Cellular localization of a plant protein PSAM 1 in arbuscular mycorrhizas of Pisum sativum

Psam 1 is a single-copy gene which is activated during early plant-fungal interaction in wild-type pea inoculated with Glomus mosseae and which codes for PSAM 1, a putative protein of 108 amino acids. A synthetic peptide was designed in an antigenic region of this protein to produce a polyclonal antibody against PSAM 1 and to investigate its cellular localization. Western blot analysis revealed that a polypeptide of about 14.5 kDa accumulated more in mycorrhizal than non-mycorrhizal pea roots. The PSAM 1 antigen was immunolocated in planta in arbuscule-containing cells of mycorrhizal roots and especially in the cytoplasm surrounding young arbuscules in cortical cells, which suggests that its accumulation is somehow related to the symbiotic state of these cells. Received: 27 June 1998 / Accepted: 27 July 1998     

Temporal and subcellular localization of PR-1 proteins in tomato stem tissues infected by virulent and avirulent isolates of Phytophthora capsici

Summary. Immunoblot analysis and immunogold labeling of PR-1 protein (pathogenesis-related protein 1) in tomato (Lycopersicon esculentum Mill.) were performed to examine the temporal and spatial expression patterns of PR-1 protein induced by Phytophthora capsici infection. Soluble proteins with molecular masses of 10, 17, 25, 27 and 75 kDa were induced and accumulated in P. capsici-infected stem tissues during the compatible and incompatible interactions. Western blot analysis revealed that expression of PR-1 protein (17 kDa), at 12 to 24 h after inoculation, occurred earlier in the incompatible than in the compatible interaction. Immunogold labeling of PR-1 proteins occurred over cell walls and cytoplasm of the host and the oomycete pathogen and at the interface between host and oomycete cell walls at 24 h after inoculation in the compatible interaction. In the incompatible interaction, numerous PR-1 proteins accumulated predominantly over oomycete cell walls and at the interface between host and oomycete cell walls. The quantity of PR-1 proteins deposited in both host and oomycete cells was much less in the compatible than the incompatible interaction. Healthy tomato stem tissue was nearly free of immunogold labeling of PR-1 proteins. Received October 9, 2001 Accepted January 18, 2002     

Cytosolic localization in tomato mesophyll cells of a novel glutamine synthetase induced in response to bacterial infection or phosphinothricin treatment

In tomato (Lycopersicon esculentum Mill.) leaves, the predominant glutamine synthetase (GS; EC 6.3.1.2) is chloroplastic (GS2; 45 kDa) whereas the cytosolic isoform (GS1; 39 kDa) is represented as a minor enzyme. Following either infection by Pseudomonas syringae pv. tomato (Pst) or treatment with phosphinothricin (PPT), a GS inhibitor, GS1 accumulated in the leaves. In contrast to healthy control leaves, where GS1 was restricted to the veins, in infected and PPT-treated leaves the GS1 polypeptide was also detected in the leaf blade; moreover, it was more abundant than GS2. Different immunological approaches were therefore used to investigate whether or not the GS1 polypeptide expressed in Pst-infected and PPT-treated tomato leaves was distributed among different tissues and subcellular compartments in the same way as the constitutive GS1 expressed in healthy leaves. By tissue-printing analysis, a similar GS immunostaining was observed in epidermis, mesophyll and phloem of leaflet midrib cross-sections of control, infected and PPT-treated leaves. Immunocytochemical localization revealed that GS protein was present in the chloroplast of mesophyll cells and the cytoplasm of phloem cells in healthy leaves; however, in Pst-infected or PPT-treated leaves, a strong labelling was observed in the cytoplasm of mesophyll cells. Two-dimensional analysis of GS polypeptides showed that, in addition to the constitutive GS1, a GS1 polypeptide different in charge was present in tomato leaflets after microbial infection or herbicide treatment. All these results indicate that a novel cytosolic GS is induced in mesophyll cells of Pst-infected or PPT-treated leaves. A possible role for this new cytosolic GS in the remobilization of leaf nitrogen during infection is proposed. Received: 16 January 1998 / Accepted 21 April 1998     

A taxon-specific oligonucleotide probe for temperate zone soil isolates of Glomus mosseae

 The 5.8 S subunit and flanking internal transcribed spacer (ITS) regions in nuclear ribosomal DNA (rDNA) from spores of Glomus mosseae FL156 and UK118 were amplified by polymerase chain reaction (PCR) using ITS1 and ITS4 as primers. The amplification product from template DNA of UK118 was cloned and sequenced (569 bp); the amplified DNA from FL156 was sequenced directly (582 bp). There was a 95% sequence similarity between DNAs amplified from the two isolates; in contrast, major dissimilarities with partial sequences of seven other glomalean taxa were observed. Four oligonucleotide sequences unique to Glomus mosseae were identified as potential primers. Their specificity to Glomus mosseae was assessed by PCR amplification of genomic DNA from spores from 36 glomalean fungi: 13 isolates of Glomus mosseae, two Glomus monosporum, 10 other Glomus isolates, and 11 other glomalean taxa from each of four other genera. The Glomus mosseae isolates were from a broad range of temperate zone agricultural soils. Oligonucleotide pair GMOS1 : GMOS2 primed specific amplification of an oligonucleotide sequence (approximately 400 bp) present in all Glomus mosseae isolates and two isolates of the closely related Glomus monosporum. This primer pair did not prime PCR when the template consisted of DNA from any of the other glomalean fungi or any of the nonmycorrhizal controls. In addition, a 24-mer oligonucleotide, designated GMOS5, hybridized with Glomus mosseae and Glomus monosporum DNA amplified by PCR using primer pairs ITS1 : ITS4 and GMOS1 : GMOS2. Colony-blot assays showed that GMOS5 hybridized to 100% and 97% of E. coli pUC19 clones of amplification products from Glomus mosseae FL156 and UK118 DNA templates, respectively, indicating that nearly all clones contained an homologous sequence. GMOS5 was used successfully to detect specifically Glomus mosseae in DNA extracted from colonized sudan grass (Sorghum sudanense L.) roots and amplified by PCR using the primer pair GMOS1 : GMOS2. The results confirm several previous indications that Glomus mosseae and Glomus monosporum are indistinguishable taxonomic entities. Accepted: 14 February 1998     

Arbuscular mycorrhizal fungi affect both penetration and further life stage development of root-knot nematodes in tomato

The root-knot nematode Meloidogyne incognita poses a worldwide threat to agriculture, with an increasing demand for alternative control options since most common nematicides are being withdrawn due to environmental concerns. The biocontrol potential of arbuscular mycorrhizal fungi (AMF) against plant-parasitic nematodes has been demonstrated, but the modes of action remain to be unraveled. In this study, M. incognita penetration of second-stage juveniles at 4, 8 and 12 days after inoculation was compared in tomato roots (Solanum lycopersicum cv. Marmande) pre-colonized or not by the AMF Glomus mosseae. Further life stage development of the juveniles was also observed in both control and mycorrhizal roots at 12 days, 3 weeks and 4 weeks after inoculation by means of acid fuchsin staining. Penetration was significantly lower in mycorrhizal roots, with a reduction up to 32%. Significantly lower numbers of third- and fourth-stage juveniles and females accumulated in mycorrhizal roots, at a slower rate than in control roots. The results show for the first time that G. mosseae continuously suppresses root-knot nematodes throughout their entire early infection phase of root penetration and subsequent life stage development.     

A procedure for the establishment of Glomus mosseae in dual culture with Ri T-DNA-transformed carrot roots

 A stepwise procedure was investigated to determine the optimal conditions for the establishment of Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe in dual in vitro culture with Ri T-DNA-transformed roots of Daucus carota L. Glomus mosseae spores germinated best in 10 mm Tris or MES-buffered medium at pH values just above neutral. Growth of hyphae from germinated spores was much greater in the presence of Tris than MES, eg. 8 mm versus 4 mm per spore for Tris and MES, respectively, at pH 7.2. Roots exhibited a broad pH optimum for growth of 6.0–7.0 in both MES and Tris, but did not grow well above pH 7.5. In addition, purified gelling agent, gellan gum, was utilized to lower the P concentration of media. With these factors combined, mycorrhizas were successfully established in 14% of dual cultures. Accepted: 5 March 1997     

Chitosanase and chitinase activities in tomato roots during interactions with arbuscular mycorrhizal fungi or Phytophthora parasitica

New chitosanase acidic isoforms have been shown in Glomus mosseae-colonized tomato roots and their induction, together with the previously described mycorrhiza-related chitinase isoform, has been further corroborated in plants colonized with another Glomus species (G. intraradices),as well as in tomato roots colonized in vitro by Giaspora rosea. The induction of these chitosanase isoforms appears as a specific response to the arbuscular mycorrhizal (AM) symbiosis, and does not correspond to unspecific defence mechanisms, since these isoforms were not induced by the pathogen Phytophthora parasitica. Analysis by isoelectrofocusing showed two closely migrating chitinase isoforms, specific to mycorrhizal plants colonized either with G. mosseae or G. intraradices, and their isoelectric points were estimated to be 4.5 and 4.7. The estimated molecular mass of chitosanases was 20 kDa, and after isoelectrofocusing, the chitosanase activities were detected along the acidic pH range (6.5-3.5). Constitutive and induced isoforms were also investigated during a time-course study. In some experiments, chitin and chitosan were embedded together as substrates in polyacrylamide gels with the aim of studying the capacity of some isoforms to display both chitinase and chitosanase activities. In extracts from plants colonized with either G. mosseae or G. intraradices, some constitutive chitinases and the previously described mycorrhiza-related chitinase isoform, appeared to display chitosanase activity, while this bifunctional character was not found for the chitinases from non-mycorrhizal tissue, nor in Phytophthora-infected plants. These results suggest some diversity in the chitinase activities concerning substrate specificity in mycorrhizal plants. The possible implications of these observations in the functioning of the symbiosis is discussed.Key words: Arbuscular mycorrhizas, chitinases, chitosanases, Phytophthora parasitica, tomato, Lycoperiscon esculentum.      

Purification and characterization of a glucokinase from young tomato (Lycopersicon esculentum L. Mill.) fruit

In order to clearly establish the properties of the enzymes responsible for hexose phosphorylation we have undertaken the separation and characterization of these enzymes present in tomato fruit (Martinez-Barajas and Randall 1996). This report describes the partial purification and characterization of glucokinase (EC. 2.7.1.1) from young green tomato fruit. The procedure yielded a 360-fold enrichment of glucokinase. Tomato fruit glucokinase is a monomer with a molecular mass of 53 kDa. Glucokinase activity was optimal between pH 7.5 and 8.5, preferred ATP as the phosphate donor (K _m = 0.223 mM) and exhibited low activity with GTP or UTP. The tomato fruit glucokinase showed highest affinity for glucose (K _m = 65 μM). Activity observed with glucose was 4-fold greater than with mannose and 50-fold greater than with fructose. The tomato fruit glucokinase was sensitive to product inhibition by ADP (K _i = 36 μM). Little inhibition was observed with glucose 6-phosphate (up to 15 mM) at pH 8.0; however, at pH 7.0 glucokinase activity was inhibited 30–50% by physiological concentrations of glucose 6-phosphate. Received: 4 October 1997 / Accepted: 10 January 1998     

The gene family coding for the light-harvesting polypeptides of Photosystem I of the red alga Galdieria sulphuraria*

Recently [Marquardt et al. (2000) Gene 255: 257–265], we isolated a gene encoding a polypeptide of the light-harvesting complex of Photosystem I (LHC I) of the red alga Galdieria sulphuraria. By screening a G. sulphuraria cDNA library with a DNA probe coding for the conserved first transmembrane helix of this protein we isolated four additional genes coding for LHC I polypeptides. The deduced preproteins had calculated molecular masses of 24.6–25.6 kDa and isoelectric points of 8.09–9.82. N-terminal sequencing of a LHC I polypeptide isolated by gel electrophoresis allowed us to determine the cleavage site of the transit peptide of one of the deduced polypeptides. The mature protein has a calculated molecular mass of 20.6 kDa and an isoelectric point of 7.76. The genes were amplified from nuclear G. sulphuraria DNA by polymerase chain reaction (PCR) using oligonucleotides annealing in the regions of the start and stop codons as primers. All genomic sequences were 80–300 base pairs longer than the PCR products obtained from the respective cDNA clones, pointing to the existence of 1–5 introns per gene. The G. sulphuraria genes form a homogeneous gene family with overall pairwise amino acid identities of 46.0–56.6%. Homology to two diatom, one cryptophytic and two higher plant light-harvesting polypeptides was lower with pairwise identities of 21.1–34.1%. Only one diatom polypeptide showed a higher degree of identity of up to −39.3%. This revised version was published online in June 2006 with corrections to the Cover Date.     

The NADH-dependent Fe3+-chelate reductases of tomato roots

The NADH-dependent Fe³⁺-chelate reductase (NFCHR) of tomato (Lycopersicon esculentum L.) roots, a strategy I species, was investigated. The Fe³⁺-citrate reductase (FeCitR) assay was strongly inhibited by p-hydroxymercuribenzoic acid (PHMB); moreover, the inhibitor was found to be more specific to the FeCitR assay than to the Fe³⁺-EDTA reductase assay, which was catalyzed by at least another reductase of 46 kDa. After high-speed centrifugation of tomato root membranes, high FeCitR activities were detected in pellets and lower activities in supernatants. After two-phase partitioning of microsomes, FeCitR activity (91 nmol · min⁻¹ · mg⁻¹) was less active in the upper phase (plasma membrane) than in the lower phase (277 nmol · min⁻¹ · mg⁻¹). However, only the activity of the plasma-membrane-associated NFCHR (FeCitR) was significantly enhanced (2.6-fold) in iron-deficient tomato plants, whereas that of NFCHR in non-plasma-membrane rich fractions was unaffected by this treatment. The NFCHR obtained from lysophosphatidylcholine-solubilized plasma membrane was present as a 200-kDa protein complex following fast protein liquid chromatography on Superdex 200, or as a 28-kDa form following Blue Sepharose CL-6B chromatography. Both preparations were more active following iron starvation. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that the 28-kDa protein purified from solubilized tomato microsomes or supernatant fractions by a final Mono Q step consisted of a single band of 32 kDa. Tomato root NFCHR resembled the NFCHR of maize (a strategy II plant, P Bagnaresi and P Pupillo, 1995, J Exp Bot 46: 1497–1503) in several properties: relative molecular mass, hydrophilicity, chromatographic behaviour, sensitivity to mercurials, specificity for electron donors and acceptors (e.g. cytochrome c), and a ferricyanide reductase-to-FeCitR ratio of 2.5. Preincubation with NADH partially protected NFCHR from PHMB-induced inactivation. Our data show that strategy I and II plants seem to share similar NFCHR proteins, which appear to belong to the cytochrome b ₅ reductase flavoprotein group. Received: 6 November 1996 / Accepted: 21 January 1997     

Evidence of protein-tyrosine kinase activity in Catharanthus roseus roots transformed by Agrobacterium rhizogenes

Homogenate fractions (soluble and particulate) from transformed roots of Catharanthus roseus (L.) G. Don showed several phosphorylated proteins when incubated with γ-[³²P]ATP. The phosphorylation in the proteins of 55, 40, 25, 18 and 10 kDa in the particulate fraction and 63 kDa in the soluble fraction was resistant to alkali treatment. Several proteins in both fractions gave a positive signal with monoclonal antiphosphotyrosine antibodies. In-situ phosphorylation in both fractions showed several proteins that cross-reacted with the antiphosphotyrosine antibodies. Tyrosine kinase activity was detected using an exogenous substrate RR-SRC, a synthetic peptide derived from the amino acid sequence surrounding the phosphorylation site in pp60^src. This activity was inhibited by genistein, a tyrosine kinase inhibitor. These results indicate, for the first time, the presence of protein-tyrosine kinase (EC 2.7.1.112) activity in transformed plant tissues. Received: 29 March 1997 / Accepted: 21 May 1997     

The relationship between density of Glomus mosseae propagules and the initiation and spread of arbuscular mycorrhizas in cotton roots

 The experiments aimed to determine the relationship between density of propagules in soil and initiation and spread of an arbuscular mycorrhizal fungus in the cotton roots. As few as 10 propagules of Glomus mosseae in approximately 95 g soil located in a band 25 cm below the soil surface established mycorrhizas in more than 80% of cotton roots at the point of inoculation within 36 days. Secondary spread was initiated 10–13 days after primary colonisation in treatments inoculated with one, 10 or 100 propagules. Spread of mycorrhizas within the root system was rapid from 100 propagules and was slower with fewer propagules. Accepted: 26 August 1999     

Differential display analysis of RNA accumulation in arbuscular mycorrhiza of pea and isolation of a novel symbiosis-regulated plant gene

Differential RNA display was used to analyze gene expression during the early steps of mycorrhiza development on Pisum sativum following inoculation with Glomus mosseae. Seven out of 118 differentially displayed cDNA fragments were subcloned and sequenced. One fragment corresponded to part of the fungal 25S ribosomal RNA gene and a second one showed similarity to a human Alu element. The others were derived from plant genes of unknown function. One of the fragments was used for the isolation of a full-length cDNA clone. It corresponded to a single-copy gene (psam1) which is induced during early symbiotic interactions, and codes for a putative transmembrane protein. Northern and RNA dot blot analyses revealed enhanced accumulation of psam1 RNA after inoculation with G. mosseae of wild-type pea and an isogenic mutant deficient for nodule development (Nod⁻, Myc⁺). Received: 3 March 1997 / Accepted: 12 May 1997     

Assessment of arbuscular mycorrhizal fungi diversity in the rhizosphere of <Emphasis Type="Italic">Viola calaminaria</Emphasis> and effect of these fungi on heavy metal uptake by clover

 The ability of arbuscular mycorrhizal (AM) fungi from a metal-tolerant plant (Viola calaminaria, violet) to colonise and reduce metal uptake by a non-tolerant plant (Trifolium subterraneum, subterranean clover) in comparison to a metal-tolerant AM fungus isolated from a non-tolerant plant was studied. AM spores from the violet rhizosphere and from violet roots were characterised by polymerase chain reaction (PCR) amplification of the SSU rDNA, and sequencing. Subterranean clover was grown in pots containing a soil supplemented with Cd and Zn salts and inoculated either with a mixture of spores extracted from the violet rhizosphere or with spores of a Cd-tolerant Glomus mosseae P2 (BEG 69), or non-inoculated. The diversity of fungi, including AM fungi, colonising clover roots was assessed and analysed using terminal-restriction fragment length polymorphism. At least four different Glomus species were found in the violet rhizosphere. After 8 weeks in a growth chamber, colonisation of clover roots with spores from the violet rhizosphere increased Cd and Zn concentrations in clover roots without significantly affecting the concentrations of metals in the shoot and plant growth. G. mosseae P2 reduced plant growth and slightly increased the Cd concentration. Only one AM fungus (Glomus b) from the violet rhizosphere colonised clover roots, but other fungi were present. AM fungi from heavy metal-contaminated soils and associated with metal-tolerant plants may be effective in accumulating heavy metals in roots in a non-toxic form. Accepted: 7 July 2000     

Purification, characterization, and immunolocalization of hydroxycinnamoyl-CoA: tyramine N-(hydroxycinnamoyl)transferase from opium poppy

A development-specific and elicitor-inducible acyltransferase [hydroxycinnamoyl-CoA: tyramine N-(hydroxycinnamoyl)transferase (THT; EC 2.3.1.110)] that catalyzes the transfer of hydroxycinnamic acids from hydroxycinnamoyl-CoA esters to hydroxyphenethylamines was purified 988-fold to apparent homogeneity from opium poppy (Papaver somniferum L.) cell-suspension cultures. The purification procedure, which resulted in a 6.8% yield, involved hydrophobic interaction and anion-exchange chromatography, followed by affinity chromatography on Reactive Yellow-3-Agarose using the acyl donor (feruloyl-CoA) as eluent. Purified THT had an isoelectric point of 5.2, a native molecular mass of approximately 50 kDa, and consisted of two apparently identical 25-kDa subunits as determined by two-dimensional polyacrylamide gel electrophoresis. The purified enzyme was able to synthesize a variety of amides due to a relatively low specificity for cinnamoyl-CoA derivatives and hydroxyphenethylamines. The best substrates were feruloyl-CoA (VK _m ⁻¹13.4 mkat g⁻¹ M⁻¹) and tyramine (VK _m ⁻¹6.57 mkat g⁻¹ M⁻¹). The THT activity increased during development of opium poppy seedlings, occurred at high levels in roots and stems of mature plants, and was induced in cell-suspension cultures after treatment with a pathogen-derived elicitor. Immunoblot analysis using THT mouse polyclonal antibodies did not always show a correlation between THT polypeptide and enzyme activity levels. For example, despite low THT activity in leaves, an abundant 25-kDa immunoreactive polypeptide was detected. Immunohistochemical localization showed that THT polypeptides occur in cortical and xylem parenchyma, immature xylem vessel elements, root periderm, anthers, ovules, and the inner layer of the seed coat, but are most abundant in phloem sieve-tube members in roots, stems, leaves, and anther filaments. Received: 19 January 1999 / Accepted: 3 March 1999     

Two xylanase genes of the vascular wilt pathogen Fusarium oxysporum are differentially expressed during infection of tomato plants

Two genes encoding putative family F xylanases from the tomato vascular wilt pathogen Fusarium oxysporum f.sp. lycopersici have been cloned and sequenced. The two genes, designated xyl2 and xyl3, encode proteins with calculated molecular masses of 33 and 39.3 kDa and isoelectric points of 8.9 and 6.7, respectively. The predicted amino acid sequences show significant homology to other family F xylanases. XYL3 contains a cellulose-binding domain in its N-terminal region. Southern analysis suggested that xyl2 and xyl3 homologs are also present in other formae speciales of F. oxysporum. Both genes were expressed during growth on oat spelt xylan and tomato vascular tissue in vitro. RT-PCR revealed that xyl3 is expressed in roots and in the lower stems of tomato plants infected by F. oxysporum f.sp. lycopersici throughout the whole disease cycle, whereas xyl2 is only expressed during the final stages of disease. Received: 1 June 1998 / Accepted: 25 December 1998     

Role of the modification in root exudation induced by arbuscular mycorrhizal colonization on the intraradical growth of Phytophthora nicotianae in tomato

We studied the role of modification in root exudation induced by colonization with Glomus intraradices and Glomus mosseae in the growth of Phytophthora nicotianae in tomato roots. Plants were grown in a compartmentalized plant growth system and were either inoculated with the AM fungi or received exudates from mycorrhizal plants, with the corresponding controls. Three weeks after planting, the plants were inoculated or not with P. nicotianae growing from an adjacent compartment. At harvest, P. nicotianae biomass was significantly reduced in roots colonized with G. intraradices or G. mosseae in comparison to non-colonized roots. Conversely, pathogen biomass was similar in non-colonized roots supplied with exudates collected from mycorrhizal or non-mycorrhizal roots, or with water. We cannot rule out that a mycorrhiza-mediated modification in root exudation may take place, but our results did not support that a change in pathogen chemotactic responses to host root exudates may be involved in the inhibition of P. nicotianae.     

Increases in α-mannosidase activity in the arbuscular mycorrhizal symbiosis of Allium schoenoprasum

 Mycorrhizal and nonmycorrhizal roots of Allium schoenoprasum were tested for activities of α-mannosidase, β-glucosidase and arabinosidase. Mannosidase activity was higher by a factor of two in mycorrhizal than in nonmycorrhizal root extracts. The apparent molecular weight of the enzyme was 152 kDa and its K_M was 1.25 mM in colonized roots and 1.85 mM in uncolonized roots. α-Mannosidase activity was further characterized by an acid pH optimum and Zn²⁺ dependency. No significant differences could be found between mycorrhizal and nonmycorrhizal roots for β-glucosidase and arabinosidase activities. Accepted: 28 August 1995     

Genes involved in resistance to powdery mildew in barley differentially modulate root colonization by the mycorrhizal fungus Glomus mosseae

 Arbuscular mycorrhizal fungi (AMF) and Erysiphe graminis are obligate biotrophic fungi with different outcomes in their interaction with plants, different targeted host tissues, but similar patterns of development and infection processes. These similarities raise the question of whether the two types of biotrophic fungal infections have common features in their regulation. To investigate this question, we compared a number of Ror and Rar barley mutants susceptible to E.graminis f. sp. hordei, as well as their resistant progenitors, for susceptibility to infection by the AMF Glomus mosseae. The two powdery mildew-resistant lines BC Ingrid and Sultan presented a similar reduction in G. mosseae development within roots when compared to the wildtype cultivar Ingrid, indicating a systemic effect of the altered genes in the plant. Ror and Rar mutants, in which susceptibility to powdery mildew is restored, showed increased resistance to AM fungal development in their roots when compared to their progenitors, which suggests that corresponding mutations must have affected genes which differentially modulate symbiotic and pathogenic biotrophic plant-fungus interactions. Accepted: 16 September 1999