Mycoparasite Pythium Oligandrum Research Articles

Novel EIicitin from Pythium oligandrum Confers Disease Resistance against Phytophthora capsici in Solanaceae Plants.

The mycoparasite Pythium oligandrum is a nonpathogenic oomycete that can boost plant immune responses. Elicitins are microbe-associated molecular patterns (MAMPs) specifically produced by oomycetes that activate plant defense. Here, we identified a novel elicitin, PoEli8, from P. oligandrum that exhibits immunity-inducing activity in plants. In vitro-purified PoEli8 induced strong innate immune responses and enhanced resistance to the oomycete pathogen Phytophthora capsici in Solanaceae plants, including Nicotiana benthamiana, tomato, and pepper. Cell death and reactive oxygen species (ROS) accumulation triggered by the PoEli8 protein were dependent on the plant coreceptors receptor-like kinases (RLKs) BAK1 and SOBIR1. Furthermore, REli from N. benthamiana, a cell surface receptor-like protein (RLP) was implicated in the perception of PoEli8 in N. benthamiana. These results indicate the potential value of PoEli8 as a bioactive formula to protect Solanaceae plants against Phytophthora.

Interactions between the mycoparasite Pythium oligandrum and two types of sclerotia of plant-pathogenic fungi

The interactions between Pythium oligandrum hyphae and two types of sclerotia, i.e. the plano-convexoid sclerotium of Botrytis cinerea and the tuberoid sclerotium of Sclerotinia minor, were investigated by ultrastructural and cytochemical experiments. In the mycoparasitism of P. oligandrum, some differences in relation to sclerotium anatomy and the role of the rind layer in preventing invasion are documented. Both types of sclerotia showed neither alterations of the heavily melanised rind walls, nor direct rind wall penetration by P. oligandrum. This oomycete successfully entered B. cinerea sclerotia only through breaches at the junction of rind cells and corresponding to gaps in melanin deposits. On the other hand, none of these breaches was observed in the small sclerotia of S. minor, and P. oligandrum ingress in the sclerotia stopped at the inner rind layer. After the penetration of B. cinerea sclerotia by the mycoparasite, it extensively colonised the cortical and medulla areas by intercellular growth. The invaded tissues displayed pronounced alterations as well as some disorganisation of tissue in places. Colonisation was associated with severe chitin degradations of all host walls, which occurred even at some distance from P. oligandrum hyphae. The observation of wall thickenings in some P. oligandrum-hyphae suggests that the sclerotial cells constitute a harsh environment unsuitable for survival of the mycoparasite. These wall thickenings could be interpreted as P. oligandrum defence-like reactions.

Oligandrin, the elicitin-like protein produced by the mycoparasite Pythium oligandrum, induces systemic resistance to Fusarium crown and root rot in tomato plants

Oligandrin, the elicitin-like protein produced by the mycoparasite Pythium oligandrum, crab shell chitosan and crude glucans, isolated from P. oligandrum cell walls were applied to decapitated tomato plants and evaluated for their potential to induce defence mechanisms in root tissues infected by Fusarium oxysporum f. sp. radicis-lycopersici. A significant decrease in disease incidence was monitored in oligandrin- and chitosan-treated plants as compared to water-treated plants whereas glucans from P. oligandrum cell walls failed to induce a resistance response. Ultrastructural investigations of the infected root tissues from water-treated (control) plants showed a rapid colonization of all tissues including the vascular stele. In root tissues from oligandrin-treated plants, restriction of fungal growth to the outer root tissues, decrease in pathogen viability and formation of aggregated deposits, which often accumulated at the surface of invading hyphae, were the most striking features of the reaction. In chitosan-treated plants, the main response was the formation of enlarged wall appositions at sites of attempted penetration. These wall appositions were found to vary greatly in their appearance from multi-textured to multi-layered structures and to contain large amounts of callose. The use of the WGA/ovomucoid-gold complex provided evidence that the wall-bound chitin component in Fusarium cells colonizing roots of oligandrin-treated tomato plants was not substantially altered even over cell walls of hyphae showing obvious signs of degradation. Evidence is provided in this study that oligandrin has the ability to induce systemic resistance in tomato. Exogenous, foliar applications of the fungal protein sensitize susceptible tomato plants to react more rapidly and more efficiently to F. o. f. sp. radicis-lycopersici attack, mainly through the massive accumulation of fungitoxic compounds at sites of attempted pathogen penetration. Although cell wall modifications do not represent the central core of the oligandrin-mediated host response in tomato, they are part of the multicomponent defence system elaborated to fend off Fusarium invasion.

Treatment with the Mycoparasite Pythium oligandrum Triggers Induction of Defense-Related Reactions in Tomato Roots When Challenged with Fusarium oxysporum f. sp. radicis-lycopersici

ABSTRACT The influence exerted by the mycoparasite Pythium oligandrum in triggering plant defense reactions was investigated using an experimental system in which tomato plants were infected with the crown and root rot pathogen Fusarium oxysporum f. sp. radicis-lycopersici. To assess the antagonistic potential of P. oligandrum against F. oxysporum f. sp. radicis-lycopersici, the interaction between the two fungi was studied by scanning and transmission electron microscopy (SEM and TEM, respectively). SEM investigations of the interaction region between the fungi demonstrated that collapse and loss of turgor of F. oxysporum f. sp. radicis-lycopersici hyphae began soon after close contact was established with P. oligandrum. Ultrastructural observations confirmed that intimate contact between hyphae of P. oligandrum and cells of the pathogen resulted in a series of disturbances, including generalized disorganization of the host cytoplasm, retraction of the plasmalemma, and, finally, complete loss of the protoplasm. Cytochemical labeling of chitin with wheat germ agglutinin (WGA)/ovomucoid-gold complex showed that, except in the area of hyphal penetration, the chitin component of the host cell walls was structurally preserved at a time when the host cytoplasm had undergone complete disorganization. Interestingly, the same antagonistic process was observed in planta. The specific labeling patterns obtained with the exoglucanase-gold and WGA-ovomucoid-gold complexes confirmed that P. oligandrum successfully penetrated invading cells of the pathogen without causing substantial cell wall alterations, shown by the intense labeling of chitin. Cytological investigations of samples from P. oligandrum-inoculated tomato roots revealed that the fungus was able to colonize root tissues without inducing extensive cell damage. However, there was a novel finding concerning the structural alteration of the invading hyphae, evidenced by the frequent occurrence of empty fungal shells in root tissues. Pythium ingress in root tissues was associated with host metabolic changes, culminating in the elaboration of structural barriers at sites of potential fungal penetration. Striking differences in the extent of F. oxysporum f. sp. radicis-lycopersici colonization were observed between P. oligandrum-inoculated and control tomato plants. In control roots, the pathogen multiplied abundantly through much of the tissues, whereas in P. oligandrum-colonized roots pathogen growth was restricted to the outermost root tissues. This restricted pattern of pathogen colonization was accompanied by deposition of newly formed barriers beyond the infection sites. These host reactions appeared to be amplified compared to those seen in nonchallenged P. oligandrum-infected plants. Most hyphae of the pathogen that penetrated the epidermis exhibited considerable changes. Wall appositions contained large amounts of callose, in addition to be infiltrated with phenolic compounds. The labeling pattern obtained with gold-complexed laccase showed that phenolics were widely distributed in Fusarium-challenged P. oligandrum-inoculated tomato roots. Such compounds accumulated in the host cell walls and intercellular spaces. The wall-bound chitin component in Fusarium hyphae colonizing P. oligandrum-inoculated roots was preserved at a time when hyphae had undergone substantial degradation. These observations provide the first convincing evidence that P. oligandrum has the potential to induce plant defense reactions in addition to acting as a mycoparasite.

Behaviour of zoospore cysts of the mycoparasite Pythium oligandrum in relation to their potential for biocontrol of plant pathogens

Several in vitro features of zoospore cysts of Pythium oligandrum suggest their potential value for biocontrol, in place of oospores which germinate slowly and erratically. Cysts germinated rapidly (20–30 min) in water at 20–25 °C. They also germinated, but more slowly, at water potentials down to −6·6 MPa (PEG 8000 solutions) or −2·65 MPa (mannitol solutions), whereas mycelia did not grow at −2·1 MPa. Cysts could be stored for at least 3 wk in PEG at 4–7°, and germinated when the PEG was diluted and temperature was raised to 20°. Germinating cysts rapidly (<1 h) parasitized hyphae of Fusarium oxysporum and F. culmorum in the absence of exogenous nutrients. When cysts of a metalaxyl-tolerant mutant of P. oligandrum were added to moist, unsterile soil at 20°, the fungus was recovered at one-third of the original level at 28 d, and was still detected at 80 d. Cysts and motile zoospores were studied for mycoparasitic activity on water-agar films in vitro . Cysts germinated readily on colonies of F. oxysporum and Pythium aphanidermatum , and in the presence of spores of Idriella bolleyi , but poorly on colonies of F. culmorum , which seemed to produce an inhibitor. Motile zoospores were not attracted to hyphae but if they encysted on hyphae they showed significant germ-tube emergence towards the host; pre-encysted spores germinated with random orientation. A higher incidence of parasitism from motile spores than from cysts indicated a need for anchorage (adhesion during encystment) in the conditions of the assays, implicating mechanical pressure in mycoparasitism. Sporelings needed to grow some distance (about 30 m) before they parasitized efficiently; this might involve transition from a germ-tube to a vegetative hypha, because it was similar to the maximum growth (26–27 m) from cysts at water potentials that inhibited hyphal growth. These findings extend previous work on zoospore behaviour in Pythium and Phytophthora spp.

Susceptibility of Pythium spp and other fungi to antagonism by the mycorparasite Phytium Oligandrum

Phytium spp and isolates within species differed in susceptibility to the mycoparasite Pythium oligandrum Drechs., as evidenced by their degree of inhibition by it on cellulose and ability to support its growth across their colonies on agar. Yet no Phythium sp. was highly susceptible to it, and P. graminicola Subramanian was highly resistant. No evidence was found that P. oligandrum produces toxins active against other fungi. In liquid culture, P. oligandrum grew on undisturbed colonies of Phialophora sp. (highly susceptible) but not P. ultimum Trow or Fusarium culmorum (W. G. Sm.) Sacc. (moderately resistant) and not Rhizoctonia solani Kuhn (highly resistant). It grew in culture filtrates of Phialophora sp., P. ultimum and F. culmorum, utilizing organic nitrogen and thiamine released by these fungi, but not in culture filtrates of R. solani. It grew on mycelial macerates of all these fungi, though poorly on those of R. solani. Resistance to parasitism by P. oligandrum seems to reside, at least partly, in low levels of nutrient release from host hyphae.

Aspects of antagonism by the mycoparasite Pythium oligandrum

Pythium oligandrum was antagonistic towards Pythium ultimum, Mycocentrospora acerina and Rhizoctonia solani through an interaction resembling hyphal interference. The significance of this and of necrotrophic mycoparasitism are assessed in relation to the ecology of P. oligandrum. It is suggested that these processes are primarily mechanisms for effecting resource capture from less combative species; benefits arising from direct exploitation of host hyphae being of subsidiary importance.

Effects of the mycoparasite Pythium oligandrum on cellulolysis and sclerotium production by Rhizoctonia solani

In vitro growth experiments show that cellulolytic ability and sclerotium production in Rhizoctonia solani are markedly reduced by co-inoculation with Pythium oligandrum, whereas the effect of P. ultimum on R. solani is negligible. The reduction in cellulolytic ability and sclerotium formation is probably due to the mycoparasitism of R. solani by P. oligandrum and possibly also to competition for nutrients.

Biological control of Pythium ultimum–induced damping‐off by treating cress seed with the mycoparasite Pythium oligandrum

Incorporation of the aggressive mycoparasite Pythium oligandrum into a carboxymethyl cellulose‐based seed coating decreased damping‐off of cress seedlings by Pythium ultimum in both naturally infested soil and artificially infested sand. This effect is attributed to the germination of P. oligandrum oospores on the seed surface with subsequent generation of a protectant mycelium around the seedling. P. oligandrum oospores survived storage periods of 10—20 weeks at zero relative humidity both on glass slides and within seed coatings. The mycoparasite also survived an 18–month burial in natural soil, probably in the form of oospores.The implications of these observations for biocontrol of seedling diseases by P. oligandrum are discussed.