AAL-Toxin-Deficient Mutants of Alternaria alternata Tomato Pathotype by Restriction Enzyme-Mediated Integration

Positive selection and functional diversification of transcription factor Cmr1 homologs in Alternaria

Transcription factor Cmr1 (Colletotrichum melanin regulation 1) and its homologs in several plant fungal pathogens are the regulators of the 1,8-dihydroxynaphthalene (DHN)-melanin biosynthesis pathway and have evolved functional diversification in morphology and pathogenicity. The fungal genus Alternaria comprises the group of "black fungi" that are rich in DHN-melanin in the primary cell wall and septa of the conidia. Some Alternaria species cause many economically important plant diseases worldwide. However, the evolution and function of Cmr1 homologs in Alternaria remain poorly understood. Here, we identified a total of forty-two Cmr1 homologs from forty-two Alternaria spp. and all contained one additional diverse fungal specific transcription factor motif. Phylogenetic analysis indicated the division of these homologs into five major clades and three branches. Dated phylogeny showed the A and D clades diverged latest and earliest, respectively. Molecular evolutionary analyses revealed that three amino acid sites of Cmr1 homologs in Alternaria were the targets of positive selection. Asmr1, the homolog of Cmr1 in the potato early blight pathogen, Alternaria solani was amplified and displayed the sequence conservation at the amino acid level in different A. solani isolates. Asmr1 was further confirmed to have the transcriptional activation activity and was upregulated during the early stage of potato infection. Deletion of asmr1 led to the decreased melanin content and pathogenicity, deformed conidial morphology, and responses to cell wall and fungicide stresses in A. solani. These results suggest positive selection and functional divergence have played a role in the evolution of Cmr1 homologs in Alternaria. KEY POINTS: • Cmr1 homologs were under positive selection in Alternaria species • Asmr1 is a functional transcription factor, involved in spore development, melanin biosynthesis, pathogenicity, and responses to cell wall and fungicide stresses in A. solani • Cmr1 might be used as a potential taxonomic marker of the genus Alternaria.

Mechanism of Fumonisin Self-Resistance: Fusarium verticillioides Contains Four Fumonisin B1-Insensitive-Ceramide Synthases

Fusarium verticillioides produces fumonisins, which are mycotoxins inhibiting sphingolipid biosynthesis in humans, animals, and other eukaryotes. Fumonisins are presumed virulence factors of plant pathogens, but may also play a role in interactions between competing fungi. We observed higher resistance to added fumonisin B1 (FB1) in fumonisin-producing Fusarium verticillioides than in nonproducing F. graminearum, and likewise between isolates of Aspergillus and Alternaria differing in production of sphinganine-analog toxins. It has been reported that in F. verticillioides, ceramide synthase encoded in the fumonisin biosynthetic gene cluster is responsible for self-resistance. We reinvestigated the role of FUM17 and FUM18 by generating a double mutant strain in a fum1 background. Nearly unchanged resistance to added FB1 was observed compared to the parental fum1 strain. A recently developed fumonisin-sensitive baker's yeast strain allowed for the testing of candidate ceramide synthases by heterologous expression. The overexpression of the yeast LAC1 gene, but not LAG1, increased fumonisin resistance. High-level resistance was conferred by FUM18, but not by FUM17. Likewise, strong resistance to FB1 was caused by overexpression of the presumed F. verticillioides "housekeeping" ceramide synthases CER1, CER2, and CER3, located outside the fumonisin cluster, indicating that F. verticillioides possesses a redundant set of insensitive targets as a self-resistance mechanism.

Impact of physical factors on bio-control potential of Lawsonia inermis leaf extract and bio-formulations as fungicides

The present study is carried out to ascertain the effect of different physical factors (sunlight, temperature, pH) and storage conditions on the antimicrobial efficacy of Lawsonia inermis leaf extracts and bio-formulation against the Alternaria alternata. In addition, the phytotoxic potential of 100% alcoholic crude extract as well as the acetone fraction of young leaves of Lawsonia inermis was also checked on seed germination of chilli (Capsicum annuum). Results showed that there was no adverse effect of wet heat (50–100 °C) and dry heat (40–90 °C) on extract and bio-formulation efficacy. Storage for 6 and 12 months had no adverse effect on extract and bio-formulation efficacy and the antifungal activity was observed similar to freshly prepared extract. We have used concentrations of 5,10, 15, 20 and 25 mg/ml to perform a phytotoxicity assay. The measurement of phytotoxicity was done by using the Standard blotter method and the result revealed that 5, 10 and 15 mg/ml concentration of the extract was non phytotoxic and were further used for in vivo experiments. These plant extracts and bio-formulations have extensive antimicrobial potential to be explored for application in sustainable agriculture.

Comparative Transcriptomics and Gene Knockout Reveal Virulence Factors of Neofusicoccum parvum in Walnut

Neofusicoccum parvum can cause stem and branch blight of walnut (Juglans spp.), resulting in great economic losses and ecological damage. A total of two strains of N. parvum were subjected to RNA-sequencing after being fed on different substrates, sterile water (K1/K2), and walnut (T1/T2), and the function of ABC1 was verified by gene knockout. There were 1,834, 338, and 878 differentially expressed genes (DEGs) between the K1 vs. K2, T1 vs. K1, and T2 vs. K2 comparison groups, respectively. The expression changes in thirty DEGs were verified by fluorescent quantitative PCR. These thirty DEGs showed the same expression patterns under both RNA-seq and PCR. In addition, ΔNpABC1 showed weaker virulence due to gene knockout, and the complementary strain NpABC1c showed the same virulence as the wild-type strain. Compared to the wild-type and complemented strains, the relative growth of ΔNpABC1 was significantly decreased when grown with H2O2, NaCl, Congo red, chloramphenicol, MnSO4, and CuSO4. The disease index of walnuts infected by the mutants was significantly lower than those infected by the wild-type and complementary strains. This result indicates that ABC1 gene is required for the stress response and virulence of N. parvum and may be involved in heavy metal resistance.

Mutations Found in the Asc1 Gene That Confer Susceptibility to the AAL-Toxin in Ancestral Tomatoes from Peru and Mexico

Tomato susceptibility/resistance to stem canker disease caused by Alternaria alternata f. sp. lycopersici and its pathogenic factor AAL-toxin is determined by the presence of the Asc1 gene. Several cultivars of commercial tomato (Solanum lycopersicum var. lycopersicum, SLL) are reported to have a mutation in Asc1, resulting in their susceptibility to AAL-toxin. We evaluated 119 ancestral tomato accessions including S. pimpinellifolium (SP), S. lycopersicum var. cerasiforme (SLC) and S. lycopersicum var. lycopersicum “jitomate criollo” (SLJ) for AAL-toxin susceptibility. Three accessions, SP PER018805, SLC PER018894, and SLJ M5-3, were susceptible to AAL-toxin. SLC PER018894 and SLJ M5-3 had a two-nucleotide deletion (nt 854_855del) in Asc1 identical to that found in SLL cv. Aichi-first. Another mutation (nt 931_932insT) that may confer AAL-toxin susceptibility was identified in SP PER018805. In the phylogenetic tree based on the 18 COSII sequences, a clade (S3) is composed of SP, including the AAL-toxin susceptible PER018805, and SLC. AAL-toxin susceptible SLC PER018894 and SLJ M5-3 were in Clade S2 with SLL cultivars. As SLC is thought to be the ancestor of SLL, and SLJ is an intermediate tomato between SLC and SLL, Asc1s with/without the mutation seem to have been inherited throughout the history of tomato domestication and breeding.

Overexpression of FBR41 enhances resistance to sphinganine analog mycotoxin-induced cell death and Alternaria stem canker in tomato

Fumonisin B1 (FB1) and Alternaria alternate f. sp. lycopersici (AAL)-toxin are classified as sphinganine analog mycotoxins (SAMTs), which induce programmed cell death (PCD) in plants and pose health threat to humans who consume the contaminated crop products. Herein, Fumonisin B1 Resistant41 (FBR41), a dominant mutant allele, was identified by map-based cloning of Arabidopsis FB1-resistant mutant fbr41, then ectopically expressed in AAL-toxin sensitive tomato (Solanum lycopersicum) cultivar. FBR41-overexpressing tomato plants exhibited less severe cell death phenotype upon AAL-toxin treatment. Analysis of free sphingoid bases showed that both fbr41 and FBR41-overexpressing tomato plants accumulated less sphinganine and phytosphingosine upon FB1 and AAL-toxin treatment, respectively. Alternaria stem canker is a disease caused by AAL and responsible for severe economic losses in tomato production, and FBR41-overexpressing tomato plants exhibited enhanced resistance to AAL with decreased fungal biomass and less cell death, which was accompanied by attenuated accumulation of free sphingoid bases and jasmonate (JA). Taken together, our results indicate that FBR41 is potential in inhibiting SAMT-induced PCD and controlling Alternaria stem canker in tomato.

Alternaria host-specific (HSTs) toxins: An overview of chemical characterization, target sites, regulation and their toxic effects

Alternaria causes pathogenic disease on various economically important crops having saprophytic to endophytic lifecycle. Pathogenic fungi of Alternaria species produce many primary and secondary metabolites (SMs). Alternaria species produce more than 70 mycotoxins. Several species of Alternaria produce various phytotoxins that are host-specific (HSTs) and non-host-specific (nHSTs). These toxins have various negative impacts on cell organelles including chloroplast, mitochondria, plasma membrane, nucleus, Golgi bodies, etc. Non-host-specific toxins such as tentoxin (TEN), Alternaric acid, alternariol (AOH), alternariol 9-monomethyl ether (AME), brefeldin A (dehydro-), Alternuene (ALT), Altertoxin-I, Altertoxin-II, Altertoxin-III, zinniol, tenuazonic acid (TeA), curvularin and alterotoxin (ATX) I, II, III are known toxins produced by Alternaria species. In other hand, Alternaria species produce numerous HSTs such as AK-, AF-, ACT-, AM-, AAL- and ACR-toxin, maculosin, destruxin A, B, etc. are host-specific and classified into different family groups. These mycotoxins are low molecular weight secondary metabolites with various chemical structures. All the HSTs have different mode of actions, biochemical reactions, and signaling mechanisms to causes diseases in the host plants. These HSTs have devastating effects on host plant tissues by affecting biochemical and genetic modifications. Host-specific mycotoxins such as AK-toxin, AF-toxin, and AC-toxin have the devastating effect on plants which causes DNA breakage, cytotoxic, apoptotic cell death, interrupting plant physiology by mitochondrial oxidative phosphorylation and affect membrane permeability. This article will elucidate an understanding of the disease mechanism caused by several Alternaria HSTs on host plants and also the pathways of the toxins and how they caused disease in plants.

Gene deletion of Corynespora cassiicola cassiicolin Cas1 suppresses virulence in the rubber tree

Corynespora cassiicola is an ascomycete fungus causing important damages in a wide range of plant hosts, including rubber tree. The small secreted protein cassiicolin is suspected to play a role in the onset of the disease in rubber tree, based on toxicity and gene expression profiles. However, its exact contribution to virulence, compared to other putative effectors, remains unclear. We created a deletion mutant targeting the cassiicolin gene Cas1 from the highly aggressive isolate CCP. Wild-type CCP and mutant ccpΔcas1 did not differ in terms of mycelium growth, sporulation, and germination rate in vitro. Cas1 gene deletion induced a complete loss of virulence on the susceptible clones PB260 and IRCA631, as revealed by inoculation experiments on intact (non-detached) leaves. However, residual symptoms persisted when inoculations were conducted on detached leaves, notably with longer incubation times. Complementation with exogenous cassiicolin restored the mutant capacity to colonize the leaf tissues. We also compared the toxicity of CCP and ccpΔcas1 culture filtrates, through electrolyte leakage measurements on abraded detached leaves, over a range of clones as well as an F1 population derived from the cross between the clones PB260 (susceptible) and RRIM600 (tolerant). On average, filtrate toxicity was lower but not fully suppressed in ccpΔcas1 compared to CCP, with clone-dependent variations. The two QTL, previously found associated with sensitivity to CPP filtrate or to the purified cassiicolin, were no longer detected with the mutant filtrate, while new QTL were revealed. Our results demonstrate that: (1) cassiicolin is a necrotrophic effector conferring virulence to the CCP isolate in susceptible rubber clones and (2) other effectors produced by CCP contribute to residual filtrate toxicity and virulence in senescing/wounded tissues. These other effectors may be involved in saprotrophy rather than necrotrophy.

Alternaria Toxins: Potential Virulence Factors and Genes Related to Pathogenesis

Alternaria is an important fungus to study due to their different life style from saprophytes to endophytes and a very successful fungal pathogen that causes diseases to a number of economically important crops. Alternaria species have been well-characterized for the production of different host-specific toxins (HSTs) and non-host specific toxins (nHSTs) which depend upon their physiological and morphological stages. The pathogenicity of Alternaria species depends on host susceptibility or resistance as well as quantitative production of HSTs and nHSTs. These toxins are chemically low molecular weight secondary metabolites (SMs). The effects of toxins are mainly on different parts of cells like mitochondria, chloroplast, plasma membrane, Golgi complex, nucleus, etc. Alternaria species produce several nHSTs such as brefeldin A, tenuazonic acid, tentoxin, and zinniol. HSTs that act in very low concentrations affect only certain plant varieties or genotype and play a role in determining the host range of specificity of plant pathogens. The commonly known HSTs are AAL-, AK-, AM-, AF-, ACR-, and ACT-toxins which are named by their host specificity and these toxins are classified into different family groups. The HSTs are differentiated on the basis of bio-statistical and other molecular analyses. All these toxins have different mode of action, biochemical reactions and signaling mechanisms to cause diseases. Different species of Alternaria produced toxins which reveal its biochemical and genetic effects on itself as well as on its host cells tissues. The genes responsible for the production of HSTs are found on the conditionally dispensable chromosomes (CDCs) which have been well characterized. Different bio-statistical methods like basic local alignment search tool (BLAST) data analysis used for the annotation of gene prediction, pathogenicity-related genes may provide surprising knowledge in present and future.

Agrobacterium tumefaciens-mediated transformation of Valsa mali: an efficient tool for random insertion mutagenesis

Valsa mali is a causal agent of apple and pear trees canker disease, which is a destructive disease that causes serious economic losses in eastern Asia, especially in China. The lack of an efficient transformation system for Valsa mali retards its investigation, which poses difficulties to control the disease. In this research, a transformation system for this pathogen was established for the first time using A. tumefaciens-mediated transformation (ATMT), with the optimal transformation conditions as follows: 10(6)/mL conidia suspension, cocultivation temperature 22°C, cocultivation time 72 hours, and 200  μ M acetosyringone (AS) in the inductive medium. The average transformation efficiency was 1015.00 ± 37.35 transformants per 10(6) recipient conidia. Thirty transformants were randomly selected for further confirmation and the results showed the presence of T-DNA in all hygromycin B resistant transformants and also revealed random and single gene integration with genetic stability. Compared with wild-type strain, those transformants exhibited various differences in morphology, conidia production, and conidia germination ability. In addition, pathogenicity assays revealed that 14 transformants had mitigated pathogenicity, while one had enhanced infection ability. The results suggest that ATMT of V. mali is a useful tool to gain novel insight into this economically important pathogen at molecular levels.

The fusarium mycotoxin deoxynivalenol can inhibit plant apoptosis-like programmed cell death

The Fusarium genus of fungi is responsible for commercially devastating crop diseases and the contamination of cereals with harmful mycotoxins. Fusarium mycotoxins aid infection, establishment, and spread of the fungus within the host plant. We investigated the effects of the Fusarium mycotoxin deoxynivalenol (DON) on the viability of Arabidopsis cells. Although it is known to trigger apoptosis in animal cells, DON treatment at low concentrations surprisingly did not kill these cells. On the contrary, we found that DON inhibited apoptosis-like programmed cell death (PCD) in Arabidopsis cells subjected to abiotic stress treatment in a manner independent of mitochondrial cytochrome c release. This suggested that Fusarium may utilise mycotoxins to suppress plant apoptosis-like PCD. To test this, we infected Arabidopsis cells with a wild type and a DON-minus mutant strain of F. graminearum and found that only the DON producing strain could inhibit death induced by heat treatment. These results indicate that mycotoxins may be capable of disarming plant apoptosis-like PCD and thereby suggest a novel way that some fungi can influence plant cell fate.

Disruption of sphingolipid biosynthesis in Nicotiana benthamiana activates salicylic acid-dependent responses and compromises resistance to Alternaria alternata f. sp. lycopersici

Sphingolipids play an important role in signal transduction pathways that regulate physiological functions and stress responses in eukaryotes. In plants, recent evidence suggests that their metabolic precursors, the long-chain bases (LCBs) act as bioactive molecules in the immune response. Interestingly, the virulence of two unrelated necrotrophic fungi, Fusarium verticillioides and Alternaria alternata, which are pathogens of maize and tomato plants, respectively, depends on the production of sphinganine-analog mycotoxins (SAMs). These metabolites inhibit de novo synthesis of sphingolipids in their hosts causing accumulation of LCBs, which are key regulators of programmed cell death. Therefore, to gain more insight into the role of sphingolipids in plant immunity against SAM-producing necrotrophic fungi, we disrupted sphingolipid metabolism in Nicotiana benthamiana through virus-induced gene silencing (VIGS) of the serine palmitoyltransfersase (SPT). This enzyme catalyzes the first reaction in LCB synthesis. VIGS of SPT profoundly affected N. benthamiana development as well as LCB composition of sphingolipids. While total levels of phytosphingosine decreased, sphinganine and sphingosine levels increased in SPT-silenced plants, compared with control plants. Plant immunity was also affected as silenced plants accumulated salicylic acid (SA), constitutively expressed the SA-inducible NbPR-1 gene and showed increased susceptibility to the necrotroph A. alternata f. sp. lycopersici. In contrast, expression of NbPR-2 and NbPR-3 genes was delayed in silenced plants upon fungal infection. Our results strongly suggest that LCBs modulate the SA-dependent responses and provide a working model of the potential role of SAMs from necrotrophic fungi to disrupt the plant host response to foster colonization.

A polyketide synthase gene, ACRTS2, is responsible for biosynthesis of host-selective ACR-toxin in the rough lemon pathotype of Alternaria alternata

The rough lemon pathotype of Alternaria alternata produces host-selective ACR-toxin and causes Alternaria leaf spot disease of rough lemon (Citrus jambhiri). The structure of ACR-toxin I (MW = 496) consists of a polyketide with an α-dihydropyrone ring in a 19-carbon polyalcohol. Genes responsible for toxin production were localized to a 1.5-Mb chromosome in the genome of the rough lemon pathotype. Sequence analysis of this chromosome revealed an 8,338-bp open reading frame, ACRTS2, that was present only in the genomes of ACR-toxin-producing isolates. ACRTS2 is predicted to encode a putative polyketide synthase of 2,513 amino acids and belongs to the fungal reducing type I polyketide synthases. Typical polyketide functional domains were identified in the predicted amino acid sequence, including β-ketoacyl synthase, acyl transferase, methyl transferase, dehydratase, β-ketoreductase, and phosphopantetheine attachment site domains. Combined use of homologous recombination-mediated gene disruption and RNA silencing allowed examination of the functional role of multiple paralogs in ACR-toxin production. ACRTS2 was found to be essential for ACR-toxin production and pathogenicity of the rough lemon pathotype of A. alternata.

Ethylene signaling pathway and MAPK cascades are required for AAL toxin-induced programmed cell death

Programmed cell death (PCD), known as hypersensitive response cell death, has an important role in plant defense response. The signaling pathway of PCD remains unknown. We employed AAL toxin and Nicotiana umbratica to analysis plant PCD. AAL toxin is a pathogenicity factor of the necrotrophic pathogen Alternaria alternata f. sp. lycopersici. N. umbratica is sensitive to AAL toxin, susceptible to pathogens, and effective in Tobacco rattle virus-based virus-induced gene silencing (VIGS). VIGS analyses indicated that AAL toxin-triggered cell death (ACD) is dependent upon the mitogen-activated protein (MAP) kinase kinase MEK2, which is upstream of both salicylic acid-induced protein kinase (SIPK) and wound-induced protein kinase (WIPK) responsible for ethylene (ET) synthesis. ET treatment of MEK2-silenced N. umbratica re-established ACD. In SIPK- and WIPK-silenced N. umbratica, ACD was compromised and ET accumulation was not observed. However, in contrast to the case of MEK2-silenced plants, ET treatment did not induce cell death in SIPK- and WIPK-silenced plants. This work showed that ET-dependent pathway and MAP kinase cascades are required in ACD. Our results suggested that MEK2-SIPK/WIPK cascades have roles in ET biosynthesis; however, SIPK and WIPK have other roles in ET signaling or another pathway leading to cell death by AAL toxin.

Role of the pathotype-specific ACRTS1 gene encoding a hydroxylase involved in the biosynthesis of host-selective ACR-toxin in the rough lemon pathotype of Alternaria alternata

The rough lemon pathotype of Alternaria alternata produces host-selective ACR-toxin and causes Alternaria leaf spot disease of the rootstock species rough lemon (Citrus jambhiri) and Rangpur lime (C. limonia). Genes controlling toxin production were localized to a 1.5-Mb chromosome carrying the ACR-toxin biosynthesis gene cluster (ACRT) in the genome of the rough lemon pathotype. A genomic BAC clone containing a portion of the ACRT cluster was sequenced which allowed identification of three open reading frames present only in the genomes of ACR-toxin producing isolates. We studied the functional role of one of these open reading frames, ACRTS1 encoding a putative hydroxylase, in ACR-toxin production by homologous recombination-mediated gene disruption. There are at least three copies of ACRTS1 gene in the genome and disruption of two copies of this gene significantly reduced ACR-toxin production as well as pathogenicity; however, transcription of ACRTS1 and production of ACR-toxin were not completely eliminated due to remaining functional copies of the gene. RNA-silencing was used to knock down the remaining ACRTS1 transcripts to levels undetectable by reverse transcription-polymerase chain reaction. The silenced transformants did not produce detectable ACR-toxin and were not pathogenic. These results indicate that ACRTS1 is an essential gene in ACR-toxin biosynthesis in the rough lemon pathotype of A. alternata and is required for full virulence of this fungus.

Sphingolipids and plant defense/disease: the "death" connection and beyond

Sphingolipids comprise a major class of structural materials and lipid signaling molecules in all eukaryotic cells. Over the past two decades, there has been a phenomenal growth in the study of sphingolipids (i.e., sphingobiology) at an average rate of ∼1000 research articles per year. Sphingolipid studies in plants, though accounting for only a small fraction (∼6%) of the total number of publications, have also enjoyed proportionally rapid growth in the past decade. Concomitant with the growth of sphingobiology, there has also been tremendous progress in our understanding of the molecular mechanisms of plant innate immunity. In this review, we (i) cross examine and analyze the major findings that establish and strengthen the intimate connections between sphingolipid metabolism and plant programmed cell death (PCD) associated with plant defense or disease; (ii) highlight and compare key bioactive sphingolipids involved in the regulation of plant PCD and possibly defense; (iii) discuss the potential role of sphingolipids in polarized membrane/protein trafficking and formation of lipid rafts as subdomains of cell membranes in relation to plant defense; and (iv) where possible, attempt to identify potential parallels for immunity-related mechanisms involving sphingolipids across kingdoms.

Horizontal transfer of supernumerary chromosomes in fungi

Several species of filamentous fungi contain so-called dispensable or supernumerary chromosomes. These chromosomes are dispensable for the fungus to survive, but may carry genes required for specialized functions, such as infection of a host plant. It has been shown that at least some dispensable chromosomes are able to transfer horizontally (i.e., in the absence of a sexual cycle) from one fungal strain to another. In this paper, we describe a method by which this can be shown. Horizontal chromosome transfer (HCT) occurs during co-incubation of two strains. To document the actual occurrence of HCT, it is necessary to select for HCT progeny. This is accomplished by transforming two different drug-resistance genes into the two parent strains before their co-incubation. In one of the strains (the "donor"), a drug-resistance gene should be integrated in a chromosome of which the propensity for HCT is under investigation. In the "tester" or "recipient" strain, another drug-resistance gene should be integrated somewhere in the core genome. In this way, after co-incubation, HCT progeny can be selected on plates containing both drugs. HCT can be initiated with equal amounts of asexual spores of both strains, plated on regular growth medium for the particular fungus, followed by incubation until new asexual spores are formed. The new asexual spores are then harvested and plated on plates containing both drugs. Double drug-resistant colonies that appear should carry at least one chromosome from each parental strain. Finally, double drug-resistant strains need to be analysed to assess whether HCT has actually occurred. This can be done by various genome mapping methods, like CHEF-gels, AFLP, RFLP, PCR markers, optical maps, or even complete genome sequencing.

Methods for investigating the UPR in filamentous fungi

Filamentous fungi have a high-capacity secretory system and are therefore widely exploited for the industrial production of native and heterologous proteins. However, in most cases, the yields of nonfungal proteins are significantly lower than those obtained for fungal proteins. One well-studied bottleneck appears to be the result of slow or aberrant folding of heterologous proteins in the ER during the early stages of secretion within the endoplasmic reticulum, leading to stress responses in the host, including the unfolded protein response (UPR). Most of the key elements constituting the signal transduction pathway of the UPR in Saccharomyces cerevisiae have been identified in filamentous fungi, including the central activation mechanism of the pathway, that is, the stress-induced splicing of an unconventional (nonspliceosomal) intron in orthologs of the HAC1 mRNA. This splicing event relieves a translational block in the HAC1 mRNA, allowing for the translation of the bZIP transcription factor Hac1p that regulates the expression of UPR target genes. The UPR is involved in regulating the folding, yield, and delivery of secretory proteins and that has consequences for fungal lifestyles, including virulence and biotechnology. The recent releases of genome sequences of several species of filamentous fungi and the availability of DNA arrays, GeneChips, and deep sequencing methodologies have provided an unprecedented resource for exploring expression profiles in response to secretion stresses. Furthermore, genome-wide investigation of translation profiles through polysome analyses is possible, and here, we outline methods for the use of such techniques with filamentous fungi and, principally, Aspergillus niger. We also describe methods for the batch and controlled cultivation of A. niger and for the replacement and study of its hacA gene, which provides either a UPR-deficient strain or a constitutively activated UPR strain for comparative analysis with its wild type. Although we focus on A. niger, the utility of the hacA-deletion strategy is also described for use in investigating the virulence of the plant pathogen Alternaria brassicicola.

When and how to kill a plant cell: infection strategies of plant pathogenic fungi

Fungi cause severe diseases on a broad range of crop and ornamental plants, leading to significant economical losses. Plant pathogenic fungi exhibit a huge variability in their mode of infection, differentiation and function of infection structures and nutritional strategy. In this review, advances in understanding mechanisms of biotrophy, necrotrophy and hemibiotrophic lifestyles are described. Special emphasis is given to the biotrophy-necrotrophy switch of hemibiotrophic pathogens, and to biosynthesis, chemical diversity and mode of action of various fungal toxins produced during the infection process.

SGT1 positively regulates the process of plant cell death during both compatible and incompatible plant-pathogen interactions

SGT1 (suppressor of G2 allele of Skp1), an interactor of SCF (Skp1-Cullin-F-box) ubiquitin ligase complexes that mediate protein degradation, plays an important role at both G1-S and G2-M cell cycle transitions in yeast, and is highly conserved throughout eukaryotes. Plant SGT1 is required for both resistance (R) gene-mediated disease resistance and nonhost resistance to certain pathogens. Using virus-induced gene silencing (VIGS) in Nicotiana benthamiana, we demonstrate that SGT1 positively regulates the process of cell death during both host and nonhost interactions with various pathovars of Pseudomonas syringae. Silencing of NbSGT1 in N. benthamiana plants delays the induction of hypersensitive response (HR)-mediated cell death against nonhost pathogens and the development of disease-associated cell death caused by the host pathogen P. syringae pv. tabaci. Our results further demonstrate that NbSGT1 is required for Erwinia carotovora- and Sclerotinia sclerotiorum-induced disease-associated cell death. Overexpression of NbSGT1 in N. benthamiana accelerates the development of HR during R gene-mediated disease resistance and nonhost resistance. Our data also indicate that SGT1 is required for pathogen-induced cell death, but is not always necessary for the restriction of bacterial multiplication in planta. Therefore, we conclude that SGT1 is an essential component affecting the process of cell death during both compatible and incompatible plant-pathogen interactions.

Identification and function of a polyketide synthase gene responsible for 1,8-dihydroxynaphthalene-melanin pigment biosynthesis in Ascochyta rabiei

Ascochyta rabiei produces and accumulates one of the well-known fungal polyketides, 1,8-dihydroxynaphthalene-melanin pigment (DHN-melanin), in asexual and sexual fruiting bodies. Degenerate PCR primers were used to isolate an ArPKS1 of A. rabiei encoding a polypeptide with high similarity to polyketide synthase (PKS) involved in biosynthesis of DHN-melanin in other ascomycetous fungi. Site-directed mutagenesis of ArPKS1 in A. rabiei generated melanin-deficient pycnidial mutants but caused no significant reduction of pathogenicity to chickpea. Pycnidiospores in ArPKS1-mutant pycnidia showed higher sensitivity to UV light exposure compared to pycnidiospores in melanized pycnidia of the wild-type progenitor isolate. Integration of an orthologous PKS1 gene from Bipolaris oryzae into the genome of the mutants complemented the dysfunctional ArPKS1 gene. This study demonstrated that A. rabiei uses a DHN-melanin pathway for pigmentation of pycnidia and this molecule may protect pycnidiospores from UV irradiation.

ACTTS3 encoding a polyketide synthase is essential for the biosynthesis of ACT-toxin and pathogenicity in the tangerine pathotype of Alternaria alternata

The tangerine pathotype of Alternaria alternata produces host-selective ACT-toxin and causes Alternaria brown spot disease of tangerine and tangerine hybrids. Sequence analysis of a genomic BAC clone identified part of the ACT-toxin TOX (ACTT) gene cluster, and knockout experiments have implicated several open reading frames (ORF) contained within the cluster in the biosynthesis of ACT-toxin. One of the ORF, designated ACTTS3, encoding a putative polyketide synthase, was isolated by rapid amplification of cDNA ends and genomic/reverse transcription-polymerase chain reactions using the specific primers designed from the BAC sequences. The 7,374-bp ORF encodes a polyketide synthase with putative beta-ketoacyl synthase, acyltransferase, methyltransferase, beta-ketoacyl reductase, and phosphopantetheine attachment site domains. Genomic Southern blots demonstrated that ACTTS3 is present on the smallest chromosome in the tangerine pathotype of A. alternata, and the presence of ACTTS3 is highly correlated with ACT-toxin production and pathogenicity. Targeted gene disruption of two copies of ACTTS3 led to a complete loss of ACT-toxin production and pathogenicity. These results indicate that ACTTS3 is an essential gene for ACT-toxin biosynthesis in the tangerine pathotype of A. alternata and is required for pathogenicity of this fungus.

Development of a transformation system in the swainsonine producing, slow growing endophytic fungus, Undifilum oxytropis

Undifilum oxytropis (Phylum: Ascomycota; Family: Pleosporaceae) is a slow growing endophytic fungus that produces a toxic alkaloid, swainsonine. This endophyte resides in locoweeds, which are perennial flowering legumes. Consumption of this fungus by grazing animals induces a neurological disorder called locoism. The alkaloid swainsonine, an alpha-mannosidase inhibitor, is responsible for the field toxicity related to locoism. Little is known about the biosynthetic pathway of swainsonine in endophytic fungi. Genetic manipulation of endophytic fungi is important to better understand biochemical pathways involved in alkaloid synthesis, but no transformation system has been available for studying such enzymes in Undifilum. In this study we report the development of protoplast and transformation system for U. oxytropis. Fungal mycelia required for generating protoplasts were grown in liquid culture, then harvested and processed with various enzymes. Protoplasts were transformed with a fungal specific vector driving the expression of Enhanced Green Florescent Protein (EGFP). The quality of transformed protoplasts and transformation efficiency were monitored during the process. In all cases, resistance to antibiotic hygromycin B was maintained. Such manipulation will open avenues for future research to decipher fungal metabolic pathways.

Role of the host-selective ACT-toxin synthesis gene ACTTS2 encoding an enoyl-reductase in pathogenicity of the tangerine pathotype of Alternaria alternata

ABSTRACT The tangerine pathotype of Alternaria alternata produces host-selective ACT-toxin and causes Alternaria brown spot disease of tangerines and tangerine hybrids. Sequence analysis of a genomic BAC clone identified a previously uncharacterized portion of the ACT-toxin biosynthesis gene cluster (ACTT). A 1,034-bp gene encoding a putative enoyl-reductase was identified by using rapid amplification of cDNA ends and polymerase chain reaction and designated ACTTS2. Genomic Southern blots demonstrated that ACTTS2 is present only in ACT-toxin producers and is carried on a 1.9 Mb conditionally dispensable chromosome by the tangerine pathotype. Targeted gene disruption of ACTTS2 led to a reduction in ACT-toxin production and pathogenicity, and transcriptional knockdown of ACTTS2 using RNA silencing resulted in complete loss of ACT-toxin production and pathogenicity. These results indicate that ACTTS2 is an essential gene for ACT-toxin biosynthesis in the tangerine pathotype of A. alternata and is required for pathogenicity of this fungus.

Horizontal chromosome transfer, a mechanism for the evolution and differentiation of a plant-pathogenic fungus

The tomato pathotype of Alternaria alternata produces host-specific AAL toxin and causes Alternaria stem canker on tomato. A polyketide synthetase (PKS) gene, ALT1, which is involved in AAL toxin biosynthesis, resides on a 1.0-Mb conditionally dispensable chromosome (CDC) found only in the pathogenic and AAL toxin-producing strains. Genomic sequences of ALT1 and another PKS gene, both of which reside on the CDC in the tomato pathotype strains, were compared to those of tomato pathotype strains collected worldwide. This revealed that the sequences of both CDC genes were identical among five A. alternata tomato pathotype strains having different geographical origins. On the other hand, the sequences of other genes located on chromosomes other than the CDC are not identical in each strain, indicating that the origin of the CDC might be different from that of other chromosomes in the tomato pathotype. Telomere fingerprinting and restriction fragment length polymorphism analyses of the A. alternata strains also indicated that the CDCs in the tomato pathotype strains were identical, although the genetic backgrounds of the strains differed. A hybrid strain between two different pathotypes was shown to harbor the CDCs derived from both parental strains with an expanded range of pathogenicity, indicating that CDCs can be transmitted from one strain to another and stably maintained in the new genome. We propose a hypothesis whereby the ability to produce AAL toxin and to infect a plant could potentially be distributed among A. alternata strains by horizontal transfer of an entire pathogenicity chromosome. This could provide a possible mechanism by which new pathogens arise in nature.

The jasmonate signaling pathway in tomato regulates susceptibility to a toxin-dependent necrotrophic pathogen

The plant hormone, jasmonic acid (JA), is known to have a critical role in both resistance and susceptibility against bacterial and fungal pathogen attack. However, little is known about the involvement of JA in the interactions between plants and toxigenic necrotrophic fungal pathogens. Using the tomato pathotype of Alternaria alternata (Aa) and its AAL-toxin/tomato interaction as a model system, we demonstrate a possible role for JA in susceptibility of plants against pathogens, which utilize host-specific toxins as virulence effectors. Disease development and in planta growth of the tomato pathotype of Aa were decreased in the def1 mutant, defective in biosynthesis of JA, compared with the wild-type (WT) cultivar. Exogenous methyl jasmonate (MeJA) application restored pathogen disease symptoms to the def1 mutant and led to increased disease in the WT. On the other hand, necrotic cell death was similarly induced by AAL-toxin both on def1 and WT, and MeJA application to the tomatoes did not affect the degree of cell death by the toxin. These results indicate that the JA-dependent signaling pathway is not involved in host basal defense responses against the tomato pathotype of Aa, but rather might affect pathogen acceptability via a toxin-independent manner. Data further suggest that JA has a promotional effect on susceptibility of tomato to toxigenic and necrotrophic pathogens, such that pathogens might utilize the JA signaling pathway for successful infection.

Improved degradation of organophosphate dichlorvos by Trichoderma atroviride transformants generated by restriction enzyme-mediated integration (REMI)

A simple technique, REMI (restriction enzyme-mediated integration), was used to construct transformants of Trichoderma atroviride with improved capability of degrading organophosphate pesticide dichlorvos. Linearized DNA of plasmid pV2 bearing the hygromycin B phosphotransferase (hph) gene was inserted into chromosomes of wild strain T23 and transformation was confirmed by PCR and Southern blot analysis, respectively. Of 247 transformants, 76% showed improved dichlorvos degradation ability as compared to the parent strain T23 based on the least significant difference (LSD) test at p=0.01. Among them, 8 transformants exhibited 30% higher in degradation rate than the parent isolate. The highest dichlorvos degradation rate of the transformants was up to 96%. This study provided an effective approach for improving organophosphate pesticide-degrading capability of T. atroviride.

Mannitol metabolism in the phytopathogenic fungus Alternaria alternata

Mannitol metabolism in fungi is thought to occur through a mannitol cycle first described in 1978. In this cycle, mannitol 1-phosphate 5-dehydrogenase (EC 1.1.1.17) was proposed to reduce fructose 6-phosphate into mannitol 1-phosphate, followed by dephosphorylation by a mannitol 1-phosphatase (EC 3.1.3.22) resulting in inorganic phosphate and mannitol. Mannitol would be converted back to fructose by the enzyme mannitol dehydrogenase (EC 1.1.1.138). Although mannitol 1-phosphate 5-dehydrogenase was proposed as the major biosynthetic enzyme and mannitol dehydrogenase as a degradative enzyme, both enzymes catalyze their respective reverse reactions. To date the cycle has not been confirmed through genetic analysis. We conducted enzyme assays that confirmed the presence of these enzymes in a tobacco isolate of Alternaria alternata. Using a degenerate primer strategy, we isolated the genes encoding the enzymes and used targeted gene disruption to create mutants deficient in mannitol 1-phosphate 5-dehydrogenase, mannitol dehydrogenase, or both. PCR analysis confirmed gene disruption in the mutants, and enzyme assays demonstrated a lack of enzymatic activity for each enzyme. GC-MS experiments showed that a mutant deficient in both enzymes did not produce mannitol. Mutants deficient in mannitol 1-phosphate 5-dehydrogenase or mannitol dehydrogenase alone produced 11.5 and 65.7 %, respectively, of wild type levels. All mutants grew on mannitol as a sole carbon source, however, the double mutant and mutant deficient in mannitol 1-phosphate 5-dehydrogenase grew poorly. Our data demonstrate that mannitol 1-phosphate 5-dehydrogenase and mannitol dehydrogenase are essential enzymes in mannitol metabolism in A. alternata, but do not support mannitol metabolism operating as a cycle.

Genetic and Genomic Dissection of the Cochliobolus heterostrophus Tox1 Locus Controlling Biosynthesis of the Polyketide Virulence Factor T‐toxin

Fungal pathogenesis to plants is an intricate developmental process requiring biological components found in most fungi, as well as factors that are unique to fungal taxa that participate in particular fungus-plant interactions. The host-selective polyketide toxin known as T-toxin produced by Cochliobolus heterostrophus race T, a highly virulent pathogen of maize, is an intriguing example of the latter type of virulence determinant. The Tox1 locus, which controls biosynthesis of T-toxin, originally defined as a single genetic locus, it is, in fact, two exceedingly complex loci on two chromosomes that are reciprocally translocated with respect to their counterparts in weakly pathogenic race O. Race O lacks the Tox1 locus and does not produce T-toxin. Highly virulent race T was first recognized when it caused an epidemic of Southern Corn Leaf Blight, which devastated the US corn crop in 1970. The evolutionary origin of the Tox1 locus remains unknown.

G protein signaling mediates developmental processes and pathogenesis of Alternaria alternata

A G protein alpha subunit gene (AGA1) has been cloned and characterized from a toxigenic and necrotrophic Alternaria alternata pathogen. Targeted disruption of AGA1 in the apple pathotype of A. alternata gave rise to mutants that differed in colony and conidial morphology as well as sporulation. The conidia of wild type and deltaAGA1 mutants showed equal germination on cellulose membranes. However, wild-type germ tubes formed readily from different points around the conidia, grew randomly, and were often branched, whereas those of the mutants formed only at one or both ends of the conidia and tended to grow in straight paths. Targeted disruption of AGA1 also resulted in reduction of pathogenicity on apple leaves, although the mutant produced host-specific AM-toxin, a fungal secondary metabolite associated with pathogenicity of the pathogen, at levels similar to the wild-type strain. Measurement of the intracellular cAMP levels of the mutant revealed that it was consistently higher than that of the wild type, indicating that AGA1 negatively regulates cAMP levels similar to mammalian Galphai systems. These results indicate that the signal transduction pathway represented by AGA1 appears to be involved in developmental pathways leading to sporulation and pathogenesis of A. alternata.

A high throughput targeted gene disruption method for Alternaria brassicicola functional genomics using linear minimal element (LME) constructs

Alternaria brassicicola causes black spot disease of cultivated Brassicas and has been used consistently as a necrotrophic fungal pathogen for studies with Arabidopsis. In A. brassicicola, mutant generation has been the most rate-limiting step for the functional analysis of individual genes due to low efficiency of both transformation and targeted integration. To improve the targeted gene disruption efficiency as well as to expedite gene disruption construct production, we used a short linear construct with minimal elements, an antibiotic resistance selectable marker gene, and a 250- to 600-bp-long partial target gene. The linear minimal element (LME) constructs consistently produced stable transformants for diverse categories of genes. Typically, 100% of the transformants were targeted gene disruption mutants when using the LME constructs, compared with inconsistent transformation and usually less than 10% targeted gene disruption with circular plasmid disruption constructs. Each mutant displayed a unique molecular signature thought to originate from endogenous exonuclease activities in fungal cells. Our data suggests that a DNA double-stranded break repair mechanism (DSBR) functions to increase targeting efficiency. This method is advantageous for high throughput gene disruption, overexpression, and reporter gene introduction within target genes, especially for asexual filamentous fungi where genetic approaches are unfavorable.

Use of REMI andAgrobacterium-mediated transformation to identify pathogenicity mutants of the biocontrol fungus,Coniothyrium minitans

Restriction enzyme mediated integration (REMI) and Agrobacterium-mediated transformation (ATMT) were used to transform protoplasts or germinated conidia of the mycoparasite Coniothyrium minitans to hygromycin resistance. Using REMI, up to 32 transformants mug DNA(-1) were obtained, while 37.8 transformants 5 x 10(5) germlings(-1) were obtained using ATMT. Single-copy integrations occurred in 8% and 40% of REMI and ATMT transformants, respectively. A novel microtitre plate-based test was developed to expedite screening of 4000 REMI and ATMT C. minitans transformants. Nine pathogenicity mutants that displayed reduced or no pathogenicity on sclerotia of Sclerotinia sclerotiorum were identified.

Development of a Robust Screening Method for Pathogenicity of Colletotrichum spp. on Strawberry Seedlings Enabling Forward Genetic Studies

Generation and screening for nonpathogenic mutants is a popular tool for identifying pathogenicity-related genes. Successful application of this technique for plant fungal pathosystems requires reliable and rapid screening procedures. This study reports on the development of a rapid in vitro bioassay enabling large-scale screening and isolation of nonpathogenic mutants of Colletotrichum gloeosporioides and C. acutatum on strawberry seedlings. Inoculation was carried out on strawberry seedlings at two different developmental stages: 12-week-old (young) and 15-week-old (older) seedlings. A comparison was made between two inoculation techniques, (i) foliar dip and (ii) root soak, at two incubation temperatures (19 and 25°C). Mortality of young seedlings was observed 4 days after inoculation with both species, reaching 50% within 10 days, using both techniques at 25°C. However, mortality of older seedlings was delayed by 4 days compared with that in the young seedlings when using the root-soak method. Disease development decreased in young and older seedlings at the lower temperature. This method also was reliable in determining pathogenicity of the cucurbit-specific C. magna that did not cause disease symptoms on strawberry by either inoculation method. The proposed method enabled screening of more than 980 restriction enzyme-mediated integration mutants resulting in a selection of five reduced-virulence isolates. Initial characterization of some of these mutants revealed large differences in germination and appressorial formation compared with pathogenic isolates.

The role and regulation of programmed cell death in plant-pathogen interactions

It is commonly known that animal pathogens often target and suppress programmed cell death (pcd) pathway components to manipulate their hosts. In contrast, plant pathogens often trigger pcd. In cases in which plant pcd accompanies disease resistance, an event called the hypersensitive response, the plant surveillance system has learned to detect pathogen-secreted molecules in order to mount a defence response. In plants without genetic disease resistance, these secreted molecules serve as virulence factors that act through largely unknown mechanisms. Recent studies suggest that plant bacterial pathogens also secrete antiapoptotic proteins to promote their virulence. In contrast, a number of fungal pathogens secrete pcd-promoting molecules that are critical virulence factors. Here, we review recent progress in determining the role and regulation of plant pcd responses that accompany both resistance and susceptible interactions. We also review progress in discerning the mechanisms by which plant pcd occurs during these different interactions.

Expression of THR1, a 1,3,8-Trihydroxynaphthalene Reductase Gene Involved in Melanin Biosynthesis in the Phytopathogenic Fungus Bipolaris oryzae, is Enhanced by Near-Ultraviolet Radiation

1,3,8-Trihydroxynaphthalene (1,3,8-THN) reductase is involved in the production of fungal dihydroxynaphthalene (DHN) melanin. We isolated and characterized THR1, a gene encoding 1,3,8-THN reductase, from the phytopathogenic fungus Bipolaris oryzae. Sequence analysis showed that THR1 encodes a putative protein of 267 amino acids having a molecular weight of 28.5 kDa and 68-98% sequence identity to other fungal 1,3,8-THN reductases. Targeted disruption of the THR1 gene showed that it is essential for melanin biosynthesis in B. oryzae. Northern blot analysis showed that THR1 transcripts are constitutively expressed during normal growth but are specifically enhanced by near-ultraviolet (NUV) radiation in a dose-dependent manner. These results indicate that THR1 expression is transcriptionally enhanced by NUV radiation in B. oryzae.

Cloning, functional analysis and expression of a scytalone dehydratase gene ( SCD1) involved in melanin biosynthesis of the phytopathogenic fungus Bipolaris oryzae

Scytalone dehydratase is involved in the production of fungal dihydroxynaphthalene melanin. We isolated and characterized SCD1, a gene encoding scytalone dehydratase, from the phytopathogenic fungus Bipolaris oryzae. Sequence analysis showed that SCD1 encodes a putative protein that has 185 amino acids, a molecular weight of 21 kDa and 51-75% sequence identity to other fungal scytalone dehydratases. Targeted disruption of SCD1 showed that this gene is necessary for melanin biosynthesis in B. oryzae. Northern blot analysis showed that SCD1 transcripts are specifically enhanced by near-ultraviolet (300-400 nm) radiation.

Cercosporin-deficient mutants by plasmid tagging in the asexual fungus Cercospora nicotianae

We have successfully adapted plasmid insertion and restriction enzyme-mediated integration (REMI) to produce cercosporin toxin-deficient mutants in the asexual phytopathogenic fungus Cercospora nicotianae. The use of pre-linearized plasmid or restriction enzymes in the transformation procedure significantly decreased the transformation frequency, but promoted a complicated and undefined mode of plasmid integration that leads to mutations in the C. nicotianae genome. Vector DNA generally integrated in multiple copies, and no increase in single-copy insertion was observed when enzymes were added to the transformation mixture. Out of 1873 transformants tested, 39 putative cercosporin toxin biosynthesis ( ctb) mutants were recovered that showed altered levels of cercosporin production. Seven ctb mutants were recovered using pre-linearized plasmids without the addition of enzymes, and these were considered to be non-REMI mutants. The correlation between a specific insertion and a mutant phenotype was confirmed using rescued plasmids as gene disruption vectors in the wild-type strain. Six out of fifteen rescued plasmids tested yielded cercosporin-deficient transformants when re-introduced into the wild-type strain, suggesting a link between the insertion site and the cercosporin-deficient phenotype. Sequence analysis of a fragment flanking the insert site recovered from one insertion mutant showed it to be disrupted in sequences with high homology to the acyl transferase domain of polyketide synthases from other fungi. Disruption of this polyketide synthase gene ( CTB1) using a rescued plasmid resulted in mutants that were defective in cercosporin production. Thus, we provide the first molecular evidence that cercosporin is synthesized via a polyketide pathway as previously hypothesized.

Alternaria spp.: from general saprophyte to specific parasite

SUMMARY Alternaria species are mainly saprophytic fungi. However, some species have acquired pathogenic capacities collectively causing disease over a broad host range. This review summarizes the knowledge on pathogenic strategies employed by the fungus to plunder the host. Furthermore, strategies employed by potential host plants in order to ward off an attack are discussed.

TAXONOMY

Alternaria spp. kingdom Fungi, subkingdom Eumycotera, phylum Fungi Imperfecti (a non-phylogenetic or artificial phylum of fungi without known sexual stages whose members may or may not be related; taxonomy does not reflect relationships), form class Hypomycetes, Form order Moniliales, form family Dematiaceae, genus Alternaria. Some species of Alternaria are the asexual anamorph of the ascomycete Pleospora while others are speculated to be anamorphs of Leptosphaeria.

HOST RANGE

Most Alternaria species are common saprophytes that derive energy as a result of cellulytic activity and are found in a variety of habitats as ubiquitous agents of decay. Some species are plant pathogens that cause a range of economically important diseases like stem cancer, leaf blight or leaf spot on a large variety of crops. Latent infections can occur and result in post-harvest diseases or damping-off in case of infected seed. Useful Website: <http://ag.arizona.edu/PLP/alternaria/online.htm>

Insertional mutagenesis of a fungal biocontrol agent led to discovery of a rare cellobiose lipid with antifungal activity

Insertional mutagenesis was applied for the first time to a fungal biocontrol agent, Pseudozyma flocculosa, in an attempt to obtain mutants with altered antagonistic properties. Transformants were obtained via DNA-mediated transformation. Molecular analyses of the transformants revealed that multiple copies of the plasmid were integrated in tandem at one to many chromosomal loci. The transformants were screened for their biocontrol properties using standard bioassays, and the 160 tested transformants were classified into four groups: group I mutants (22 transformants) showed a stronger antagonistic effect than the wild type (WT) while those of group II (107 transformants) had a comparable antagonistic effect; group III mutants (17 transformants) had a decreased antagonistic effect relative to WT and group IV mutants (14 transformants) had lost their biocontrol properties. Culture extracts of the mutants (group IV) and WT were analyzed and compared for the presence of active metabolites which were then separated by solid-phase extraction and purified using conventional methods. Nuclear magnetic resonance experiments and analytical studies on a metabolite specifically produced by the WT revealed the presence of 2-(2',4'-diacetoxy-5'-carboxy-pentanoyl) octadecyl cellobioside (flocculosin), a novel glycolipid with strong antifungal properties; the production of this compound would account for the biocontrol activity of P. flocculosa.

Host-selective toxins and avirulence determinants: what's in a name?

Host-selective toxins, a group of structurally complex and chemically diverse metabolites produced by plant pathogenic strains of certain fungal species, function as essential determinants of pathogenicity or virulence. Investigations into the molecular and biochemical responses to these disease determinants reveal responses typically associated with host defense and incompatibility induced by avirulence determinants. The characteristic responses that unify these disparate disease phenotypes are numerous, yet the evidence implicating a causal relationship of these responses, whether induced by host-selective toxins or avirulence factors, in determining the consequences of the host-pathogen interaction is equivocal. This review summarizes some examples of the action of host-selective toxins to illustrate the similarity in responses with those to avirulence determinants.

Overexpression of the tomato Asc-1 gene mediates high insensitivity to AAL toxins and fumonisin B1 in tomato hairy roots and confers resistance to Alternaria alternata f. sp. lycopersici in Nicotiana umbratica plants

The sphinganine-analog mycotoxins (SAMs) fumonisin B1 and AAL toxins are inhibitors of eukaryotic sphinganine N-acyltransferase in vitro. Treatment of eukaryotes with SAMs generally results in an accumulation of sphingoid base precursors and a depletion of complex sphingolipids. The asc,asc genotypes of tomato (Lycopersicon esculentum) and Nicotiana umbratica are sensitive to SAMs and host of the AAL toxin-producing fungus Alternaria alternata f. sp. lycopersici. Codominant insensitivity to SAMs in tomato is mediated by the Asc-1 gene, and sensitivity is associated with a frame-shift mutation present in asc-1. We investigated the function of Asc-1 in mediating insensitivity to SAMs and resistance to the fungus by overexpression of asc-1 and Asc-1. In this study, it is shown that overexpression of these genes did not lead to visual symptoms in tomato hairy roots and N. umbratica plants. Overexpression of asc-1 did not influence the (in)sensitivity to SAMs. Overexpression of Asc-1 in SAM-sensitive hairy roots and N. umbratica plants, however, mediated a high insensitivity to SAMs and resistance to plant infection by Alternaria alternata f. sp. lycopersici.

Host-selective toxins as agents of cell death in plant-fungus interactions

Summary Host-selective toxins are known determinants of compatibility in plant-fungus interactions and provide a powerful model for understanding the specificity of these associations. The identification of genes required for toxin biosynthesis has shown that the genes are unique to the toxin producing species and are clustered in complex loci. These loci may have been acquired by horizontal gene transfer. Many, if not all, host-selective toxins act by disrupting biochemical processes and in several cases the resulting cell death has the characteristics of programmed cell death. This ability to make dead tissue from living has enabled these facultative saprophytic fungi to become plant pathogens.

Pathogenicity genes of phytopathogenic fungi

Summary Recently many fungal genes have been identified that, when disrupted, result in strains with a reduction or total loss of disease symptoms. Such pathogenicity genes are the subject of this review. The large number of pathogenicity genes identified is due to the application of tagged mutagenesis techniques (random or targeted). Genes have been identified with roles in the formation of infection structures, cell wall degradation, overcoming or avoiding plant defences, responding to the host environment, production of toxins, and in signal cascades. Additionally, genes with no database matches and with 'novel' functions have also been found. Improved technologies for mutation analysis and for sequencing and analysing fungal genomes hold promise for identifying many more pathogenicity genes.

Endopolygalacturonase is essential for citrus black rot caused by Alternaria citri but not brown spot caused by Alternaria alternata

Alternaria citri, the cause of Alternaria black rot, and Alternaria alternata rough lemon pathotype, the cause of Alternaria brown spot, are morphologically indistinguishable pathogens of citrus: one causes rot by macerating tissues and the other causes necrotic spots by producing a host-selective toxin. To evaluate the role of endopolygalacturonase (endoPG) in pathogenicity of these two Alternaria spp. pathogens, their genes for endoPG were mutated by gene targeting. The endoPGs produced by these fungi have similar biochemical properties, and the genes are highly similar (99.6% nucleotide identity). The phenotypes of the mutants, however, are completely different. An endoPG mutant of A. citri was significantly reduced in its ability to cause black rot symptoms on citrus as well as in the maceration of potato tissue and could not colonize citrus peel segments. In contrast, an endoPG mutant of A. alternata was unchanged in pathogenicity. The results indicate that a cell wall-degrading enzyme can play different roles in the pathogenicity of fungal pathogens. The role of a cell wall-degrading enzyme depends upon the type of disease but not the taxonomy of the fungus.