Isolation and characterization of a chitinase-secreting mutant of mortierella pusilla with altered interaction with the mycoparasite piptocephalis virginiana

Mortierella pusilla is a susceptible host and supports good growth of the mycoparasite, Piptocephalis virginiana. Uninucleate spores of M. pusilla were sUbjected to N-methyl-N'-nitro-nitrosoguanidine (MNNG). To attain a high mutation frequency , a 1o-minute exposure to 10 mg/ml MNNG was used and lead to the survival of about 10 % of the spores. The exposed spores then were plated on chitin or milk plates. Approximately 30,000 colonies were examined after mutagenesis on the screening media. A strain, MUT23 , with abnormal slow growth morphology was found to delay parasitism by £. virginiana. The particular morphology was not due to auxotrophy, because this strain displayed normal hyphae when glucose was used as the sole carbon source. One interesting phenomenon was that MUT23 showed an extensive clearing zone around the colony on colloidal chitin agar after 20-25 d. On the same conditions, wild type strain did not show this phenotype. In addition, the MUT23 strain produced the same normal hypha as the wild type strain when it was grown on colloidal chitin agar. The MUT23 was also able to produce more spores on colloidal chitin agar than on malt-yeast extract and minimal media. The parasite germ tubes formed appressoria at the point of contact on the cell surface of wild type and MUT23 grown for 6 days cell surface but not on the cel surface of MUT23 grown for 2 days. Thus, interaction between MUT23 strain and the mycoparasite was dependent on MUT23 age. The effect of MUT23 filtrate on germination of the parasite was tested. Lysis of germinated spores of the parasite were observed in concentrated MUT23 filtered solution. MUT23 was compared to the wild type strain for their chitinase production in sUbmerged culture. The chitinase isozymes of both wild type and MUT23 were shown by immunoblotting. Eight distinct chitinase molecules were detected. MUT23 showed markedly higher chitinase activity than the wild type cultured in chitin-containing medium. Maximum chitinase activities of MUT23 were 13.5 fold higher at 20 day of the culture then that of wild type.

Characterization of the chitinolytic system during the mycoparasitic interaction between Trichoderma aggressivum f. aggressivum and different host strains of Agaricus bisporus /

Green mould is a serious disease of commercially grown mushrooms, the causal agent being attributed to the filamentous soil fungus Triclzodenna aggressivum f. aggressivu11l and T. aggressivum f. ellropaellm. Found worldwide, and capable of devastating crops, this disease has caused millions of dollars in lost revenue within the mushroom industry. One mechanism used by TricllOdenlla spp. in the antagonism of other fungi, is the secretion of lytic enzymes such as chitinases, which actively degrade a host's cell wall. Therefore, the intent of this study was to examine the production of chitinase enzymes during the host-parasite interaction of Agaricus bisporus (commercial mushroom) and Triclzodemza aggressivum, focusing specifically on chitinase involvement in the differential resistance of white, off-white, and brown commercial mushroom strains. Chitinases isolated from cultures of A. bisporus and T. aggressivu11l grown together and separately, were identified following native PAGE, and analysis of fluorescence based on specific enzymatic cleavage of 4-methylumbelliferyl glucoside substrates. Results indicate that the interaction between T. aggressivulll and A. bisporus involves a complex enzyme battle. It was determined that T. aggressivum produces a number of chitinases that appear to correlate to those isolated in previous studies using biocontrol strains of T. Izarziallilm. A 122 kDa N-acetylglucosaminidase of T. aggressivu11l revealed the highest and most variable activity, and is therefore believed to be an important predictor of antifungal activity. Furthermore, results indicate that brown strain resistance of mushrooms may be related to high levels of a 96 kDa N-acetylglucosaminidase, which showed elevated activity in both solitary and dual cultures with T. aggressivum. Overall, each host-parasite combination produced unique enzyme profiles, with the majority of the differences seen between day 0 and day 6 for the extracellular chitinases. Therefore, it was concluded that the antagonistic behaviour of T. aggressivli1ll does not involve a typical response, always producing the same types and levels of enzymes, but that mycoparasitism, specifically in the form of chitinase production, may be induced and regulated based on the host presented.

A comparative study of chitin synthase activity in two Mortierella species

An in vitro investigation of some important factors controlling the activity of chitin synthase in cell-free extracts of two Mortierella species has been carried out. Mixed membrane fractions from mycelial homogenates of Mortierella candelabrum and Mortierella pusilla were found to catalyse the transfer of N-acetylglucosamine from UDP-N-acetylglucosamine into an insoluble product characterized as chitin by its insolubility in weak acid and alkali, and the release of glucosamine and diacetylchitobiose on hydrolysis with a strong acid and chitinase, respectively. Apparent Km values for UDP-GlcNAc were 1.8 mM and 2.0 mM for M. pusilla and ~ candelabrum, respectively. Polyoxin D was found to be a very potent competitive inhibitor with values of the constant of inhibition, Ki' for both species about three orders of magnitude lower than theKm for UDP-GlcNAc. A divalent cation, Mg+2 , Mn+2 or Co+2 , was required for activity. N-acetylglucosamine, the monomer of chitin, stimulated the activity of the enzyme. The crude enzyme preparation of ~ candelabrum, unlike that of ~ pusilla, showed an absolute requirement for both Mg+2 and N-acetylglucosamine. Large differences in response to exogenous proteases were noted in the ratio of active to inactive chitin synthase of the two species. A fifteen fold or greater increase was obtained after treatment with acid protease (from Aspergillussaitoi) as compared to a two- to four-fold activation of the M. pusilla membrane preparation treated similarly. During storage at 4°C over 48 hours, an endogenous activation of chitin synthase of ~ pus ilIa was achieved, comparable to that obtained by exogenous protease treatment. The high speed supernatant of both species inhibited the chitin synthase activity of the mixed membrane fractions. The inhibitor of ~ pus ilIa was effective against the pre-activated enzyme whereas that of M. candelabrum inhibited the activated enzyme. Several possibilities are discussed as to the role of the different factors regulating the enzyme activity. The suggestion is made from the properties of chitin synthase in the two species that in vivo a delicate balance exists between the activation and inactivation of the enzyme which is responsible for the pattern of wall growth of each fungus.

Immuno-gluorescence study of five zygomycetous fungi with two chitin-binding probes

A polyclonal antiserum was prepared against a purified microsomal chitinase isolated from the fungus Choanephora cucurbitarum. Indirect immunofluorescence was used to localize chitinase at various developmental stages of five zygomycetous fungi and during abiotrophic mycoparasite interaction with a susceptible and resistant host. This was compared to localization of oligomers of N-acetylglucosamine with the lectin wheat germ agglutinin (WGA). Dotimmunoblot and Western blot techniques revealed that the anti-serum reacted strongly with the antigen from which it was derived. Cross reactivity of the antiserum was found with WGA and another chitin binding lectin, Phyto/acca americana agglutinin (PAA). Immuno-fluorescence results showed the direct involvement of chitinase in spore swelling, germination, sporangium development and response during mechanical injury. There appeared to be no involvement of chitinase during apical hyphal growth or new branch initiation in any of the fungi tested despite mild proteolysis and permeabilization of the cell surface prior to labelling. Binding with WGA revealed similar patterns of fluorescence to that of chitinase localization but differed by showing fluorescence and therefore chitin localization at the apex and new branch initiation when tested at different developmental stages. There was no difference between chitinase localization and binding with WGA in a susceptible host and resistant host challenged with the mycoparasite, Piptocephalis virginiana. Differences in binding ability of antichitinase and lectin WGA suggests that the latter is not a suitable indicator for indirect localization of the lytic enzyme, chitinase.