Infections with various types of organisms stimulate transcription from a short promoter fragment of the potato gst1 gene

Synthetic promoters consisting of defined cis-acting elements link multiple signaling pathways to probenazole-inducible system

Probenazole (3-allyloxy-1,2-benzisothiazole-1,1-dioxide, PBZ), the active component of Oryzemate, could induce systemic acquired resistance (SAR) in plants through the induction of salicylic acid (SA) biosynthesis. As a widely used chemical inducer, PBZ is a good prospect for establishing a new chemical-inducible system. We first designed artificially synthetic promoters with tandem copies of a single type of cis-element (SARE, JERE, GCC, GST1, HSRE, and W-box) that could mediate the expression of the β-glucuronidase (GUS) reporter gene in plants upon PBZ treatment. Then we combined different types of elements in order to improve inducibility in the PBZ-inducible system. On the other hand, we were surprised to find that the cis-elements, which are responsive to jasmonic acid (JA) and ethylene, also responded to PBZ, implying that SA, JA, and ethylene pathways also would play important roles in PBZ's action. Further analysis demonstrated that PBZ also induced early events of innate immunity via a signaling pathway in which Ca(2+) influx and mitogen-activated protein kinase (MAPK) activity were involved. We constructed synthesized artificial promoters to establish a PBZ chemical-inducible system, and preliminarily explored SA, JA, ethylene, calcium, and MAPK signaling pathways via PBZ-inducible system, which could provide an insight for in-depth study.

Identification of transcriptional regulatory nodes in soybean defense networks using transient co-transactivation assays

Plant responses to major environmental stressors, such as insect feeding, not only occur via the functions of defense genes but also involve a series of regulatory factors. Our previous transcriptome studies proposed that, in addition to two defense-related genes, GmVSPβ and GmN:IFR, a high proportion of transcription factors (TFs) participate in the incompatible soybean-common cutworm interaction networks. However, the regulatory mechanisms and effects of these TFs on those induced defense-related genes remain unknown. In the present work, we isolated and identified 12 genes encoding MYB, WRKY, NAC, bZIP, and DREB TFs from a common cutworm-induced cDNA library of a resistant soybean line. Sequence analysis of the promoters of three co-expressed genes, including GmVSPα, GmVSPβ, and GmN:IFR, revealed the enrichment of various TF-binding sites for defense and stress responses. To further identify the regulatory nodes composed of these TFs and defense gene promoters, we performed extensive transient co-transactivation assays to directly test the transcriptional activity of the 12 TFs binding at different levels to the three co-expressed gene promoters. The results showed that all 12 TFs were able to transactivate the GmVSPβ and GmN:IFR promoters. GmbZIP110 and GmMYB75 functioned as distinct regulators of GmVSPα/β and GmN:IFR expression, respectively, while GmWRKY39 acted as a common central regulator of GmVSPα/β and GmN:IFR expression. These corresponding TFs play crucial roles in coordinated plant defense regulation, which provides valuable information for understanding the molecular mechanisms involved in insect-induced transcriptional regulation in soybean. More importantly, the identified TFs and suitable promoters can be used to engineer insect-resistant plants in molecular breeding studies.

The interactome of soybean GmWRKY53 using yeast 2-hybrid library screening to saturation

Soybean GmWRKY53 functions in both biotic and abiotic stress signaling. Using GmWRKY53 as a bait yeast 2-hybrid library screening to saturation isolated multiple independent fragments for many interacting proteins, enabling delineation of minimal interacting domains and computation of a confidence score. Multiple independent clones coding for the LATE ELONGATED HYPOCOTYL clock protein GmLCL2 (MYB114) were isolated and the binding site for GmWRKY53 was mapped to 90 amino acids separate from the MYB domain. This suggests a direct input from the clock on GmWRKY53 activity. The GmWRKY53-interacting proteins also included 3 water stress-inducible AP2/ERF transcription factors. One of these (Glyma03g26310) is one of the most strongly water stress induced genes in soybean roots, suggesting that GmWRKY53/ERF complexes regulate water stress responses.

Local and systemic mycorrhiza-induced protection against the ectoparasitic nematode Xiphinema index involves priming of defence gene responses in grapevine

The ectoparasitic dagger nematode (Xiphinema index), vector of Grapevine fanleaf virus (GFLV), provokes gall formation and can cause severe damage to the root system of grapevines. Mycorrhiza formation by Glomus (syn. Rhizophagus) intraradices BEG141 reduced both gall formation on roots of the grapevine rootstock SO4 (Vitis berlandieri×V. riparia) and nematode number in the surrounding soil. Suppressive effects increased with time and were greater when the nematode was post-inoculated rather than co-inoculated with the arbuscular mycorrhizal (AM) fungus. Using a split-root system, decreased X. index development was shown in mycorrhizal and non-mycorrhizal parts of mycorrhizal root systems, indicating that both local and systemic induced bioprotection mechanisms were active against the ectoparasitic nematode. Expression analyses of ESTs (expressed sequence tags) generated in an SSH (subtractive suppressive hybridization) library, representing plant genes up-regulated during mycorrhiza-induced control of X. index, and of described grapevine defence genes showed activation of chitinase 1b, pathogenesis-related 10, glutathione S-transferase, stilbene synthase 1, 5-enolpyruvyl shikimate-3-phosphate synthase, and a heat shock proein 70-interacting protein in association with the observed local and/or systemic induced bioprotection against the nematode. Overall, the data suggest priming of grapevine defence responses by the AM fungus and transmission of a plant-mediated signal to non-mycorrhizal tissues. Grapevine gene responses during AM-induced local and systemic bioprotection against X. index point to biological processes that are related either to direct effects on the nematode or to protection against nematode-imposed stress to maintain root tissue integrity.

Symbiosis-related pea genes modulate fungal and plant gene expression during the arbuscule stage of mycorrhiza with Glomus intraradices

The arbuscular mycorrhiza association results from a successful interaction between genomes of the plant and fungal symbiotic partners. In this study, we analyzed the effect of inactivation of late-stage symbiosis-related pea genes on symbiosis-associated fungal and plant molecular responses in order to gain insight into their role in the functional mycorrhizal association. The expression of a subset of ten fungal and eight plant genes, previously reported to be activated during mycorrhiza development, was compared in Glomus intraradices-inoculated wild-type and isogenic genotypes of pea mutated for the PsSym36, PsSym33, and PsSym40 genes where arbuscule formation is inhibited or fungal turnover modulated, respectively. Microdissection was used to corroborate arbuscule-related fungal gene expression. Molecular responses varied between pea genotypes and with fungal development. Most of the fungal genes were downregulated when arbuscule formation was defective, and several were upregulated with more rapid fungal development. Some of the plant genes were also affected by inactivation of the PsSym36, PsSym33, and PsSym40 loci, but in a more time-dependent way during root colonization by G. intraradices. Results indicate a role of the late-stage symbiosis-related pea genes not only in mycorrhiza development but also in the symbiotic functioning of arbuscule-containing cells.

Expression profiling of fungal genes during arbuscular mycorrhiza symbiosis establishment using direct fluorescent in situ RT-PCR

Expression profiling of fungal genes in the arbuscular mycorrhiza (AM) symbiosis has been based on studies of RNA extracted from fungal tissue or mycorrhizal roots, giving only a general picture of overall transcript levels in the targeted tissues. Information about the spatial distribution of transcripts within AM fungal structures during different developmental stages is essential to a better understanding of fungal activity in symbiotic interactions with host roots and to determine molecular events involved in establishment and functioning of the AM symbiosis. The obligate biotrophic nature of AM fungi is a challenge for developing new molecular methods to identify and localize their activity in situ. The direct fluorescent in situ (DIFIS) RT-PCR procedure described here represents a novel tool for spatial mapping of AM fungal gene expression simultaneously prior to root penetration, within fungal tissues in the host root and in the extraradical stage of fungal development.In order to enhance detection sensitivity of the in situ RT-PCR technique and enable localization of low abundance mRNA, we have adopted direct fluorescent labeling of primers for the amplification step to overcome the problem of low detection associated with digoxigenin or biotin-labeled primers and to avoid the multiplicity of steps associated with immunological detection. Signal detection has also been greatly improved by eliminating autofluorescence of AM fungal and root tissues using confocal microscopy.

Physiological roles of glutathione s-transferases in soybean root nodules

Glutathione S-transferases (GSTs) are ubiquitous enzymes that catalyze the conjugation of toxic xenobiotics and oxidatively produced compounds to reduced glutathione, which facilitates their metabolism, sequestration, or removal. We report here that soybean (Glycine max) root nodules contain at least 14 forms of GST, with GST9 being most prevalent, as measured by both real-time reverse transcription-polymerase chain reaction and identification of peptides in glutathione-affinity purified extracts. GST8 was prevalent in stems and uninfected roots, whereas GST2/10 prevailed in leaves. Purified, recombinant GSTs were shown to have wide-ranging kinetic properties, suggesting that the suite of GSTs could provide physiological flexibility to deal with numerous stresses. Levels of GST9 increased with aging, suggesting a role related to senescence. RNA interference studies of nodules on composite plants showed that a down-regulation of GST9 led to a decrease in nitrogenase (acetylene reduction) activity and an increase in oxidatively damaged proteins. These findings indicate that GSTs are abundant in nodules and likely function to provide antioxidant defenses that are critical to support nitrogen fixation.

Activation of the pathogen-inducible Gst1 promoter of potato after elicitation by Venturia inaequalis and Erwinia amylovora in transgenic apple (Malus x domestica)

Rather than using a constitutive promoter to drive transgenes for resistance against fungal and bacterial diseases in genetic engineering of apple (Malus x domestica) cultivars, a promoter induced only after infection was preferred. The ability of the Pgst1 promoter from potato (Solanum tuberosum L.) to drive expression of the gusA reporter gene was determined in two genotypes of apple: the fruit cultivar Royal Gala and the M.26 rootstock. beta-Glucuronidase activity in the transgenic lines grown in a growth chamber was determined quantitatively using fluorometric assays and compared to the activity in Cauliflower Mosaic Virus (CaMV) 35S promoter-driven transgenic lines. In both apple genotypes, the Pgst1 promoter exhibited a low level of expression after bacterial and fungal inoculation compared to the level obtained with the PCaMV35S promoter (15% and 8% respectively). The Pgst1 promoter was systematically activated in apple at the site of infection with a fungal pathogen. It was also activated after treatment with salicylic acid, but not after wounding. Taken together, these data show that, although the Pgst1 promoter is less active than the PCaMV35S promoter in apple, its pathogen responsiveness could be useful in driving the expression of transgenes to promote bacterial and fungal disease resistance.

Spatial monitoring of gene activity in extraradical and intraradical developmental stages of arbuscular mycorrhizal fungi by direct fluorescent in situ RT-PCR

Gene expression profiling based on tissue extracts gives only limited information about genes associated with complex developmental processes such as those implicated in fungal interactions with plant roots during arbuscular mycorrhiza development and function. To overcome this drawback, a direct fluorescent in situ RT-PCR methodology was developed for spatial mapping of gene expression in different presymbiotic and symbiotic structures of an arbuscular mycorrhizal fungus. Transcript detection was optimized by targeting the LSU rRNA gene of Glomus intraradices and monitoring expression of a stearoyl-CoA-desaturase gene that is consistently expressed at high levels in spores, hyphae, arbuscules and vesicles. This method was further validated by localizing expression of fungal peptidylprolyl isomerase and superoxide dismutase genes, which are expressed to different extents in fungal structures. Direct fluorescent in situ RT-PCR offers new perspectives for the sensitive analysis of fungal developmental processes that occur during functional differentiation in symbiotic arbuscular mycorrhiza interactions.

Identification of plant genes regulated in resistant potato Solanum sparsipilum during the early stages of infection by Globodera pallida

Using a complementary (c)DNA-amplified fragment length polymorphism (AFLP) approach, we investigated differential gene expression linked to resistance mechanisms during the incompatible potato - Globodera pallida interaction. Expression was compared between a resistant and a susceptible potato clone, inoculated or not inoculated with G. pallida. These clones were issued from a cross between the resistant Solanum sparsipilum spl329.18 accession and the susceptible dihaploid S. tuberosum Caspar H3, and carried, respectively, resistant and susceptible alleles at the resistance quantitative trait loci (QTLs). Analysis was done on root fragments picked up at 4 time points, during a period of 6 days after infection, from penetration of the nematode in the root to degradation of the feeding site in resistant plants. A total of 2560 transcript-derived fragments (TDFs) were analyzed, resulting in the detection of 46 TDFs that were up- or downregulated. The number of TDFs that were up- or downregulated increased with time after inoculation. The majority of TDFs were upregulated at only 1 or 2 time points in response to infection. After isolation and sequencing of the TDFs of interest, a subset of 36 sequences were identified, among which 22 matched plant sequences and 2 matched nematode sequences. Some of the TDFs that matched plant genes showed clear homologies to genes involved in cell-cycle regulation, transcription regulation, resistance downstream signalling pathways, and defense mechanisms. Other sequences with homologies to plant genes of unknown function or without any significant similarity to known proteins were also found. Although not exhaustive, these results represent the most extensive list of genes with altered RNA levels after the incompatible G. pallida-potato interaction that has been published to date. The function of these genes could provide insight into resistance or plant defense mechanisms during incompatible potato-cyst nematode interactions.

Identification of heavy metal-induced genes encoding glutathione S-transferases in the arbuscular mycorrhizal fungus Glomus intraradices

Arbuscular mycorrhizal fungi are able to alleviate the stress for plants caused by heavy metal contamination of soil. To analyze the molecular response of arbuscular mycorrhizal fungi to these pollutants, a subtractive cDNA library was constructed using RNA from Glomus intraradices extraradical hyphae of a root organ culture treated with a mixture of Cd, Zn, and Cu. Screening by reverse Northern blot analysis indicated that, among 308 clones, 17% correspond to genes up-regulated by heavy metals. Sequence analysis of part of the clones resulted, amongst others, in the identification of six genes putatively coding for glutathione S-transferases belonging to two different classes of these enzymes. Expression analyses indicated that the genes are differentially expressed during fungal development and that their RNA accumulation dramatically increases in extraradical hyphae grown in a heavy metal-containing solution.

A novel wound-responsive cis-element, VWRE, of the vascular system-specific expression of a tobacco peroxidase gene, tpoxN1

The wound-induced expression of tpoxN1, encoding a tobacco peroxidase, is unique because of its vascular system-specific expression and insensitivity to known wound-signal compounds such as jasmonic acid, ethylene, and plant hormones [Sasaki et al. (2002) Plant Cell Physiol 43:108-117]. To study the mechanism of expression, the 2-kbp tpoxN1 promoter region and successive 5'-deletion of the promoter were introduced as GUS fusion genes into tobacco plants. Analysis of GUS activity in transgenic plants indicated that a vascular system-specific and wound-responsive cis-element (VWRE) is present at the -239/-200 region of the promoter. Gel mobility shift assays suggested that a nuclear factor(s) prepared from wounded tobacco stems binds a 14-bp sequence (-229/-215) in the -239/-200 region in a sequence-specific manner. A mutation in this 14-bp region of the -239 promoter fragment resulted in a considerable decrease in wound-responsive GUS activity in transgenic plants. An 11-bp sequence, which completely overlaps with the 14-bp sequence, was found in the 5' distal region (-420/-410) and is thought to contribute to the wound-induced expression together with the 14-bp. The -114-bp core promoter of the tpoxN1 gene was indispensable for wound-induced expression, indicating that the 14-bp region is a novel wound-responsive cis-element VWRE, which may work cooperatively with other factors in the promoter.

Signaling in the arbuscular mycorrhizal symbiosis

Many microorganisms form symbioses with plants that range, on a continuous scale, from parasitic to mutualistic. Among these, the most widespread mutualistic symbiosis is the arbuscular mycorrhiza, formed between arbuscular mycorrhizal (AM) fungi and vascular flowering plants. These associations occur in terrestrial ecosystems throughout the world and have a global impact on plant phosphorus nutrition. The arbuscular mycorrhiza is an endosymbiosis in which the fungus inhabits the root cortical cells and obtains carbon provided by the plant while it transfers mineral nutrients from the soil to the cortical cells. Development of the symbiosis involves the differentiation of both symbionts to create novel symbiotic interfaces within the root cells. The aim of this review is to explore the current understanding of the signals and signaling pathways used by the symbionts for the development of the AM symbiosis. Although the signal molecules used for initial communication are not yet known, recent studies point to their existence. Within the plant, there is evidence of arbuscular mycorrhiza-specific signals and of systemic signaling that influences phosphate-starvation responses and root development. The landmark cloning of three plant signaling proteins required for the development of the symbiosis has provided the first insights into a signaling pathway that is used by AM fungi and by rhizobia for their symbiotic associations with legumes.

Expression profiling of up-regulated plant and fungal genes in early and late stages of Medicago truncatula-Glomus mosseae interactions

Suppression subtractive hybridization (SSH), expression profiling and EST sequencing identified 12 plant genes and six fungal genes that are expressed in the arbuscular mycorrhizal symbiosis between Medicago truncatula and Glomus mosseae. All the plant genes and three of the fungal genes were up-regulated in symbiotic tissues. Expression of 15 of the genes is described for the first time in mycorrhizal roots and two are novel sequences. Six M. truncatula genes were also activated during appressorium formation at the root surface, suggesting a role in this early stage of mycorrhiza establishment, whilst the other six plant genes were only induced in the late stages of mycorrhization and could be involved in the development or functioning of the symbiosis. Phosphate fertilization had no significant influence on expression of any of the plant genes. Expression profiling of G. mosseae genes indicated that two of them may be associated with appressorium development on roots and one with arbuscule formation or function. The other three fungal genes were expressed throughout the life-cycle of G. mosseae.

Identification of mycorrhiza-regulated genes with arbuscule development-related expression profile

Suppressive subtractive hybridisation was applied to the analysis of late stage arbuscular mycorrhizal development in pea. 96 cDNA clones were amplified and 81, which carried fragments more than 200 nt in size, were sequence analysed. Among 67 unique fragments, 10 showed no homology and 10 were similar to sequences with unknown function. RNA accumulation of the corresponding 67 genes was analysed by hybridisation of macro-arrays. The cDNAs used as probes were derived from roots of wild type and late mutant pea genotypes, inoculated or not with the AM fungus Glomus mosseae. After calibration, a more than 2.5-fold mycorrhiza-induced RNA accumulation was detected in two independent experiments in the wild type for 25 genes, 22 of which seemed to be induced specifically during late stage AM development. Differential expression for 7 genes was confirmed by RT-PCR using RNA from mycorrhiza and from controls of a different pea cultivar. In order to confirm arbuscule-related expression, the Medicago truncatula EST data base was screened for homologous sequences with putative mycorrhiza-related expression and among a number of sequences with significant similarities, a family of trypsin inhibitor genes could be identified. Mycorrhiza-induced RNA accumulation was verified for five members by real-time PCR and arbuscule-related activation of the promoter could be shown in transgenic roots for one of the genes, MtTi 1.

Construction and validation of cDNA-based Mt6k-RIT macro- and microarrays to explore root endosymbioses in the model legume Medicago truncatula

To construct macro- and microarray tools suitable for expression profiling in root endosymbioses of the model legume Medicago truncatula, we PCR-amplified a total of 6048 cDNA probes representing genes expressed in uninfected roots, mycorrhizal roots and young root nodules [Nucleic Acids Res. 30 (2002) 5579]. Including additional probes for either tissue-specific or constitutively expressed control genes, 5651 successfully amplified gene-specific probes were used to grid macro- and to spot microarrays designated Mt6k-RIT (M. truncatula 6k root interaction transcriptome). Subsequent to a technical validation of microarray printing, we performed two pilot expression profiling experiments using Cy-labeled targets from Sinorhizobium meliloti-induced root nodules and Glomus intraradices-colonized arbuscular mycorrhizal roots. These targets detected marker genes for nodule and arbuscular mycorrhiza development, amongst them different nodule-specific leghemoglobin and nodulin genes as well as a mycorrhiza-specific phosphate transporter gene. In addition, we identified several dozens of genes that have so far not been reported to be differentially expressed in nodules or arbuscular mycorrhiza thus demonstrating that Mt6k-RIT arrays serve as useful tools for an identification of genes relevant for legume root endosymbioses. A comprehensive profiling of such candidate genes will be very helpful to the development of breeding strategies and for the improvement of cultivation management targeted at increasing legume use in sustainable agricultural systems.

Common gene expression in Medicago truncatula roots in response to Pseudomonas fluorescens colonization, mycorrhiza development and nodulation

•  Beneficial rhizosphere microorganisms may share similar molecular steps during root colonization. To test this hypothesis, we compared Medicago truncatula Gaertn. gene expression in roots colonized, or not colonized, by Glomus mosseae BEG12, Pseudomonas fluorescens C7R12 or Sinorhizobium meliloti 2011. •  Pseudomonas fluorescens C7R12 formed colonies on the surface of M. truncatula roots and colonized root tissues intercellularly and intracellularly in a way similar to that previously described for other plants. •  Semiquantitative reverse transcriptase polymerase chain reaction of a set of 12 mycorrhiza upregulated M. truncatula genes revealed different expression profiles in roots 3 weeks after inoculation with P. fluorescens or S. meliloti. Pseudomonas fluorescens colonization activated seven of the plant genes while nodulated root systems showed increased expression in only three genes and five appeared to be downregulated. •  This first report of similar gene induction by a fluorescent pseudomonad and a mycorrhizal fungus in roots supports the hypothesis that some plant cell programmes may be shared during root colonization by these beneficial microorganisms. Less similarity existed in expression of the gene set with nodulation by S. meliloti.

Transcriptional changes in response to arbuscular mycorrhiza development in the model plant Medicago truncatula

Significant changes in root morphology and physiology during arbuscular mycorrhiza (AM) development are likely to be controlled by specific gene expression pattern in the host plant. Until now, little was known about transcriptional changes which occur AM-exclusively; that is, they do not occur during other root-microbe associations, nor are they induced by improved phosphate nutrition. In order to identify such AM-exclusive gene inductions of Medicago truncatula, we used a pool of different RNA samples as subtractor population in a suppressive subtractive hybridization (SSH) experiment. This approach resulted in the identification of a number of new AM-regulated genes. None of these genes were expressed in nonmycorrhiza roots or leaves. Electronic data obtained by comparison of the cDNA sequences to expressed sequence tag (EST) sequences from a wide range of cDNA libraries in the M. truncatula EST database (Gene Index, MtGI) support the mycorrhiza specificity of the corresponding genes, because sequences in the MtGI that were found to match the identified SSH-cDNA sequences originated exclusively from AM cDNA libraries. The promoter of one of those genes, MtGst1, showing similarities to plant glutathione-S-transferase (GST) encoding genes, was cloned and used in reporter gene studies. In contrast to studies with the potato GST gene PRP, MtGst 1 promoter activity was detected in all zones of the root cortex colonized by Glomus intraradices, but nowhere else.

Exploring root symbiotic programs in the model legume Medicago truncatula using EST analysis

We report on a large-scale expressed sequence tag (EST) sequencing and analysis program aimed at characterizing the sets of genes expressed in roots of the model legume Medicago truncatula during interactions with either of two microsymbionts, the nitrogen-fixing bacterium Sinorhizobium meliloti or the arbuscular mycorrhizal fungus Glomus intraradices. We have designed specific tools for in silico analysis of EST data, in relation to chimeric cDNA detection, EST clustering, encoded protein prediction, and detection of differential expression. Our 21 473 5'- and 3'-ESTs could be grouped into 6359 EST clusters, corresponding to distinct virtual genes, along with 52 498 other M.truncatula ESTs available in the dbEST (NCBI) database that were recruited in the process. These clusters were manually annotated, using a specifically developed annotation interface. Analysis of EST cluster distribution in various M.truncatula cDNA libraries, supported by a refined R test to evaluate statistical significance and by 'electronic northern' representation, enabled us to identify a large number of novel genes predicted to be up- or down-regulated during either symbiotic root interaction. These in silico analyses provide a first global view of the genetic programs for root symbioses in M.truncatula. A searchable database has been built and can be accessed through a public interface.

Tandemly duplicated Safener-induced glutathione S-transferase genes from Triticum tauschii contribute to genome- and organ-specific expression in hexaploid wheat

Glutathione S-transferase (GST) gene expression was examined in several Triticum species, differing in genome constitution and ploidy level, to determine genome contribution to GST expression in cultivated, hexaploid bread wheat (Triticum aestivum). Two tandemly duplicated tau class GST genes (TtGSTU1 and TtGSTU2) were isolated from a single bacterial artificial chromosome clone in a library constructed from the diploid wheat and D genome progenitor to cultivated wheat, Triticum tauschii. The genes are very similar in genomic structure and their encoded proteins are 95% identical. Gene-specific reverse transcriptase-polymerase chain reaction analysis revealed differential transcript accumulation of TtGSTU1 and TtGSTU2 in roots and shoots. Expression of both genes was induced by herbicide safeners, 2,4-dichlorophenoxyacetic acid and abscisic acid, in the shoots of T. tauschii; however, expression of TtGSTU1 was always higher than TtGSTU2. In untreated seedlings, TtGSTU1 was expressed in both shoots and roots, whereas TtGSTU2 expression was only detected in roots. RNA gel-blot analysis of ditelosomic, aneuploid lines that are deficient for 6AS, 6BS, or 6DS chromosome arms of cultivated, hexaploid bread wheat showed differential genome contribution to safener-induced GST expression in shoots compared with roots. The GST genes from the D genome of hexaploid wheat contribute most to safener-induced expression in the shoots, whereas GSTs from the B and D genomes contribute to safener-induced expression in the roots.

A novel root-specific gene, MIC-3, with increased expression in nematode-resistant cotton (Gossypium hirsutum L.) after root-knot nematode infection

A full-length cDNA, MIC-3, has been identified from a lambda ZAPII cDNA library constructed from the mRNA of nematode-resistant cotton (Gossypium hirsutum L.) roots after infection with root-knot nematode (Meloidogyne incognita). The putative open reading frame of MIC-3 encoded a protein of 141 amino acids with a calculated molecular mass of 15.3 kDa. Seven alternative polyadenylation sites have been identified for the MIC-3 transcripts, and the major transcripts are the longest ones. The MIC-3 gene contains a single intron within its coding region and belongs to a novel, multi-gene family containing up to six members. Expression of MIC-3 is root localized and specifically enhanced in the nematode induced, immature galls of resistant cotton line M-249, suggesting that MIC-3 may play a critical role in the resistance response to root-knot nematode.

Gene expression in nematode feeding sites

The feeding sites induced by sedentary root-endoparasitic nematodes have long fascinated researchers. Nematode feeding sites are constructed from plant cells, modified by the nematode to feed itself. Powerful new techniques are allowing us to begin to elucidate the molecular mechanisms that produce the ultrastructural features in nematode feeding cells. Many plant genes that are expressed in feeding sites produced by different nematodes have been identified in several plant species. Nematode-responsive plant genes can now be grouped in categories related to plant developmental pathways and their roles in the making of a feeding site can be illuminated. The black box of how nematodes bring about such elaborate cell differentiation in the plant is also starting to open. Although the information is far from complete, the groundwork is set so that the functions of the plant and nematode genes in feeding site development can begin to be assessed.

Use of the T-RFLP technique to assess spatial and temporal changes in the bacterial community structure within an agricultural soil planted with transgenic and non-transgenic potato plants

The aim of this study was to examine whether the terminal restriction fragment length polymorphism (T-RFLP) analysis represents an appropriate technique for monitoring highly diverse soil bacterial communities, i.e. to assess spatial and/or temporal effects on bacterial community structure. The T-RFLP method, a recently described fingerprinting technique, is based on terminal restriction fragment length polymorphisms between distinct small-subunit rRNA gene sequence types. This technique permits an automated quantification of the fluorescence signal intensities of the individual terminal restriction fragments (T-RFs) in a given community fingerprint pattern. The indigenous bacterial communities of three soil plots located within an agricultural field of 110 m(2) were compared. The first site was planted with non-transgenic potato plants, while the other two were planted with transgenic GUS and Barnase/Barstar potato plants, respectively. Once prior to planting and three times after planting, seven parallel samples were taken from each of the three soil plots. The T-RFLP analysis resulted in very complex but highly reproducible community fingerprint patterns. The percentage abundance values of defined T-RFs were calculated for the seven parallel samples of the respective soil plot. A multivariate analysis of variance was used to test T-RFLP data sets for significant differences. The statistical treatments clearly revealed spatial and temporal effects, as well as spacextime interaction effects, on the structural composition of the bacterial communities. T-RFs which showed the highest correlations to the discriminant factors were not those T-RFs which showed the largest single variations between the seven-sample means of individual plots. In summary, the T-RFLP technique, although a polymerase chain reaction-based method, proved to be a suitable technique for monitoring highly diverse soil microbial communities for changes over space and/or time.

MOLECULAR AND CELLULAR ASPECTS OF THE ARBUSCULAR MYCORRHIZAL SYMBIOSIS

Arbuscular mycorrhizae are symbiotic associations formed between a wide range of plant species including angiosperms, gymnosperms, pteridophytes, and some bryophytes, and a limited range of fungi belonging to a single order, the Glomales. The symbiosis develops in the plant roots where the fungus colonizes the apoplast and cells of the cortex to access carbon supplied by the plant. The fungal contribution to the symbiosis is complex, but a major aspect includes the transfer of mineral nutrients, particularly phosphate from the soil to the plant. Development of this highly compatible association requires the coordinate molecular and cellular differentiation of both symbionts to form specialized interfaces over which bi-directional nutrient transfer occurs. Recent insights into the molecular events underlying these aspects of the symbiosis are discussed.

THE TOMATO-CLADOSPORIUM FULVUM INTERACTION: A Versatile Experimental System to Study Plant-Pathogen Interactions

Over the past 20 years, the interaction between the biotrophic fungal pathogen Cladosporium fulvum and tomato has developed into a versatile experimental system for molecular plant pathology and resistance breeding. This interaction provided the resources for cloning of fungal avirulence genes for the first time and interesting clues on recognition of their extracellular products by tomato, as well as mechanisms employed by the fungus to circumvent this recognition. A wealth of information has become available on the structure and genomic organization of Cf resistance genes. The occurrence of many clustered Cf homologues allows the generation of new genes with additional recognitional specificities by reshuffling. It is anticipated that potentially all proteins secreted by C. fulvum are recognized by one or more individuals in a population of tomato genotypes, a hypothesis that has been experimentally confirmed. The future challenge will be to elucidate the mechanisms of perception of avirulence factors and the subsequent signaling eventually leading to activation of host defense responses.

Fungal avirulence genes: structure and possible functions

Avirulence (Avr) genes exist in many fungi that share a gene-for-gene relationship with their host plant. They represent unique genetic determinants that prevent fungi from causing disease on plants that possess matching resistance (R) genes. Interaction between elicitors (primary or secondary products of Avr genes) and host receptors in resistant plants causes induction of various defense responses often involving a hypersensitive response. Avr genes have been successfully isolated by reverse genetics and positional cloning. Five cultivar-specific Avr genes (Avr4, Avr9, and Ecp2 from Cladosporium fulvum; nip1 from Rhynchosporium secalis; and Avr2-YAMO from Magnaporthe grisea) and three species-specific Avr genes (PWL1 and PWL2 from M. grisea and inf1 from Phytophthora infestans) have been cloned. Isolation of additional Avr genes from these fungi, but also from other fungi such as Uromyces vignae, Melampsora lini, Phytophthora sojae, and Leptosphaeria maculans, is in progress. Molecular analyses of nonfunctional Avr gene alleles show that these originate from deletions or mutations in the open reading frame or the promoter sequence of an Avr gene. Although intrinsic biological functions of most Avr gene products are still unknown, recent studies have shown that two Avr genes, nip1 and Ecp2, encode products that are important pathogenicity factors. All fungal Avr genes cloned so far have been demonstrated or predicted to encode extracellular proteins. Current studies focus on unraveling the mechanisms of perception of avirulence factors by plant receptors. The exploitation of Avr genes and the matching R genes in engineered resistance is also discussed.

Induction of defense-related responses in Cf9 tomato cells by the AVR9 elicitor peptide of cladosporium fulvum is developmentally regulated

The AVR9 elicitor from the fungal pathogen Cladosporium fulvum induces defense-related responses, including cell death, specifically in tomato (Lycopersicon esculentum Mill.) plants that carry the Cf-9 resistance gene. To study biochemical mechanisms of resistance in detail, suspension cultures of tomato cells that carry the Cf-9 resistance gene were initiated. Treatment of cells with various elicitors, except AVR9, induced an oxidative burst, ion fluxes, and expression of defense-related genes. Agrobacterium tumefaciens-mediated transformation of Cf9 tomato leaf discs with Avr9-containing constructs resulted efficiently in transgenic callus formation. Although transgenic callus tissue showed normal regeneration capacity, transgenic plants expressing both the Cf-9 and the Avr9 genes were never obtained. Transgenic F1 seedlings that were generated from crosses between tomato plants expressing the Avr9 gene and wild-type Cf9 plants died within a few weeks. However, callus cultures that were initiated on cotyledons from these seedlings could be maintained for at least 3 months and developed similarly to callus cultures that contained only the Cf-9 or the Avr9 gene. It is concluded, therefore, that induction of defense responses in Cf9 tomato cells by the AVR9 elicitor is developmentally regulated and is absent in callus tissue and cell-suspension cultures, which consists of undifferentiated cells. These results are significant for the use of suspension-cultured cells to investigate signal transduction cascades.

Genetic engineering of potato for broad-spectrum protection against virus infection

Transgenic potato plants expressing mutant alleles of PLRV ORF4, the gene for the movement protein pr17 of this luteovirus, were generated for broad-range protection against virus infection. When tested for protection against infection by PLRV, all transgenic lines showed a significant reduction of virus antigen. Potato lines accumulating N- or C-terminally extended PLRV pr17 mutant proteins were resistant to infection by the unrelated potato viruses PVY and PVX. Transgenic lines that did not express protein despite high transcript levels failed to exhibit virus resistance.