BSPP Presidential Meeting 1999

Biotic Interactions in Plant-pathogen Associations

Session VII - Biological Control: across taxon

Ecological interactions between plant pathogens and biocontrol bacteria
Brion K. Duffy & Genevive Dfago

Phytomedizin, Institut fr Pflanzenwissenschaften, ETH-Z, Zrich Switzerland.
E-Mail:
brion.duffy@ipw.agrl.ethz.ch

Root-colonizing Pseudomonas fluorescens produce 2,4-diacetylphloroglucinol (PHL) which is a potent growth inhibitor of the tomato pathogen Fusarium oxysporum f.sp. radicis-lycopersici. PHL production by Pseudomonas is a primary mechanism in the biological control of Fusarium root rot of tomato in hydroponic production systems. Interestingly though, we recently found that a phyto-/mycotoxin produced by Fusarium, fusaric acid, specifically represses bacterial biosynthesis of PHL. In other words, the pathogen protects itself by blocking the main biocontrol mechanism of its antagonist. The end result is that the biocontrol efficacy of Pseudomonas is greatly reduced. This is the first, and so far only, example of direct signalling from a fungus to a bacterial antagonist.

Two approaches we have taken to short-circuit this defence mechanism of the pathogen have promise for stabilizing biocontrol of Fusarium diseases.

By amending hydroponic nutrient solution with certain trace-elements (i.e., zinc), Fusarium is no longer able to produce fusaric acid. This creates an environment where Pseudomonas is then able to produce PHL and control tomato root rot.

By screening an ecologically and genetically diverse collection of PHL-producing Pseudomonas, we found that strains sensitive or resistant to fusaric acid repression fall into two distinct groups. Sensitive strains (e.g. CHA0 from Morens, CH) produce PHL plus another antibiotic, pyoluteorin. Resistant strains (e.g., F113 from Ireland) produce only PHL. We can now select biocontrol bacteria not only for having PHL biosynthetic genes (using a PCR probe), but also for the ability to produce PHL in specific environments. This presents an excellent opportunity for prescription biocontrol; using fusaric acid resistant bacteria to control Fusarium diseases.

Together with the group of Dieter Haas and Christoph Keel (UniL), we have begun to elucidate the molecular mechanisms behind fusaric acids action on PHL biosynthesis. Early data implicate the bacterial autorepressor phlF as the target site, since PhlF mutants are no longer sensitive to the toxin. This suggests that the fungus has hijacked the bacteriums own regulatory network. We are currently looking at the possible co-evolution of biocontrol pseudomonads and pathogenic fusaria. Analysis of biosynthetic gene diversity indicates that an evolutionary divergence among PHL-producing pseudomonads may have been driven by fusaric acid sensitivity.


Interactions in the rhizosphere between plant parasitic nematodes and nematophagous fungi.
Brian R. Kerry.

Entomology & Nematology Department, IACR-Rothamsted, Harpenden, Herts AL5 2JQ, UK.
E-Mail:
brian.kerry@bbsrc.ac.uk

The cereal cyst nematode, Heterodera avenae, has been controlled in monocultures of susceptible crops because of the build up of nematophagous fungi that parasitise females and eggs of the nematode in the cereal rhizosphere and prevent populations increasing. This natural control caused by Nematophthora gynophila and Verticillium chlamydosporium is only effective after the fourth or fifth cereal crop and it has proved too difficult to manipulate these fungi with methods that are practical for a grower. Attention has moved away from the manipulation of suppressive soils to the application of selected fungal isolates as biological control agents. Research at Rothamsted has concentrated on the tri-trophic interactions between root-knot nematodes (Meloidogyne spp.), the facultative parasite V. chlamydosporium and vegetable crops. Root-knot nematodes are major nematode pests that cause about $70b crop losses per annum. They are major problems on vegetable crops in many countries and methyl bromide is frequently used to control then on these high value crops. The planned removal of methyl bromide from the market before 2005 because it depletes the ozone layer has increased efforts to develop alternative strategies for the management of root-knot nematodes. Verticillium chlamydosporium is a widespread fungus that parasitises the eggs of cyst and root-knot nematodes. The fungus colonises the rhizosphere of some plants and switches to become a nematode parasite when the females and egg-masses appear on roots. The host plant has a marked effect on these interactions and affects the growth of the fungus on roots and the numbers of nematodes infected. Although a semi-selective medium has been developed to isolate the fungus from soil and on roots, there is a need for more discriminating techniques. Molecular methods based on PCR techniques have been developed that enable the fungus to be monitored after its release and for variation between isolates to be studied. Both molecular and immunological methods are being developed to visualise the fungus in the rhizosphere and enable detailed studies of its ecology to be made. Although significant control of different species of root-knot nematodes has been achieved with applications of V. chlamydosporium in small plots, consistent control in a range of conditions will only be possible when the key interactions in the rhizosphere that affect the efficacy of the fungus are understood.


Biological control as a component of an integrated approach to managing lettuce big-vein.
John A. Walsh & Judith M. Bambridge.

Plant Pathology and Microbiology Department, Horticulture Research International, Wellesbourne, Warwick, CV35 9EF, UK. E-Mail: john.walsh@hri.ac.uk.

Lettuce big-vein virus is the most damaging virus affecting lettuce production in Europe. It is transmitted by the soil-borne, root-infecting, fungal, obligate parasite Olpidium brassicae. Intensification of production, lack of, or reduced rotations along with the increased use of irrigation have increased losses caused by lettuce big vein virus and other viruses vectored by root-infecting fungi and protists.

We are in the process of developing an integrated strategy for the control of big-vein disease. The components of this strategy include improved hygiene, reduced fungicide applications, disease avoidance through vector and pathogen detection, plant resistance and biological control.

The criteria for selecting different potential biological control agents for testing was based on their possible across taxon interactions with the fungal vector O. brassicae and/or the virus itself. These criteria will be outlined for each of the agents tested.

The effects of some of the biological control agents on three different aspects of big-vein disease development will be presented.

This work was funded by the Ministry of Agriculture, Fisheries and Food, UK.


Plant pathogen - herbivore interactions and their effects on weeds
1
Paul E. Hatcher, & 2Nigel D. Paul.

1Department of Agricultural Botany, School of Plant Sciences, The University of Reading, Reading RG6 6AU, UK.
E-Mail:
p.e.hatcher@rdg.ac.uk
2
Division of Biology, I.E.N.S., Lancaster University, Lancaster, LA1 4YQ, UK of Lancaster, UK

Plant pathogenic fungi and insects are among the most abundant consumers of plants, and thus it is not surprising that they should occur together on their hosts. As the study of plant-consumer relationships has mainly been the concern of either the plant pathologist or entomologist, little attempt has been made to consider insect-fungus interactions, which span these disciplines.

The study of insect-fungus interactions is complicated because along with two-way interactions, a new, indirect interaction, the effect of the fungus on the insect (and vice-versa) as mediated by the host plant needs to be considered.

There has been interest in using insect-fungus combinations for the biological control of weeds, and we illustrate this with our study on the interactions between the chrysomelid beetle Gastrophysa viridula, the rust fungus Uromyces rumicis and their hosts Rumex crispus and Rumex obtusifolius.

Gastrophysa viridula grazing significantly inhibited U. rumicis infection around the site of feeding damage and also inhibited infection in undamaged leaves, the latter a systemic effect. A similar systemic effect was observed in field experiments with natural infection by the rust. Likewise, U. rumicis infection of leaves reduced the suitability of leaves for G. viridula: mortality of larvae was reduced, larval development was delayed, and the fecundity of adults was reduced in insects feeding on infected leaves.

However, the effect of this insect-fungus combination on the host plant was significantly greater than was expected from these negative interactions. The combination of insect and fungus often had an additive effect (equivalent to the sum of the effects of insect and fungus alone) on the host. We suggest that this is due to resource partitioning between insect and fungus, with the fungus confined to older leaves, due to younger leaves being resistant to the rust, and G. viridula selecting to feed on these young, uninfected leaves.

We contrast this system with another on the related Emex australis, where the insect and fungus (which negatively affect each other) were unable to escape the effects of each other, with consequently less damage caused to the plant than expected (Shivas & Scott 1993).

Key references

Hatcher, P.E. (1995) Three-way interactions between plant pathogenic fungi, herbivorous insects and their host plants. Biological Reviews 70, 639 - 694.

Hatcher, P.E. & Ayres, P.G. (1997) Indirect interactions between insect herbivores and pathogenic fungi on leaves. In: Multitrophic Interactions in Terrestrial Systems (eds A.C. Gange & V.K. Brown), pp.133 - 149. Blackwell Science, Oxford.

Shivas, R.G. & Scott, J.K. (1993) Effect of the stem blight pathogen, Phomopsis emicis, and the weevil, Perapion antiquum, on the weed Emex australis. Annals of Applied Biology 122, 617 - 622.


The role of hyperparasites in host plant - parasitic fungi relationships
Levente Kiss.

Plant Protection Institute of the Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 102, Hungary.
E-Mail:
LKISS@NKI.HU

Traditionally, the interactions between plant parasitic fungi and host plants are regarded as closed, two-species systems. However, both parasites and their hosts are, in fact, components of complex multitrophic interactions in which parasitic fungi are often attacked and killed by hyperparasites or other antagonists. Parasites, by definition, have a negative effect on host fitness, so hyperparasitism should be favourable for plants infected with parasites. However, studies on the possible role of hyperparasites in the natural control of plant parasites are completely missing from the literature. There are a few quantitative studies even on the natural occurrence of hyperparasitism that represents only the first step towards evaluating the impact of hyperparasites on host fungal and plant populations in nature. This paper synthesizes the current knowledge on structural, physiological and evolutionary aspects of host-parasite-hyperparasite relationships. It is concluded that hyperparasites certainly add further complexities to host-parasite interactions, but detailed investigations are needed to elucidate their significance, if any, in the natural control of fungal plant parasites.