Molecular characterization of PVAS3: an asparagine synthetase gene from common bean prevailing in developing organs

In common bean, asparagine synthetase (AS; EC 6.3.5.4) is encoded by three members of a multigene family called PVAS1, PVAS2 and PVAS3. Two of these genes, PVAS1 and PVAS2, have been extensively studied, but little is known about PVAS3, remaining unclear whether PVAS3 function is redundant to the other AS or if it plays a specific role in Phaseolus vulgaris metabolism. In this work, we used a molecular approach to characterize PVAS3 expression and to gain some knowledge about its physiological function. We showed that, in contrast to PVAS1 and PVAS2, PVAS3 was expressed in all organs analyzed. Interestingly, PVAS3 was the AS gene most highly expressed in nodules, leaves and pods at the earliest stages of development, and its expression decreased as these organs developed. Expression of PVAS3 parallels the accumulation of AS protein and the asparagine content during the earliest stages of nodule, leaf and pod development, suggesting an important role for PVAS3 in the synthesis of asparagine in that period. Furthermore, PVAS3 was not repressed by light, as most class-II AS genes. Surprisingly, fertilization of nodulated plants with nitrate or ammonium, conditions that induce PVAS1 and PVAS2 and the shift from ureides to amide synthesis, repressed the expression of PVAS3 in nodules and roots. The possible implications of this regulation are discussed.

Multiple resistance traits control Plum pox virus infection in Arabidopsis thaliana

Twelve Arabidopsis accessions were challenged with Plum pox potyvirus (PPV) isolates representative of the four PPV strains. Each accession supported local and systemic infection by at least some of the PPV isolates, but high variability was observed in the behavior of the five PPV isolates or the 12 Arabidopsis accessions. Resistance to local infection or long-distance movement occurred in about 40% of all the accession-isolate combinations analyzed. Except for Nd-1, all accessions showed resistance to local infection by PPV-SoC; in the Landsberg erecta (Ler) accession, this resistance was compromised by sgt1 and rar1 mutations, suggesting that it could be controlled by an R gene-mediated resistance pathway. While most of the susceptible accessions were symptomless, PPV induced severe symptoms on inflorescences in C24, Ler, and Bay-0 as early as 15 days after inoculation. Genetic analyses indicated that these interaction phenotypes are controlled by different genetic systems. The restriction of long-distance movement of PPV-El Amar and of another member of genus Potyvirus, Lettuce mosaic virus, in Col-0 requires the RTM genes, indicating for the first time that the RTM system may provide a broad range, potyvirus-specific protection against systemic infection. The restriction to PPV-PS long-distance movement in Cvi-1 is controlled by a single recessive gene, designated rpv1, which was mapped to chromosome 1. The nuclear inclusion polymerase b-capsid protein region of the viral genome appears to be responsible for the ability of PPV-R to overcome rpv1-mediated resistance.

Identification of a plum pox virus CI-interacting protein from chloroplast that has a negative effect in virus infection

The cylindrical inclusion (CI) protein of potyviruses is involved in virus replication and cell-to-cell movement. These two processes should rely on multiple plant-virus interactions; however, little is known about the host factors that are involved in, or that may interfere with, CI functions. By using a yeast two-hybrid system, the CI protein from Plum pox virus (PPV) was found to interact with the photosystem I PSI-K protein, the product of the gene psaK, of Nicotiana benthamiana. Coexpression of PPV CI was shown to cause a decrease in the accumulation level of PSI-K transiently expressed in N. benthamiana leaves. To test the biological relevance of this interaction, we have analyzed the infection of PPV in N. benthamiana plants in which psaK gene expression has been silenced by RNA interference, as well as in Arabidopsis thaliana psaK knockout plants. Our results show that downregulation of the psaK gene leads to higher PPV accumulation, suggesting a role for the CI-PSI-K interaction in PPV infection.

Identification of secret agent as the O-GlcNAc transferase that participates in Plum pox virus infection

Serine and threonine of many nuclear and cytoplasmic proteins are posttranslationally modified with O-linked N-acetylglucosamine (O-GlcNAc). This modification is made by O-linked N-acetylglucosamine transferases (OGTs). Genetic and biochemical data have demonstrated the existence of two OGTs of Arabidopsis thaliana, SECRET AGENT (SEC) and SPINDLY (SPY), with at least partly overlapping functions, but there is little information on their target proteins. The N terminus of the capsid protein (CP) of Plum pox virus (PPV) isolated from Nicotiana clevelandii is O-GlcNAc modified. We show here that O-GlcNAc modification of PPV CP also takes place in other plant hosts, N. benthamiana and Arabidopsis. PPV was able to infect the Arabidopsis OGT mutants sec-1, sec-2, and spy-3, but at early times of the infection, both rate of virus spread and accumulation were reduced in sec-1 and sec-2 relative to spy-3 and wild-type plants. By matrix-assisted laser desorption ionization-time of flight mass spectrometry, we determined that a 39-residue tryptic peptide from the N terminus of CP of PPV purified from the spy-3 mutant, but not sec-1 or sec-2, was O-GlcNAc modified, suggesting that SEC but not SPY modifies the capsid. While our results indicate that O-GlcNAc modification of PPV CP by SEC is not essential for infection, they show that the modification has a role(s) in the process.

Antibiotic activities of peptides, hydrogen peroxide and peroxynitrite in plant defence

Genes encoding plant antibiotic peptides show expression patterns that are consistent with a defence role. Transgenic over-expression of defence peptide genes is potentially useful to engineer resistance of plants to relevant pathogens. Pathogen mutants that are sensitive to plant peptides in vitro have been obtained and a decrease of their virulence in planta has been observed, which is consistent with their hypothetical defence role. A similar approach has been followed to elucidate the potential direct anti-microbial role of hydrogen peroxide. Additionally, a scavenger of peroxynitrite has been used to investigate its involvement in plant defence.

Effects of urate, a natural inhibitor of peroxynitrite-mediated toxicity, in the response of Arabidopsis thaliana to the bacterial pathogen Pseudomonas syringae

Urate, a natural peroxynitrite scavenger, has been used to investigate the possible role of peroxynitrite during plant-pathogen interactions. Urate greatly reduced lesion formation in Arabidopsis leaves treated with an abiotic peroxynitrite-generating system or with a peroxynitrite solution, indicating that it can act as an effective scavenger in planta. In the interaction with the avirulent Pseudomonas syringae pv. phaseolicola (avrRPM1+), cell death in the inoculated area was strongly reduced by urate, without compromising disease resistance. In contrast, urate promoted discrete cell death in response to an isogenic Pseudomonas syringae (avrRPM1-), which did not trigger an HR when inoculated alone, and it induced resistance and arrest of pathogen growth. Scavenging of peroxynitrite did not modify the response of Arabidopsis to an avirulent strain of Xanthomonas campestris pv campestris, that showed a high resistance to NO and peroxynitrite. Our data indicate that peroxynitrite plays a significant role in the responses of plants to Pseudomonas syringae.

Plant defense peptides

Eight families of antimicrobial peptides, ranging in size from 2 to 9 kD, have been identified in plants. These are thionins, defensins, so-called lipid transfer proteins, hevein- and knottin-like peptides, MBP1, IbAMP, and the recently reported snakins. All of them have compact structures that are stabilized by 2-6 disulfide bridges. They are part of both permanent and inducible defense barriers. Transgenic overexpression of the corresponding genes leads to enhanced tolerance to pathogens, and peptide-sensitive pathogen mutants have reduced virulence.

Processing of thionin precursors in barley leaves by a vacuolar proteinase

Thionins are synthesized as precursors with a signal peptide and a long C-terminal acidic peptide that is post-translationally processed. A fusion protein including the maltose-binding protein from Escherichia coli (MalE), thionin DG3 from barley leaves, and its acidic C-terminal peptide has been used to obtain antibodies that recognize both domains of the precursor. In barley leaf sections, mature thionins accumulated in the vacuolar content, while the acidic peptide was not detected in any cell fraction. Brefeldin A and monensin inhibited processing of the precursor but its export from the microsomal fraction was not inhibited. Both purified vacuoles and an acid (pH 5.5) extract from leaves processed the fusion protein into a MalE-thionin and an acidic peptide fragment. A 70-kDa proteinase that effected this cleavage was purified from the acid extract. Processing of the fusion protein by both lysed vacuoles and the purified proteinase was inhibited by Zn2+ and by Cu2+, but not by inhibitors of the previously described vacuolar processing thiol or aspartic proteinases. In vivo processing of the thionin precursor in leaf sections was also inhibited by Zn2+ and Cu2+. Variants of the fusion protein with altered processing sites that represented those of thionin precursors from different taxa were readily processed by the proteinase, whereas changing the polarity of either the C-terminal or N-terminal residues of the processing site prevented cleavage by the proteinase.

A novel embryo-specific barley cDNA clone encodes a protein with homologies to bacterial glucose and ribitol dehydrogenase

In order to analyze the genetic programme expressed during the early stages of embryogenesis a cDNA clone bank was constructed from desiccation-tolerant excised barley embryos 18 d after pollination (D. Bartels et al., 1988, Planta 175, 485-492). One of the selected cDNA clones pG31 encodes a transcript of 1300 nucleotides and a protein of 31 kDa, both are specifically expressed in developing embryos and are not detected in other tissues. The expression of the pG31 mRNA is not modulated by the plant hormone cis-abscisic acid but it ceases to be expressed in germinating embryos. The protein sequence deduced from the pG31 transcript shows substantial sequence homologies to bacterial glucose dehydrogenase and ribitol dehydrogenase. Biochemical analysis indicates that glucose dehydrogenase activity is present in protein extracts from embryos 18 d after pollination. This glucose dehydrogenase activity is inhibited by antiserum raised against the recombinant pG31 protein. These findings provide evidence for the discovery of a novel pathway in carbohydrate metabolism acting specifically during embryogenesis.

Purification and substrate inactivation of xanthine dehydrogenase from Chlamydomonas reinhardtii

Xanthine dehydrogenase (XDH) from the unicellular green alga Chlamydomonas reinhardtii has been purified to electrophoretic homogeneity by a procedure which includes several conventional steps (gel filtration, anion exchange chromatography and preparative gel electrophoresis). The purified protein exhibited a specific activity of 5.7 units/mg protein (turnover number = 1.9 .10(3) min-1) and a remarkable instability at room temperature. Spectral properties were identical to those reported for other xanthine-oxidizing enzymes with absorption maxima in the 420-450 nm region and a shoulder at 556 nm characteristic of molybdoflavoproteins containing iron-sulfur centers. Chlamydomonas XDH was irreversibly inactivated upon incubation of enzyme with its physiological electron donors xanthine and hypoxanthine, in the absence of NAD+, its physiological electron acceptor. As deduced from spectral changes in the 400-500 nm region, xanthine addition provoked enzyme reduction which was followed by inactivation. This irreversible inactivation also took place either under anaerobic conditions or whenever oxygen or any of its derivatives were excluded. Adenine, 8-azaxanthine and acetaldehyde which could act as reducing substrates of XDH were also able to inactivate it upon incubation. The same inactivating effect was observed with NADH and NADPH, electron donors for the diaphorase activity associated with xanthine dehydrogenase. In addition, partial activities of XDH were differently affected by xanthine incubation. We conclude that xanthine dehydrogenase inactivation by substrate is due to an irreversible process affecting mainly molybdenum center and that sequential and uninterrupted electron flow from xanthine to NAD+ is essential to maintain the enzyme in its active form.

Purification and molecular properties of urate oxidase from Chlamydomonas reinhardtii

Urate oxidase (urate: oxygen oxidoreductase, EC 1.7.3.3) from the unicellular green alga Chlamydomonas reinhardtii has been purified to electrophoretic and immunological homogeneity by a procedure which includes as main steps ammonium sulfate fractionation, gel filtration, ion exchange and xanthine-agarose affinity chromatography. The native enzyme has a relative molecular mass (Mr) of 124,000 and consists of four identical or similar-sized subunits of Mr 31,000 each. The enzyme has a Stokes's radius of 3.87 nm, a sedimentation coefficient of 6.8 S and an f/f0 of 1.23, and exhibits its maximal absorption at 276 nm. Optimum pH was 8.5 and maximum activity was shown at 40 degrees C, with an activation energy of 53 kJ.mol-1 and a Q10 of 1.96. Absorption spectrum of native reduced enzyme showed two transient maxima at 392 and 570 nm, very similar to those of metal-urate complexes, which disappeared in the presence of cyanide. Inhibition by cyanide and neocuproin, but not by salicylhydroxamic acid, strongly suggests that copper is the metal involved in enzymatic urate oxidation. By using a sensitive photokinetic method for copper determination, a content of 4 mol of copper per mol of enzyme has been found.