Plant diseases caused by various pathogens, like for example viruses, bacteria, and fungi, may lead to substantial yield losses in growing cultivated plants. In order to control fungal diseases, nowadays fungicides are intensively used in agricultural production. Despite such means of control, a substantial portion of the possible yield is lost as a result of diseases. For a while now, there have been efforts to use cultivated plants having a natural resistance against significant fungal pathogens within the scope of integrated plant protection in order to reduce said yield deficits on the one hand and the use of fungicides in general on the other hand. Besides the classical cultivation methods for producing plants having a natural resistance, genetic engineering approaches, wherein resistances are supposed to be introduced selectively into significant cultivated plants, for example by means of introducing external resistance genes or manipulating endogenous gene expression in the plants, have been playing an increasingly important role in recent years.
Various mechanisms of resistance can be distinguished among the naturally occurring resistances. The so-called preformed “non-host” resistance describes the observation that an entire plant species is resistant to a specific pathogen. This phenomenon, which has not be understood yet, is probably based on structural or chemical properties of the plant species. Said properties can be, for example, the thickness of the cuticle, the presence of inhibitory substances, or the limited availability of nutrients.
In contrast, active mechanisms of resistance comprise such reactions and mechanisms which are triggered in the host plant by the attacking pathogen. Normally, the latter mechanism of resistance is of greater significance. However, it has to be noted that a clear distinction between the active resistance mechanisms and preformed resistance is not possible in all cases (Heitefuss, R. (2001), Naturwissenschaften, 88, 273-283).
Furthermore, differences with respect to the host/pathogen interaction have to be considered. For instance, obligate biotrophic pathogens require living host tissue. Thus, rapid cell death in the host, as is triggered by the so-called hypersensitive reaction (HR), can be a significant component in resistance against biotrophic pathogens. In contrast, perthotrophic pathogens cause cell death in the host, which is required for further development of the pathogen on the destroyed tissue.
It has to be emphasized that plants are resistant to a vast majority of potential pathogens, i.e. a specific plant species can only be attacked successfully by a limited number of pathogens. The failure of a successful attack by a non-pathogen is the result of the “non-host” resistance mentioned in the above.
The prerequisite for a successful attack of a plant species by a pathogen is to be seen in the so-called basic compatibility, which probably has developed as a result of co-evolution of the plant host and the potential pathogens. An attack will only be successful if the pathogen has factors allowing to overcome the basic resistance of the plant species.
Correspondingly, specific plant species and cultivars of a species, respectively, will be resistant or susceptible to a specific pathogen depending on their genotype. The different resistance mechanisms responsible for resistance or susceptibility of a plant species and its cultivars, respectively, to specific pathogens will be illustrated exemplarily for the mildew pathogen (Blumeria graminis), which infects various different grass species.
The mildew fungus as a species comprises various formae speciales, depending on whether the respective mildew fungus infects, for example, wheat or barley. In case barley is infected, it will be by Blumeria graminis f. sp. hordei, while in case wheat is infected, it will be by Blumeria graminis f. sp. tritici. Moreover, different races or pathotypes, to which different cultivars of the host species exhibit different resistances, can be identified within the different formae speciales.
In the following, the different resistance mechanisms of barley against mildew pathogens will be illustrated as this host/pathogen system has been best studied. The findings obtained therefrom can, however, also be transferred to other mildew/host systems, like for example the infection of wheat by mildew pathogens mentioned in the above. Other plant species infected by mildew pathogens comprise, for example, Arabidopsis thaliana, Hordeum vulgare (barley), Triticum aestivum and T. durum (wheat), Secale cereale (rye), Avena sativa (oat), Lycopersicon spp. (tomato), Vitis spp. (wine), Cucumis spp. (cucumber), Cucurbita spp. (pumpkin), Pisum spp. (pea), Prunus spp. (peach), Solanum tuberosum (potato), Rosa spp. (rose), Fragaria ananassa (strawberry), Rhododendron spp. (azalea), Malus domestica (apple), and Nicotiana tabacum (tobacco).
Blumeria graminis f. sp. hordei exclusively attacks the epidermal cell layer of barley leaves. The fungus mechanically and enzymatically penetrates the cell wall via a penetration peg (i.e. penetration hypha), which consists of conidia, i.e. asexually formed spores. A successful infection of barley leaves is achieved if the haustorium, which is the fungal organ of nutrition, has developed.
There are two distinct genetic mechanisms to be distinguished, which render barley resistant to mildew. The first mechanism is based on the so-called “gene-by-gene” concept. In this mechanism, resistance is achieved in that a dominantly acting resistance gene renders the plants resistant to only such fungal isolates which carry the corresponding avirulence gene. In most cases, this so-called race-specific resistance, wherein a barley cultivar is resistant only to selected mildew isolates, is characterized by the hypersensitive reaction (HR), i.e. the host cells of the infection site die off (Heitefuss, R., vide supra).
In contrast to this, the second mechanism imparts a broad spectrum resistance to all known isolates of a forma specialis of the mildew fungus and is characterized by the absence of the so-called Mlo wild-type gene. Mlo is a presumably negative regulator of the pathogen defense (Devoto, A. et al. (1999), J. Biol. Chem., 274, 34993-35004). The function of this mechanism is also depending on at least two further genes, Ror1 and Ror2 (Freialdenhoven, A. et al. (1996), Plant Cell, 8, 5-14). Resistance or incompatibility, as is mediated by recessive mlo resistance alleles, is generally not characterized by the occurrence of an HR. Rather, the only observable cellular effect, which becomes visible during defense of the plant against the attacking fungus, is the formation of a subcellular cell wall apposition, which is referred to as papilla and forms directly below the fungal penetration hypha, the so-called appressorium. In this type of non-race-specific resistance, which is mediated by recessive mlo alleles, the penetration attempts of the fungus are inhibited at the stage of papilla formation, i.e. a haustorium, which is essential for establishing an efficient infection, is not even developed.
Pathogen-induced papilla formation is also observed in other Gramineae species, which indicates that non-race-specific resistance, as is known for the barley/mildew system, also occurs in other plant species. Another sign for this is the fact that Mlo proteins occur in other species, like for example in Arabidopsis thaliana or Oryza sativa. 
As in case of non-race-specific resistance a barley cultivar is resistant to various different mildew isolates or several barley cultivars are resistant to various different mildew isolates of Blumeria graminis f. sp. hordei (and as, due to the functional equivalence of the Mlo proteins in the various plant species in which they occur, this probably also applies to said plants), these plants have considerable advantages and are of particular interest as compared to those plants having only race-specific resistance. There is thus a need for further plants or plant cells having such non-race-specific resistance to fungal pathogens like, for example, mildew.