Molecular mechanisms of early plant pattern-triggered immune signaling

Many plant-associated microbes are pathogens that impair plant growth and reproduction. Plants respond to infection using a two-branched innate immune system. The first branch recognizes and responds to molecules common to many classes of microbes, including non-pathogens. The second responds to pathogen virulence factors, either directly or through their effects on host targets. These plant immune systems, and the pathogen molecules to which they respond, provide extraordinary insights into molecular recognition, cell biology and evolution across biological kingdoms. A detailed understanding of plant immune function will underpin crop improvement for food, fibre and biofuels production.

Higher eukaryotes sense microbes through the perception of pathogen-associated molecular patterns (PAMPs). Arabidopsis plants detect a variety of PAMPs including conserved domains of bacterial flagellin and of bacterial EF-Tu. Here, we show that flagellin and EF-Tu activate a common set of signaling events and defense responses but without clear synergistic effects. Treatment with either PAMP results in increased binding sites for both PAMPs. We used this finding in a targeted reverse-genetic approach to identify a receptor kinase essential for EF-Tu perception, which we called EFR. Nicotiana benthamiana, a plant unable to perceive EF-Tu, acquires EF-Tu binding sites and responsiveness upon transient expression of EFR. Arabidopsis efr mutants show enhanced susceptibility to the bacterium Agrobacterium tumefaciens, as revealed by a higher efficiency of T-DNA transformation. These results demonstrate that EFR is the EF-Tu receptor and that plant defense responses induced by PAMPs such as EF-Tu reduce transformation by Agrobacterium.