, our results suggest that the RhACS1 and RhACS2 promoters respond to abiotic stresses in a developmentally regulated and spatially specific manner.

ACC deaminase from plant growth-promoting bacteria affects crown gall development

In addition to the well-known roles of indoleacetic acid and cytokinin in crown gall formation, the plant hormone ethylene also plays an important role in this process. Many plant growth-promoting bacteria (PGPB) encode the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, which can degrade ACC, the immediate precursor of ethylene in plants, to α-ketobutyrate and ammonia and thereby lower plant ethylene levels. To study the effect of ACC deaminase on crown gall development, an ACC deaminase gene from the PGPB Pseudomonas putida UW4 was introduced into Agrobacterium tumefaciens C58, so that the effect of ACC deaminase activity on tumour formation in tomato and castor bean plants could be assessed. Plants were also coinoculated with A. tumefaciens C58 and P. putida UW4 or P. putida UW4-acdS– (an ACC deaminase minus mutant strain). In both types of experiments, it was observed that the presence of ACC deaminase generally inhibited tumour development on both tomato and castor bean plants.

Differential regulation of Ku gene expression in etiolated mung bean hypocotyls by auxins

Plant Ku genes were identified very recently in Arabidopsis thaliana, and their roles in repair of double-stranded break DNA and maintenance of telomere integrity were scrutinized. In this study, the cDNAs encoding Ku70 (VrKu70) and Ku80 (VrKu80) were isolated from mung bean (Vigna radiata L.) hypocotyls. Both genes were expressed widely among different tissues of mung bean with the highest levels in hypocotyls and leaves. The VrKu gene expression was stimulated by exogenous auxins in a concentration- and time-dependent manner. The stimulation could be abolished by auxin transport inhibitors, N-(1-naphthyl) phthalamic acid and 2,3,5-triiodobenzoic acid implicating that exogenous auxins triggered the effects following their uptake by the cells. Further analysis using specific inhibitors of auxin signaling showed that the stimulation of VrKu expression by 2,4-dichlorophenoxyacetic acid (2,4-D) was suppressed by intracellular Ca(2+) chelators, calmodulin antagonists, and calcium/calmodulin dependent protein kinase inhibitors, suggesting the involvement of calmodulin in the signaling pathway. On the other hand, exogenous indole-3-acetic acid (IAA) and alpha-naphthalene acetic acid (NAA) stimulated VrKu expression through the mitogen-activated protein kinase/extracellular signal-regulated kinase pathway. Altogether, it is thus proposed that 2,4-D and IAA (or NAA) regulate the expression of VrKu through two distinct pathways.

Characterization of a strong, constitutive mung bean (Vigna radiata L.) promoter with a complex mode of regulation in planta

We report the cloning and characterization in tobacco and Arabidopsis of a Vigna radiata L. (mung bean) promoter that controls the expression of VR-ACS1, an auxin-inducible ACC synthase gene. The VR-ACS1 promoter exhibits a very unusual behavior when studied in plants different from its original host, mung bean. GUS and luciferase in situ assays of transgenic plants containing VR-ACS1 promoter fusions show strong constitutive reporter gene expression throughout tobacco and Arabidopsis development. In vitro quantitative analyses show that transgenic plants harboring VR-ACS1 promoter-reporter constructs have on average 4-6 fold higher protein and activity levels of both reporter genes than plants transformed with comparable CaMV 35S promoter fusions. Similar transcript levels are present in VR-ACS1 and CaMV 35S promoter lines, suggesting that the high levels of gene product observed for the VR-ACS1 promoter are the combined result of transcriptional and translational activation. All tested deletion constructs retaining the core promoter region can drive strong constitutive promoter activity in transgenic plants. This is in contrast to mung bean, where expression of the native VR-ACS1 gene is almost undetectable in plants grown under normal conditions, but is rapidly and highly induced by a variety of stimuli. The constitutive behavior of the VR-ACS1 promoter in heterologous hosts is surprising, suggesting that the control mechanisms active in mung bean are impaired in tobacco and Arabidopsis. The 'aberrant' behavior of the VR-ACS1 promoter is further emphasized by its failure to respond to auxin and cycloheximide in heterologous hosts. VR-ACS1 promoter regulatory mechanisms seem to be different from all previously characterized auxin-inducible promoters.

Cytokinin-induced hypocotyl elongation in light-grown Arabidopsis plants with inhibited ethylene action or indole-3-acetic acid transport

Cytokinins inhibit hypocotyl elongation in darkness but have no obvious effect on hypocotyl length in the light. However, we found that cytokinins do promote hypocotyl elongation in the light when ethylene action is blocked. A 50% increase in Arabidopsis thaliana (L.) Heynh. hypocotyl length was observed in response to N6-benzyladenine (BA) treatment in the presence of Ag+. The level of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid was strongly increased, indicating that ethylene biosynthesis was up-regulated by treatment with cytokinin. Furthermore, the effects of cytokinins on hypocotyl elongation were also tested using a series of mutants in the cascade of the ethylene-signal pathway. In the ethylene-insensitive mutants etr1-3 and ein2-1, cytokinin treatment resulted in hypocotyl lengths comparable to those of wild-type seedlings treated with both Ag+ and BA. A similar phenotypical response to cytokinin was observed when auxin transport was blocked by alpha-naphthylphthalamic acid (NPA). Applied cytokinin largely restored cell elongation in the basal and middle parts of the hypocotyls of NPA-treated seedlings and at the same time abolished the NPA-induced decrease in indole-3-acetic acid levels. Our data support the hypothesis that, in the light, cytokinins interact with the ethylene-signalling pathway and conditionally up-regulate ethylene and auxin synthesis.

Ethylene Synthesis and Auxin Augmentation in Pistil Tissues are Important for Egg Cell Differentiation after Pollination in Maize

The role of ethylene and auxin in stigma-to-ovule signalling was investigated in maize (Zea mays L.). Maturation of the egg cells in an ear was stimulated before actual fertilization by the application of fresh pollen grains or quartz sand to fully receptive stigmas. Ethylene emission by maize ears increased in response to those treatments. Silks and ovaries were involved in ethylene synthesis after pollen or sand was shed over the silks. The content of ethylene precursor [1-aminocyclopropane-1-carboxylic acid (ACC)] increased in both pistil parts soon after pollination. ACC rise was delayed by 4 h in the ovaries, and by 8 h in the silks after mock-pollination with sand. The auxin level increased rapidly in the silks and ovaries after pollination, and it was very high in the pollinated silks due to the high indole-3-acetic acid (IAA) content of pollen grains. IAA rise also appeared in the silks and ovaries after treatment with sand but it was delayed by 8 h. Application of ACC (10 microM) or IAA (6 microM) solutions to non-pollinated silks stimulated maturation of the egg cells. Moreover, the response of the egg cells to pollination was cancelled by l-alpha-(2-aminoethoxyvinyl)-glycine, alpha-aminoisobutyric acid or 2,3,5-triiodobenzoic acid applied to the silks before pollination. Thus ethylene synthesis and polar auxin transport in the silks pollinated with fresh pollen were necessary to evoke accelerated differentiation of the egg cells in maize ovules. Differences in pistil responses found between true- and mock-pollination suggest that signalling pathways are at least partially different for the reception of pollen grains and sand crystals on maize stigma.

Ethylene biosynthesis in detached young persimmon fruit is initiated in calyx and modulated by water loss from the fruit

Persimmon (Diospyros kaki Thunb.) fruit are usually classified as climacteric fruit; however, unlike typical climacteric fruits, persimmon fruit exhibit a unique characteristic in that the younger the stage of fruit detached, the greater the level of ethylene produced. To investigate ethylene induction mechanisms in detached young persimmon fruit, we cloned three cDNAs encoding 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (DK-ACS1, 2, and -3) and two encoding ACC oxidase (DK-ACO1 and -2) genes involved in ethylene biosynthesis, and we analyzed their expression in various fruit tissues. Ethylene production was induced within a few days of detachment in all fruit tissues tested, accompanied by temporally and spatially coordinated expression of all the DK-ACS and DK-ACO genes. In all tissues except the calyx, treatment with 1-methylcyclopropene, an inhibitor of ethylene action, suppressed ethylene production and ethylene biosynthesis-related gene expression. In the calyx, one ACC synthase gene (DK-ACS2) exhibited increased mRNA accumulation accompanied by a large quantity of ethylene production, and treatment of the fruit with 1-methylcyclopropene did not prevent either the accumulation of DK-ACS2 transcripts or ethylene induction. Furthermore, the alleviation of water loss from the fruit significantly delayed the onset of ethylene production and the expression of DK-ACS2 in the calyx. These results indicate that ethylene biosynthesis in detached young persimmon fruit is initially induced in calyx and is modulated by water loss through transcriptional activation of DK-ACS2. The ethylene produced in the calyx subsequently diffuses to other fruit tissues and acts as a secondary signal that stimulates autocatalytic ethylene biosynthesis in these tissues, leading to a burst of ethylene production.

IAA and N(6)-benzyladenine inhibit ethylene-regulated expression of ACC oxidase and ACC synthase genes in mungbean hypocotyls

Regulation of expression of 1-aminocyclopropane-1-carboxylate (ACC) oxidase (VR-ACO1) and ACC synthase (VR-ACS1) genes by ethylene, indole-3-acetic acid (IAA), and N(6)-benzyladenine (BA), was investigated in mungbean hypocotyl tissues. Exogenous ethylene markedly increased transcript level of VR-ACO1 and reduced that of VR-ACS1, whereas aminooxyacetic acid (AOA), an inhibitor of ethylene biosynthesis, decreased the level of VR-ACO1 mRNA and increased that of VR-ACS1, indicating that expression of VR-ACO1 and VR-ACS1 genes are under positive and negative feedback control by ethylene, respectively. However, IAA treatment reduced the level of VR-ACO1 transcripts and increased that of VR-ACS1, although the hormone greatly induced ethylene production. We have demonstrated that, in a system that separates the effect of IAA proper from the effect of IAA-induced ethylene, the amount of IAA-induced ethylene was enough to cause accumulation of VR-ACO1 mRNA and decrease of VR-ACS1 mRNA. We have also shown that the responsiveness of VR-ACO1 and VR-ACS1 to exogenous ethylene was greatly reduced in the presence of IAA. In addition, BA abolished ethylene responsiveness with respect to expression of VR-ACO1 and VR-ACS1. Based on these results, we suggest that IAA and BA inhibit ethylene action, resulting in suppression of VR-ACO1 expression and induction of VR-ACS1 expression.

Cloning of an auxin-responsive 1-aminocyclopropane-1-carboxylate synthase gene (CMe-ACS2) from melon and the expression of ACS genes in etiolated melon seedlings and melon fruits

Two cDNA fragments (pCMe-ACS2 and 3) encoding auxin-responsive 1-aminocyclopropane-1-carboxylate synthase (ACS; EC.4.4.1.14) have been isolated from melon, and the expression patterns of the genes in etiolated melon seedlings and melon fruit have been determined by RT-PCR analysis. The deduced amino acid sequences of pCMe-ACS2 and 3 were homologous to those of AT-ACS6 and 4, which were auxin-responsive ACS genes of Arabidopsis. Both CMe-ACS2 and 3 were auxin-responsive ACS genes and their expressions in roots and hypocotyls were induced by treatment with indole acetic acid (IAA, 100 µM). The mRNA level of CMe-ACS2 in the fruit increased after pollination. Those of both CMe-ACS2 and 3 temporarily increased in the mesocarp tissues at the preclimacteric stage (from day 3 to day 5 after harvest) during ripening, while that of CMe-ACS3 was lower than that of CMe-ACS2. The increase in the mRNA level of CMe-ACS1 (wound- and ripening-induced gene, T. Miki, M. Yamamoto, N. Nakagawa, O. Ogura, H. Mori, H. Imaseki, T. Sato, Nucleotide sequence of a cDNA for 1-aminocyclopropane-1-carboxylate synthase from melon fruits, Plant Physiol. 107 (1995) 297-298.) in the mesocarp tissue was not observed until 5 days after harvest. A genomic DNA encoding CMe-ACS2 was isolated and its nucleotide sequence was determined. Nucleotide sequences resembling the auxin-responsive elements (AuxRE) D1 and D4 (the TGTCTC element) in the GH3 gene from soybean, and the auxin-responsive domain (AuxRD) B in PS-IAA4/5 from pea were found in the 5'-flanking region of the CMe-ACS2 gene.

The regulation of 1-aminocyclopropane-1-carboxylic acid synthase gene expression during the transition from system-1 to system-2 ethylene synthesis in tomato

1-Aminocyclopropane-1-carboxylic acid synthase (ACS) is one of the key regulatory enzymes involved in the synthesis of the hormone ethylene and is encoded by a multigene family containing at least eight members in tomato (Lycopersicon esculentum). Increased ethylene production accompanies ripening in tomato, and this coincides with a change in the regulation of ethylene synthesis from auto-inhibitory to autostimulatory. The signaling pathways that operate to bring about this transition from so-called system-1 to system-2 ethylene production are unknown, and we have begun to address these by investigating the regulation of ACS expression during ripening. Transcripts corresponding to four ACS genes, LEACS1A, LEACS2, LEACS4, and LEACS6, were detected in tomato fruit, and expression analysis using the ripening inhibitor (rin) mutant in combination with ethylene treatments and the Never-ripe (Nr) mutant has demonstrated that each is regulated in a unique way. A proposed model suggests that system-1 ethylene is regulated by the expression of LEACS1A and LEACS6. In fruit a transition period occurs in which the RIN gene plays a pivotal role leading to increased expression of LEACS1A and induction of LEACS4. System-2 ethylene synthesis is subsequently initiated and maintained by ethylene-dependent induction of LEACS2.

Differential expression of three members of the 1-aminocyclopropane-1-carboxylate synthase gene family in carnation

We investigated the expression patterns of three 1-aminocyclopropane-1-carboxylate (ACC) synthase genes in carnation (Dianthus caryophyllus cv White Sim) under conditions previously shown to induce ethylene biosynthesis. These included treatment of flowers with 2,4-dichlorophenoxyacetic acid, ethylene, LiCl, cycloheximide, and natural and pollination-induced flower senescence. Accumulation of ACC synthase transcripts in leaves following mechanical wounding and treatment with 2,4-dichlorophenoxyacetic acid or LiCl was also determined by RNA gel-blot analysis. As in other species, the carnation ACC synthase genes were found to be differentially regulated in a tissue-specific manner. DCACS2 and DCACS3 were preferentially expressed in styles, whereas DCACS1 mRNA was most abundant in petals. Cycloheximide did not induce increased accumulation of ACC synthase transcripts in carnation flowers, whereas the expression of ACC synthase was up-regulated by auxin, ethylene, LiCl, pollination, and senescence in a floral-organ-specific manner. Expression of the three ACC synthases identified in carnation did not correspond to elevated ethylene biosynthesis from wounded or auxin-treated leaves, and there are likely additional members of the carnation ACC synthase gene family responsible for ACC synthase expression in vegetative tissues.

Differential expression and internal feedback regulation of 1-aminocyclopropane-1-carboxylate synthase, 1-aminocyclopropane-1-carboxylate oxidase, and ethylene receptor genes in tomato fruit during development and ripening

We investigated the feedback regulation of ethylene biosynthesis in tomato (Lycopersicon esculentum) fruit with respect to the transition from system 1 to system 2 ethylene production. The abundance of LE-ACS2, LE-ACS4, and NR mRNAs increased in the ripening fruit concomitant with a burst in ethylene production. These increases in mRNAs with ripening were prevented to a large extent by treatment with 1-methylcyclopropene (MCP), an ethylene action inhibitor. Transcripts for the LE-ACS6 gene, which accumulated in preclimacteric fruit but not in untreated ripening fruit, did accumulate in ripening fruit treated with MCP. Treatment of young fruit with propylene prevented the accumulation of transcripts for this gene. LE-ACS1A, LE-ACS3, and TAE1 genes were expressed constitutively in the fruit throughout development and ripening irrespective of whether the fruit was treated with MCP or propylene. The transcripts for LE-ACO1 and LE-ACO4 genes already existed in preclimacteric fruit and increased greatly when ripening commenced. These increases in LE-ACO mRNA with ripening were also prevented by treatment with MCP. The results suggest that in tomato fruit the preclimacteric system 1 ethylene is possibly mediated via constitutively expressed LE-ACS1A and LE-ACS3 and negatively feedback-regulated LE-ACS6 genes with preexisting LE-ACO1 and LE-ACO4 mRNAs. At the onset of the climacteric stage, it shifts to system 2 ethylene, with a large accumulation of LE-ACS2, LE-ACS4, LE-ACO1, and LE-ACO4 mRNAs as a result of a positive feedback regulation. This transition from system 1 to system 2 ethylene production might be related to the accumulated level of NR mRNA.