Guarding the green: pathways to stomatal immunity

Guard cells regulate plant gas exchange and transpiration by modulation of stomatal aperture upon integrating external cues like photosynthetic effective illumination, CO2 levels and water availability and internal signals like abscisic acid (ABA). Being pores, stomata constitute a natural entry site for potentially harmful microbes. To prevent microbial invasion, stomata close upon perception of microbe-associated molecular patterns (MAMPs), and this represents an important layer of active immunity at the preinvasive level. The signaling pathways leading to stomatal closure triggered by biotic and abiotic stresses employ several common components, such as reactive oxygen species, calcium, kinases, and hormones, suggesting considerable intersection between MAMP- and ABA-induced stomatal closures, which we will discuss in this review.

Production of fungal antibiotics using polymeric solid supports in solid-state and liquid fermentation

The use of inert absorbent polymeric supports for cellular attachment in solid-state fungal fermentation influenced growth, morphology, and production of bioactive secondary metabolites. Two filamentous fungi exemplified the utility of this approach to facilitate the discovery of new antimicrobial compounds. Cylindrocarpon sp. LL-Cyan426 produced pyrrocidines A and B and Acremonium sp. LL-Cyan416 produced acremonidins A-E when grown on agar bearing moist polyester-cellulose paper and generated distinctly different metabolite profiles than the conventional shaken or stationary liquid fermentations. Differences were also apparent when tenfold concentrated methanol extracts from these fermentations were tested against antibiotic-susceptible and antibiotic-resistant Gram-positive bacteria, and zones of inhibition were compared. Shaken broth cultures of Acremonium sp. or Cylindrocarpon sp. showed complex HPLC patterns, lower levels of target compounds, and high levels of unwanted compounds and medium components, while agar/solid support cultures showed significantly increased yields of pyrrocidines A and B and acremonidins A-E, respectively. This method, mixed-phase fermentation (fermentation with an inert solid support bearing liquid medium), exploited the increase in surface area available for fungal growth on the supports and the tendency of some microorganisms to adhere to solid surfaces, possibly mimicking their natural growth habits. The production of dimeric anthraquinones by Penicillium sp. LL-WF159 was investigated in liquid fermentation using various inert polymeric immobilization supports composed of polypropylene, polypropylene cellulose, polyester-cellulose, or polyurethane. This culture produced rugulosin, skyrin, flavomannin, and a new bisanthracene, WF159-A, after fermentation in the presence and absence of polymeric supports for mycelial attachment. The physical nature of the different support systems influenced culture morphology and relative metabolite yields, as determined by HPLC analysis and measurement of antimicrobial activity. The application of such immobilized-cell fermentation methods under solid and liquid conditions facilitated the discovery of new antibiotic compounds, and offers new approaches to fungal fermentation for natural product discovery.

Nuclear behavior in fungal hyphae

A characteristic feature of fungal hypha is the presence of large number of nuclei in a common cytoplasmic environment. Where it has been examined, the coenocytic mycelium is commonly heterokaryotic. The nuclei cooperate, compete or combat. It is proposed that in addition to their classical role in heredity, supernumerary nuclei in filamentous fungi serve as store house for nitrogen and phosphorus in the form of DNA which is degraded by regulated autophagy. The breakdown products recycled, giving hyphal tips the capability of persistent extension and foraging in new areas.

Research on the Rust Fungi During the Twentieth Century

▪ Abstract  Interest in the rust fungi derives from their success as plant pathogens. For example, the epidemic on coffee had serious economic and social impacts on diverse cultures. During the century, research on the rust germling shifted from a study of germling development, including a search for the signals that induce differentiation, to an examination of the genes expressed during host colonization. Research on host resistance was most influenced by Stakman, who studied the genetics and epidemiology of rust disease. His innovations enabled Flor to propose the gene-for-gene hypothesis, a concept that stimulated development of resistant crops, and led to research that gradually shifted during the century to an examination of the molecular basis of rust genetics.

Infection structures of fungal plant pathogens - a cytological and physiological evaluation

Many fungi differentiate specific infection structures in order to infect the host plant. The spore attaches to the host surface, the cuticle, and the germ tube may recognize suitable penetration sites, over which an appressorium is formed. Additional wall layers in appressoria of many fungi suggest that this structure supports increasing pressure during the penetration process. During appressorium formation, synthesis of polymer-degrading enzymes is often initiated. Cutinases, cellulases and pectin-degrading enzymes can be formed in a developmentally controlled or adaptive, i.e. substrate-dependent, fashion. The penetration hypha develops below the appressorium. This hypha has a new wall structure and exhibits features which serve to breach the plant cell wall. However, at present it is not clear whether penetration hyphae arising from appressoria are more efficient in penetration or induce less damage than hyphae which penetrate without detectable special adaptations. The infection hypha differentiates within the host. During differentiation a characteristic set of enzymes is synthesized to enable successful establishment of the host-pathogen relationship. If, as in most cases, multiple forms of cell wall-degrading enzymes are formed by the pathogen, mutagenesis or deletion of a gene encoding one of these enzymes very often has no effect on pathogenicity or even virulence. Proof is missing very often that an enzyme is needed at the right time and at the right site of infection. Events occurring during differentiation of fungal infection structures are reviewed with special emphasis on Magnaporthe grisea, Colletotrichum spp., and rust fungi, and common features which may be of importance to the success of infection are discussed. CONTENTS Summary 193 I. Introduction 193 II. Spore and germ tube 195 III. The appressorium 199 IV. The penetration hypha 201 V. The infection hypha 204 VI. Future prospects 208 Acknowledgements 208 References 208.

Characterization of an infection structure-specific gene from the rust fungus Uromyces appendiculatus

Uredospores of the plant pathogen, Uromyces appendiculatus, infect leaves of the bean plant, Phaseolus vulgaris, through stomata. Physical stimuli provided by the stomate induce differentiation of the germ tube to form a series of infection structures involved in host colonization. Contact between the uredospores and the oil-collodion membranes induces formation of infection structures in the absence of the host. This report describes the characterization of a Uromyces gene, INF24, that is induced by the physical stimulus of an oil-collodion membrane. INF24 contains a 450-bp open reading frame which encodes a 16.4-kDa polypeptide. The N terminus of the INF24-encoded protein, and the C terminus of human single-stranded DNA-binding protein are both glycine-rich and share homology.

Acrolein Induces Differentiation of Infection Structures in the Wheat Stem Rust Fungus

A chemical factor from wheat stem rust uredospores that induces infection structure formation has been identified as acrolein (2-propenal). This compound is the active component of distillates from uredospore extracts previously shown to induce infection structure formation.

Infection structures from rust urediospores: effect of RNA and protein synthesis inhibitors

Urediospores of Puccinia graminis tritici, floated on buffer, produce infection structures when subjected briefly to 30 degrees C soon after germination. Inhibitors of RNA synthesis interfere with the difierentiation of infection structures if present during this heat treatment. Inhibitors of protein synthesis prevent differentiation if present following heat treatment. Apparently infection structure formation is accompanied by synthesis of RNA, and the completion of infection structure development requires protein synthesis.