The researchers, on the other hand, observed that when PEN2 is missing, the plants become more susceptible not only to grass powdery mildew fungi but also other pests ?for example, the pathogens causing late potato blight. PEN2 is therefore a basic component of the plant's immune system with a broad range of effects.
However if PEN2 is missing, the plant is not completely helpless against fungal diseases. There is still another line of defence which they have to get through. If PEN2 is missing, the plant takes a drastic step: the cell dies together with its attacker, which protects the neighbouring plant tissue from infection.
In this deadly line of defence, very different proteins play a key role ?particularly EDS1, PAD4 and SAG101. They were already known to researchers in other species of plants, which identify molecular traits only present in parasites by using immune receptors both on the cell surface and inside the cell. Only if this second mechanism also fails can the originally non-virulent grass powdery mildew fungus colonise the plant.
The Max Planck research has now demonstrated that the nonhost resistance of plants develops out of a defence system with at least two steps. These steps determine whether a plant is susceptible to a disease or not. The redundancy of the defence layers and the wide-ranging effects of PEN2 explain why, in nature, nonhost resistance is a durable and broadly effective defence mechanism. If a building block is missing from one defence layer, its function will be taken over by components of the next layer.