Ultrastructure and germinability of Verticillium dahliae microsclerotia parasitized by Talaromyces flavus on agar medium and in treated soil

The interactions between microsclerotia (ms) of the fungal plant pathogen Verticillium dahliae and the mycoparasite Talaromyces flavus were followed in soil and on agar medium. Germinability of ms, which had been incubated for 14 days in soil treated with 0.5% of a T. flavus ‐ wheat bran preparation, decreased from 84% to 17%, as compared with 81% and 74% in untreated soil and in soil treated with a sterilized biocontrol preparation respectively. Germinability of ms which had been buried in treated soil for 4 days decreased to 70%, all ms being parasitized by T. flavus. Upon transfer of the ms to untreated soil for 10 more days, germinability decreased further to 20%, indicating that T. flavus continued to parasitize sclerotia in the untreated soil. Scanning electron micrographs showed heavy fungal colonization and typical T. flavus conidia on the surface of the ms buried in the treated soil, but not in control soils. Transmission electron micrographs of ms incubated with T. flavus on agar revealed parsitism involving invasion of some host cells by means of small penetration pegs; the host cell walls were mainly lysed at their site of contact with the parasite hyphal tips. Further colonization of the ms cells occurred simultaneously with the degradation of the invaded host cell contents, rather than the cell walls. Mycoparasitism of V. dahliae ms by T. flavus hyphae may be involved in the biological control of verticillium wilt disease.     

An atypical forkhead‐containing transcription factor SsFKH1 is involved in sclerotial formation and is essential for pathogenicity in Sclerotinia sclerotiorum

Sclerotinia sclerotiorum (Lib.) de Bary is a necrotrophic plant pathogen with a worldwide distribution. The sclerotia of S. sclerotiorum are pigmented multicellular structures formed from the aggregation of vegetative hyphae. These survival structures play a central role in the life and infection cycles of this pathogen. Here, we characterized an atypical forkhead (FKH)‐box‐containing protein, SsFKH1, involved in sclerotial development and virulence. To investigate the role of SsFkh1 in S. sclerotiorum, the partial sequence of SsFkh1 was cloned and RNA interference (RNAi)‐based gene silencing was employed to alter the expression of SsFkh1. RNA‐silenced mutants with significantly reduced SsFkh1 RNA levels exhibited slow hyphal growth and sclerotial developmental defects. In addition, the expression levels of a set of putative melanin biosynthesis‐related laccase genes and a polyketide synthase‐encoding gene were significantly down‐regulated in silenced strains. Disease assays demonstrated that pathogenicity in RNAi‐silenced strains was significantly compromised with the development of a smaller infection lesion on tomato leaves. Collectively, the results suggest that SsFkh1 is involved in hyphal growth, virulence and sclerotial formation in S. sclerotiorum.     

The 1st step initiation essential for allergen‐specific IgE antibody production upon the 2nd step: Induction of non‐specific IgE+ small B cells containing secondly‐sensitized allergen‐specific ones in mice firstly‐sensitized with an allergen

There was a significant amount of non‐specific, but not of allergen (e.g., papain, mite feces and four kinds of pollen)‐specific, IgE antibodies (Abs) in the sera of normal mice. An i.n. injection of each allergen without adjuvant into mice caused an increase in total IgE Ab titers with a similar time course in the serum. However, the stage of initiation of allergy varied from allergen to allergen. Submandibular lymph node cells from normal mice contained papain‐, but not mite feces‐ or pollen‐specific IgE⁺ cells and an i.n. injection of papain induced papain‐specific IgE Abs in the serum. In contrast, one (i.n.) or two (i.n. and s.c) injections of mite feces induced neither mite feces‐specific IgE⁺ cells in the lymph nodes nor mite feces‐specific IgE Abs in the serum. I.n. sensitization with cedar pollen induced cedar pollen‐specific IgE⁺ small B cells in the lymph nodes on Day 10, when non‐specific IgE Ab titers reached a peak in the serum, implying induction of related allergen‐specific IgE⁺ small cells as well. In fact, a second (s.c.) injection of ragweed (or cedar) pollen into mice sensitized i.n. once with cedar (or ragweed) pollen, but not with mite feces, induced a large amount of ragweed (or cedar) pollen‐specific IgE Abs in the serum. These results indicate that when firstly‐sensitized non‐specific IgE⁺ small B cells in mouse lymph nodes include some secondly‐sensitized allergen‐specific ones, mice produce IgE Abs specific for the secondly‐injected allergen.

     

Genetic alteration of UDP‐rhamnose metabolism in Botrytis cinerea leads to the accumulation of UDP‐KDG that adversely affects development and pathogenicity

Botrytis cinerea is a model plant‐pathogenic fungus that causes grey mould and rot diseases in a wide range of agriculturally important crops. A previous study has identified two enzymes and corresponding genes (bcdh, bcer) that are involved in the biochemical transformation of uridine diphosphate (UDP)‐glucose, the major fungal wall nucleotide sugar precursor, to UDP‐rhamnose. We report here that deletion of bcdh, the first biosynthetic gene in the metabolic pathway, or of bcer, the second gene in the pathway, abolishes the production of rhamnose‐containing glycans in these mutant strains. Deletion of bcdh or double deletion of both bcdh and bcer has no apparent effect on fungal development or pathogenicity. Interestingly, deletion of the bcer gene alone adversely affects fungal development, giving rise to altered hyphal growth and morphology, as well as reduced sporulation, sclerotia production and virulence. Treatments with wall stressors suggest the alteration of cell wall integrity. Analysis of nucleotide sugars reveals the accumulation of the UDP‐rhamnose pathway intermediate UDP‐4‐keto‐6‐deoxy‐glucose (UDP‐KDG) in hyphae of the Δbcer strain. UDP‐KDG could not be detected in hyphae of the wild‐type strain, indicating fast conversion to UDP‐rhamnose by the BcEr enzyme. The correlation between high UDP‐KDG and modified cell wall and developmental defects raises the possibility that high levels of UDP‐KDG result in deleterious effects on cell wall composition, and hence on virulence. This is the first report demonstrating that the accumulation of a minor nucleotide sugar intermediate has such a profound and adverse effect on a fungus. The ability to identify molecules that inhibit Er (also known as NRS/ER) enzymes or mimic UDP‐KDG may lead to the development of new antifungal drugs.