The tomato I gene for Fusarium wilt resistance encodes an atypical leucine‐rich repeat receptor‐like protein whose function is nevertheless dependent on SOBIR 1 and SERK 3/ BAK 1

Summary We have identified the tomato I gene for resistance to the Fusarium wilt fungus Fusarium oxysporum f. sp. lycopersici ( Fol ) and show that it encodes a membrane‐anchored leucine‐rich repeat receptor‐like protein ( LRR ‐ RLP ). Unlike most other LRR ‐ RLP genes involved in plant defence, the I gene is not a member of a gene cluster and contains introns in its coding sequence. The I gene encodes a loopout domain larger than those in most other LRR ‐ RLP s, with a distinct composition rich in serine and threonine residues. The I protein also lacks a basic cytosolic domain. Instead, this domain is rich in aromatic residues that could form a second transmembrane domain. The I protein recognises the Fol Avr1 effector protein, but, unlike many other LRR ‐ RLP s, recognition specificity is determined in the C‐terminal half of the protein by polymorphic amino acid residues in the LRR s just preceding the loopout domain and in the loopout domain itself. Despite these differences, we show that I/Avr1‐dependent necrosis in Nicotiana benthamiana depends on the LRR receptor‐like kinases ( RLK s) SERK 3/ BAK 1 and SOBIR 1. Sequence comparisons revealed that the I protein and other LRR ‐ RLP s involved in plant defence all carry residues in their last LRR and C‐terminal LRR capping domain that are conserved with SERK 3/ BAK 1‐interacting residues in the same relative positions in the LRR ‐ RLK s BRI 1 and PSKR 1. Tyrosine mutations of two of these conserved residues, Q922 and T925, abolished I/Avr1‐dependent necrosis in N. benthamiana , consistent with similar mutations in BRI 1 and PSKR 1 preventing their interaction with SERK 3/ BAK 1.