[Show abstract][Hide abstract] ABSTRACT: Microarray technology was used to identify the genes associated with disease defence responses in the model legume Medicago truncatula. Transcript profiles from M. truncatula cv. Jemalong genotype A17 leaves inoculated with Colletotrichum trifolii and Erysiphe pisi and roots infected with Phytophthora medicaginis were compared to identify the genes expressed in response to all three pathogens and genes unique to an interaction. The A17 genotype is resistant to C. trifolii and E. pisi, exhibiting a hypersensitive response after inoculation, and is moderately susceptible to P. medicaginis. Among the most strongly up-regulated genes in all three interactions were those encoding a hevein-like protein, thaumatin-like protein (TLP) and members of the pathogenesis response (PR)10 family. Transcripts of genes for enzymes in the phenylpropanoid pathway leading to the production of isoflavonoid phytoalexins increased dramatically in response to inoculation with the foliar pathogens. In P. medicaginis-inoculated roots, transcripts of genes in the phenylpropanoid pathway peaked at 5 days post-inoculation, when symptoms became visible. Transcript accumulation of three PR10 family members, a TLP and chalcone synthase (CHS) was assessed in M. truncatula genotype R108 plants. The R108 plants are resistant to C. trifolii and moderately susceptible to E. pisi and P. medicaginis. Transcript accumulation paralleled the stages of pathogen development. To evaluate the role of a TLP, a PR10 family member and CHS in disease resistance, transgenic R108 plants containing interfering RNA (RNAi) constructs were produced. Reduced expression of PR10 and TLP had no effect on the disease phenotype, whereas reduced expression of CHS resulted in increased susceptibility to necrotrophic pathogens.
[Show abstract][Hide abstract] ABSTRACT: Arabidopsis UDP-sugar pyrophosphorylase (AtUSP) is a broad substrate enzyme that synthesizes nucleotide sugars. The products of the AtUSP reaction can act as precursors for the synthesis of glycolipids, glycoproteins, and cell wall components including pectin and hemicellulose. AtUSP has no close homologs in Arabidopsis and its biological function has not been clearly defined. We identified two T-DNA insertional mutant lines for AtUSP, usp-1 and usp-2. No homozygous individuals were identified and progeny from plants heterozygous for usp-1 or usp-2 showed a 1:1 segregation ratio under selection. Despite decreased levels of both AtUSP transcript and USP activity (UDP-GlcA-->GlcA-1-P), heterozygous plants were indistinguishable from wild type at all stages of development. Reciprocal test crosses indicated the source of the segregation distortion was lack of transmission through the male gametophyte. Analysis of pollen tetrads from usp-1 in the quartet background revealed a 2:2 ratio of normal:collapsed pollen grains. The collapsed pollen grains were not viable as determined by Alexander's viability and DAPI staining, and pollen germination tests. The pollen phenotype of usp-1 was complemented by transformation of usp-1 with the AtUSP cDNA sequence. Surface and ultrastructural analyses of pollen from wild-type and usp mutants demonstrated that the mutation had no apparent effect on the outer wall (exine) but prevented the synthesis of the pectocellulosic inner wall (intine). Evidence presented here shows that AtUSP has a critical role in pollen development.
[Show abstract][Hide abstract] ABSTRACT: At5g52560, a homolog of pea (Pisum sativum) UDP-sugar pyrophosphorylase (PsUSP) was functionally annotated by expression in Escherichia coli and subsequent characterization of substrate specificity and kinetic properties. Arabidopsis contains a single USP gene (AtUSP) and evaluation of gene databases suggests that USP is unique to plants. The 69 kDa AtUSP gene product exhibited high activity with Glc-1-P, GlcA-1-P and Gal-1-P, but low activity with GlcNAc-1-P, Fuc-1-P, Man-1-P, inositol-1-P or Glc-6-P. AtUSP was activated by magnesium and preferred UTP as co-substrate. Apparent K(m) values for GlcA-1-P, Glc-1-P and UTP were 0.13 mM, 0.42 mM and 0.14 mM, respectively. In the reverse direction (pyrophosphorolysis), the apparent K(m) values for UDP-GlcA, UDP-Glc and pyrophosphate were 0.56 mM, 0.72 mM and 0.15 mM, respectively. USP enzyme activity (UDP-GlcA --> GlcA-1-P) was detected in Arabidopsis tissues with highest activity found in the inflorescence. As determined by semi-quantitative RT-PCR, AtUSP transcript is widely expressed with high levels detected in the inflorescence. To evaluate tissue-specific expression of AtUSP, histochemical GUS staining of plants transformed with AtUSPprom:GUS constructs was performed. In 7-day-old seedlings, GUS staining was detected in cotyledons, trichomes and vascular tissues of the primary root. In the inflorescence of older plants, high levels of GUS staining were detected in cauline leaves, the epidermis of the stem and in pollen. In silico analysis of AtUSP expression in developing pollen indicates that transcript levels increase as development proceeds from the uninucleate to the tricellular stage. The results suggest that AtUSP plays an important role in pollen development in Arabidopsis.
Plant Physiology and Biochemistry 04/2006; 44(4):171-80. · 2.78 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: During soybean [Glycine max (L.) Merrill] embryo development, cell wall polysaccharides (CWPs) derived from UDP-glucuronic acid (UDP-GlcA) (uronic acids, arabinose, xylose) exhibited a linear increase during the period of 25-45 days after flowering (daf). At embryo maturity, CWPs derived from UDP-GlcA accounted for 39% of total CWPs. To ascertain the relative importance of the nucleotide sugar oxidation (NSO) and the myo-inositol oxidation (MIO) pathways to UDP-GlcA biosynthesis, UDP-glucose (UDP-Glc) dehydrogenase (UDP-Glc DH, EC 188.8.131.52) and UDP-glucuronic acid pyrophosphorylase (UDP-GlcA PPase, EC 184.108.40.206) activities, respectively, were measured in desalted extracts of developing embryos. UDP-Glc DH and UDP-GlcA PPase activities, expressed on a per seed basis, increased 3.5- and 3.9-fold, respectively, during the period of 25-45 daf. However, UDP-GlcA PPase activity was 35-50-fold greater than UDP-Glc DH activity. The soybean UDP-sugar pyrophosphorylase gene (USP1), a homolog of pea USP, and a candidate gene for UDP-GlcA PPase, was cloned and the recombinant enzyme characterized. Recombinant soybean USP1 (71 kDa) exhibited high activity with glucuronic acid 1-phosphate (GlcA-1-P), glucose 1-phosphate (Glc-1-P) and galactose 1-phosphate (Gal-1-P), but low activity with mannose 1-phosphate (Man-1-P), N-acetylglucosamine 1-phosphate and Glc-6-P. Determination of kinetic constants indicated that USP1 has a higher affinity for GlcA-1-P (Km= 0.14 ± 0.02 mM) than for Glc-1-P (Km= 0.23 ± 0.02 mM). Semiquantitative RT-PCR was used to measure transcript levels of the UDP-glucose DH (UGD) and USP gene families in developing soybean embryos. Transcript levels, normalized to the 18S rRNA controls, were greater for UGD than USP throughout embryo development. The possibility that USP serves as UDP-GlcA PPase, the terminal enzyme of the MIO pathway, is discussed.
Physiologia Plantarum 01/2006; · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Long-chain acyl-CoA synthetase (LACS) activities are encoded by a family of at least nine genes in Arabidopsis (Arabidopsis thaliana). These enzymes have roles in lipid synthesis, fatty acid catabolism, and the transport of fatty acids between subcellular compartments. Here, we show that the LACS2 gene (At1g49430) is expressed in young, rapidly expanding tissues, and in leaves expression is limited to cells of the adaxial and abaxial epidermal layers, suggesting that the LACS2 enzyme may act in the synthesis of cutin or cuticular waxes. A lacs2 null mutant was isolated by reverse genetics. Leaves of mutant plants supported pollen germination and released chlorophyll faster than wild-type leaves when immersed in 80% ethanol, indicating a defect in the cuticular barrier. The composition of surface waxes extracted from lacs2 leaves was similar to the wild type, and the total wax load was higher than the wild type (111.4 microg/dm(2) versus 76.4 microg/dm(2), respectively). However, the thickness of the cutin layer on the abaxial surface of lacs2 leaves was only 22.3 +/- 1.7 nm compared with 33.0 +/- 2.0 nm for the wild type. In vitro assays showed that 16-hydroxypalmitate was an excellent substrate for recombinant LACS2 enzyme. We conclude that the LACS2 isozyme catalyzes the synthesis of omega-hydroxy fatty acyl-CoA intermediates in the pathway to cutin synthesis. The lacs2 phenotype, like the phenotypes of some other cutin mutants, is very pleiotropic, causing reduced leaf size and plant growth, reduced seed production, and lower rates of seedling germination and establishment. The LACS2 gene and the corresponding lacs2 mutant will help in future studies of the cutin synthesis pathway and in understanding the consequences of reduced cutin production on many aspects of plant biology.
The Plant Cell 04/2004; 16(3):629-42. · 9.25 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In plants and other eukaryotes, long-chain acyl-CoAs are assumed to be imported into peroxisomes for beta-oxidation by an ATP binding cassette (ABC) transporter. However, two genes in Arabidopsis thaliana, LACS6 and LACS7, encode peroxisomal long-chain acyl-CoA synthetase (LACS) isozymes. To investigate the biochemical and biological roles of peroxisomal LACS, we identified T-DNA knockout mutants for both genes. The single-mutant lines, lacs6-1 and lacs7-1, were indistinguishable from the wild type in germination, growth, and reproductive development. By contrast, the lacs6-1 lacs7-1 double mutant was specifically defective in seed lipid mobilization and required exogenous sucrose for seedling establishment. This phenotype is similar to the A. thaliana pxa1 mutants deficient in the peroxisomal ABC transporter and other mutants deficient in beta-oxidation. Our results demonstrate that peroxisomal LACS activity and the PXA1 transporter are essential for early seedling growth. The peroxisomal LACS activity would be necessary if the PXA1 transporter delivered unesterified fatty acids into the peroxisomal matrix. Alternatively, PXA1 and LACS6/LACS7 may act in parallel pathways that are both required to ensure adequate delivery of acyl-CoA substrates for beta-oxidation and successful seedling establishment.
The Plant Cell 03/2004; 16(2):394-405. · 9.25 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Acyl-coenzyme A (CoA) synthetases (ACSs, EC 220.127.116.11) catalyze the formation of fatty acyl-CoAs from free fatty acid, ATP, and CoA. Essentially all de novo fatty acid synthesis occurs in the plastid. Fatty acids destined for membrane glycerolipid and triacylglycerol synthesis in the endoplasmic reticulum must be first activated to acyl-CoAs via an ACS. Within a family of nine ACS genes from Arabidopsis, we identified a chloroplast isoform, LACS9. LACS9 is highly expressed in developing seeds and young rosette leaves. Both in vitro chloroplast import assays and transient expression of a green fluorescent protein fusion indicated that the LACS9 protein is localized in the plastid envelope. A T-DNA knockout mutant (lacs9-1) was identified by reverse genetics and these mutant plants were indistinguishable from wild type in growth and appearance. Analysis of leaf lipids provided no evidence for compromised export of acyl groups from chloroplasts. However, direct assays demonstrated that lacs9-1 plants contained only 10% of the chloroplast long-chain ACS activity found for wild type. The residual long-chain ACS activity in mutant chloroplasts was comparable with calculated rates of fatty acid synthesis. Although another isozyme contributes to the activation of fatty acids during their export from the chloroplast, LACS9 is a major chloroplast ACS.
[Show abstract][Hide abstract] ABSTRACT: One of the most prominent and important topics in modern agricultural biotechnology is the manipulation of oilseed triacylglycerol composition. Towards this goal, we have sought to identify and characterize acyl-CoA synthetases (ACSs), which play an important role in both de novo synthesis and modification of existing lipids. We have identified and cloned 20 different genes that bear strong sequence homology to known ACSs from other organisms. Through sequence comparisons and functional characterization, we have identified several members of this group that encode ACSs, while the other genes fall into the broader category of genes for AMP-binding proteins (AMPBPs). Distinguishing ACSs from AMPBPs will simplify our efforts to understand the role of ACS in triacylglycerol metabolism.
Biochemical Society Transactions 01/2001; 28(6):955-7. · 2.59 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: One of the major goals of modern plant biotechnology is to manipulate lipid metabolism in oilseed crops to produce new and improved edible and industrial vegetable oils. Lipids constitute the structural components of cellular membranes and act as sources of energy for the germinating seed and are therefore essential to plant cell function. Both de novo synthesis and modification of existing lipids are dependent on the activity of acyl-CoA synthetases (ACSs). To date, ACSs have been recalcitrant to traditional methods of purification due to their association with membranes. In our laboratory, several isoforms of ACSs have been identified in Arabidopsis thaliana. Reverse genetics allowed us to identify a mutant containing a transfer DNA-interrupted ACS gene. Results will be presented that describe the isolation and characterization of this mutant. The elucidation of the specific roles of ACSs will lead to a greater understanding of plant lipid metabolism.
Biochemical Society Transactions 01/2001; 28(6):957-8. · 2.59 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Alfalfa (Medicago sativa L.) is an agronomically important forage, but digestibility of stem cell wall material is low. Because the tetraploid genome of alfalfa complicates genetic dissection of complex pathways, the diploid M. truncatula Gaertn. could serve as a model for stem cell wall development in alfalfa. We compared stem morphology, chemical composition (protein, soluble carbohydrates, cell wall polysaccharides, and lignin), and in vitro ruminal cell wall polysaccharide digestibility of two alfalfa clones (Regen-SY27 and 718) and four M. truncatula inbred lines (A17, A20, DZA315.16, and R108) in a replicated growth chamber experiment. Stem tissue development and cell wall lignification observed by light microscopy were similar between the species. While differences in stem morphology, composition, and digestibility were observed among the germplasms, there was overlap between the alfalfa and M. truncatula germplasms for all traits except protein concentration, which was greater for the two alfalfa clones. Younger stem internodes (top third of the stem) of both species had a higher protein concentration and greater cell wall polysaccharide digestibility, and lower cell wall concentration than older internodes (bottom third of stem). Based on the data presented here, it appears that M. truncatula is a suitable model for stem development, composition, and digestibility of alfalfa.