The role of fungal appressoria in plant infection

Re-routing MAP kinase signaling for penetration peg formation in predator yeasts

Saccharomycopsis yeasts are natural organic sulfur auxotrophs due to lack of genes required for the uptake and assimilation of sulfate/sulfite. Starvation for methionine induces a shift to a predatory, mycoparasitic life strategy that is unique amongst ascomycetous yeasts. Similar to fungal plant pathogens that separated from Saccharomycopsis more than 400 million years ago, a specialized infection structure called penetration peg is used for prey cell invasion. Penetration pegs are highly enriched with chitin. Here we demonstrate that an ancient and conserved MAP kinase signaling pathway regulates penetration peg formation and successful predation in the predator yeast S. schoenii. Deletion of the MAP kinase gene SsKIL1, a homolog of the Saccharomyces cerevisiae ScKSS1/ScFUS3 and the rice blast Magnaporthe oryzae MoPMK1 genes, as well as deletion of the transcription factor SsSTE12 generate non-pathogenic mutants that fail to form penetration pegs. Comparative global transcriptome analyses using RNAseq indicate loss of the SsKil1-SsSte12-dependent predation response in the mutant strains, while a methionine starvation response is still executed. Within the promoter sequences of genes upregulated during predation we identified a cis-regulatory element similar to the ScSte12 pheromone response element. Our results indicate that, re-routing MAP-kinase signaling by re-wiring Ste12 transcriptional control towards predation specific genes contributed to the parallel evolution of this predacious behaviour in predator yeasts. Consequently, we found that SsSTE12 is dispensable for mating.

The galactose metabolism genes UGE1 and UGM1 are novel virulence factors of the maize anthracnose fungus Colletotrichum graminicola

Fungal cell walls represent the frontline contact with the host and play a prime role in pathogenesis. While the roles of the cell wall polymers like chitin and branched β-glucan are well understood in vegetative and pathogenic development, that of the most prominent galactose-containing polymers galactosaminogalactan and fungal-type galactomannan is unknown in plant pathogenic fungi. Mining the genome of the maize pathogen Colletotrichum graminicola identified the single-copy key galactose metabolism genes UGE1 and UGM1, encoding a UDP-glucose-4-epimerase and UDP-galactopyranose mutase, respectively. UGE1 is thought to be required for biosynthesis of both polymers, whereas UGM1 is specifically required for fungal-type galactomannan formation. Promoter:eGFP fusion strains revealed that both genes are expressed in vegetative and in pathogenic hyphae at all stages of pathogenesis. Targeted deletion of UGE1 and UGM1, and fluorescence-labeling of galactosaminogalactan and fungal-type galactomannan confirmed that Δuge1 mutants were unable to synthesize either of these polymers, and Δugm1 mutants did not exhibit fungal-type galactomannan. Appressoria of Δuge1, but not of Δugm1 mutants, were defective in adhesion, highlighting a function of galactosaminogalactan in the establishment of these infection cells on hydrophobic surfaces. Both Δuge1 and Δugm1 mutants showed cell wall defects in older vegetative hyphae and severely reduced appressorial penetration competence. On intact leaves of Zea mays, both mutants showed strongly reduced disease symptom severity, indicating that UGE1 and UGM1 represent novel virulence factors of C. graminicola.

The serine-threonine protein kinase Snf1 orchestrates the expression of plant cell wall-degrading enzymes and is required for full virulence of the maize pathogen Colletotrichum graminicola

Colletotrichum graminicola, the causal agent of maize leaf anthracnose and stalk rot, differentiates a pressurized infection cell called an appressorium in order to invade the epidermal cell, and subsequently forms biotrophic and necrotrophic hyphae to colonize the host tissue. While the role of force in appressorial penetration is established (Bechinger et al., 1999), the involvement of cell wall-degrading enzymes (CWDEs) in this process and in tissue colonization is poorly understood, due to the enormous number and functional redundancy of these enzymes. The serine/threonine protein kinase gene SNF1 identified in Sucrose Non-Fermenting yeast mutants mediates de-repression of catabolite-repressed genes, including many genes encoding CWDEs. In this study, we identified and functionally characterized the SNF1 homolog of C. graminicola. Δsnf1 mutants showed reduced vegetative growth and asexual sporulation rates on media containing polymeric carbon sources. Microscopy revealed reduced efficacies in appressorial penetration of cuticle and epidermal cell wall, and formation of unusual medusa-like biotrophic hyphae by Δsnf1 mutants. Severe and moderate virulence reductions were observed on intact and wounded leaves, respectively. Employing RNA-sequencing we show for the first time that more than 2,500 genes are directly or indirectly controlled by Snf1 in necrotrophic hyphae of a plant pathogenic fungus, many of which encode xylan- and cellulose-degrading enzymes. The data presented show that Snf1 is a global regulator of gene expression and is required for full virulence.

Highly efficient gene knockout system in the maize pathogen Colletotrichum graminicola using Agrobacterium tumefaciens-mediated transformation (ATMT)

Colletotrichum graminicola, a hemibiotrophic pathogenic fungus, is the causal agent of anthracnose of maize, which causes significant yield losses worldwide, especially in warm and humid maize production regions. An efficient targeted genes knockout protocol is crucial to explore molecular mechanisms of fungal virulence to the host. In this study, we established a gene knockout transformation system by employing Agrobacterium tumefaciens-mediated transformation to knockout genes in M 1.001 strain of C. graminicola. The conidia germination status, induction medium type, and ratio of Agrobacterium cell and conidia suspension were optimized for the knockout of CgBRN1(OR352905), a gene relating to the fungal melanin biosynthesis pathway. Additionally, CgPKS18 (OR352906) and CgCDC25 (OR352903) were knocked out to test the applicability of the gene knockout transformation system. In this established system, transformation efficiency was 176 transformants per 1 × 105 conidia and the homologous recombination efficiency was 53.3 to 75%. Furthermore, disease index, lesion number and lesion size caused by the three above-mentioned mutant strains were found to be reduced significantly compared to the wild-type strain, which indicated reduction in fungal virulence due to the lack of those genes.

Hyphopodium-Specific Signaling Is Required for Plant Infection by Verticillium dahliae

For successful colonization, fungal pathogens have evolved specialized infection structures to overcome the barriers present in host plants. The morphology of infection structures and pathogenic mechanisms are diverse according to host specificity. Verticillium dahliae, a soil-borne phytopathogenic fungus, generates hyphopodium with a penetration peg on cotton roots while developing appressoria, that are typically associated with leaf infection on lettuce and fiber flax roots. In this study, we isolated the pathogenic fungus, V. dahliae (Vda^Sm), from Verticillium wilt eggplants and generated a GFP-labeled isolate to explore the colonization process of Vda^Sm on eggplants. We found that the formation of hyphopodium with penetration peg is crucial for the initial colonization of Vda^Sm on eggplant roots, indicating that the colonization processes on eggplant and cotton share a similar feature. Furthermore, we demonstrated that the VdNoxB/VdPls1-dependent Ca²⁺ elevation activating VdCrz1 signaling is a common genetic pathway to regulate infection-related development in V. dahliae. Our results indicated that VdNoxB/VdPls1-dependent pathway may be a desirable target to develop effective fungicides, to protect crops from V. dahliae infection by interrupting the formation of specialized infection structures.

Appressoria-Small but Incredibly Powerful Structures in Plant-Pathogen Interactions

Plant-pathogenic fungi are responsible for many of the most severe crop diseases in the world and remain very challenging to control. Improving current protection strategies or designating new measures based on an overall understanding of molecular host-pathogen interaction mechanisms could be helpful for disease management. The attachment and penetration of the plant surface are the most important events among diverse plant-fungi interactions. Fungi evolved as small but incredibly powerful infection structure appressoria to facilitate attachment and penetration. Appressoria are indispensable for many diseases, such as rusts, powdery mildews, and blast diseases, as well as devastating oomycete diseases. Investigation into the formation of plant-pathogen appressoria contributes to improving the understanding of the molecular mechanisms of plant-pathogen interactions. Fungal host attachment is a vital step of fungal pathogenesis. Here, we review recent advances in the molecular mechanisms regulating the formation of appressoria. Additionally, some biocontrol agents were revealed to act on appressorium. The regulation of fungal adhesion during the infective process by acting on appressoria formation is expected to prevent the occurrence of crop disease caused by some pathogenic fungi.

Identification and Observation of Infection Processes of Colletotrichum Species Associated with Pearl Plum Anthracnose in Guangxi, China

Pearl plum (Prunus salicina Lindl.) is mainly cultivated in Tian'e County in Guangxi Province, southern China. Anthracnose is a devastating disease on pearl plum, causing extensive leaf blight. Diseased leaves were sampled from 21 orchards in Tian'e County. Isolates were first screened for ones resembling Colletotrichum, and 21 representative isolates were selected for sequencing of portions of the ribosomal internal transcribed spacer (ITS), the intergenic region of apn2 and MAT1-2-1 genes (ApMAT), actin (ACT), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), calmodulin (CAL), chitin synthase (CHS-1), and β-tubulin 2 (TUB2). Based on colony, conidial, and appressorial morphology and sequence analyses, the Colletotrichum isolates associated with pearl plum anthracnose were identified as four species: Colletotrichum fructicola (16 isolates), C. gloeosporioides (3 isolates), C. cigarro (1 isolate), and C. siamense (1 isolate). The results of pathogenicity tests showed that isolates of all four species were pathogenic to wounded leaves of pearl plum seedlings. In this study, we microscopically observed the infection processes of isolates of these four species on attached pearl plum leaves. For C. cigarro and C. siamense, the entire infection processes took 120 h; for C. fructicola and C. gloeosporioides, it only took 72 h. This is the first report of C. fructicola and C. cigarro causing anthracnose on pearl plum worldwide, and also the first report of C. siamense causing anthracnose on pearl plum in China.

Cytological observation and transcriptome analysis reveal dynamic changes of Rhizoctonia solani colonization on leaf sheath and different genes recruited between the resistant and susceptible genotypes in rice

Sheath blight, caused by Rhizoctonia solani, is a big threat to the global rice production. To characterize the early development of R. solani on rice leaf and leaf sheath, two genotypes, GD66 (a resistant genotype) and Lemont (a susceptible genotype), were observed using four cytological techniques: the whole-mount eosin B-staining confocal laser scanning microscopy (WE-CLSM), stereoscopy, fluorescence microscopy, and plastic semi-thin sectioning after in vitro inoculation. WE-CLSM observation showed that, at 12 h post-inoculation (hpi), the amount of hyphae increased dramatically on leaf and sheath surface, the infection cushions occurred and maintained at a huge number from about 18 to 36 hpi, and then the infection cushions disappeared gradually from about 42 to 72 hpi. Interestingly, R. solani could not only colonize on the abaxial surfaces of leaf sheath but also invade the paraxial side of the leaf sheath, which shows a different behavior from that of leaf. RNA sequencing detected 6,234 differentially expressed genes (DEGs) for Lemont and 7,784 DEGs for GD66 at 24 hpi, and 2,523 DEGs for Lemont and 2,719 DEGs for GD66 at 48 hpi, suggesting that GD66 is recruiting more genes in fighting against the pathogen. Among DEGs, resistant genes, such as OsRLCK5, Xa21, and Pid2, displayed higher expression in the resistant genotype than the susceptible genotype at both 24 and 48 hpi, which were validated by quantitative reverse transcription-PCR. Our results indicated that the resistance phenotype of GD66 was the consequence of recruiting a series of resistance genes involved in different regulatory pathways. WE-CLSM is a powerful technique for uncovering the mechanism of R. solani invading rice and for detecting rice sheath blight-resistant germplasm.

Role of the cAMP signaling pathway in the dissemination and development on pepper fruit anthracnose disease caused by Colletotrichum scovillei

The ascomycete fungus Colletotrichum scovillei causes severe anthracnose disease on the fruit of sweet pepper and chili pepper (Capsicum annuum L.) worldwide. Understanding the biology of C. scovillei would improve the management of fruit anthracnose diseases. The cyclic adenosine monophosphate (cAMP) signaling pathway regulates diverse cellular and physiological processes in several foliar fungal pathogens. We investigated the roles of the cAMP signaling pathway in C. scovillei using pharmaceutical and genetic approaches. Exogenous cAMP was found to increase conidiation, appressorium formation, and anthracnose disease development in C. scovillei. CsAc1, CsCap1, and CsPdeH, which regulate the intracellular cAMP level, were deleted by homology-dependent gene replacement. Expectedly, the intracellular cAMP level was significantly decreased in ΔCsac1 and ΔCscap1 but increased in ΔCspdeh. All three deletion mutants exhibited serious defects in multiple fungal developments and pathogenicity, suggesting regulation of the intracellular cAMP level is important for C. scovillei. Notably, exogenous cAMP recovered the defect of ΔCsac1 in appressorium development, but not penetration, which was further recovered by adding CaCl2. This result suggests that CsAc1 is associated with both the cAMP and Ca2+ signaling pathways in C. scovillei. ΔCscap1 produced morphologically abnormal conidia with reduced tolerance to thermal stress. ΔCspdeh was completely defective in conidiation in C. scovillei, unlike other foliar pathogens. Taken together, these results demonstrate the importance of cAMP signaling in anthracnose disease caused by C. scovillei.

The Small Ras Superfamily GTPase Rho4 of the Maize Anthracnose Fungus Colletotrichum graminicola Is Required for β-1,3-glucan Synthesis, Cell Wall Integrity, and Full Virulence

Small Ras superfamily GTPases are highly conserved regulatory factors of fungal cell wall biosynthesis and morphogenesis. Previous experiments have shown that the Rho4-like protein of the maize anthracnose fungus Colletotrichum graminicola, formerly erroneously annotated as a Rho1 protein, physically interacts with the β-1,3-glucan synthase Gls1 (Lange et al., 2014; Curr. Genet. 60:343–350). Here, we show that Rho4 is required for β-1,3-glucan synthesis. Accordingly, Δrho4 strains formed distorted vegetative hyphae with swellings, and exhibited strongly reduced rates of hyphal growth and defects in asexual sporulation. Moreover, on host cuticles, conidia of Δrho4 strains formed long hyphae with hyphopodia, rather than short germ tubes with appressoria. Hyphopodia of Δrho4 strains exhibited penetration defects and often germinated laterally, indicative of cell wall weaknesses. In planta differentiated infection hyphae of Δrho4 strains were fringy, and anthracnose disease symptoms caused by these strains on intact and wounded maize leaf segments were significantly weaker than those caused by the WT strain. A retarded disease symptom development was confirmed by qPCR analyses. Collectively, we identified the Ras GTPase Rho4 as a new virulence factor of C. graminicola.

Pest categorisation of Colletotrichum aenigma, C. alienum, C. perseae, C. siamense and C. theobromicola

The EFSA Plant Health Panel performed a pest categorisation of Colletotrichum aenigma, C. alienum, C. perseae, C. siamense and C. theobromicola, five clearly defined fungi of the C. gloeosporioides complex causing anthracnose. The pathogens are widely distributed in at least three continents. C. aenigma and C. siamense are reported from Italy and C. alienum from Portugal, including the Madeira Islands, with a restricted distribution. C. perseae and C. theobromicola are not known to be present in the EU. However, there is uncertainty on the status of the pathogens worldwide and in the EU because of the taxonomic re-evaluation of the genus Colletotrichum and the lack of specific surveys. The pathogens are not included in Commission Implementing Regulation (EU) 2019/2072 and there are no reports of interceptions in the EU. With the exception of C. perseae, which has a very limited number of hosts, the other four Colletotrichum species have relatively wide host ranges. Therefore, this pest categorisation focused on those hosts for which there is robust evidence that the pathogens were formally identified by a combination of morphology, pathogenicity and multilocus sequence analysis. Host plants for planting and fresh fruits are the main entry pathways into the EU. Host availability and climate suitability factors occurring in some parts of the EU are favourable for the establishment of the pathogens. No yield losses have been reported so far in the EU but in non-EU areas of their current distribution, the pathogens have a direct impact on cultivated hosts that are also relevant for the EU. Phytosanitary measures are available to prevent the further introduction and spread of C. aenigma, C. alienum and C. siamense into the EU as well as the introduction and spread of C. perseae and C. theobromicola. C. aenigma, C. alienum, C. perseae, C. siamense and C. theobromicola satisfy the criteria that are within the remit of EFSA to assess for these species to be regarded as potential Union quarantine pests.

The appressorium at a glance

Many plant pathogenic fungi have the capacity to infect their plant hosts using specialised cells called appressoria. These structures act as a gateway between the fungus and host, allowing entry to internal tissues. Appressoria apply enormous physical force to rupture the plant surface, or use a battery of enzymes to digest the cuticle and plant cell wall. Appressoria also facilitate focal secretion of effectors at the point of plant infection to suppress plant immunity. These infection cells develop in response to the physical characteristics of the leaf surface, starvation stress and signals from the plant. Appressorium morphogenesis has been linked to septin-mediated reorganisation of F-actin and microtubule networks of the cytoskeleton, and remodelling of the fungal cell wall. In this Cell Science at a Glance and accompanying poster, we highlight recent advances in our understanding of the mechanisms of appressorium-mediated infection, and compare development on the leaf surface to the biology of invasive growth by pathogenic fungi. Finally, we outline key gaps in our current knowledge of appressorium cell biology.

Antifungal Potential of Bacillus velezensis CE 100 for the Control of Different Colletotrichum Species through Isolation of Active Dipeptide, Cyclo-(D-phenylalanyl-D-prolyl)

Colletotrichum species are important fungal pathogens causing anthracnose of tropical and subtropical fruit and vegetable crops. Dual culture assay indicated that Bacillus velezensis CE 100 was a strong antagonist against C. acutatum, C. coccodes, C. dematium, and C. gloeosporioides. The volatile organic compounds produced by B. velezensis CE 100 affected mycelial growth of Colletotrichum species tested in our study and caused twisted hyphal structures of all these fungal species. Chloroform crude compounds of B. velezensis CE 100 inhibited four Colletotrichum species in a concentration-dependent manner and induced severe damage in hyphal morphology of these fungal pathogens, including swelling, bulging, and multiple branching. Moreover, the active cyclic dipeptide, cyclo-(D-phenylalanyl-D-prolyl), was isolated from chloroform crude extract and identified by nuclear magnetic resonance (NMR) and mass spectrometry. The inhibitory effect of cyclo-(D-phenylalanyl-D-prolyl) on conidial germination of C. gloeosporioides occurred in a concentration-dependent manner. The conidial germination rate was completely inhibited by a concentration of 3 mg/mL of cyclo-(D-phenylalanyl-D-prolyl). Scanning electron micrographs revealed that the exposure to cyclic dipeptide resulted in seriously deformed hyphae and conidia with shriveled surfaces in dipeptide-treated C. gloeosporioides. Therefore, active dipeptide-producing B. velezensis CE 100 is a promising biocontrol agent for Colletotrichum species causing anthracnose.

The Life Cycle and Ultrastructure of the Host Response of the Smut Pathogen Anthracocystis destruens on Broomcorn Millet

Broomcorn millet smut caused by the fungus Anthracocystis destruens is one of the most destructive diseases in broomcorn millet production. The life cycle of A. destruens and host defense responses against A. destruens remain elusive. Here we investigated the disease symptom development and the parasitic process of A. destruens as well as the ultrastructure of the host-pathogen interface. The results showed that there are four typical symptoms of broomcorn millet smut, which are blackfly, cluster leaves, hedgehog head, and incomplete fruiting. A. destruens colonizes all tissues of broomcorn millet but produces teliospores only in the inflorescence. After infection, A. destruens proliferates in the host, likely in a systemic manner. Ultrastructural study of the infected inflorescence showed that the pathogen grows intercellularly and intracellularly within the host. The host activates defense response to prevent pathogen infection, accumulation of callose analogs and highly electron-dense deposits to resist A. destruens infection.

Aspartate Transaminase AST2 Involved in Sporulation and Necrotrophic Pathogenesis in the Hemibiotrophs Magnaporthe oryzae and Colletotrichum graminicola

Aspartate family includes five additional amino acids other than aspartate, among which most except aspartate have been reported for their action in pathogenesis by amino acid biosynthesis. However, how aspartate, the initial substrate of this family metabolic pathway, is involved in pathogenesis remains unknown. Here, we focused on aspartate transaminase (AST) that catalyzes transamination reaction between glutamate-aspartate in Magnaporthe oryzae. Three MoAST genes were bioinformatically analyzed, of which MoAST2 was uniquely upregulated when invasive hyphae switched to necrotrophic pathogenesis. MoAST2 deletion (ΔMoast2) caused a drastic reduction in conidiogenesis and appressorium formation. Particularly, ΔMoast2 was observed to be proliferated at the biotrophic phase but inhibited at the necrotrophic stage, and with invisible symptoms detected, suggesting a critical role in necrotrophic phase. Glutamate family restored the ΔMoast2 defects but aspartate family did not, inferring that transamination occurs from aspartate to glutamine. MoAST2 is cytosolic and possessed H₂O₂ stress tolerance. In parallel, Colletotrichum graminicola AST2, CgAST2 was proven to be a player in necrotrophic anthracnose development. Therefore, conserved AST2 is qualified to be a drug target for disease control.

Fungal Pathogenesis-Related Cell Wall Biogenesis, with Emphasis on the Maize Anthracnose Fungus Colletotrichum graminicola

The genus Colletotrichum harbors many plant pathogenic species, several of which cause significant yield losses in the field and post harvest. Typically, in order to infect their host plants, spores germinate, differentiate a pressurized infection cell, and display a hemibiotrophic lifestyle after plant invasion. Several factors required for virulence or pathogenicity have been identified in different Colletotrichum species, and adaptation of cell wall biogenesis to distinct stages of pathogenesis has been identified as a major pre-requisite for the establishment of a compatible parasitic fungus-plant interaction. Here, we highlight aspects of fungal cell wall biogenesis during plant infection, with emphasis on the maize leaf anthracnose and stalk rot fungus, Colletotrichum graminicola.

Thinopyrum intermedium TiAP1 interacts with a chitin deacetylase from Blumeria graminis f. sp. tritici and increases the resistance to Bgt in wheat

The biotrophic fungal pathogen Blumeria graminis f. sp. tritici (Bgt) is a crucial factor causing reduction in global wheat production. Wild wheat relatives, for example Thinopyrum intermedium, is one of the wild-used parents in wheat disease-resistant breeding. From T. intermedium line, we identified the aspartic protease gene, TiAP1, which is involved in resistance against Bgt. TiAP1 is a secreted protein that accumulates in large amounts at the infection sites of Bgt and extends to the intercellular space. Yeast two-hybrid, luciferase complementation imaging and bimolecular florescent complimentary analysis showed that TiAP1 interacted with the chitin deacetylase (BgtCDA1) of Bgt. The yeast expression, purification and in vitro test confirmed the chitin deacetylase activity of BgtCDA1. The bombardment and VIGS-mediated host-induced gene silencing showed that BgtCDA1 promotes the invasion of Bgt. Transcriptome analysis showed the cell wall xylan metabolism, lignin biosynthesis-related and defence genes involved in the signal transduction were up-regulated in the transgenic TiAP1 wheat induced by Bgt. The TiAP1 in wheat may inactivate the deacetylation function of BgtCDA1, cause chitin oligomers expose to wheat chitin receptor, then trigger the wheat immune response to inhibit the growth and penetration of Bgt, and thereby enhance the resistance of wheat to pathogens.

Distinct Transcriptional Programs Underlie Differences in Virulence of Isolates on Host Plants in a Fungal Pathogen, Colletotrichum gloeosporioides

Susceptible host plants challenged by fungal pathogens can display different types of lesions, which can be attributed to environmental factors affecting the nature of interactions between the host and pathogen. During our survey of apple anthracnose in Korea, two distinct types of disease symptoms, designated as progressive (PS) and static symptoms (SS), were recognized. PS is a typical, rapidly enlarging symptom of apple anthracnose, while SS is a small, dark speck that does not expand further until the harvesting season. Isolation and genotyping of pathogens from disease lesions suggested that all of them belong to Colletotrichum gloeosporioides, a well-known causal agent of apple anthracnose. Two types of isolates were comparable in growth on media, spore germination and appressorium formation, virulence test on fruits at various temperature conditions. Furthermore, they were analyzed at the molecular level by a phylogenetic tree, RNA-seq, and expression of virulence gene. However, the SS isolates were defective in appressorium-mediated penetration into the underlying substratum. RNA-seq analysis of PS and SS isolates showed that distinct transcriptional programs underlie the development of different types of anthracnose symptoms in host plants. One downregulated gene in SS encoded isocitrate lyase is essential for disease development via its involvement in the glyoxylate cycle. It partly explains why SS is less virulent than PS on host plants. Overall, our work challenges the traditional view on the development of different lesion types and provides valuable insights into variations that exist in the pathogen population.

Recent advances in Penicillium expansum infection mechanisms and current methods in controlling P. expansum in postharvest apples

One of the most significant challenges associated with postharvest apple deterioration is the blue mold caused by Penicillium expansum, which leads to considerable economic losses to apple production industries. Apple fruits are susceptible to mold infection owing to their high nutrient and water content, and current physical control methods can delay but cannot completely inhibit P. expansum growth. Biological control methods present promising alternatives; however, they are not always cost effective and have application restrictions. P. expansum infection not only enhances disease pathogenicity, but also inhibits the expression of host-related defense genes. The implementation of new ways to investigate and control P. expansum are expected with the advent of omics technology. Advances in these techniques, together with molecular biology approaches such as targeted gene deletion and whole genome sequencing, will lead to a better understanding of the P. expansum infectious machinery. Here, we review the progress of research on the blue mold disease caused by P. expansum in apples, including physiological and molecular infection mechanisms, as well as various methods to control this common plant pathogen.

Occurrence and infective potential of Colletotrichum gloeosporioides isolates associated to Citrus limon var Eureka

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37 fungi from damaged tissues of Italian lemon were obtained.

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D1/D2, ITS1-5.8S-ITS2 and COX1 phylogenetic trees allow identify ten fungi genera.

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Colletotrichum isolates were characterized by species-specific PCR, rep-PCR and multilocus analysis.

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Colletotrichum isolates had leaves´s infection percentages between 17 to 67%.

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This the first report of C. gloeosporioides on Italian lemon in Mexico.

A collection of 37 fungi associated to Italian lemon plants with disease symptoms, was obtained. Ten genera including Aspergillus, Alternaria, Nigrospora, Lasiodiplodia, Dothideomycetes, Pleurostoma, Setosphaeria, Penicillium, Fusarium and Colletotrichum were identified by using ITS1–5.8S–ITS2, D1/D2 26S and COX1 loci. The last three genera were abundant on the damaged fruits, being Colletotrichum the more abundant (32.4 %). CaInt2 and CgInt primers support the identity of these isolates as C. gloeosporioides. Variability, inferred by rep-PCR and multilocus sequence analysis shows genetic differences among the C. gloeosporioides isolates. Infective profile evaluated in Colletotrichum isolates shows different leave infection percentages (26 to 60 %). SEM analysis showed mycelium, spores and appressoria on the leaves of selected Colletotrichum isolates. Specifically, the AL-05 and AL-13 isolates showed a high chitin deacetylase activity (CDA) peaking at 1.2 U/mg protein in AL-13. This is the first report on C. gloeosporioides infecting Italian lemon leaves in Mexico.

Diversity and Function of Appressoria

Endophytic, saprobic, and pathogenic fungi have evolved elaborate strategies to obtain nutrients from plants. Among the diverse plant-fungi interactions, the most crucial event is the attachment and penetration of the plant surface. Appressoria, specialized infection structures, have been evolved to facilitate this purpose. In this review, we describe the diversity of these appressoria and classify them into two main groups: single-celled appressoria (proto-appressoria, hyaline appressoria, melanized (dark) appressoria) and compound appressoria. The ultrastructure of appressoria, their initiation, their formation, and their function in fungi are discussed. We reviewed the molecular mechanisms regulating the formation and function of appressoria, their strategies to evade host defenses, and the related genomics and transcriptomics. The current review provides a foundation for comprehensive studies regarding their evolution and diversity in different fungal groups.

Structure, Assembly and Function of Cuticle from Mechanical Perspective with Special Focus on Perianth

This review is devoted to the structure, assembly and function of cuticle. The topics are discussed from the mechanical perspective and whenever the data are available a special attention is paid to the cuticle of perianth organs, i.e., sepals, petals or tepals. The cuticle covering these organs is special in both its structure and function and some of these peculiarities are related to the cuticle mechanics. In particular, strengthening of the perianth surface is often provided by a folded cuticle that functionally resembles profiled plates, while on the surface of the petal epidermis of some plants, the cuticle is the only integral continuous layer. The perianth cuticle is distinguished also by those aspects of its mechanics and development that need further studies. In particular, more investigations are needed to explain the formation and maintenance of cuticle folding, which is typical for the perianth epidermis, and also to elucidate the mechanical properties and behavior of the perianth cuticle in situ. Gaps in our knowledge are partly due to technical problems caused by very small thicknesses of the perianth cuticle but modern tools may help to overcome these obstacles.

The infection cushion of Botrytis cinerea: a fungal 'weapon' of plant-biomass destruction

The necrotrophic plant-pathogen fungus Botrytis cinerea produces multicellular appressoria dedicated to plant penetration, named infection cushions (IC). A microarray analysis was performed to identify genes upregulated in mature IC. The expression data were validated by RT-qPCR analysis performed in vitro and in planta, proteomic analysis of the IC secretome and biochemical assays. 1231 upregulated genes and 79 up-accumulated proteins were identified. The data support the secretion of effectors by IC: phytotoxins, ROS, proteases, cutinases, plant cell wall-degrading enzymes and plant cell death-inducing proteins. Parallel upregulation of sugar transport and sugar catabolism-encoding genes would indicate a role of IC in nutrition. The data also reveal a substantial remodelling of the IC cell wall and suggest a role for melanin and chitosan in IC function. Lastly, mutagenesis of two upregulated genes in IC identified secreted fasciclin-like proteins as actors in the pathogenesis of B. cinerea. These results support the role of IC in plant penetration and also introduce other unexpected functions for this fungal organ, in colonization, necrotrophy and nutrition of the pathogen.

Involvement of the Mitochondrial Protein Tyrosine Phosphatase PTPM1 in the Promotion of Conidiation, Development, and Pathogenicity in Colletotrichum graminicola

The phosphorylation status of proteins, which is determined by protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs), governs many cellular actions. In fungal pathogens, phosphorylation-mediated signal transduction has been considered to be one of the most important mechanisms in pathogenicity. Colletotrichum graminicola is an economically important corn pathogen. However, whether phosphorylation is involved in its pathogenicity is unknown. A mitochondrial protein tyrosine phosphatase gene, designated CgPTPM1, was deduced in C. graminicola through the use of bioinformatics and confirmed by enzyme activity assays and observation of its subcellular localization. We then created a CgPTPM1 deletion mutant (ΔCgPTPM1) to analyze its biological function. The results indicated that the loss of CgPTPM1 dramatically affected the formation of conidia and the development and differentiation into appressoria. However, the colony growth and conidial morphology of the ΔCgPTPM1 strains were unaffected. Importantly, the ΔCgPTPM1 mutant strains exhibited an obvious reduction of virulence, and the delayed infected hyphae failed to expand in the host cells. In comparison with the wild-type, ΔCgPTPM1 accumulated a larger amount of H2O2 and was sensitive to exogenous H2O2. Interestingly, the host cells infected by the mutant also exhibited an increased accumulation of H2O2 around the infection sites. Since the expression of the CgHYR1, CgGST1, CgGLR1, CgGSH1 and CgPAP1 genes was upregulated with the H2O2 treatment, our results suggest that the mitochondrial protein tyrosine phosphatase PTPM1 plays an essential role in promoting the pathogenicity of C. graminicola by regulating the excessive in vivo and in vitro production of H2O2.

Breakpoint: Cell Wall and Glycoproteins and their Crucial Role in the Phytopathogenic Fungi Infection

The cell wall that surrounds fungal cells is essential for their survival, provides protection against physical and chemical stresses, and plays relevant roles during infection. In general, the fungal cell wall is composed of an outer layer of glycoprotein and an inner skeletal layer of β-glucans or α- glucans and chitin. Chitin synthase genes have been shown to be important for septum formation, cell division and virulence. In the same way, chitin can act as a potent elicitor to activate defense response in several plant species; however, the fungi can convert chitin to chitosan during plant infection to evade plant defense mechanisms. Moreover, α-1,3-Glucan, a non-degradable polysaccharide in plants, represents a key feature in fungal cell walls formed in plants and plays a protective role for this fungus against plant lytic enzymes. A similar case is with β-1,3- and β-1,6-glucan which are essential for infection, structure rigidity and pathogenicity during fungal infection. Cell wall glycoproteins are also vital to fungi. They have been associated with conidial separation, the increase of chitin in conidial cell walls, germination, appressorium formation, as well as osmotic and cell wall stress and virulence; however, the specific roles of glycoproteins in filamentous fungi remain unknown. Fungi that can respond to environmental stimuli distinguish these signals and relay them through intracellular signaling pathways to change the cell wall composition. They play a crucial role in appressorium formation and penetration, and release cell wall degrading enzymes, which determine the outcome of the interaction with the host. In this review, we highlight the interaction of phypatophogen cell wall and signaling pathways with its host and their contribution to fungal pathogenesis.

Incompatibility between proliferation and plant invasion is mediated by a regulator of appressorium formation in the corn smut fungus Ustilago maydis

Plant pathogenic fungi often developed specialized infection structures to breach the outer surface of a host plant. These structures, called appressoria, lead the invasion of the plant by the fungal hyphae. Studies in different phytopathogenic fungi showed that appressorium formation seems to be subordinated to the cell cycle. This subordination ensures the loading in the invading hypha of the correct genetic information to proceed with plant infection. However, how the cell cycle transmits its condition to the genetic program controlling appressorium formation and promoting the plant's invasion is unknown. Our results have uncovered how this process occurs for the appressorium of Ustilago maydis, the agent responsible for corn smut disease. Here, we described that the complex Clb2-cyclin-dependent kinase (Cdk)1, one of the master regulators of G2/M cell cycle progression in U. maydis, interacts and controls the subcellular localization of Biz1, a transcriptional factor required for the activation of the appressorium formation. Besides, Biz1 can arrest the cell cycle by down-regulation of the gene encoding a second b-cyclin Clb1 also required for the G2/M transition. These results revealed a negative feedback loop between appressorium formation and cell cycle progression in U. maydis, which serves as a "toggle switch" to control the fungal decision between infecting the plant or proliferating out of the plant.

UhAVR1, an HR-Triggering Avirulence Effector of Ustilago hordei, Is Secreted via the ER-Golgi Pathway, Localizes to the Cytosol of Barley Cells during in Planta-Expression, and Contributes to Virulence Early in Infection

The basidiomycete Ustilago hordei causes covered smut disease of barley and oats. Virulence effectors promoting infection and supporting pathogen lifestyle have been described for this fungus. Genetically, six avirulence genes are known and one codes for UhAVR1, the only proven avirulence effector identified in smuts to date that triggers complete immunity in barley cultivars carrying resistance gene Ruh1. A prerequisite for resistance breeding is understanding the host targets and molecular function of UhAVR1. Analysis of this effector upon natural infection of barley coleoptiles using teliospores showed that UhAVR1 is expressed during the early stages of fungal infection where it leads to HR triggering in resistant cultivars or performs its virulence function in susceptible cultivars. Fungal secretion of UhAVR1 is directed by its signal peptide and occurs via the BrefeldinA-sensitive ER-Golgi pathway in cell culture away from its host. Transient in planta expression of UhAVR1 in barley and a nonhost, Nicotiana benthamiana, supports a cytosolic localization. Delivery of UhAVR1 via foxtail mosaic virus or Pseudomonas species in both barley and N. benthamiana reveals a role in suppressing components common to both plant systems of Effector- and Pattern-Triggered Immunity, including necrosis triggered by Agrobacterium-delivered cell death inducers.

Disruption of an adenylate-forming reductase required for conidiation, increases virulence of the insect pathogenic fungus Metarhizium acridum by enhancing cuticle invasion

BACKGROUND

Metarhizium acridum, is a specific acridid pathogen developed for use against the migratory locust (Locusta migratoria manilensis). Adenylate-forming reductases (AFRs) include enzymes that are involved in natural product biosynthesis. Here, we genetically characterize the functions of a class IV AFR in M. acridum (MaAfr^IV ) on fungal development and virulence.

RESULTS

Gene expression analyses indicated MaAfr^IV was induced on locust wings early during the infection process. Surprisingly, loss of MaAfr^IV increased virulence (25.20% decrease in the median lethal time) against the locust in topical bioassays but was no different than the wild type when the cuticle was bypassed by direct infection of conidia into the insect hemocoel. Virulence markers including protease (Pr1) expression and appressorial turgor pressure were higher in the mutant than the parent strain. No difference was seen in the expression of host immune genes (Toll pathway) or in polyphenol oxidase (PPO) activity in locusts infected by the ΔMaAfr^IV or wild type strains. However, the ΔMaAfr^IV strain was unable to successfully sporulate on dead cadavers.

CONCLUSION

Disruption of MaAfr^IV increased fungal virulence by promoting insect cuticle invasion without altering host immune response or fungal immune evasion. Although loss of MaAfr^IV conferred an apparent benefit to the fungus in terms of enhanced virulence, a significant trade-off was seen in the inability of the fungus to sporulate on the cadaver. As conidiation on the cadaver is essential for subsequent propagation in the environment, loss of MaAfr^IV can reduce the engineering strains survivability in the field and improve the safety. © 2019 Society of Chemical Industry.

Disclosure of the Molecular Mechanism of Wheat Leaf Spot Disease Caused by Bipolaris sorokiniana through Comparative Transcriptome and Metabolomics Analysis

Wheat yield is greatly reduced because of the occurrence of leaf spot diseases. Bipolaris sorokiniana is the main pathogenic fungus in leaf spot disease. In this study, B. sorokiniana from wheat leaf (W-B. sorokiniana) showed much stronger pathogenicity toward wheat than endophytic B. sorokiniana from Pogostemon cablin (P-B. sorokiniana). The transcriptomes and metabolomics of the two B. sorokiniana strains and transcriptomes of B. sorokiniana-infected wheat leaves were comparatively analyzed. In addition, the expression levels of unigenes related to pathogenicity, toxicity, and cell wall degradation were predicted and validated by quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis. Results indicated that pathogenicity-related genes, especially the gene encoding loss-of-pathogenicity B (LopB) protein, cell wall-degrading enzymes (particularly glycosyl hydrolase-related genes), and killer and Ptr necrosis toxin-producing related unigenes in the W-B. sorokiniana played important roles in the pathogenicity of W-B. sorokiniana toward wheat. The down-regulation of cell wall protein, photosystem peptide, and rubisco protein suggested impairment of the phytosynthetic system and cell wall of B. sorokiniana-infected wheat. The up-regulation of hydrolase inhibitor, NAC (including NAM, ATAF1 and CUC2) transcriptional factor, and peroxidase in infected wheat tissues suggests their important roles in the defensive response of wheat to W-B. sorokiniana. This is the first report providing a comparison of the transcriptome and metabolome between the pathogenic and endophytic B. sorokiniana strains, thus providing a molecular clue for the pathogenic mechanism of W-B. sorokiniana toward wheat and wheat's defensive response mechanism to W-B. sorokiniana. Our study could offer molecular clues for controlling the hazard of leaf spot and root rot diseases in wheat, thus improving wheat yield in the future.

Deficiencies in the Mitochondrial Electron Transport Chain Affect Redox Poise and Resistance Toward Colletotrichum higginsianum

To investigate if and how the integrity of the mitochondrial electron transport chain (mETC) influences susceptibility of Arabidopsis toward Colletotrichum higginsianum, we have selected previously characterized mutants with defects at different stages of the mETC, namely, the complex I mutant ndufs4, the complex II mutant sdh2-1, the complex III mutant ucr8-1, and a mutant of the uncoupling protein ucp1-2. Relative to wild type, the selected complex I, II, and III mutants showed decreased total respiration, increased alternative respiration, as well as increased redox charge of the NADP(H) pool and decreased redox charge of the NAD(H) pool in the dark. In the light, mETC mutants accumulated free amino acids, albeit to varying degrees. Glycine and serine, which are involved in carbon recycling from photorespiration, and N-rich amino acids were predominantly increased in mETC mutants compared to the wild type. Taking together the physiological phenotypes of all examined mutants, our results suggest a connection between the limitation in the re-oxidation of reducing equivalents in the mitochondrial matrix and the induction of nitrate assimilation into free amino acids in the cytosol, which seems to be engaged as an additional sink for reducing power. The sdh2-1 mutant was less susceptible to C. higginsianum and did not show hampered salicylic acid (SA) accumulation as previously reported for SDH1 knock-down plants. The ROS burst remained unaffected in sdh2-1, emonstrating that subunit SDH2 is not involved in the control of ROS production and SA signaling by complex II. Moreover, the ndufs4 mutant showed only 20% of C. higginsianum colonization compared to wild type, with the ROS burst and the production of callose papillae being significantly increased compared to wild type. This indicates that a restriction of respiratory metabolism can positively affect pre-penetration resistance of Arabidopsis. Taking metabolite profiling data from all investigated mETC mutants, a strong positive correlation of resistance toward C. higginsianum with NADPH pool size, pyruvate contents, and other metabolites associated with redox poise and energy charge was evident, which fosters the hypothesis that limitations in the mETC can support resistance at post-penetration stages by improving the availability of metabolic power.

Appressorium: The Breakthrough in Dikarya

Phytopathogenic and mycorrhizal fungi often penetrate living hosts by using appressoria and related structures. The differentiation of similar structures in saprotrophic fungi to penetrate dead plant biomass has seldom been investigated and has been reported only in the model fungus Podospora anserina. Here, we report on the ability of many saprotrophs from a large range of taxa to produce appressoria on cellophane. Most Ascomycota and Basidiomycota were able to form appressoria. In contrast, none of the three investigated Mucoromycotina was able to differentiate such structures. The ability of filamentous fungi to differentiate appressoria no longer belongs solely to pathogenic or mutualistic fungi, and this raises the question of the evolutionary origin of the appressorium in Eumycetes.

Differences in the Characteristics and Pathogenicity of Colletotrichum camelliae and C. fructicola Isolated From the Tea Plant [Camellia sinensis (L.) O. Kuntze]

Colletotrichum, the causative agent of anthracnose, is an important pathogen that invades the tea plant (Camellia sinensis). In this study, 38 isolates were obtained from the diseased leaves of tea plants collected in different areas of Zhejiang Province, China. A combination of multigene (ITS, ACT, GAPDH, TUB2, CAL, and GS) and morphology analyses showed that the 38 strains belonged to two different species, namely, C. camelliae (CC), and C. fructicola (CF). Pathogenicity tests revealed that CC was more invasive than CF. In vitro inoculation experiments demonstrated that CC formed acervuli at 72 hpi and developed appressoria on wound edges, but CF did not develop these structures. Under treatment with catechins and caffeine, the growth inhibition rates of CF were remarkably higher than those of CC, indicating that the nonpathogenic species CF was more vulnerable to catechins and caffeine. Growth condition testing indicated that CF grew at a wide temperature range of 15-35°C and that the optimum temperature for CC growth was 25°C. Growth of both CC and CF did not differ between acidic and weakly alkaline environments (pH 5-8), but the growth of CC was significantly reduced at pH values of 9 and 10. Furthermore, the PacC/RIM101 gene, which associated with pathogenicity, was identified from CC and CF genomes, and its expression was suppressed in the hyphae of both species under pH value of 5 and 10, and much lower expression level was detected in CC than that in CF at pH 6. These results indicated that temperature has more important effect than pH for the growth of two Colletotrichum species. In conclusion, the inhibition by secondary metabolite is an important reason why the pathogenicity by CC and CF are different to tea plant, although the environmental factors including pH and temperature effect the growth of two Colletotrichum species.

Commonalities and Differences in Controlling Multipartite Intracellular Infections of Legume Roots by Symbiotic Microbes

Legumes have the almost unique ability to establish symbiotic associations with rhizobia and arbuscular mycorrhizal fungi. Forward and reverse genetics have identified a large number of genes that are required for either or both interactions. However, and in sharp contrast to natural soils, these interactions have been almost exclusively investigated under laboratory conditions by using separate inoculation systems, whereas both symbionts are simultaneously present in the field. Considering our recent understanding of the individual symbioses, the community is now promisingly positioned to co-inoculate plants with two or more microbes in order to understand mechanistically how legumes efficiently balance, regulate and potentially separate these symbioses and other endophytic microbes within the same root. Here, we discuss a number of key control layers that should be considered when assessing tri- or multipartite beneficial interactions and that may contribute to colonization patterns in legume roots.

High species diversity in Colletotrichum associated with citrus diseases in Europe

Species of Colletotrichum are considered important plant pathogens, saprobes, and endophytes on a wide range of plant hosts. Several species are well-known on citrus, either as agents of pre- or post-harvest infections, such as anthracnose, postbloom fruit drop, tear stain and stem-end rot on fruit, or as wither-tip of twigs. In this study we explored the occurrence, diversity and pathogenicity of Colletotrichum spp. associated with Citrus and allied genera in European orchards, nurseries and gardens. Surveys were carried out during 2015 and 2016 in Greece, Italy, Malta, Portugal and Spain. A total of 174 Colletotrichum strains were isolated from symptomatic leaves, fruits, petals and twigs. A multi-locus phylogeny was established based on seven genomic loci (ITS, GAPDH, ACT, CAL, CHS-1, HIS3 and TUB2), and the morphological characters of the isolates determined. Preliminary pathogenicity tests were performed on orange fruits with representative isolates. Colletotrichum strains were identified as members of three major species complexes. Colletotrichum gloeosporioides s.str. and two novel species (C. helleniense and C. hystricis) were identified in the C. gloeosporioides species complex. Colletotrichum karstii, C. novae-zelandiae and two novel species (C. catinaense and C. limonicola) in the C. boninense species complex, and C. acutatum s.str. was also isolated as member of C. acutatum species complex. Colletotrichum gloeosporioides and C. karstii were the predominant species of Colletotrichum isolated. This study represents the first report of C. acutatum on citrus in Europe, and the first detection of C. novae-zelandiae from outside New Zealand. Pathogenicity tests revealed C. gloeosporioides s.str. to be the most virulent species on fruits. The present study improves our understanding of species associated with several disease symptoms on citrus fruits and plants, and provides useful information for effective disease management.

Comparative secretome analysis of Colletotrichum falcatum identifies a cerato-platanin protein (EPL1) as a potential pathogen-associated molecular pattern (PAMP) inducing systemic resistance in sugarcane

Colletotrichum falcatum, an intriguing hemibiotrophic fungal pathogen causes red rot, a devastating disease of sugarcane. Repeated in vitro subculturing of C. falcatum under dark condition alters morphology and reduces virulence of the culture. Hitherto, no information is available on this phenomenon at molecular level. In this study, the in vitro secretome of C. falcatum cultured under light and dark conditions was analyzed using 2-DE coupled with MALDI TOF/TOF MS. Comparative analysis identified nine differentially abundant proteins. Among them, seven proteins were less abundant in the dark-cultured C. falcatum, wherein only two protein species of a cerato-platanin protein called EPL1 (eliciting plant response-like protein) were found to be highly abundant. Transcriptional expression of candidate high abundant proteins was profiled during host-pathogen interaction using qRT-PCR. Comprehensively, this comparative secretome analysis identified five putative effectors, two pathogenicity-related proteins and one pathogen-associated molecular pattern (PAMP) of C. falcatum. Functional characterization of three distinct domains of the PAMP (EPL1) showed that the major cerato-platanin domain (EPL1∆N1-92) is exclusively essential for inducing defense and hypersensitive response (HR) in sugarcane and tobacco, respectively. Further, priming with EPL1∆N1-92 protein induced systemic resistance and significantly suppressed the red rot severity in sugarcane.

BIOLOGICAL SIGNIFICANCE

Being the first secretomic investigation of C. falcatum, this study has identified five potential effectors, two pathogenicity-related proteins and a PAMP. Although many reports have highlighted the influence of light on pathogenicity, this study has established a direct link between light and expression of effectors, for the first time. This study has presented the influence of a novel N-terminal domain of EPL1 in physical and biological properties and established the functional role of major cerato-platanin domain of EPL1 as a potential elicitor inducing systemic resistance in sugarcane. Comprehensively, the study has identified proteins that putatively contribute to virulence of C. falcatum and for the first time, demonstrated the potential role of EPL1 in inducing PAMP-triggered immunity (PTI) in sugarcane.

Sugar Accumulation in Leaves of Arabidopsis sweet11/sweet12 Double Mutants Enhances Priming of the Salicylic Acid-Mediated Defense Response

In compatible interactions, biotrophic microbial phytopathogens rely on the supply of assimilates by the colonized host tissue. It has been found in rice that phloem localized SWEET sucrose transporters can be reprogrammed by bacterial effectors to establish compatibility. We observed that sweet11/sweet12 double mutants, but not single mutants, exhibited increased resistance toward the fungal hemibiotroph Colletotrichum higginsianum (Ch), both in the biotrophic and the necrotrophic colonization phase. We therefore investigated if the phloem localized transporters AtSWEET11 and AtSWEET12 represent additive susceptibility factors in the interaction of Arabidopsis with Ch. AtSWEET12-YFP fusion protein driven by the endogenous promoter strongly accumulated at Ch infection sites and in the vasculature upon challenge with Ch. However, susceptibility of sweet12 single mutants to Ch was comparable to wild type, indicating that the accumulation of AtSWEET12 at Ch infection sites does not play a major role for compatibility. AtSWEET12-YFP reporter protein was not detectable at the plant-pathogen interface, suggesting that AtSWEET12 is not targeted by Ch effectors. AtSWEET11-YFP accumulation in pAtSWEET11:AtSWEET11-YFP plants were similar in Ch infected and mock control leaves. A close inspection of major carbohydrate metabolism in non-infected control plants revealed that soluble sugar and starch content were substantially elevated in sweet11/sweet12 double mutants during the entire diurnal cycle, that diurnal soluble sugar turnover was increased more than twofold in sweet11/sweet12, and that accumulation of free hexoses and sucrose was strongly expedited in double mutant leaves compared to wild type and both single mutants during the course of Ch infection. After 2 days of treatment, free and conjugated SA levels were significantly increased in infected and mock control leaves of sweet11/sweet12 relative to all other genotypes, respectively. Induced genes in mock treated sweet11/sweet12 leaves were highly significantly enriched for several GO terms associated with SA signaling and response compared to mock treated wild-type leaves, indicating sugar-mediated priming of the SA pathway in the double mutant. Infection assays with salicylic acid deficient sweet11/sweet12/sid2 triple mutants demonstrated that reduced susceptibility observed in sweet11/sweet12 was entirely dependent on the SA pathway. We suggest a model how defects in phloem loading of sucrose can influence SA priming and hence, compatibility.

FvSNF1, the sucrose non-fermenting protein kinase gene of Fusarium virguliforme, is required for cell-wall-degrading enzymes expression and sudden death syndrome development in soybean

Fusarium virguliforme is a soil-borne pathogenic fungus that causes sudden death syndrome (SDS) in soybean. Its pathogenicity is believed to require the activity of cell-wall-degrading enzymes (CWDEs). The sucrose non-fermenting protein kinase 1 gene (SNF1) is a key component of the glucose de-repression pathway in yeast, and a regulator of gene expression for CWDEs in some plant pathogenic fungi. To elucidate the functional role of the SNF1 homolog in F. virguliforme, FvSNF1 was disrupted using a split-marker strategy. Disruption of FvSNF1 in F. virguliforme abolishes galactose utilization and causes poor growth on xylose, arabinose and sucrose. However, the resulting Fvsnf1 mutant grew similar to wild-type and ectopic transformants on glucose, fructose, maltose, or pectin as the main source of carbon. The Fvsnf1 mutant displayed no expression of the gene-encoding galactose oxidase (GAO), a secretory enzyme that catalyzes oxidation of D-galactose. It also exhibited a significant reduction in the expression of several CWDE-coding genes in contrast to the wild-type strain. Greenhouse pathogenicity assays revealed that the Fvsnf1 mutant was severely impaired in its ability to cause SDS on challenged soybean plants. Microscopy and microtome studies on infected roots showed that the Fvsnf1 mutant was defective in colonizing vascular tissue of infected plants. Cross and longitudinal sections of infected roots stained with fluorescein-labeled wheat germ agglutinin and Congo red showed that the Fvsnf1 mutant failed to colonize the xylem vessels and phloem tissue at later stages of infection. Quantification of the fungal biomass in inoculated roots further confirmed a reduced colonization of roots by the Fvsnf1 mutant when compared to the wild type. These findings suggest that FvSNF1 regulates the expression of CWDEs in F. virguliforme, thus affecting the virulence of the fungus on soybean.

TeA is a key virulence factor for Alternaria alternata (Fr.) Keissler infection of its host

A toxin-deficient mutant strain, HP001 mutant of Alternaria alternata, whose mycelium is unable to infect its host, produces little tenuazonic acid (TeA) toxin. How TeA plays a role in initiating host infection by A. alternata remains unclear. In this research we use Imaging-PAM based on chlorophyll fluorescence parameters and transmission electron microscopy to explore the role of TeA toxin during the infection process of A. alternata. Photosystem II damage began even before wild type mycelium infected the leaves of its host, croftonweed (Ageratina adenophora). Compared with the wild type, HP001 mutant produces morphologically different colonies, hyphae with thinner cell walls, has higher reactive oxygen species (ROS) content and lower peroxidase activity, and fails to form appressoria on the host surface. Adding TeA toxin allows the mutant to partially recover these characters and more closely resemble the wild type. Additionally, we found that the mutant is able to elicit disease symptoms when its mycelium is placed on leaves whose epidermis has been manually removed, which indicates that TeA may be determinant in the fungus recognition of its plant host. Lack of TeA toxin appears responsible for the loss of pathogenicity of the HP001 mutant. As a key virulence factor, TeA toxin not only damages the host plant but also is involved in maintaining ROS content, host recognition, inducing appressoria to infect the host and for allowing completion of the infection process.

Cell wall composition and penetration resistance against the fungal pathogen Colletotrichum higginsianum are affected by impaired starch turnover in Arabidopsis mutants

Penetration resistance represents the first level of plant defense against phytopathogenic fungi. Here, we report that the starch-deficient Arabidopsis thaliana phosphoglucomutase (pgm) mutant has impaired penetration resistance against the hemibiotrophic fungus Colletotrichum higginsianum. We could not determine any changes in leaf cutin and epicuticular wax composition or indolic glucosinolate levels, but detected complex alterations in the cell wall monosaccharide composition of pgm. Notably, other mutants deficient in starch biosynthesis (adg1) or mobilization (sex1) had similarly affected cell wall composition and penetration resistance. Glycome profiling analysis showed that both overall cell wall polysaccharide extractability and relative extractability of specific pectin and xylan epitopes were affected in pgm, suggesting extensive structural changes in pgm cell walls. Screening of mutants with alterations in content or modification of specific cell wall monosaccharides indicated an important function of pectic polymers for penetration resistance and hyphal growth of C. higginsianum during the biotrophic interaction phase. While mutants with affected pectic rhamnogalacturonan-I (mur8) were hypersusceptible, penetration frequency and morphology of fungal hyphae were impaired on pmr5 pmr6 mutants with increased pectin levels. Our results reveal a strong impact of starch metabolism on cell wall composition and suggest a link between carbohydrate availability, cell wall pectin and penetration resistance.

Modulation of oxidative responses by a virulent isolate of Colletotrichum fructicola in apple leaves

Apple bitter rot (ABR) and Glomerella leaf spot (GLS) can be caused by Colletotrichum fructicola. Although both diseases can occur simultaneously in orchards, some isolates show clear organ specialization. Thus, this work was aimed to compare microscopically the development of preinfective structures of ABR- and GLS isolates and their impact on the enzymatic oxidant defense system during the leaf infection process. On leaves, conidial germlings of GLS-isolate formed appressoria mostly sessile. In contrast, those of ABR-isolate were pedicellate and formed multiple melanized appressoria probably as a sign of unsuccessful infection attempts. Neither ABR- nor GLS isolate triggered hypersensitive response in apple leaves. In overall, the activity of scavenging enzymes was higher and long-lasting in leaves inoculated by GLS- than by ABR isolate and control. Guaiacol peroxidase, catalase, and glutathione reductase had activity peaks within 24 h after inoculation (HAI). Ascorbate peroxidase activity was higher only in GLS-infected leaves at 6 HAI, while superoxide dismutase remained unaltered. A lower level of hydrogen peroxide (H2O2) was determined in GLS-infected plants at 48 HAI, but the electrolyte leakage markedly increased. Disease symptoms in leaves were only caused by GLS-isolate. Results suggest that the virulent isolate coordinately downregulates the oxidative plant defense responses enabling its successful establishment in apple leaves.

Root Endophyte Colletotrichum tofieldiae Confers Plant Fitness Benefits that Are Phosphate Status Dependent

A staggering diversity of endophytic fungi associate with healthy plants in nature, but it is usually unclear whether these represent stochastic encounters or provide host fitness benefits. Although most characterized species of the fungal genus Colletotrichum are destructive pathogens, we show here that C. tofieldiae (Ct) is an endemic endophyte in natural Arabidopsis thaliana populations in central Spain. Colonization by Ct initiates in roots but can also spread systemically into shoots. Ct transfers the macronutrient phosphorus to shoots, promotes plant growth, and increases fertility only under phosphorus-deficient conditions, a nutrient status that might have facilitated the transition from pathogenic to beneficial lifestyles. The host's phosphate starvation response (PSR) system controls Ct root colonization and is needed for plant growth promotion (PGP). PGP also requires PEN2-dependent indole glucosinolate metabolism, a component of innate immune responses, indicating a functional link between innate immunity and the PSR system during beneficial interactions with Ct.