Plants and animals recognize microbial invaders by detecting pathogen-associated molecular patterns (PAMPs) such as flagellin. However, the importance of flagellin perception for disease resistance has, until now, not been demonstrated. Here we show that treatment of plants with flg22, a peptide representing the elicitor-active epitope of flagellin, induces the expression of numerous defence-related genes and triggers resistance to pathogenic bacteria in wild-type plants, but not in plants carrying mutations in the flagellin receptor gene FLS2. This induced resistance seems to be independent of salicylic acid, jasmonic acid and ethylene signalling. Wild-type and fls2 mutants both display enhanced resistance when treated with crude bacterial extracts, even devoid of elicitor-active flagellin, indicating the existence of functional perception systems for PAMPs other than flagellin. Although fls2 mutant plants are as susceptible as the wild type when bacteria are infiltrated into leaves, they are more susceptible to the pathogen Pseudomonas syringae pv. tomato DC3000 when it is sprayed on the leaf surface. Thus, flagellin perception restricts bacterial invasion, probably at an early step, and contributes to the plant's disease resistance.

in the long run. However, migration rate in our experiment was density independent and migration was confined to the two nearest neighbors, whereas it is known that the dynamics of a metapopulation can vary depending on the exact form of density dependence (31) and scheme of migration (11). Moreover, growth rates of Drosophila (and most insects, microbes, and fishes) are higher than those of mammals and birds, which are generally of greater concern for conservation. The intrinsic growth rates of subpopulations are also known to interact strongly with migration rate in producing observed metapopulation dynamics (12). Therefore, due caution should be exercised when extrapolating our results to natural populations.