Department of Plant Biology, Royal Veterinary and Agricultural University, Thorvaidsensvej 40, DK-1871 Frederiksberg C, Denmark

Background and objectives
In barley and other grasses, induced or acquired resistance against biotrophic pathogens is a well-known phenomenon which has been studied intensively. In contrast, induced resistance against necrotrophic pathogens in monocotyledonous plants has apparently not been examined to any great extent, with a few exceptions such as the rice-Pyricularia oryzae system.

In a previous report [1], the severity of barley net blotch caused by the necrotrophic pathogen Drechslera teres and hyphal growth in the host tissue was shown to be strongly reduced when leaves were pre-treated with conidial suspensions of either of the two non-barley pathogens Bipolaris maydis from maize or Septoria nodorum from wheat. The results suggested that induced resistance was involved, but detailed studies of the mechanisms responsible were not carried out. Such investigations are, however, important when evaluating an organism to be used in disease control. If induced resistance is responsible for disease reductions, valuable information can be obtained on how defence of the plant is expressed.

The present study aims to investigate whether induced resistance is responsible for inhibiting D. teres in barley after inoculation with B. maydis and S. nodorum. This was done by quantitative histological examination of the primary infection stages of D. teres, by qualitative studies of the later development of this fungus, and by studies of the expression of defence response genes in the host after inoculation with B. maydis and S. nodorum.

Materials and methods
B. maydis and S. nodorum were applied to leaves of the barley cultivar Lenka 24 h before D. teres. The primary infection processes of D. teres were investigated by light microscopy of epidermal strips made from the first leaf 3, 12 and 24 h after inoculation with the pathogen. The later developmental stages of D. teres were examined in whole cleared, as well as in sectioned leaf material after staining with Toluidine Blue 0, 1, 3, 5, 7 and 9 days after inoculation with the pathogen. Defence response gene activation was studied by Northern blotting of RNA extracted from leaves inoculated with B. maydis or S. nodorum alone.

Results and conclusions
Collectively, our data suggest that induced resistance is the principal mechanism responsible for impeding the pathogen. The enhancement of resistance in the host was primarily manifested during penetration by D. teres and after penetration where growth of D. teres ceased soon after development of infection vesicles. Thus, 24 h after pretreatment with B. maydis or S. nodorum, the penetration frequency from D. teres appressoria was reduced from 42.7% in the controls to 9.5 and 14.8%, respectively. The reductions were associated with increased formation of fluorescent papillae (early defence reaction). The post-penetrational inhibition of D. teres completely stopped fungal growth and was apparently linked to an enhancement of multicellular hypersensitive responses after pre-treatment with B. maydis and S. nodorum (late defence reaction). Papillae formation and multicellular hypersensitive reactions were also observed in control plants, but were generally inadequate to stop fungal progress. The Northern blots support the conclusion that induced resistance is involved in suppression of D. teres by increased formation of papillae and hypersensitive reactions. Thus the blots showed strong expression of several defence response genes which are involved in these reactions in barley attacked by Erysiphe graminis.

1. Jorgensen HJL, Andresen H, Smedegaard-Petersen V, 1996. Phytopathology 86, 602-607.