Department of Biological and Nutritional Sciences, University of Newcastle, Newcastle upon Tyne NE1 7RU, UK

Background and objectives
Bipolaris sorokiniana (teleomorph Cochliobolus sativus) is a relatively unspecialized pathogen of wheat and barley with the ability to attack all parts of its hosts. How it causes disease and how cereals resist it are poorly understood; resistance is likely to involve multiple factors. One factor that can affect the ability of a soilborne pathogen to attack a plant is its response to the plant's roots. Chemicals released from roots support the growth and control the development of rhizosphere microorganisms [1]. We therefore investigated whether root exudates of wheat cultivars might influence their susceptibility to B. sorokiniana. In preliminary investigations, we grew wheat seedlings in water with fungal spores added. Seedlings of cultivar Sonalicca remained relatively healthy, whereas those of cultivar Mohtar 83 became chlorotic and died. The results suggested that these cultivars were differentially affected by toxin(s) produced by the fungus (which is known to produce toxins [2]), prompting us to compare their toxin sensitivity.

Materials and methods
Root exudates were prepared by growing seedlings in sterile water [3]. A drop placed on a microscope slide was inoculated with fungal spores, whose germination and germ-tube growth were observed. To assess further growth, mycelial plugs were placed on filter paper circles soaked in root exudates or water, which were placed on water agar in petri dishes. Subsequently, newly-formed spores were washed off the filter paper and counted. The chemical composition of root exudates was analysed by HPLC. For toxin tests, seeds were incubated on filter paper soaked in fungal culture filtrate prepared as in [2], or filtrate-impregnated filter paper was applied to seedling leaves.

Results and conclusions
On the basis of studies of the disease susceptibility of wheat cultivars obtained from the Yemen, cultivars Mohtar 83 and Sonalicca were selected as model susceptible (S) and resistant (R) cultivars, respectively. B. sorokiniana spores germinated more rapidly in S exudate than in distilled water, but more slowly in R exudate. Final percentage germination was nearly 100% in S exudate or water, but only about 60% in R exudate. Germ tubes grew more rapidly in S exudate than in water, but more slowly in R exudate. The fungus produced more spores when allowed to grow on S exudate than on sterile water, but fewer on R exudate. Overall, there were considerably higher concentrations of sugars and amino acids in S than in R exudate, and the proportions of individual sugars and amino acids differed. Because the most abundant sugar and amino acid in S exudate were maltose and asparagine, we tested the effects of these compounds on spore germination. Addition to water of maltose, asparagine, or both at levels similar to those in S exudate had no significant effect on spore germination. Addition of asparagine and maltose to R exudate, however, overcame its inhibitory effect, raising germination to a level similar to that obtained with S exudate. Pathogen culture filtrates killed about half the S seedlings exposed to them, but had little effect on R seed germination. They inhibited growth of roots and shoots of surviving S and R seedlings. Culture filtrates caused necrosis when applied to S seedling leaves but had little effect on R seedling leaves. Toxicity was reduced by neutralization of the filtrate.

Our results are consistent with the hypothesis that differences among wheat cultivars in root exudate composition and in sensitivity to compounds produced by the pathogen contribute to differences in disease susceptibility. It may be possible to use these differences to evaluate particular components of resistance in breeding studies. It will be important to examine whether our observations can be generalized to other cultivars.

1. Shepherd T, 1994. In Blakeman JP, Williamson B, eds, Ecology of Plant Pathogens. CAB International, Wallingford, pp. 39-62.
2. Carlson H, Nilsson P, Jansson H-B, Odham G, 1991. Journal of Microbiological Methods 13, 259-269.
3. Jansson H-B, Johansson T, Nordbring-Hertz B et al., 1988. Transactions of the British Mycological Society 90, 647-650.