Ethylene promotes submergence-induced expression of OsABA8ox1, a gene that encodes ABA 8'-hydroxylase in rice

A rapid decrease of the plant hormone ABA under submergence is thought to be a prerequisite for the enhanced elongation of submerged shoots of rice (Oryza sativa L.). Here, we report that the level of phaseic acid (PA), an oxidized form of ABA, increased with decreasing ABA level during submergence. The oxidation of ABA to PA is catalyzed by ABA 8'-hydroxylase, which is possibly encoded by three genes (OsABA8ox1, -2 and -3) in rice. The ABA 8'-hydroxylase activity was confirmed in microsomes from yeast expressing OsABA8ox1. OsABA8ox1-green fluorescent protein (GFP) fusion protein in onion cells was localized to the endoplasmic reticulum. The mRNA level of OsABA8ox1, but not the mRNA levels of other OsABA8ox genes, increased dramatically within 1 h after submergence. On the other hand, the mRNA levels of genes involved in ABA biosynthesis (OsZEP and OsNCEDs) decreased after 1-2 h of submergence. Treatment of aerobic seedlings with ethylene and its precursor, 1-aminocyclopropane-1-carboxylate (ACC), rapidly induced the expression of OsABA8ox1, but the ethylene treatment did not strongly affect the expression of ABA biosynthetic genes. Moreover, pre-treatment with 1-methylcyclopropene (1-MCP), a potent inhibitor of ethylene action, partially suppressed induction of OsABA8ox1 expression under submergence. The ABA level was found to be negatively correlated with OsABA8ox1 expression under ACC or 1-MCP treatment. Together, these results indicate that the rapid decrease in ABA levels in submerged rice shoots is controlled partly by ethylene-induced expression of OsABA8ox1 and partly by ethylene-independent suppression of genes involved in the biosynthesis of ABA.     

Abscisic acid regulates TSRF1-mediated resistance to Ralstonia solanacearum by modifying the expression of GCC box-containing genes in tobacco

Although recent studies have established a significant regulatoryrole for abscisic acid (ABA) and ethylene response factor (ERF)proteins in plant pathogen resistance, it is not clear whetherand how ABA performs this role. Previously, it was reportedthat an ERF protein, TSRF1, activates the expression of GCCbox-containing genes and significantly enhances the resistanceto Ralstonia solanacearum in both tobacco and tomato plants.Here, it is reported that TSRF1-regulated pathogen resistanceis modified by ABA application. TSRF1 activates the expressionof ABA biosynthesis-related genes, resulting in the increaseof ABA biosynthesis, which further stimulates ethylene production.More interestingly, ABA application decreases, while the inhibitorof ABA biosynthesis fluridone increases, the TSRF1-enhancedresistance to R. solanacearum. This observation is further supportedby the finding that ABA and fluridone reversibly modify theability of TSRF1 to bind the ethylene-responsive GCC box, consequentlyaltering the expression of element-controlled genes. These resultstherefore establish that TSRF1-regulated resistance to R. solanacearumcan be modified in tobacco by ABA. Key words: Abscisic acid, ERF protein TSRF1, GCC box-containing genes, Ralstonia solanacearum, tobacco     

Regulation of genes associated with auxin, ethylene and ABA pathways by 2,4-dichlorophenoxyacetic acid in Arabidopsis

The chemical 2,4-dichlorophenoxyacetic acid (2,4-D) regulates plant growth and development and mimics auxins in exhibiting a biphasic mode of action. Although gene regulation in response to the natural auxin indole acetic acid (IAA) has been examined, the molecular mode of action of 2,4-D is poorly understood. Data from biochemical studies, (Grossmann (2000) Mode of action of auxin herbicides: a new ending to a long, drawn out story. Trends Plant Sci 5:506–508) proposed that at high concentrations, auxins and auxinic herbicides induced the plant hormones ethylene and abscisic acid (ABA), leading to inhibited plant growth and senescence. Further, in a recent gene expression study (Raghavan et al. (2005) Effect of herbicidal application of 2,4-dichlorophenoxyacetic acid in Arabidopsis. Funct Integr Genomics 5:4–17), we have confirmed that at high concentrations, 2,4-D induced the expression of the gene NCED1, which encodes 9-cis-epoxycarotenoid dioxygenase, a key regulatory enzyme of ABA biosynthesis. To understand the concentration-dependent mode of action of 2,4-D, we further examined the regulation of whole genome of Arabidopsis in response to a range of 2,4-D concentrations from 0.001 to 1.0 mM, using the ATH1-121501 Arabidopsis whole genome microarray developed by Affymetrix. Results of this study indicated that 2,4-D induced the expression of auxin-response genes (IAA1, IAA13, IAA19) at both auxinic and herbicidal levels of application, whereas the TIR1 and ASK1 genes, which are associated with ubiquitin-mediated auxin signalling, were down-regulated in response to low concentrations of 2,4-D application. It was also observed that in response to low concentrations of 2,4-D, ethylene biosynthesis was induced, as suggested by the up-regulation of genes encoding 1-aminocyclopropane-1-carboxylic acid (ACC) synthase and ACC oxidase. Although genes involved in ethylene biosynthesis were not regulated in response to 0.1 and 1.0 mM 2,4-D, ethylene signalling was induced as indicated by the down-regulation of CTR1 and ERS, both of which play a key role in the ethylene signalling pathway. In response to 1.0 mM 2,4-D, both ABA biosynthesis and signalling were induced, in contrast to the response to lower concentrations of 2,4-D where ABA biosynthesis was suppressed. We present a comprehensive model indicating a molecular mode of action for 2,4-D in Arabidopsis and the effects of this growth regulator on the auxin, ethylene and abscisic acid pathways. Experiment station: Plant Biotechnology Centre, Primary Industries Research Victoria, Department of Primary Industries, La Trobe University, Bundoora, Victoria 3086, and the Victorian Microarray Technology Consortium (VMTC).     

Wounding of potato tubers induces increases in ABA biosynthesis and catabolism and alters expression of ABA metabolic genes

The effects of physical wounding on ABA biosynthesis and catabolism and expression of genes encoding key ABA metabolic enzymes were determined in potato tubers. An increase in ABA and ABA metabolite content was observed 48 h after wounding and remained elevated through 96 h. Wounding induced dramatic increases in the expression of the ABA metabolic genes encoding zeaxanthin epoxidase (ZEP), 9-cis-epoxycarotenoid dioxygenase (NCED), and ABA-8′-hydroxylase. Although the patterns of wound-induced expression of individual genes varied, increased gene expression was observed within 3 h of wounding and remained elevated through 96 h. An apparent correlation between expression of the gene encoding ZEP and the increase in ABA content suggested that the wound-induced increase in ABA biosynthesis was regulated by both substrate availability and increased NCED activity. Suppression of wound-induced jasmonic acid accumulation by rinsing the wounded tissue with water did not inhibit the subsequent increase in ABA content. Exogenous ethylene completely suppressed the wound-induced increase in ABA content and dramatically reduced wound-induced up-regulation of ABA metabolic genes. This study is the first to identify the molecular bases for increased ABA accumulation following physical trauma in potato tubers and highlights the complex physiological interactions between various wound-induced hormones.     

Assessment of the Germination Potential of Brassica oleracea Seeds Treated with Karrikin 1 and Cyanide,Which Modify the Ethylene Biosynthetic Pathway

The relationship between ethylene and cyanide (HCN) and karrikin 1 (KAR1) in dormancy release was studied in secondary dormant Brassica oleracea L. (Chinese cabbage) seeds. Freshly harvested seeds of Brassica oleracea usually have poor germination potential. Karrikin1 (KAR1) and cyanide (HCN) are able to stimulate seed germination. However, the stimulatory effects of these two chemicals depend on the activation of the ethylene biosynthesis pathway and on ethylene perception. In this study, KAR1 and HCN application increased the activity of ethylene and of two ethylene biosynthesis enzymes, ACC synthase (ACS) and ACC oxidase (ACO). KAR1 and HCN collectively promoted the accumulation of 1 aminocyclopropane-1-carboxylic acid (ACC). In the presence of NO (nitric oxide) and KAR1, ACS and ACO activities reached their maximum levels after 36 and 42 h, respectively. Ethylene inhibitors suppressed seed germination by approximately 55%, whereas the respiratory inhibitors SHAM and NaN3 inhibited seed germination by 5–10% in the presence of HCN and KAR1. KAR1 and HCN collectively reduced the abscisic acid (ABA) content in seeds, increased the gibberellic acid (GA) content and released seed dormancy. The expression of ethylene biosynthesis genes and ethylene receptor genes (BOACO1, BOACS1, BOACS3, BOACS4, BOACS5, BOACS7, BOACS9, BOACS11, BOETR1 and BOETR2) provided further evidence of the involvement of ethylene in KAR1 and HCN-induced germination. BOACO1, BOACS1, BOACS5, BOACS7, BOACS9, BOACS11, BOETR1 and BOETR2 genes were up regulated in the presence of KAR1 and HCN, while the remaining genes were down regulated. The expression of various ethylene biosynthesis and ethylene receptor genes suggested functional diversification and variations in seed sensitivity in the presence of KAR1 and HCN. Therefore, in the current study, KAR1 and HCN application effectively induced the germination of B. oleracea seeds (approximately 97% germination rate) after 6 days by modifying the ethylene biosynthetic pathway.

     

Interactions between abscisic acid and ethylene signaling cascades

We screened for mutations that either enhanced or suppressed the abscisic acid (ABA)-resistant seed germination phenotype of the Arabidopsis abi1-1 mutant. Alleles of the constitutive ethylene response mutant ctr1 and ethylene-insensitive mutant ein2 were recovered as enhancer and suppressor mutations, respectively. Using these and other ethylene response mutants, we showed that the ethylene signaling cascade defined by the ETR1, CTR1, and EIN2 genes inhibits ABA signaling in seeds. Furthermore, epistasis analysis between ethylene- and ABA-insensitive mutations indicated that endogenous ethylene promotes seed germination by decreasing sensitivity to endogenous ABA. In marked contrast to the situation in seeds, ein2 and etr1-1 roots were resistant to both ABA and ethylene. Our data indicate that ABA inhibition of root growth requires a functional ethylene signaling cascade, although this inhibition is apparently not mediated by an increase in ethylene biosynthesis. These results are discussed in the context of the other hormonal regulations controlling seed germination and root growth.