The Arabidopsis ABA-deficient mutant aba4 demonstrates that the major route for stress-induced ABA accumulation is via neoxanthin isomers

A novel abscisic acid (ABA)-deficient mutant, aba4, was identified in a screen for paclobutrazol-resistant germination. Compared with wild-type, the mutant showed reduced endogenous ABA levels in both dehydrated rosettes and seeds. Carotenoid composition analysis demonstrated that the defective locus affects neoxanthin synthesis. The ABA4 gene was identified by map-based cloning, and found to be a unique gene in the Arabidopsis genome. The predicted protein has four putative helical transmembrane domains and shows significant similarity to predicted proteins from tomato, rice and cyanobacteria. Constitutive expression of the ABA4 gene in Arabidopsis transgenic plants led to increased accumulation of trans-neoxanthin, indicating that the ABA4 protein has a direct role in neoxanthin synthesis. aba4 mutant phenotypes were mild compared with previously identified ABA-deficient mutants that exhibit vegetative tissue phenotypes. Indeed, ABA levels in seeds of aba4 mutants were higher than those of aba1 mutants. As aba1 mutants are also affected in a unique gene, this suggests that ABA can be produced in the aba4 mutant by an alternative pathway using violaxanthin as a substrate. It appears, therefore, that in Arabidopsis both violaxanthin and neoxanthin are in vivo substrates for 9-cis-epoxycarotenoid dioxygenases. Furthermore, significantly reduced levels of ABA were synthesized in the aba4 mutant on dehydration, demonstrating that ABA biosynthesis in response to stress must occur mainly via neoxanthin isomer precursors.     

Abscisic acid deficiency increases defence responses against Myzus persicae in Arabidopsis

Comparison of Arabidopsis thaliana (Arabidopsis) gene expression induced by Myzus persicae (green peach aphid) feeding, aphid saliva infiltration and abscisic acid (ABA) treatment showed a significant positive correlation. In particular, ABA‐regulated genes are over‐represented among genes that are induced by M. persicae saliva infiltration into Arabidopsis leaves. This suggests that the induction of ABA‐related gene expression could be an important component of the Arabidopsis–aphid interaction. Consistent with this hypothesis, M. persicae populations induced ABA production in wild‐type plants. Furthermore, aphid populations were smaller on Arabidopsis aba1‐1 mutants, which cannot synthesize ABA, and showed a significant preference for wild‐type plants compared with the mutant. Total free amino acids, which play an important role in aphid nutrition, were not altered in the aba1‐1 mutant line, but the levels of isoleucine (Ile) and tryptophan (Trp) were differentially affected by aphids in wild‐type and mutant plants. Recently, indole glucosinolates have been shown to promote aphid resistance in Arabidopsis. In this study, 4‐methoxyindol‐3‐ylmethylglucosinolate was more abundant in the aba1‐1 mutant than in wild‐type Arabidopsis, suggesting that the induction of ABA signals that decrease the accumulation of defence compounds may be beneficial for aphids.     

Molecular identification of zeaxanthin epoxidase of Nicotiana plumbaginifolia, a gene involved in abscisic acid biosynthesis and corresponding to the ABA locus of Arabidopsis thaliana.

Abscisic acid (ABA) is a plant hormone which plays an important role in seed development and dormancy and in plant response to environmental stresses. An ABA-deficient mutant of Nicotiana plumbaginifolia, aba2, was isolated by transposon tagging using the maize Activator transposon. The aba2 mutant exhibits precocious seed germination and a severe wilty phenotype. The mutant is impaired in the first step of the ABA biosynthesis pathway, the zeaxanthin epoxidation reaction. ABA2 cDNA is able to complement N.plumbaginifolia aba2 and Arabidopsis thaliana aba mutations indicating that these mutants are homologous. ABA2 cDNA encodes a chloroplast-imported protein of 72.5 kDa, sharing similarities with different mono-oxigenases and oxidases of bacterial origin and having an ADP-binding fold and an FAD-binding domain. ABA2 protein, produced in Escherichia coli, exhibits in vitro zeaxanthin epoxidase activity. This is the first report of the isolation of a gene of the ABA biosynthetic pathway. The molecular identification of ABA2 opens the possibility to study the regulation of ABA biosynthesis and its cellular location.     

Arabidopsis mutants define a central role for the xanthophyll cycle in the regulation of photosynthetic energy conversion.

A conserved regulatory mechanism protects plants against the potentially damaging effects of excessive light. Nearly all photosynthetic eukaryotes are able to dissipate excess absorbed light energy in a process that involves xanthophyll pigments. To dissect the role of xanthophylls in photoprotective energy dissipation in vivo, we isolated Arabidopsis xanthophyll cycle mutants by screening for altered nonphotochemical quenching of chlorophyll fluorescence. The npq1 mutants are unable to convert violaxanthin to zeaxanthin in excessive light, whereas the npq2 mutants accumulate zeaxanthin constitutively. The npq2 mutants are new alleles of aba1, the zeaxanthin epoxidase gene. The high levels of zeaxanthin in npq2 affected the kinetics of induction and relaxation but not the extent of nonphotochemical quenching. Genetic mapping, DNA sequencing, and complementation of npq1 demonstrated that this mutation affects the structural gene encoding violaxanthin deepoxidase. The npq1 mutant exhibited greatly reduced nonphotochemical quenching, demonstrating that violaxanthin deepoxidation is required for the bulk of rapidly reversible nonphotochemical quenching in Arabidopsis. Altered regulation of photosynthetic energy conversion in npq1 was associated with increased sensitivity to photoinhibition. These results, in conjunction with the analysis of npq mutants of Chlamydomonas, suggest that the role of the xanthophyll cycle in nonphotochemical quenching has been conserved, although different photosynthetic eukaryotes rely on the xanthophyll cycle to different extents for the dissipation of excess absorbed light energy.     

Regulation of carotenoid and ABA accumulation during the development and germination of Nicotiana plumbaginifolia seeds

Abscisic acid (ABA) is derived from epoxycarotenoid cleavage and regulates seed development and maturation. A detailed carotenoid analysis was undertaken to study the contribution of epoxycarotenoid synthesis to the regulation of ABA accumulation in Nicotiana plumbaginifolia developing seeds. Maximal accumulation of xanthophylls occurred at mid-development in wild type seeds, when total ABA levels also peaked. In contrast, in ABA-deficient mutants xanthophyll synthesis was delayed, in agreement with the retardation in seed maturation. Seed dormancy was restored in mutants impaired in the conversion of zeaxanthin into violaxanthin by zeaxanthin epoxidase (ZEP), by the introduction of the Arabidopsis AtZEP gene under the control of promoters inducing expression during later stages of seed development compared to wild type NpZEP, and in dry and imbibed seeds. Alterations in the timing and level of ZEP expression did not highly affect the temporal regulation of ABA accumulation in transgenic seeds, despite notable perturbations in xanthophyll accumulation. Therefore, major regulatory control of ABA accumulation might occur downstream of epoxycarotenoid synthesis.     

Maintenance of shoot growth by endogenous ABA: genetic assessment of the involvement of ethylene suppression

Previous work demonstrated that normal levels of endogenous abscisic acid (ABA) are required to maintain shoot growth in well-watered tomato plants independently of effects of hormone status on plant water balance. The results suggested that the impairment of shoot growth in ABA-deficient mutants is at least partly attributable to increased ethylene production. To assess the extent to which ABA maintains shoot growth by ethylene suppression, the growth of ABA-deficient (aba2-1) and ethylene-insensitive (etr1-1) single- and double-mutants of Arabidopsis was examined. To ensure that the results were independent of effects of hormone status on plant water balance, differential relative humidity regimes were used to achieve similar leaf water potentials in all genotypes and treatments. In aba2-1, shoot growth was substantially inhibited and ethylene evolution was doubled compared with the wild type, consistent with the results for tomato. In the aba2-1 etr1-1 double mutant, in which ABA was equally as deficient as in aba2-1 and shoot growth was shown to be insensitive to ethylene, shoot growth was substantially, although incompletely, restored relative to etr1-1. Treatment with ABA resulted in the complete recovery of shoot growth in aba2-1 relative to the wild type, and also significantly increased the growth of aba2-1 etr1-1 such that total leaf area and shoot fresh weight were not significantly lower than in etr1-1. In addition, ABA treatment of aba2-1 etr1-1 restored the wider leaf morphology phenotype exhibited by etr1-1. The results demonstrate that normal levels of endogenous ABA maintain shoot development, particularly leaf expansion, in well-watered Arabidopsis plants, partly by suppressing ethylene synthesis and partly by another mechanism that is independent of ethylene.     

Biochemical characterization of the aba2 and aba3 mutants in Arabidopsis thaliana.

Abscisic acid (ABA)-deficient mutants in a variety of species have been identified by screening for precocious germination and a wilty phenotype. Mutants at two new loci, aba2 and aba3, have recently been isolated in Arabidopsis thaliana (L.) Hynh. (K.M. Léon-Kloosterziel, M. Alvarez-Gil, G.J. Ruijs, S.E. Jacobsen, N.E. Olszewski, S.H. Schwartz, J.A.D. Zeevaart, M. Koornneef [1996] Plant J 10: 655-661), and the biochemical characterization of these mutants is presented here. Protein extracts from aba2 and aba3 plants displayed a greatly reduced ability to convert xanthoxin to ABA relative to the wild type. The next putative intermediate in ABA synthesis, ABA-aldehyde, was efficiently converted to ABA by extracts from aba2 but not by extracts from aba3 plants. This indicates that the aba2 mutant is blocked in the conversion of xanthoxin to ABA-aldehyde and that aba3 is impaired in the conversion of ABA-aldehyde to ABA. Extracts from the aba3 mutant also lacked additional activities that require a molybdenum cofactor (Moco). Nitrate reductase utilizes a Moco but its activity was unaffected in extracts from aba3 plants. Moco hydroxylases in animals require a desulfo moiety of the cofactor. A sulfido ligand can be added to the Moco by treatment with Na2S and dithionite. Treatment of aba3 extracts with Na2S restored ABA-aldehyde oxidase activity. Therefore, the genetic lesion in aba3 appears to be in the introduction of S into the Moco.     

The Arabidopsis aba4-1 mutant reveals a specific function for neoxanthin in protection against photooxidative stress

The aba4-1 mutant completely lacks neoxanthin but retains all other xanthophyll species. The missing neoxanthin in light-harvesting complex (Lhc) proteins is compensated for by higher levels of violaxanthin, albeit with lower capacity for photoprotection compared with proteins with wild-type levels of neoxanthin. Detached leaves of aba4-1 were more sensitive to oxidative stress than the wild type when exposed to high light and incubated in a solution of photosensitizer agents. Both treatments caused more rapid pigment bleaching and lipid oxidation in aba4-1 than wild-type plants, suggesting that neoxanthin acts as an antioxidant within the photosystem II (PSII) supercomplex in thylakoids. While neoxanthin-depleted Lhc proteins and leaves had similar sensitivity as the wild type to hydrogen peroxide and singlet oxygen, they were more sensitive to superoxide anions. aba4-1 intact plants were not more sensitive than the wild type to high-light stress, indicating the existence of compensatory mechanisms of photoprotection involving the accumulation of zeaxanthin. However, the aba4-1 npq1 double mutant, lacking zeaxanthin and neoxanthin, underwent stronger PSII photoinhibition and more extensive oxidation of pigments than the npq1 mutant, which still contains neoxanthin. We conclude that neoxanthin preserves PSII from photoinactivation and protects membrane lipids from photooxidation by reactive oxygen species. Neoxanthin appears particularly active against superoxide anions produced by the Mehler's reaction, whose rate is known to be enhanced in abiotic stress conditions.     

The ABA1 gene and carotenoid biosynthesis are required for late skotomorphogenic growth in Arabidopsis thaliana

Several plant hormones, including auxin, brassinosteroids and gibberellins, are required for skotomorphogenesis, which is the etiolated growth that seedlings undergo in the absence of light. To examine the growth of abscisic acid (ABA)-deficient mutants in the dark, we analysed several aba1 loss-of-function alleles, which are deficient in zeaxanthin epoxidase. The aba1 mutants displayed a partially de-etiolated phenotype, including reduced hypocotyl growth, cotyledon expansion and the development of true leaves, during late skotomorphogenic growth. In contrast, only small differences in hypocotyl growth were found between wild-type seedlings and ABA-deficient mutants impaired in subsequent steps of the pathway, namely nced3, aba2, aba3 and aao3. Interestingly, phenocopies of the partially de-etiolated phenotype of the aba1 mutants were obtained when wild-type seedlings were dark-grown on medium supplemented with fluridone, an inhibitor of phytoene desaturase, and hence, of carotenoid biosynthesis. ABA supplementation did not restore the normal skotomorphogenic growth of aba1 mutants or fluridone-treated wild-type plants, suggesting a direct inhibitory effect of fluridone on carotenoid biosynthesis. In addition, aba1 mutants showed impaired production of the beta-carotene-derived xanthophylls, neoxanthin, violaxanthin and antheraxanthin. Because fluridone treatment of wild-type plants phenocopied the phenotype of dark-grown aba1 mutants, impaired carotenoid biosynthesis in aba1 mutants is probably responsible for the observed skotomorphogenic phenotype. Thus, ABA1 is required for skotomorphogenic growth, and beta-carotene-derived xanthophylls are putative regulators of skotomorphogenesis.     

Effect of hydroxylated carotenoid deficiency on ABA accumulation in Arabidopsis

Abscisic acid (ABA) is a sesquiterpene compound (C₁₅) derived from C₄₀ carotenoids. The immediate carotenoid precursors for ABA biosynthesis, 9- cis -violaxanthin and 9'- cis -neoxanthin, are produced from β -carotene by a series of hydroxylation, epoxidation, and isomerization reactions. Carotenoid hydroxylase deficient mutants contain severely reduced levels of violaxanthin and neoxanthin ( < 20% of wild type level) and provide a unique system to correlate carotenoid substrate availability and ABA production in photosynthetic tissues under non-stressed conditions. Quantitative measurements indicated that ABA levels in the carotenoid hydroxylase mutants are reduced nearly 50% compared to the wild type plants under non-stressed conditions. When drought-stressed, wild type plants showed up to a 17-fold increase in ABA levels, while ABA levels in the carotenoid hydroxylase mutants were only increased 6- to 7-fold (25% of wild type drought-stressed ABA levels). Expression of AtNCED3 ( Arabidopsis thaliana nine- cis -epoxycarotenoid dioxygenase 3, the rate-limiting activity for ABA biosynthesis) was induced in the carotenoid hydroxylase mutants, but to a lesser extent than the 40-fold increase in wild type plants. Therefore, the reduced ABA accumulation in response to drought-stress is at least partially due to the attenuated increase in AtNCED3 gene expression in the carotenoid hydroxylase mutants. The remaining violaxanthin and neoxanthin in the carotenoid hydroxylase mutants can not be converted into ABA, indicating that there is probably a separate pool of violaxanthin and neoxanthin that is not accessible to the cleavage enzymes, because it is sequestered in the light-harvesting complexes.     

Regulation of hormone metabolism in Arabidopsis seeds: phytochrome regulation of abscisic acid metabolism and abscisic acid regulation of gibberellin metabolism

In a wide range of plant species, seed germination is regulated antagonistically by two plant hormones, abscisic acid (ABA) and gibberellin (GA). In the present study, we have revealed that ABA metabolism (both biosynthesis and inactivation) was phytochrome-regulated in an opposite fashion to GA metabolism during photoreversible seed germination in Arabidopsis. Endogenous ABA levels were decreased by irradiation with a red (R) light pulse in dark-imbibed seeds pre-treated with a far-red (FR) light pulse, and the reduction in ABA levels in response to R light was inhibited in a phytochrome B (PHYB)-deficient mutant. Expression of an ABA biosynthesis gene, AtNCED6, and the inactivation gene, CYP707A2, was regulated in a photoreversible manner, suggesting a key role for the genes in PHYB-mediated regulation of ABA metabolism. Abscisic acid-deficient mutants such as nced6-1, aba2-2 and aao3-4 exhibited an enhanced ability to germinate relative to wild type when imbibed in the dark after irradiation with an FR light pulse. In addition, the ability to synthesize GA was improved in the aba2-2 mutant compared with wild type during dark-imbibition after an FR light pulse. Activation of GA biosynthesis in the aba2-2 mutant was also observed during seed development. These data indicate that ABA is involved in the suppression of GA biosynthesis in both imbibed and developing seeds. Spatial expression patterns of the AtABA2 and AAO3 genes, responsible for last two steps of ABA biosynthesis, were distinct from that of the GA biosynthesis gene, AtGA3ox2, in both imbibed and developing seeds, suggesting that biosynthesis of ABA and GA in seeds occurs in different cell types.     

In Vivo Inhibition of Seed Development and Reserve Protein Accumulation in Recombinants of Abscisic Acid Biosynthesis and Responsiveness Mutants in Arabidopsis thaliana

In Arabidopsis thaliana, seed development in recombinants of the ABA-deficient aba mutant with the ABA response mutants abi1 or abi3 is compared to wild type and the monogenic parents. Aberrant seed development occurred in the aba,abi3 recombinant and was normal in aba,abi1, abi3 and aba,abi1 seeds. Embryos of the recombinant aba,abi3 seeds maintained the green color until maturity, the seeds kept a high water content, did not form the late abundant 2S and 12S storage proteins, were desiccation intolerant, and often showed viviparous germination. Application of ABA, and particularly of an ABA analog, to the roots of plants during seed development partially alleviated the aberrant phenotype. Seeds of aba,abi3 were normal when they developed on a mother plant heterozygous for Aba. In contrast to seed development, the induction of dormancy was blocked in all monogenic mutants and recombinants. Dormancy was only induced by embryonic ABA; it could not be increased by maternal ABA or ABA applied to the mother plant. It is concluded that endogenous ABA has at least two different effects in developing seeds. The nature of these responses and of the ABA response system is discussed.     

Use of infrared thermal imaging to isolate Arabidopsis mutants defective in stomatal regulation

In response to drought, plants synthesise the hormone abscisic acid (ABA), which triggers closure of the stomatal pores. This process is vital for plants to conserve water by reducing transpirational water loss. Moreover, recent studies have demonstrated the advantages of the Arabidopsis stomatal guard cell for combining genetic, molecular and biophysical approaches to characterise ABA action. However, genetic dissection of stomatal regulation has been limited by the difficulty of identifying a reliable phenotype for mutant screening. Leaf temperature can be used as an indicator to detect mutants with altered stomatal control, since transpiration causes leaf cooling. In this study, we optimised experimental conditions under which individual Arabidopsis plants with altered stomatal responses to drought can be identified by infrared thermography. These conditions were then used to perform a pilot screen for mutants that displayed a reduced ability to close their stomata and hence appeared colder than the wild type. Some of the mutants recovered were deficient in ABA accumulation, and corresponded to alleles of the ABA biosynthesis loci ABA1, ABA2 and ABA3. Interestingly, two of these novel aba2 alleles were able to intragenically complement the aba2-1 mutation. The remaining mutants showed reduced ABA responsiveness in guard cells. In addition to the previously known abi1-1 mutation, we isolated mutations at two novel loci designated as OST1 (OPEN STOMATA 1) and OST2. Remarkably, ost1 and ost2 represent, to our knowledge, the first Arabidopsis mutations altering ABA responsiveness in stomata and not in seeds.     

Involvement of endogenous abscisic acid in methyl jasmonate-induced stomatal closure in Arabidopsis

In this study, we examined the involvement of endogenous abscisic acid (ABA) in methyl jasmonate (MeJA)-induced stomatal closure using an inhibitor of ABA biosynthesis, fluridon (FLU), and an ABA-deficient Arabidopsis (Arabidopsis thaliana) mutant, aba2-2. We found that pretreatment with FLU inhibited MeJA-induced stomatal closure but not ABA-induced stomatal closure in wild-type plants. The aba2-2 mutation impaired MeJA-induced stomatal closure but not ABA-induced stomatal closure. We also investigated the effects of FLU and the aba2-2 mutation on cytosolic free calcium concentration ([Ca(2+)](cyt)) in guard cells using a Ca(2+)-reporter fluorescent protein, Yellow Cameleon 3.6. In wild-type guard cells, FLU inhibited MeJA-induced [Ca(2+)](cyt) elevation but not ABA-induced [Ca(2+)](cyt) elevation. The aba2-2 mutation did not affect ABA-elicited [Ca(2+)](cyt) elevation but suppressed MeJA-induced [Ca(2+)](cyt) elevation. We also tested the effects of the aba2-2 mutation and FLU on the expression of MeJA-inducible VEGETATIVE STORAGE PROTEIN1 (VSP1). In the aba2-2 mutant, MeJA did not induce VSP1 expression. In wild-type leaves, FLU inhibited MeJA-induced VSP1 expression. Pretreatment with ABA at 0.1 μm, which is not enough concentration to evoke ABA responses in the wild type, rescued the observed phenotypes of the aba2-2 mutant. Finally, we found that in wild-type leaves, MeJA stimulates the expression of 9-CIS-EPOXYCAROTENOID DIOXYGENASE3, which encodes a crucial enzyme in ABA biosynthesis. These results suggest that endogenous ABA could be involved in MeJA signal transduction and lead to stomatal closure in Arabidopsis guard cells.     

Endogenous abscisic acid is involved in methyl jasmonate-induced reactive oxygen species and nitric oxide production but not in cytosolic alkalization in Arabidopsis guard cells

We recently demonstrated that endogenous abscisic acid (ABA) is involved in methyl jasmonate (MeJA)-induced stomatal closure in Arabidopsis thaliana. In this study, we investigated whether endogenous ABA is involved in MeJA-induced reactive oxygen species (ROS) and nitric oxide (NO) production and cytosolic alkalization in guard cells using an ABA-deficient Arabidopsis mutant, aba2-2, and an inhibitor of ABA biosynthesis, fluridon (FLU). The aba2-2 mutation impaired MeJA-induced ROS and NO production. FLU inhibited MeJA-induced ROS production in wild-type guard cells. Pretreatment with 0.1 μM ABA, which does not induce stomatal closure in the wild type, complemented the insensitivity to MeJA of the aba2-2 mutant. However, MeJA induced cytosolic alkalization in both wild-type and aba2-2 guard cells. These results suggest that endogenous ABA is involved in MeJA-induced ROS and NO production but not in MeJA-induced cytosolic alkalization in Arabidopsis guard cells.     

Accumulation of Zeaxanthin in Abscisic Acid-Deficient Mutants of Arabidopsis Does Not Affect Chlorophyll Fluorescence Quenching or Sensitivity to Photoinhibition in Vivo

Abscisic acid (ABA)-deficient mutants of Arabidopsis do not synthesize the epoxy-xanthophylls antheraxanthin, violaxanthin, or neoxanthin. However, thylakoid membranes from these mutants contain 3-fold more zeaxanthin than wild-type plants. This increase in zeaxanthin occurs as a stoichiometric replacement of the missing violaxanthin and neoxanthin within the pigment-protein complexes of both photosystem I and photosystem II (PSII). The retention of zeaxanthin in the dark by ABA-deficient mutants sensitizes the leaves to the development of nonphotochemical quenching (NPQ) during the first 2 to 4 min following a dark-light transition. However, the increase in pool size does not result in any increase in steady-state NPQ. When we exposed wild-type and ABA-deficient mutants leaves to twice growth irradiance, the mutants developed lower maximal NPQ but suffered similar photoinhibition to wildtype, measured both as a decline in the ratio of variable to maximal fluorescence and as a loss of functional PSII centers from oxygen flash yield measurements. These results suggest that only a few of the zeaxanthin molecules present within the light-harvesting antenna of PSII may be involved in NPQ and neither the accumulation of a large pool of zeaxanthin within the antenna of PSII nor an increase in conversion of violaxanthin to zeaxanthin will necessarily enhance photoprotective energy dissipation.     

Abscisic acid induces the alcohol dehydrogenase gene in Arabidopsis.

Exogenous abscisic acid (ABA) induced the alcohol dehydrogenase gene (Adh) in Arabidopsis roots. Both the G-box-1 element and the GT/GC motifs (anaerobic response element) were required for Adh inducibility. Measurement of endogenous ABA levels during stress treatment showed that ABA levels increased during dehydration treatment but not following exposure to either hypoxia or low temperature. Arabidopsis ABA mutants (aba1 and abi2) displayed reduced Adh mRNA induction levels following either dehydration treatment or exogenous application of ABA. Low-oxygen response was slightly increased in the aba1 mutant but was unchanged in abi2. Low-temperature response was unaffected in both aba1 and abi2 mutants. Our results indicate that, although induction of the Adh gene by ABA, dehydration, and low temperature required the same cis-acting promoter elements, their regulatory pathways were at least partially separated in a combined dehydration/ABA pathway and an ABA-independent low-temperature pathway. These pathways were in turn independent of a third signal transduction pathway leading to low-oxygen response, which did not involve either ABA or the G-box-1 promoter element.