Localized adhesion of nongerminated Venturia inaequalis conidia to leaves and artificial surfaces

Hyperspectral imaging reveals small-scale water gradients in apple leaves due to minimal cuticle perforation by Venturia inaequalis conidiophores

Effects of Venturia inaequalis on water relations of apple leaves were studied under controlled conditions without limitation of water supply to elucidate their impact on the non-haustorial biotrophy of this pathogen. Leaf water relations, namely leaf water content and transpiration, were spatially resolved by hyperspectral imaging and thermography; non-imaging techniques-gravimetry, a pressure chamber, and porometry-were used for calibration and validation. Reduced stomatal transpiration 3-4 d after inoculation coincided with a transient increase of water potential. Perforation of the plant cuticle by protruding conidiophores subsequently increased cuticular transpiration even before visible symptoms occurred. With sufficient water supply, cuticular transpiration remained at elevated levels for several weeks. Infections did not affect the leaf water content before scab lesions became visible. Only hyperspectral imaging was suitable to demonstrate that a decreased leaf water content was strictly limited to sites of emerging conidiophores and that cuticle porosity increased with sporulation. Microscopy confirmed marginal cuticle injury; although perforated, it tightly surrounded the base of conidiophores throughout sporulation and restricted water loss. The role of sustained redirection of water flow to the pathogen's hyphae in the subcuticular space above epidermal cells, to facilitate the acquisition and uptake of nutrients by V. inaequalis, is discussed.

A Melanin-Deficient Isolate of Venturia inaequalis Reveals Various Roles of Melanin in Pathogen Life Cycle and Fitness

Venturia inaequalis is the ascomycetous pathogen causing apple scabs and forms dark-pigmented spores and partially melanised infection structures. Although melanin is considered to be essential for the infection of host tissue, a spontaneously occurring melanin-deficient mutant was isolated from an abaxial side of an apple leaf and can be cultivated in vitro as well as in vivo. The morphology and development of the melanin-deficient-isolate SW01 on leaves of susceptible apple plants were compared to that of the corresponding wild-type isolate HS1. White conidia of SW01 were often wrinkled when dry and significantly increased their volume in suspension. Germination and formation of germtubes and appressoria were not impaired; however, the lack of melanisation of the appressorial ring structure at the interface with the plant cuticle significantly reduced the infection success of SW01. The colonisation of leaf tissue by non-melanised subcuticular hyphae was not affected until the initiation of conidiogenesis. Non-melanised conidiophores penetrated the plant cuticle from inside less successfully than the wild type, and the release of white conidia from less solid conidiophores above the cuticle was less frequent. Melanin in the outer cell wall of V. inaequalis was not required for the survival of conidia under ambient temperature or at -20 °C storage conditions, however, promoted the tolerance of the pathogen to copper and synthetic fungicides affecting the stability and function of the fungal cell wall, plasma membrane, respiration (QoIs) and enzyme secretion, but had no effect on the sensitivity to sulphur and SDHIs. The roles of melanin in different steps of the V. inaequalis life cycle and the epidemiology of apple scabs are discussed.

The conidial coin toss: A polarized conidial adhesive in Colletotrichum graminicola

Colletotrichum graminicola is an economically significant fungal pathogen of maize. The primary infective conidia of the fungus, falcate conidia, are splash-dispersed during rain events. The adhesion of the falcate conidia triggers germination and is required for the development of infection structures. Falcate conidia are capable of immediate adhesion upon encountering the substrate. We report that rapid adhesion in C. graminicola is polarized, with a single-sided strip of adhesive material running the length of a single side (or face) of the conidium between the tips. This strip of adhesive is co-localized with dynamic transverse actin cables, and both the adhesive strip and actin cables are formed after liberation of the conidium from its conidiogenous cell but prior to adhesion to the infection court. Orientation of conidia upon contact with substrate determines whether they will rapidly adhere, and those which do not initially adhere can be induced to do so by applying force to reorient or "flip" the conidia. We propose that C. graminicola possesses an adhesive mechanism resulting in an adhesion efficiency of approximately 50% upon initial contact with substrata, and that an increase in adhesion efficiency can be induced by disturbance.

The key role of peltate glandular trichomes in symbiota comprising clavicipitaceous fungi of the genus periglandula and their host plants

Clavicipitaceous fungi producing ergot alkaloids were recently discovered to be epibiotically associated with peltate glandular trichomes of Ipomoea asarifolia and Turbina corymbosa, dicotyledonous plants of the family Convolvulaceae. Mediators of the close association between fungi and trichomes may be sesquiterpenes, main components in the volatile oil of different convolvulaceous plants. Molecular biological studies and microscopic investigations led to the observation that the trichomes do not only secrete sesquiterpenes and palmitic acid but also seem to absorb ergot alkaloids from the epibiotic fungal species of the genus Periglandula. Thus, the trichomes are likely to have a dual and key function in a metabolic dialogue between fungus and host plant.

Morphology of fungal stains on paper characterized with multi-scale and multi-sensory surface metrology

Pigmentation of paper induced by fungi is one of the most complex phenomenons because it involves living organisms growing on a heterogeneous paper substrate. A novel approach to the study of interfaces of fungi and paper in black stains produced by pigmented Dematiaceous fungi with meristematic growth was undertaken applying surface metrology techniques: confocal laser scanning microscope (CLSM) white-light, confocal chromatic aberration profilometer (LCA) and scanning electron microscope in variable pressure (SEM-VP); fungal morphology was examined with transmitted light microscopy (TLM). The role of paper topography and surface morphology in fungi-induced biodeterioration was investigated elucidating some of the dynamic interactions of fungi with paper, spatial distribution of biological deposits, inclusions in paper matrix, and patterns of fungal growth on paper thus contributing to a better understanding of biodeterioration of paper-based cultural heritage. So far, to the authors' knowledge, there are no published reports on the investigation of interfaces of bio-stains and paper utilizing surface metrology techniques.

Glycosylphosphatidylinositol-anchored proteins in Fusarium graminearum: inventory, variability, and virulence

The contribution of cell surface proteins to plant pathogenicity of fungi is not well understood. As such, the objective of this study was to investigate the functions and importance of glycosylphosphatidylinositol-anchored proteins (GPI-APs) in the wheat pathogen F. graminearum. GPI-APs are surface proteins that are attached to either the membrane or cell wall. In order to simultaneously disrupt several GPI-APs, a phosphoethanolamine transferase-encoding gene gpi7 was deleted and the resultant mutant characterized in terms of growth, development, and virulence. The Δgpi7 mutants exhibited slower radial growth rates and aberrantly shaped macroconidia. Furthermore, virulence tests and microscopic analyses indicated that Gpi7 is required for ramification of the fungus throughout the rachis of wheat heads. In parallel, bioinformatics tools were utilized to predict and inventory GPI-APs within the proteome of F. graminearum. Two of the genes identified in this screen (FGSG_01588 and FGSG_08844) displayed isolate-specific length variability as observed for other fungal cell wall adhesion genes. Nevertheless, deletion of these genes failed to reveal obvious defects in growth, development, or virulence. This research demonstrates the global importance of GPI-APs to in planta proliferation in F. graminearum, and also highlights the potential of individual GPI-APs as diagnostic markers.

Tailoring fungal morphology of Aspergillus niger MYA 135 by altering the hyphal morphology and the conidia adhesion capacity: biotechnological applications

Current problems of filamentous fungi fermentations and their further successful developments as microbial cell factories are dependent on control fungal morphology. In this connection, this work explored new experimental procedures in order to quantitatively check the potential of some culture conditions to induce a determined fungal morphology by altering both hyphal morphology and conidia adhesion capacity. The capacity of environmental conditions to modify hyphal morphology was evaluated by examining the influence of some culture conditions on the cell wall lytic potential of Aspergillus niger MYA 135. The relative value of the cell wall lytic potential was determined by measuring a cell wall lytic enzyme activity such as the mycelium-bound β-N-acetyl-D-glucosaminidase (Mb-NAGase). On the other hand, the quantitative value of conidia adhesion was considered as an index of its aggregation capacity. Concerning microscopic morphology, a highly negative correlation between the hyphal growth unit length (l_HGU) and the specific Mb-NAGase activity was found (r = -0.915, P < 0.001). In fact, the environment was able to induce highly branched mycelia only under those culture conditions compatible with specific Mb-NAGase values equal to or higher than 190 U g_dry._wt^-1. Concerning macroscopic morphology, a low conidia adhesion capacity was followed by a dispersed mycelial growth. In fact, this study showed that conidia adhesion units per ml equal to or higher than 0.50 were necessary to afford pellets formation. In addition, it was also observed that once the pellet was formed the l_HGU had an important influence on its final diameter. Finally, the biotechnological significance of such results was discussed as well.

Influence of the host contact sequence on the outcome of competition among aspergillus flavus isolates during host tissue invasion

Biological control of aflatoxin contamination by Aspergillus flavus is achieved through competitive exclusion of aflatoxin producers by atoxigenic strains. Factors dictating the extent to which competitive displacement occurs during host infection are unknown. The role of initial host contact in competition between pairs of A. flavus isolates coinfecting maize kernels was examined. Isolate success during tissue invasion and reproduction was assessed by quantification of isolate-specific single nucleotide polymorphisms using pyrosequencing. Isolates were inoculated either simultaneously or 1 h apart. Increased success during competition was conferred to the first isolate to contact the host independent of that isolate's innate competitive ability. The first-isolate advantage decreased with the conidial concentration, suggesting capture of limited resources on kernel surfaces contributes to competitive exclusion. Attempts to modify access to putative attachment sites by either coating kernels with dead conidia or washing kernels with solvents did not influence the success of the first isolate, suggesting competition for limited attachment sites on kernel surfaces does not mediate first-isolate advantage. The current study is the first to demonstrate an immediate competitive advantage conferred to A. flavus isolates upon host contact and prior to either germ tube emergence or host colonization. This suggests the timing of host contact is as important to competition during disease cycles as innate competitive ability. Early dispersal to susceptible crop components may allow maintenance within A. flavus populations of genetic types with low competitive ability during host tissue invasion.

Venturia inaequalis: the causal agent of apple scab

The fungus Venturia inaequalis infects members of the Maloideae, and causes the disease apple scab, the most important disease of apple worldwide. The early elucidation of the gene-for-gene relationship between V. inaequalis and its host Malus has intrigued plant pathologists ever since, with the identification of 17 resistance (R)-avirulence (Avr) gene pairings. The Avr gene products are presumably a subset of the total effector arsenal of V. inaequalis (predominantly proteins secreted in planta assumed to facilitate infection). The supposition that effectors from V. inaequalis act as suppressors of plant defence is supported by the ability of the pathogen to penetrate the cuticle and differentiate into large pseudoparenchymatous structures, termed stromata, in the subcuticular space, without the initiation of an effective plant defence response. If effectors can be identified that are essential for pathogenicity, the corresponding R genes will be durable and would add significant value to breeding programmes. An R gene cluster in Malus has been cloned, but no V. inaequalis effectors have been characterized at the molecular level. However, the identification of effectors is likely to be facilitated by the resolution of the whole genome sequence of V. inaequalis.

TAXONOMY

Teleomorph: Venturia inaequalis Cooke (Wint.); Kingdom Fungi; Phylum Ascomycota; Subphylum Euascomycota; Class Dothideomycetes; Family Venturiaceae; genus Venturia; species inaequalis. Anamorph: Fusicladium pomi (Fr.) Lind or Spilocaea pomi (Fr.). LIFE CYCLE: V. inaequalis is a hemibiotroph and overwinters as pseudothecia (sexual fruiting bodies) following a phase of saprobic growth in fallen leaf tissues. The primary inoculum consists of ascospores, which germinate and penetrate the cuticle. Stromata are formed above the epidermal cells but do not penetrate them. Cell wall-degrading enzymes are only produced late in the infection cycle, raising the as yet unanswered question as to how V. inaequalis gains nutrients from the host. Conidia (secondary inoculum) arise from the upper surface of the stromata, and are produced throughout the growing season, initiating multiple rounds of infection. VENTURIA INAEQUALIS AS A MODEL PATHOGEN OF A WOODY HOST: V. inaequalis can be cultured and is amenable to crossing in vitro, enabling map-based cloning strategies. It can be transformed readily, and functional analyses can be conducted by gene silencing. Expressed sequence tag collections are available to aid in gene identification. These will be complemented by the whole genome sequence, which, in turn, will contribute to the comparative analysis of different races of V. inaequalis and plant pathogens within the Dothideomycetes.

The Venturia apple pathosystem: pathogenicity mechanisms and plant defense responses

Venturia inaequalis is the causal agent of apple scab, a devastating disease of apple. We outline several unique features of this pathogen which are useful for molecular genetics studies intended to understand plant-pathogen interactions. The pathogenicity mechanisms of the pathogen and overview of apple defense responses, monogenic and polygenic resistance, and their utilization in scab resistance breeding programs are also reviewed.

Infection strategies of filamentous microbes described with the Gene Ontology

Filamentous microbes that form highly developed symbiotic associations (ranging from pathogenesis to mutualism) with their hosts include fungi, oomycetes and actinomycete bacteria. These organisms share many common features in growth, development and infection and have evolved similar strategies for neutralizing host defense responses to establish symbioses. Recent advances in sequencing technologies have led to a remarkable increase in the number of sequenced genomes of filamentous organisms. Analysis of the available genomes has provided useful information about genes that might be important for host infection and colonization. However, because many functional similarities among these organisms have arisen by convergent evolution, sequence-based genomic comparisons will miss many genes that are functionally analogous. In the absence of sequence similarity, annotating genes with standardized terms from the Gene Ontology (GO) can facilitate functional comparisons. Here, we review common strategies employed by filamentous organisms during colonization of their hosts, with reference to GO terms that best describe the processes involved.