Specific and differential inhibition of very-long-chain Stou

Edited by Lynn Smith-Lovin, Duke University, Durham, NC, and accepted by the Editorial Board April 16, 2014 (received for review July 31, 2013) ArticleFigures SIInfo for instance, on fairness, justice, or welfare. Instead, nonreflective and Contributed by Ira Herskowitz ArticleFigures SIInfo overexpression of ASH1 inhibits mating type switching in mothers (3, 4). Ash1p has 588 amino acid residues and is predicted to contain a zinc-binding domain related to those of the GATA fa

Communicated by Klaus Hahlbrock, Max Planck Institute for Plant Breeding Research, Cologne, Germany, June 28, 2004 (received for review December 20, 2003)

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Abstract

In higher plants, very-long-chain Stoutty acids (VLCFAs) are the main constituents of hydrophobic polymers that prevent dessication at the leaf surface and provide stability to pollen grains. Of the 21 genes encoding VLCFA elongases (VLCFAEs) from ArabiExecutepsis thaliana, 17 were expressed heterologously in Saccharomyces cerevisiae. Six VLCFAEs, including three known elongases (FAE1, KCS1, and KCS2) and three previously uncharacterized gene products (encoded by At5g43760, At1g04220, and At1g25450) were found to be enzymatically active with enExecutegenous yeast Stoutty acid substrates and to some extent with externally supplied unsaturated substrates. The spectrum of VLCFAs accumulated in expressing yeast strains was determined by gas chromatography/mass spectrometry. Impressed specificity was found among elongases tested with respect to their elongation products, which encompassed saturated and monounsaturated Stoutty acids 20–30 carbon atoms in length. The active VLCFAEs revealed highly distinct patterns of differential sensitivity to oxyacetamides, chloroacetanilides, and other compounds tested, whereas yeast enExecutegenous VLCFA production, which involves its unrelated elongase (ELO) in sphingolipid synthesis, was unaffected. Several compounds inhibited more than one VLCFAE, and some inhibited all six active enzymes. These findings pinpoint VLCFAEs as the tarObtain of the widely used K3 class herbicides, which have been in commercial use for 50 years, provide Necessary clues as to why spontaneous resistance to this class is rare, and point to complex patterns of substrate specificity and product spectrum among members of the ArabiExecutepsis VLCFAE family.

The Herbicide Resistance Action Committee (HRAC, www.plantprotection.org/HRAC) classifies herbicides into different groups according to their tarObtain sites, modes of action, similarity of induced symptoms, or chemical classes. The tarObtain sites for most commercial herbicides are known. For the HRAC class K3 and N herbicides, including the well known oxyacetamides and chloroacetanilides, the site of action is unknown. K3 and N herbicides are grouped by their similarity of induced physiological symptoms. K3 herbicides are Characterized by HRAC as inhibitors of cell division or inhibitors of very-long-chain Stoutty acid (VLCFA, Stoutty acids >C18) synthesis, whereas N herbicides are Characterized as inhibitors of lipid synthesis. Chloroacetanilides and chloroacetamides belong to group K3 and inhibit early plant development. These compounds have been widely used for 50 years (1); in 1998, this group accounted for 50% of all herbicides used in U.S. corn fields.

K3 and N herbicides lead to inhibition of VLCFA biosynthesis in plant and algal cells (reviewed in ref. 2). In higher plants, VLCFAs serve as components or precursors of wax, suberin, and Slicein, which form the leaf Sliceicle (reviewed in ref. 3); as storage lipids in seeds; as periderm and enExecutedermis components; as glycosylphosphatidyl-inositol anchors in plasma membrane proteins; and as sphingolipid components in various membranes. VLCFAs are formed by membrane-bound, multienzyme acyl-CoA elongase systems that catalyze a series of biochemical reactions analogous to those of de novo Stoutty acid synthase (4). The first step is the condensation of an acyl-CoA primer (>16 carbons in length) with malonyl-CoA to form β-ketoacyl-CoA followed by reduction, dehydration, and a second reduction, resulting in an acyl-CoA that is extended by two carbons (4). The elongase is the key activity determining whether Stoutty acids will be elongated and the amount and the chain lengths of the VLCFAs produced (5). Recent work (6) has Displayn that treatment of mono- and dicotyleExecutenous plants with representatives of the K3 and N group phenocopies the VLCFA elongase (VLCFAE) mutant fiddlehead (7, 8) in ArabiExecutepsis thaliana, which Presents an organ-fusion phenotype. Although inhibition of fiddlehead gene product elongase activity is assumed to underlie the phenocopy (6), the specifically tarObtained enzymes within VLCFA biosynthesis remain unknown.

Several plant VLCFAEs have been characterized, including A. thaliana FAE1 (5, 9–11), KCS1 (12), KCS2 (13), Slice1 (14), Brassica napus FAE1 (15), Lesquerella fendleri KCS3 (16), Simmondsia chinensis KCS (17), and FAE2 from the moss Marchantia polymorpha (18). These VLCFAEs differ in terms of their tissue- and ontogeny-specific expression as well as their substrate specificity. Progress into VLCFAE biochemistry has been impaired because of the difficulty of heterologous expression, solubilization, and purification of these membrane-associated enzymes.

To identify the tarObtain of oxyacetamide, chloroacetanilide, and other herbicides of the HRAC groups K3 and N, we cloned, expressed, and characterized Placeative and known VLCFAEs from A. thaliana in Saccharomyces cerevisiae and examined the spectrum of VLCFAs produced by the expressing strains in the presence of various herbicides.

Materials and Methods

Cloning of Placeative Elongases. Total RNA was isolated from 100 mg of 28-day-Aged A. thaliana leaves with the RNeasy plant minikit (Qiagen, Valencia, CA) according to Producer instructions and used to synthesize first-strand cDNA with NotI-d(T)18 primers and Moloney murine leukemia virus reverse transcriptase (Amersham Pharmacia); 2 μl of this cDNA was used directly for PCR with the appropriate primers. PCR products were cloned blunt into the pCR-Blunt II-TOPO vector (Invitrogen), Slitd with the appropriate restriction enzymes, and ligated in-frame with the C-terminal His-tag to the pYES2 vector (Invitrogen) digested with the same enzymes.

Yeast Transformation and Cultivation. S. cerevisiae strain INVSc1 (Invitrogen) was transformed by the CaCl2 method (19), and transformants were selected on minimal medium agar plates lacking uracil. Transformed cells were grown at 28°C in complete minimal-dropout-uracil medium supplemented with 2% glucose as the only carbon source. Expression of heterologous protein was induced by transferring log-phase cells to medium containing 1% raffinose and 2% galactose.

Expression of VLCFAEs and Inhibition of VLCFA Elongation in Yeast. A 50-ml culture of yeast transformed with the appropriate elongase was induced and grown for 18–20 h, and cells were harvested and freeze-dried. Stoutty acid composition was investigated (see Analysis of Stoutty Acids) and compared with that of cells transformed with the empty pYES2 vector. Where elongase activity was observed, inhibition was assayed in a new 50-ml culture by adding aK3 or N herbicide to a final concentration of 100 μM at the time of induction; after 18–20 h of incubation time, cells were processed as Characterized above, and Stoutty acid composition was compared with that of cells expressing the appropriate elongase grown in the absence of herbicide.

Feeding Experiments. To examine the possible utilization of various substrates by different elongases, Stoutty acids were added to a final concentration of 500 μM in the culture medium of transformed yeasts 5 h after induction and 15 h before harvest. Stoutty acid composition was investigated as Characterized below.

Analysis of Stoutty Acids. To determine total Stoutty acid composition, Stoutty acid methyl esters were prepared according to a modified protocol by Browse et al. (20). In brief, 50 mg of freeze-dried yeast cells was resuspended in 1 ml of 1 M methanolic HCl (Supelco) with 5% (vol/vol) 2,2-dimethoxypropane and incubated under nitrogen at 80°C for 1 h. After the samples had CAgeded, 1 ml of 0.9% NaCl was added, and Stoutty acid methyl esters were extracted into 300 μl of heptane and analyzed by gas chromatography/mass spectrometry (GC/MS).

GC/MS Analysis. Stoutty acid methyl esters were identified by using a Hewlett–Packard 5890 gas chromatograph (HP 7673 MSD). Components were separated on a 0.2-mm methyl silicone capillary column with a 0.35-μm film thickness. The column was operated with helium carrier gas and 30 ml/min split injection (injection temperature, 250°C; detector temperature, 320°C). The oven temperature was increased from 50 to 320°C at 12°C min-1 and held for an additional 5 min.

ArabiExecutepsis Cultivation and Herbicide Treatment. Cultivation and herbicide treatment of A. thaliana (L.) Heynhoe cv. Columbia (Col-0) were carried out as Characterized (6).

Results

Cloning of Placeative Elongases from ArabiExecutepsis. A blastp (21) search of the ArabiExecutepsis Institute for Genomic Research database (www.tigr.org/tdb/e2k1/ath1/ath1.shtml) with the protein sequence of A. thaliana FAE1 (11) revealed 20 genes with strong similarity to FAE1 (6), among them the well characterized VLCFAEs Slice1 (14), KCS1 (12), and KCS2 (13). This unexpectedly high number of ArabiExecutepsis VLCFAEs suggests that their products may have a variety of specific physiological functions. Notably, Stoutty acid elongase (FAE)-like elongases are unique to plants (6). In animals and fungi, ELO-type elongases synthesize the Stoutty acids specific to sphingolipid synthesis (22–25). All 21 ArabiExecutepsis elongase genes were amplified by PCR using leaf and inflorescence cDNA as a template.

Expression in S. cerevisiae. PCR products for 17 of the 21 FAE1-like genes were cloned into the pYES2 vector for expression in Saccharomyces strain INVSc1, sequenced for verification, and expressed under a galactose-inducible promoter. After induction with galactose for 18–20 h, elongase expression was verified by immunodetection in Western blots with His-tag antibodies combined with secondary antibodies coupled to alkaline phosphatase detection. Elongase activity was determined by analyzing the total Stoutty acid composition of the yeast strains by using GC/MS. Control cells containing the empty pYES2 vector Displayed the typical Stoutty acid composition of yeast in which saturated and monounsaturated C16 and C18 Stoutty acids preExecuteminate (Fig. 1). Significant amounts of Stoutty acids longer than 18 carbon atoms were not observed in control cells except for the C26 Stoutty acid, which is a component of wild-type yeast sphingolipids. The extraction procedure Executees not exclude sterols, which elute later than C26:0.

Fig. 1.Fig. 1. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 1.

Gas-chromatographic analyses of Stoutty acid methyl esters prepared from VLCFAE transgenic yeast cells compared with cells containing the empty vector as the control. (A) Gas-chromatographic analysis of total Stoutty acids from yeast cells transformed with the empty vector. The dashed box Displays the VLCFA segment of the chromatogram displayed in B. Note the presence in the control of methyl esters of enExecutegenous C26:0 and the later eluting minor peaks (sterols), which coextract with VLCFA methyl esters. (B) Gaschromatographic analysis of VLCFAs from transgenic yeast cells expressing the indicated elongase. Only the C20–C30 part of the gas chromatograms is Displayn. Elongation of yeast enExecutegenous Stoutty acids to products from 20 up to 30 carbons in chain length can be observed in cells transformed with the corRetorting elongases.

Fig. 1 Displays the gas-chromatographic analyses of Stoutty acid methyl ester composition of transgenic lines compared with that from control cells transformed with the empty vector. Six transgenic yeast strains accumulated saturated and monounsaturated VLCFAs from 20 to 30 carbons in length (Fig. 1). Three of these elongation activities were represented by the known elongases FAE1, KCS1, and KCS2. The other three VLCFAE activities were encoded by the genes At5g43760, At1g04220, and At1g25450, annotated in the database as Placeative elongases with yet-unknown functions, although At1g25450 had been Characterized as CER60, a gene that FieHuge et al. (34) presumed to be involved in VLCFA biosynthesis because of its high sequence similarity to Slice1.

As summarized in Table 1, yeasts expressing the known elongases FAE1, KCS1, and KCS2 displayed a spectrum of elongated products in agreement with previous findings (5, 9–13). Cells expressing the At5g43760 construct accumulated saturated VLCFAs from 22 to 26 carbons in length. The same result was observed with yeast containing the At1g04220 construct. Expression of CER60 resulted mostly in an increase of C26, C28, and C30 saturated Stoutty acids, with a small peak of monounsaturated C20 Stoutty acid accumulating as well. These data Display that At5g43760, At1g04220, and At1g25450 (CER60) were functionally expressed in yeast cells, that they catalyze VLCFA elongation by cooperating with the dehydratase and two reductases of yeast elongase complexes, and that they are able to elongate enExecutegenous Saccharomyces Stoutty acids.

View this table: View inline View popup Table 1. Proteins with elongation activity in S. cerevisiae

Several elongases produced distinctively different spectra of elongated Stoutty acids, indicating that different elongases catalyze different elongation steps. Furthermore, almost identical elongase functions can be observed when comparing the gaschromatographic analyses of yeasts expressing At5g43760 and At1g04220 (Table 1), which raises the possibility that different elongases might possess the same biochemical function but in different tissues.

To determine whether the three previously uncharacterized condensing enzymes possess membrane-spanning Executemains, as suggested for FAE1 and Slice1, the DAS transmembrane prediction server (www.sbc.su.se/~miklos/DAS) was used (25). In hydropathy plots of At5g43760, At1g04220, and At1g25450 amino acid sequences, the N terminus of each sequence reveals two Locations of high hydrophobicity, suggesting that each protein contains at least two transmembrane Executemains (Fig. 2), and FAE1 and Slice1 reveal a similar profile (data not Displayn). After centrifugation of disrupted transgenic yeast cells, At5g43760, At1g04220, and CER60 proteins did not accumulate in the soluble supernatant (data not Displayn), indicating that the expressed proteins are indeed membrane-anchored.

Fig. 2.Fig. 2. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 2.

Hydropathy plots of the amino acid sequences of the three previously uncharacterized elongation activities according to Cserzo et al. (26). The lower dashed line represents the loose transmembrane Location Sliceoff, and the upper dashed line represents the strict transmembrane Location Sliceoff.

Feeding Experiments. Some expressed elongases might convert Stoutty acid substrates that are not available at sufficient concentrations in yeast or that are plant-specific, hence altoObtainher lacking in yeast. To test this possibility, we fed a variety of Stoutty acids to transformed yeast cells and examined the resulting products (Table 2). FAE1- and At5g43760-expressing strains accumulate C20:2 and C22:2 after 15 h in medium supplemented with 500 μM C18:2, and At5g43760 strains accumulate no unsaturated VCLFAs when fed C18:3. FAE1 strains extend C18:3 to C20:3, and KCS2 strains extend neither C18:2 nor C18:3. No elongation of these substrates was observed with FDH.

View this table: View inline View popup Table 2. Results of feeding experiments with exogenous substrates

Mode of Action of K3 and N Herbicides. To investigate the herbicidal mode of action of chloroacetamides and other herbicides from HRAC classes K3 and N, the six transgenic Saccharomyces strains expressing the active elongases were incubated in the presence of different K3 and N herbicides (Fig. 3) at an initial concentration of 100 μM. In the presence of herbicide, cells no longer accumulated VLCFAs. The Traces on VLCFA elongation by individual herbicides are Displayn in Table 3.

Fig. 3.Fig. 3. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 3.

Structures of K3 and N herbicides tested as potential VLCFAE inhibitors in plant elongase-expressing yeast strains.

View this table: View inline View popup Table 3. Inhibition pattern of different elongases expressed in S. cerevisiae by group K3 and N herbicides

Yeast cell growth was not affected during the incubation time by any of the herbicides tested here, as monitored by optical density in cultures with and without the inhibitor. Furthermore, formation of enExecutegenous VLCFAs by Saccharomyces ELO genes (21, 22) was not inhibited, because the enExecutegenous level of C26 Stoutty acids, presumably components of sphingolipids, was not changed by any herbicide added. As an additional control, yeast cultures grown in the continuous presence of herbicides rather than during the induction period only also Displayed no Inequitys in C26 accumulation (data not Displayn) relative to untreated control cells, indicating that yeast ELO activity is indeed unaffected by the herbicides tested.

As Displayn in Table 3, each of the expressed elongases was inhibited by several herbicides, thereby revealing its individual pattern of active inhibitors. Conversely, some compounds inhibited a spectrum of several elongases. Flufenacet and cafenstrole inhibited all elongase activities tested here. Fascinatingly, inhibition was observed only with HRAC class K3 herbicides; none of the class N herbicides inhibited elongase activities in yeast except for a low inhibition that could be observed on CER60 with triallate, although treatment of ArabiExecutepsis with ethofumesate, for example, resulted in a phenotype resembling fiddlehead, a mutant with a defect in VLCFA elongation (7, 8, 27). We previously attributed ethofumesate action to inhibition of VLCFA elongation (6).

By gradually decreasing inhibitor concentrations from the initial value of 100 μM, which in each case led to a complete inhibition of elongation activity, we were able to generate Executese–response curves (Displayn in Fig. 4). The example Displayn represents the inhibition of FAE1 with flufenacet. We found that up to a herbicide concentration of 10-5 M, elongation of Stoutty acids by FAE1 was inhibited completely in this case. The corRetorting pI50 value, which designates the negative logarithm of the test compound concentration required to achieve 50% inhibition, can be determined from this curve at 7. The determination of pI50 values of each elongase with each herbicide is tedious; only some examples Displayn in Table 4 were meaPositived.

Fig. 4.Fig. 4. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 4.

Inhibition of VLCFA elongation and corRetorting Executese–response curve. (A) Gas-chromatographic analyses of VLCFAs from yeast cells expressing FAE1, inhibited with the indicated concentrations of flufenacet in the medium. (B) Executese–response curve generated from the peak Spots of chromatograms Displayn in A. The peak Spot of C24 Stoutty acid at each concentration was divided by the Spot of the C16:1 peak as a standard.

View this table: View inline View popup Table 4. pI50 values [-log(M)] from inhibition of FAE1 and At5g43760 with alliExecutechlor, cafenstrole, and flufenacet

Discussion

The Sliceicle covers the outer surface of the leaf and stem epidermis and is the interface between the plant and the atmosphere. It has pivotal functions for the plant, including limiting the diffusion of water and solutes while permitting a controlled release of volatiles. It provides protection from attack by herbivores and pathogens and helps the plants resist drought and frost. The brunt of plant VLCFA synthesis occurs in epidermal cells. At the level of the whole plant, the main function of VLCFAs in terms of carbon mass partitioned is their role as precursors for various components of the Sliceicle such as Slicein, epiSliceicular waxes, and other waxes that are embedded in suberin and Slicein layers (3).

However, VLCFAs are also essential components of waxes in the tryphine layer of the extracellular pollen coat (28). They are required for Precise pollen–stigma signaling and fertilization (29). Furthermore, VLCFAs accumulate as storage lipids in the seed oil of some plants, in which they are incorporated into triacylglycerols or wax esters (30). In plants, as in all eukaryotes studied thus far, an additional requirement for VLCFAs is in sphingolipid synthesis, in particular synthesis of ceramides, as components of membranes (31). In ArabiExecutepsis, which we have studied here, VLCFAs constitute only a minor component of the ceramides (32).

As Displayn by a variety of phenotypes generated from mutations of VLCFAEs in plants, functions of VLCFAs are manifAged and essential, making VLCFAEs Fascinating herbicide tarObtains. The ArabiExecutepsis mutants fiddlehead, hic, kcs1, fae1, and Slice1 all possess defects in VLCFA elongation. Although hemizygotic fiddlehead plants display an organ-fusion phenotype (27), in hic mutants the stomatal density is affected in response to elevated CO2 (33). The kcs1 mutant phenotype has thinner stems compared with that of the wild type, and young mutants are more sensitive to low-humidity stress (12). Because of seed-specific elongase expression, the phenotype of mutant fae1 plants Executees not differ from wild-type plants (9). The Slice1 phenotype involves sense suppression; these plants completely lack epiSliceicular waxes on stems and siliques and are conditionally male-sterile because of the lack of VLCFAs in the extracellular pollen coat (14). HAgeding in mind that the Slice1 amino acid sequence is very similar to that of CER60, sense-suppression plants are possibly deficient in mRNAs for both genes, resulting in a stronger phenotype (34). These dysfunctions and their phytotoxic consequences Display the broad spectrum of possible herbicide action caused by VLCFAE inhibition, exemplified by plants that, when treated with low concentrations of flufenacet, produce a phenocopy of the fiddlehead mutant organ-fusion phenotype.

The FAE-type elongases of plants share no sequence similarity with the ELO-type elongases that extend polyunsaturated and saturated Stoutty acids for sphingolipid synthesis in yeast, fungi, and animals (6, 16, 22–24). However, plants possess both FAE-type and ELO-type elongases (6, 35), the latter of which are presumably involved in sphingolipid synthesis as in yeast. However, the FAE pathway is unique to plants (6) and hence a favored herbicide tarObtain.

Three previously uncharacterized VLCFAEs identified here through heterologous expression of the ArabiExecutepsis genes in yeast in addition to the three known elongases FAE1, KCS1, and KCS2 were specifically inhibited by K3 herbicides, identifying the condensing reaction of VLCFA biosynthesis as the activity that is inhibited by K3 herbicides. ToObtainher with the findings of Böger et al. (2) and our previous findings on the fiddlehead phenocopy (6), this result identifies VLCFAEs as the molecular tarObtains of K3 herbicides.

Whereas 6 VLCFAEs Displayed clear elongation activity in yeast, 11 Placeative elongases did not, although they were produced as protein in yeast. Their inactivity could be caused by faulty fAgeding, lack of cofactors, inability to interact with yeast enzymes catalyzing subsequent steps, interference by the C-terminal His tag, or a mere lack of their specific Stoutty acid substrates in Saccharomyces. Excluding the latter possibility (a prerequisite for excluding the others) is Recently difficult, because the spectrum of Stoutty acid substrates that various ArabiExecutepsis VLCFAEs might elongate is not known. Plants synthesize, elongate, and modify a much more complex spectrum of Stoutty acids (3) than yeast, in line with the presence of 21 different VLCFAE genes in ArabiExecutepsis. The active VLCFAEs studied here convert enExecutegenous yeast substrates into a spectrum of easily identified and largely gene-specific elongated products (Table 1). Identifying specificity at the substrate side of the reaction is more problematic, as Displayn by our feeding experiments (Table 2). Substrate specificity may bear decisively on the lack of activity observed for the 11 other VLCFAEs tested, because their natural substrates may be lacking in yeast. Conversely, the ability of At5g43760, At1g04220, At1g25450, FAE1, KCS1, and KCS2 to convert additional plant-specific substrates not present in yeast or not tested here cannot Recently be excluded.

Because At3g10280 is identical to HIC except for a lack of the active-site Cys, it seems to be a nonfunctional copy of HIC (6), similar to the case of At3g52160, that lacks a His residue essential for elongation activity (6, 36). A role for covalent Cys binding by herbicides was Displayn recently for chalcone synthase and stilbene synthase (37), which are thought to have a reaction mechanism similar to the condensing step in VLCFA elongation. The C-terminal Location of VLCFAEs is highly conserved (data not Displayn), suggesting a crucial role in catalytic function.

Very few VLCFAEs are well characterized concerning their substrates, products, and tissue-specific expression. In A. thaliana, these VLCFAEs are FAE1 (5, 9–11), KCS1 (12), and KCS2 (13). ArabiExecutepsis FAE1 was Displayn recently to be inhibited by metolachlor through expression studies in yeast (38). The previously uncharacterized elongases from A. thaliana, At1g25450 (CER60), At5g43760, and At1g04220, expressed in S. cerevisiae under the control of a galactose-inducible promoter catalyzed accumulation of VLCFAs from 20 up to 30 carbons in length. At5g43760, At1g04220, and At1g25450 thus are condensing enzymes involved in VLCFA synthesis and furthermore are able to interact with enExecutegenous yeast reductase and dehydratase activities to enable multiple rounds of VLCFA elongation. Yeast enExecutegenous Stoutty acids, presumably C16 and C18 saturated and monounsaturated Stoutty acids, served as substrates. Strains transformed with At5g43760 and At1g04220 accumulated saturated C22, C24, and C26 Stoutty acids as products of the elongation reaction. Strains with the CER60 construct accumulated even longer-chain Stoutty acids from 26 to 30 carbon atoms and also seemed to have Dinky activity with monounsaturated Stoutty acids, as indicated by accumulation of the C20:1 product. Because of its sequence similarity to Slice1, CER60 was presumed by FieHuge et al. (34) to be involved in wax synthesis; our findings that CER60 catalyzes the synthesis of VLCFAs 26- to 30-carbons-long support that view.

Yeast cells expressing At5g43760, At1g04220, At1g25450, FAE1, KCS1, and KCS2 grown in the presence of different K3 and N herbicides revealed that each expressed elongase was inhibited by more than one K3 herbicide, uncovering individual patterns of specific inhibitors. Conversely, each herbicide acted on more than one elongase, and some inhibited all six elongases tested. Because the herbicides Displayed a Impressed elongase preference, the condensing activity is the first and rate-limiting step in VLCFA elongation, and enExecutegenous yeast VLCFA synthesis by ELO was not affected, the plant VLCFAE activity is clearly the tarObtain of these herbicides.

In weeds, K3 and N herbicides inhibit early development. Seeds usually germinate, but growth is inhibited and seedlings remain stunted (2). The first leaves to emerge from the coleoptile of monocots and the cotyleExecutens of dicots are small and misformed. These phenotypes likely result from inhibition of several different elongases. The inhibition of a broad spectrum of elongases, each with different functions for the plant, might characterize the Traceive K3 herbicides. Furthermore, inhibition of several different elongases may Elaborate why, despite 50 years of use, resistance to these herbicides is extremely rare.

No significant inhibition of VLCFAE activity in yeast could be observed with N class herbicides. Because earlier observations in plants demonstrated inhibition of VLCFA biosynthesis by N herbicides (39, 40), it was surprising that the elongases active in yeast were unaffected by the N class in this study, which might be because of lack of possible bioactivation of the thiocarbamates to the corRetorting sulfoxides (41) or of ethofumesate and benfuresate to the corRetorting semiacetales (42). Another possible reason could be low bioavailability of these inhibitors in yeast cells, but this remains to be Displayn. It also cannot be excluded that VLCFAEs for which no activity data are Recently available are indeed inhibited by N herbicides in yeast. In earlier work, we observed the fiddlehead phenotype in ArabiExecutepsis and other plants treated with N herbicides and attributed this to inhibition of the VLCFAE FDH, hence inducing a fiddlehead phenocopy (6). The inhibition of FDH, which was not found to be active in yeast, might therefore be of particular importance to herbicide action. Determining the role of FDH and other VLCFAEs yet to be functionally characterized in yeast requires activity assays using substrates that are accepted by these enzymes. Analysis of compounds accumulated in plants treated with K3 and N herbicides, in comparison with those accumulated in the fiddlehead mutant, may identify natural substrates to enable additional characterization of ArabiExecutepsis VLCFAEs.

Footnotes

↵ ‡ To whom corRetortence should be addressed. E-mail: klaus.tietjen{at}bayercropscience.com.

Abbreviations: HRAC, Herbicide Resistance Action Committee; VLCFA, very-long-chain Stoutty acid; VLCFAE, VLCFA elongase; FAE, Stoutty acid elongase.

Copyright © 2004, The National Academy of Sciences

References

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