DELLA-mediated cotyledon expansion breaks coat-imposed seed dormancy

Seed dormancy is a key adaptive trait in plants responsible for the soil seed bank. The long established hormone-balance theory describes the antagonistic roles of the dormancy promoting plant hormone abscisic acid (ABA), and the germination promoting hormone gibberellin (GA) in dormancy control. Light, temperature, and other dormancy-breaking signals function to modulate the synthesis and perception of these hormones in the seed. However, the way in which these hormones control dormancy in the imbibed seed remains unknown. Here, we show that the DELLA protein regulators of the GA response are required for dormancy and describe a model through which hormone signal integration and dormancy regulation is achieved. We demonstrate that cotyledon expansion precedes radicle emergence during Arabidopsis seed germination and that a striking correlation exists between final seedling cotyledon size and seed dormancy in the DELLA mutants. Furthermore, twelve previously characterized seed-dormancy mutants are also defective in the control of cotyledon size in a manner consistent with their effect on germination potential. We propose that DELLA-mediated, light-, temperature-, and hormone-responsive cotyledon expansion prior to radicle emergence overcomes dormancy imposed by the seed coat and underlies seed-dormancy control in Arabidopsis.

The mitochondrial electron transfer flavoprotein complex is essential for survival of Arabidopsis in extended darkness

In mammals, the electron transfer flavoprotein (ETF) is a heterodimeric protein composed of two subunits, alpha and beta, that is responsible for the oxidation of at least nine mitochondrial matrix flavoprotein dehydrogenases. Electrons accepted by ETF are further transferred to the main respiratory chain via the ETF ubiquinone oxide reductase (ETFQO). Sequence analysis of the unique Arabidopsis homologues of two subunits of ETF revealed their high similarity to both subunits of the mammalian ETF. Yeast two-hybrid experiments showed that the Arabidopsis ETFalpha and ETFbeta can form a heteromeric protein. Isolation and characterization of two independent T-DNA insertional Arabidopsis mutants of the ETFbeta gene revealed accelerated senescence and early death compared to wild-type during extended darkness. Furthermore in contrast to wild-type, the etfb mutants demonstrated a significant accumulation of several amino acids, isovaleryl CoA and phytanoyl CoA during dark-induced carbohydrate deprivation. These phenotypic characteristics of etfb mutants are broadly similar to those that we observed previously in Arabidopsis etfqo mutants, suggesting functional association between ETF and ETFQO in Arabidopsis, and confirming the essential roles of the ETF/ETFQO electron transfer complex in the catabolism of leucine and involvement in the chlorophyll degradation pathway activated during dark-induced carbohydrate deprivation.

Arabidopsis ABA INSENSITIVE4 regulates lipid mobilization in the embryo and reveals repression of seed germination by the endosperm

Regulation of seed germination requires coordinate action by the embryo and surrounding endosperm. We used Arabidopsis thaliana to establish the relative roles of embryo and endosperm in the control of seed germination and seedling establishment. We previously showed that endospermic oil reserves are used postgerminatively via gluconeogenesis to fuel seedling establishment and that lipid breakdown is repressed by abscisic acid (ABA) in embryo but not endosperm tissues. Here, we use RNA amplification to describe the transcriptome of the endosperm and compare the hormone responses of endosperm and embryo tissues. We show that the endosperm responds to both ABA and gibberellin but that ABA in particular regulates nuclear but not plastid-encoded photosynthetic gene expression in the embryo. We also show that ABA INSENSITIVE4 (ABI4) expression is confined to the embryo, accounts for the major differences in embryo response to ABA, and defines a role for ABI4 as a repressor of lipid breakdown. Furthermore, ABI5 expression in the endosperm defines a second region of altered ABA signaling in the micropylar endosperm cap. Finally, embryo and endosperm ABA signaling mutants demonstrate the spatial specificity of ABA action in seed germination. We conclude that the single cell endosperm layer plays an active role in the regulation of seed germination in Arabidopsis.

Arabidopsis ABA INSENSITIVE4 regulates lipid mobilization in the embryo and reveals repression of seed germination by the endosperm

Regulation of seed germination requires coordinate action by the embryo and surrounding endosperm. We used Arabidopsis thaliana to establish the relative roles of embryo and endosperm in the control of seed germination and seedling establishment. We previously showed that endospermic oil reserves are used postgerminatively via gluconeogenesis to fuel seedling establishment and that lipid breakdown is repressed by abscisic acid (ABA) in embryo but not endosperm tissues. Here, we use RNA amplification to describe the transcriptome of the endosperm and compare the hormone responses of endosperm and embryo tissues. We show that the endosperm responds to both ABA and gibberellin but that ABA in particular regulates nuclear but not plastid-encoded photosynthetic gene expression in the embryo. We also show that ABA INSENSITIVE4 (ABI4) expression is confined to the embryo, accounts for the major differences in embryo response to ABA, and defines a role for ABI4 as a repressor of lipid breakdown. Furthermore, ABI5 expression in the endosperm defines a second region of altered ABA signaling in the micropylar endosperm cap. Finally, embryo and endosperm ABA signaling mutants demonstrate the spatial specificity of ABA action in seed germination. We conclude that the single cell endosperm layer plays an active role in the regulation of seed germination in Arabidopsis.

Cold and light control seed germination through the bHLH transcription factor SPATULA

BACKGROUND

Plants integrate signals from the environment and use these to modify the timing of development according to seasonal cues. Seed germination is a key example of this phenomenon and in Arabidopsis is promoted by the synergistic interaction of light and low temperatures in dormant seeds. This signaling pathway is known to converge on the regulation of the gibberellin (GA) biosynthetic genes GA3 oxidase (GA3ox), whose expression is transcriptionally induced by light and cold in imbibed seeds. However, the molecular basis of this response has until now been unknown.

RESULTS

Here we show that the bHLH transcription factor SPATULA is a light-stable repressor of seed germination and mediates the germination response to temperature. Furthermore, SPT is required in dormant seeds for maintaining the repression of GA3ox transcription. We also show that the related protein PIL5 represses seed germination and GA3ox expression in the dark.

CONCLUSIONS

We conclude that SPT and PIL5 form part of a regulatory network coupling seed germination and GA3ox expression to light and temperature signaling in the seed.

Delayed embryo development in the ARABIDOPSIS TREHALOSE-6-PHOSPHATE SYNTHASE 1 mutant is associated with altered cell wall structure, decreased cell division and starch accumulation

The tps1 mutant, which is disrupted in the TREHALOSE-6-PHOSPHATE SYNTHASE 1 gene, has been previously characterized as a recessive embryo lethal. tps1 embryos do not develop past late torpedo or early cotyledon stage. We report here that at the ultrastructural, biochemical, and transcriptional levels tps1 exhibits many features typically associated with the maturation phase. The appearance of storage reserve transcripts and organelles follows the same temporal pattern in tps1 and wild-type (WT) embryos in the same silique as does accumulation of storage lipid and protein. The mutant plastids accumulate large starch granules that persist until the end of seed development, in contrast with WT plastids where starch accumulation is transient. The transcriptome of tps1 embryos shows a coordinate downregulation of genes involved in starch and sucrose degradation. Interestingly, genes involved in lipid mobilization and gluconeogenesis are induced in tps1 embryos. The cell walls of tps1 embryos show a remarkable degree of thickening at the ultrastructural level and immunodetection of cell wall components shows that altered deposition of pectins accounts for this altered morphology. Consistent with this at the transcriptome level, genes involved in sugar nucleotide and pectin metabolism are altered in the mutant. The frequency of cell division in tps1 embryos is half that of the wild type at the heart and torpedo stages. These results suggest that TPS1 may play a major role in coordinating cell wall biosynthesis and cell division with cellular metabolism during embryo development.

A spatiotemporal analysis of enzymatic activities associated with carbon metabolism in wild-type and mutant embryos of Arabidopsis using in situ histochemistry

Arabidopsis as a molecular genetic model offers many advantages for the study of seed development, but these do not extend to biochemical and enzymatic studies, which are often compromised by the limited amount of material available from the small developing embryos. A set of assays based on the coupling of an enzymatic reaction to the reduction of NAD, NADP or FAD, and subsequent reduction and precipitation of a tetrazolium salt, have been adapted to investigate 18 enzyme activities associated with carbon metabolism in developing Arabidopsis embryos. The use of organelle-specific marker enzymes demonstrates the utility of the method for detection of activities in mitochondria, plastids and peroxisomes as well as the cytosol. The temporal staining patterns obtained allow classification of the activities into three main categories based on whether they peak in the early, intermediate or late stages of maturation. An interesting switch from ATP to pyrophosphate consuming pathways occurs at the onset of the maturation phase, which involves key steps in primary carbon metabolism such as phosphofructokinase. This spatiotemporal characterization of carbon metabolism has also been applied to various mutants disrupted in embryo development including gnom (gn), acetyl-CoA carboxylase1 (acc1), schlepperless (slp), and wrinkled1 (wri1). The data obtained demonstrate that the extent to which carbon metabolism is affected in mutants is not necessarily correlated to the severity of the mutation considered. Through the advanced characterization of trehalose-6-P synthase1 (tps1) embryos, this approach finally provides new insight into the regulatory role played by trehalose metabolism in embryo development.

An Arabidopsis mutant impaired in coenzyme A biosynthesis is sugar dependent for seedling establishment

Once the plant coenzyme A (CoA) biosynthetic pathway has been elucidated by comparative genomics, it is feasible to analyze the physiological relevance of CoA biosynthesis in plant life. To this end, we have identified and characterized Arabidopsis (Arabidopsis thaliana) T-DNA knockout mutants of two CoA biosynthetic genes, HAL3A and HAL3B. The HAL3A gene encodes a 4'-phosphopantothenoyl-cysteine decarboxilase that generates 4'-phosphopantetheine. A second gene, HAL3B, whose gene product is 86% identical to that of HAL3A, is present in the Arabidopsis genome. HAL3A appears to have a predominant role over HAL3B according to their respective mRNA expression levels. The hal3a-1, hal3a-2, and hal3b mutants were viable and showed a similar growth rate as that in wild-type plants; in contrast, a hal3a-1 hal3b double mutant was embryo lethal. Unexpectedly, seedlings that were null for HAL3A and heterozygous for HAL3B (aaBb genotype) displayed a sucrose (Suc)-dependent phenotype for seedling establishment, which is in common with mutants defective in beta-oxidation. This phenotype was genetically complemented in aaBB siblings of the progeny and chemically complemented by pantethine. In contrast, seedling establishment of Aabb plants was not Suc dependent, proving a predominant role of HAL3A over HAL3B at this stage. Total fatty acid and acyl-CoA measurements of 5-d-old aaBb seedlings in medium lacking Suc revealed stalled storage lipid catabolism and impaired CoA biosynthesis; in particular, acetyl-CoA levels were reduced by approximately 80%. Taken together, these results provide in vivo evidence for the function of HAL3A and HAL3B, and they point out the critical role of CoA biosynthesis during early postgerminative growth.

The Arabidopsis thaliana multifunctional protein gene (MFP2) of peroxisomal beta-oxidation is essential for seedling establishment

The multifunctional protein (MFP) of peroxisomal beta-oxidation catalyses four separate reactions, two of which (2-trans enoyl-CoA hydratase and L-3-hydroxyacyl-CoA dehydrogenase) are core activities required for the catabolism of all fatty acids. We have isolated and characterized five Arabidopsis thaliana mutants in the MFP2 gene that is expressed predominantly in germinating seeds. Seedlings of mfp2 require an exogenous supply of sucrose for seedling establishment to occur. Analysis of mfp2-1 seedlings revealed that seed storage lipid was catabolized more slowly, long-chain acyl-CoA substrates accumulated and there was an increase in peroxisome size. Despite a reduction in the rate of beta-oxidation, mfp2 seedlings are not resistant to the herbicide 2,4-dichlorophenoxybutyric acid, which is catabolized to the auxin 2,4-dichlorophenoxyacetic acid by beta-oxidation. Acyl-CoA feeding experiments show that the MFP2 2-trans enoyl-CoA hydratase only exhibits activity against long chain (C18:0) substrates, whereas the MFP2 L-3-hydroxyacyl-CoA dehydrogenase is active on C6:0, C12:0 and C18:0 substrates. A mutation in the abnormal inflorescence meristem gene AIM1, the only homologue of MFP2, results in an abnormal inflorescence meristem phenotype in mature plants (Richmond and Bleecker, Plant Cell 11, 1999, 1911) demonstrating that the role of these genes is very different. The mfp2-1 aim1double mutant aborted during the early stages of embryo development showing that these two proteins share a common function that is essential for this key stage in the life cycle.

Chewing the fat: beta-oxidation in signalling and development

Peroxisomal beta-oxidation is involved not only in fatty acid catabolism and lipid housekeeping but also in metabolism of hormones and amino acids in plants. Recent research in model species has led to new insights into the roles of this pathway in signalling and development, in particular regarding the involvement of beta-oxidation in jasmonic acid biosynthesis. Analysis of associated processes, such as the glyoxylate cycle and redox metabolism has also highlighted the importance of integration of beta-oxidation with cytosolic and mitochondrial metabolism. Mutations that disrupt beta-oxidation can have extremely pleiotropic effects, indicating important and varied roles for this pathway throughout the plant life cycle and making this an exciting topic for future research.

Sucrose rescues seedling establishment but not germination of Arabidopsis mutants disrupted in peroxisomal fatty acid catabolism

The Arabidopsis acyl-CoA oxidase (ACX) family comprises isozymes with distinct fatty acid chain-length specificities that together catalyse the first step of peroxisomal fatty acid beta-oxidation. We have isolated and characterized T-DNA insertion mutants in the medium to long-chain (ACX1) and long-chain (ACX2) acyl-CoA oxidases, and show that the corresponding endogenous activities are decreased in the mutants. Lipid catabolism during germination and early post-germinative growth was unaltered in the acx1-1 mutant, but slightly delayed in the acx2-1 mutant, with 3-day-old acx2-1 seedlings accumulating long-chain acyl-CoAs. In acx1-1 and acx2-1, seedling growth and establishment in the absence of an exogenous supply of sucrose was unaffected. Seedlings of the double mutant acx1-1 acx2-1 were unable to catabolize seed storage lipid, and accumulated long-chain acyl-CoAs. The acx1-1 acx2-1 seedlings were also unable to establish photosynthetic competency in the absence of an exogenous carbon supply, a phenotype that is shared with a number of other Arabidopsis mutants disrupted in storage lipid breakdown. Germination frequency of the double mutant was significantly reduced compared with wild-type seeds. This was unaffected by the addition of exogenous sucrose, but was improved by dormancy-breaking treatments such as cold stratification and after-ripening. We show that the acx1-1, acx2-1 and acx1-2 acx2-1 double mutants and the ketoacyl-CoA thiolase-2 (kat2) mutant exhibit a sucrose-independent germination phenotype comparable with that reported for comatose (cts-2), a mutant in a peroxisomal ABC transporter which exhibits enhanced dormancy. This demonstrates an additional role beyond that of carbon provision for the beta-oxidation pathway during germination or in dormant seeds.

The critical role of Arabidopsis electron-transfer flavoprotein:ubiquinone oxidoreductase during dark-induced starvation

In mammals, electron-transfer flavoprotein:ubiquinone oxidoreductase (ETFQO) and electron-transfer flavoprotein (ETF) are functionally associated, and ETF accepts electrons from at least nine mitochondrial matrix flavoprotein dehydrogenases and transfers them to ubiquinone in the inner mitochondrial membrane. In addition, the mammalian ETF/ETFQO system plays a key role in beta-oxidation of fatty acids and catabolism of amino acids and choline. By contrast, nothing is known of the function of ETF and ETFQO in plants. Sequence analysis of the unique Arabidopsis thaliana homologue of ETFQO revealed high similarity to the mammalian ETFQO protein. Moreover, green fluorescent protein cellular localization experiments suggested a mitochondrial location for this protein. RNA gel blot analysis revealed that Arabidopsis ETFQO transcripts accumulated in long-term dark-treated leaves. Analysis of three independent insertional mutants of Arabidopsis ETFQO revealed a dramatic reduction in their ability to withstand extended darkness, resulting in senescence and death within 10 d after transfer, whereas wild-type plants remained viable for at least 15 d. Metabolite profiling of dark-treated leaves of the wild type and mutants revealed a dramatic decline in sugar levels. In contrast with the wild type, the mutants demonstrated a significant accumulation of several amino acids, an intermediate of Leu catabolism, and, strikingly, high-level accumulation of phytanoyl-CoA. These data demonstrate the involvement of a mitochondrial protein, ETFQO, in the catabolism of Leu and potentially of other amino acids in higher plants and also imply a novel role for this protein in the chlorophyll degradation pathway activated during dark-induced senescence and sugar starvation.

Storage reserve mobilization in germinating oilseeds: Arabidopsis as a model system

Germinating oilseeds break down fatty acids through peroxisomal beta-oxidation and convert the carbon into soluble carbohydrates through the glyoxylate cycle and gluconeogenesis. This interconversion is unique among higher eukaryotes. Using a combination of forward and reverse genetic screens, we have isolated mutants that compromise fatty acid breakdown at each step. These mutants exhibit characteristic, yet nonidentical, seedling establishment phenotypes that can be rescued by the provision of an alternative carbon source. In addition, we have recently shown that Arabidopsis seed's lipid breakdown occurs in two distinct tissues, the embryo and endosperm. The utilization of endospermic lipid reserves requires gluconeogenesis and transport of the resulting sugars to the germinating embryo. We discuss the potential of the Arabidopsis endosperm tissue as a simplified model system for the study of germination and lipid breakdown in germinating oilseeds.

The role of trehalose-6-phosphate synthase in Arabidopsis embryo development

We previously showed that trehalose-6-phosphate synthase 1 (TPS1), which catalyses the first step in trehalose synthesis, is essential for embryo maturation in Arabidopsis. The tps1 mutant embryos develop more slowly than wild type. Patterning in the tps1 embryos appears normal but they do not progress past the torpedo stage to cotyledon stage, which is when storage reserves start to accumulate in the expanding cotyledons. Our initial data led to the hypothesis that trehalose metabolism plays a key role in regulating storage reserve accumulation by allowing the embryo to respond to the dramatic increase in sucrose levels that occurs at the torpedo stage of embryo development. More recent data demonstrate that while the tps1 mutant is blocked in the developmental progression of embryos from torpedo to cotyledon stage the expression of genes involved in the accumulation of storage reserves proceeds in a similar fashion to wild type. Thus it appears that induction of metabolic processes required for accumulation of storage reserves in tps1 occurs independently of the developmental stage and instead follows a temporal programme similar to wild-type seeds in the same silique.

Fatty acid desaturases from the microalga Thalassiosira pseudonana

Analysis of a draft nuclear genome sequence of the diatom Thalassiosira pseudonana revealed the presence of 11 open reading frames showing significant similarity to functionally characterized fatty acid front-end desaturases. The corresponding genes occupy discrete chromosomal locations as determined by comparison with the recently published genome sequence. Phylogenetic analysis showed that two of the T. pseudonana desaturase (Tpdes) sequences grouped with proteobacterial desaturases that lack a fused cytochrome b5 domain. Among the nine remaining gene sequences, temporal expression analysis revealed that seven were expressed in T. pseudonana cells. One of these, TpdesN, was previously characterized as encoding a Delta11-desaturase active on palmitic acid. From the six remaining putative desaturase genes, we report here that three, TpdesI, TpdesO and TpdesK, respectively encode Delta6-, Delta5- and Delta4-desaturases involved in production of the health beneficial polyunsaturated fatty acid DHA (docosahexaenoic acid). Furthermore, we show that one of the remaining genes, TpdesB, encodes a Delta8-sphingolipid desaturase with strong preference for dihydroxylated substrates.

Non-coordinate expression of peroxisome biogenesis, beta-oxidation and glyoxylate cycle genes in mature Arabidopsis plants

The expression of three genes that encode proteins involved in peroxisome biogenesis, beta-oxidation and the glyoxylate cycle was studied in Arabidopsis plants by fusing their promoter regions to the reporter gene luciferase. Malate synthase showed an extremely restricted pattern of expression, being detected only in young seedlings and the root tips of older plants. PEX1 and 3-ketoacyl thiolase (PED1) were expressed in roots, mature leaves, stems and flowers. However, only thiolase was up-regulated by starvation. Immunoblotting confirmed that neither malate synthase nor the other unique glyoxylate cycle enzyme isocitrate lyase are expressed in senescent leaves. These results indicate that, in contrast to cucumber, pumpkin and barley, the glyoxylate cycle does not play a role in the recycling of carbon from the turnover of membrane lipids during senescence and starvation in Arabidopsis.

Reserve mobilization in the Arabidopsis endosperm fuels hypocotyl elongation in the dark, is independent of abscisic acid, and requires PHOSPHOENOLPYRUVATE CARBOXYKINASE1

Arabidopsis thaliana is used as a model system to study triacylglycerol (TAG) accumulation and seed germination in oilseeds. Here, we consider the partitioning of these lipid reserves between embryo and endosperm tissues in the mature seed. The Arabidopsis endosperm accumulates significant quantities of storage lipid, and this is effectively catabolized upon germination. This lipid differs in composition from that in the embryo and has a specific function during germination. Removing the endosperm from the wild-type seeds resulted in a reduction in hypocotyl elongation in the dark, demonstrating a role for endospermic TAG reserves in fueling skotomorphogenesis. Seedlings of two allelic gluconeogenically compromised phosphoenolpyruvate carboxykinase1 (pck1) mutants show a reduction in hypocotyl length in the dark compared with the wild type, but this is not further reduced by removing the endosperm. The short hypocotyl phenotypes were completely reversed by the provision of an exogenous supply of sucrose. The PCK1 gene is expressed in both embryo and endosperm, and the induction of PCK1:beta-glucuronidase at radicle emergence occurs in a robust, wave-like manner around the embryo suggestive of the action of a diffusing signal. Strikingly, the induction of PCK1 promoter reporter constructs and measurements of lipid breakdown demonstrate that whereas lipid mobilization in the embryo is inhibited by abscisic acid (ABA), no effect is seen in the endosperm. This insensitivity of endosperm tissues is not specific to lipid breakdown because hydrolysis of the seed coat cell walls also proceeded in the presence of concentrations of ABA that effectively inhibit radicle emergence. Both processes still required gibberellins, however. These results suggest a model whereby the breakdown of seed carbon reserves is regulated in a tissue-specific manner and shed new light on phytohormonal regulation of the germination process.

An Arabidopsis mutant disrupted in valine catabolism is also compromised in peroxisomal fatty acid beta-oxidation

Characterisation of the Arabidopsis dbr5 mutant, which was isolated on the basis of 2,4-dichlorophenoxybutyric acid (2,4-DB) resistance, revealed that it is disrupted in the CHY1 gene. CHY1 encodes a peroxisomal protein that is 43% identical to the mammalian beta-hydroxyisobutryl-CoA hydrolase of valine catabolism. We show that 2,4-DB resistance and the associated sucrose dependent seedling growth are due to a large activity decrease of 3-ketoacyl-CoA thiolase, which is involved in peroxisomal fatty acid beta-oxidation. (14)C-feeding studies demonstrate that dbr5 and chy1 seedlings are reduced in valine catabolism. These data support the hypothesis that CHY1 plays a key role in peroxisomal valine catabolism and that disruption of this enzyme results in accumulation of a toxic intermediate, methacrylyl-CoA, that inhibits 3-ketoacyl-CoA thiolase activity and thus blocks peroxisomal beta-oxidation. We also show that CHY1 is repressed in seedlings grown on sugars, which suggests that branched chain amino acid catabolism is transcriptionally regulated by nutritional status.

Identification of a fatty acid Δ11-desaturase from the microalgaThalassiosira pseudonana1

A set of genomic DNA sequences putatively encoding front-end desaturases were identified by in silico analysis of the draft genome of the marine microalga Thalassiosira pseudonana. Among these candidate genes, an open reading frame named TpdesN was found to be full-length, intronless, and constitutively expressed during cell cultivation. The predicted amino acid sequence of the corresponding protein, TpDESN, exhibited typical features of desaturases involved in the production of polyunsaturated fatty acids (PUFAs) in algae, i.e. a cytochrome b5-like domain at the N-terminus and three conserved histidine-rich motifs in the desaturase domain. Expression of TpDESN in Saccharomyces cerevisiae revealed that this enzyme was not involved in PUFA synthesis, but specifically desaturated palmitic acid 16:0 to 16:1Delta11. To our knowledge, until this report, Delta11-desaturase activity had only been detected in insect cells.

Acetate non-utilizing mutants of Arabidopsis: evidence that organic acids influence carbohydrate perception in germinating seedlings

A phenotypic screen was employed to isolate Arabidopsis plants that are deficient in their ability to utilize or sense acetate. The screening strategy, based on resistance to the toxic acetate analogue monofluoroacetic acid, was adapted from one that has been used successfully to identify important metabolic and regulatory genes involved in acetate metabolism in fungi. Following conventions established from the fungal work, the mutants were called acn mutants for acetate non-utilization. Three highly resistant plant lines were the focus of genetic and physiological studies. Mutant acn1 appears to be a true acetate non-utilizing mutant, as it displays increased sensitivity to exogenous acetate. The progeny of the original acn2 mutant did not germinate, even in the presence of sucrose as an exogenous carbon source. The germination of seeds from the F3 generation depended on the sucrose concentration in the medium. Only a small proportion of seeds germinated in the absence of exogenous sucrose and in the presence of 100 mM sucrose, but up to 70% of seeds germinated on 20 mM sucrose. Mutant acn3 exhibited sensitivity to exogenous sucrose, showing significant chlorosis on medium containing 20 mM sucrose, but no chlorosis when grown in the absence of exogenous sucrose. This phenotype was alleviated if acetate was provided. The acn mutants demonstrate that disrupting organic acid utilization can have profound affects on carbohydrate metabolism.

Identification of a very long chain polyunsaturated fatty acid Δ4-desaturase from the microalga Pavlova lutheri 1

Pavlova lutheri, a marine microalga, is rich in the very long chain polyunsaturated fatty acids (VLCPUFAs) eicosapentaenoic (20:5n-3) and docosahexaenoic (22:6n-3) acids. Using an expressed sequence tag approach, we isolated a cDNA designated Pldes1, and encoding an amino acid sequence showing high similarity with polyunsaturated fatty acid front-end desaturases. Heterologous expression in yeast demonstrated that PlDES1 desaturated 22:5n-3 and 22:4n-6 into 22:6n-3 and 22:5n-6 respectively, and was equally active on both substrates. Thus, PlDES1 is a novel VLCPUFA Delta4-desaturase. Pldes1 expression is four-fold higher during the mid-exponential phase of growth compared to late exponential and stationary phases.

Arabidopsis mutants in short- and medium-chain acyl-CoA oxidase activities accumulate acyl-CoAs and reveal that fatty acid beta-oxidation is essential for embryo development

The short-chain acyl-CoA oxidase (ACX4) is one of a family of ACX genes that together catalyze the first step of peroxisomal fatty acid beta-oxidation during early, postgerminative growth in oilseed species. Here we have isolated and characterized an Arabidopsis thaliana mutant containing a T-DNA insert in ACX4. In acx4 seedlings, short-chain acyl-CoA oxidase activity was reduced by greater than 98%, whereas medium-chain activity was unchanged from wild type levels. Despite the almost complete loss of short-chain activity, lipid catabolism and seedling growth and establishment were unaltered in the acx4 mutant. However, the acx4 seedlings accumulated high levels (31 mol %) of short-chain acyl-CoAs and showed resistance to 2,4-dichlorophenoxybutyric acid, which is converted to the herbicide and auxin analogue 2,4-dichlorophenoxyacetic acid by beta-oxidation. A mutant in medium-chain length acyl-CoA activity (acx3) (1) shows a similar phenotype to acx4, and we show here that acx3 seedlings accumulate medium-chain length acyl-CoAs (16.4 mol %). The acx3 and acx4 mutants were crossed together, and remarkably, the acx3acx4 double mutants aborted during the first phase of embryo development. We propose that acx3acx4 double mutants are nonviable because they have a complete block in short-chain acyl-CoA oxidase activity. This is the first demonstration of the effects of eliminating (short-chain) beta-oxidation capacity in plants and shows that a functional beta-oxidation cycle is essential in the early stages of embryo development.

Trehalose metabolism: a regulatory role for trehalose-6-phosphate?

Trehalose is a disaccharide that was initially thought to be rare in plants but now appears to be ubiquitous. A recent study has established that the initial step in trehalose synthesis is essential in Arabidopsis. Evidence is emerging that the precursor of trehalose (trehalose-6-phosphate) is an important regulatory molecule. In yeast, trehalose-6-phosphate regulates sugar influx into glycolysis. In plants, trehalose-6-phosphate also appears to regulate sugar metabolism, but the underlying mechanism is unresolved and may be substantially different from that in yeast.