The Rx1_'Gpa2 locus in potato: a molecular and genetic framework for engineering novel NLR genes

Plants are constantly exposed to a diverse array of pathogens and parasites that attempt to invade leafs, stems, or roots by various mechanisms. To sense foreign invaders, plants have evolved a cell autonomous immune system consisting of specific receptor-like proteins, including nucleotide binding domain and leucine-rich repeat containing proteins (NLRs), which confer host specific resistance upon recognition of pathogen elicitors. The close homologs Gpa2 and Rx1 confer resistance in potato to taxonomically unrelated pathogens: the cyst nematode Globodera pallida and Potato virus X (PVX), respectively. This provides us with a model system to study evolutionary and molecular aspects involved in pathogen recognition and NLR activation in plants. Our results demonstrate that complex NLR loci provide a genetic framework in which intergenic sequence exchange between homologous genes is allowed, but also point to functional constraints that act on the generation of effective novel NLR proteins. Sequence exchange results often in gain or loss of function phenotypes due to incompatibility between functional domains involved in regulating the molecular switch function of these proteins. However, functionality can be restored by modulating the sensitivity of the protein. This knowledge contributes to a better understanding of NLR evolution, but provides us also with a functional framework for engineering novel NLR genes based on gene shuffling and targeted mutagenesis.