In BRI1 the residues interacting with coreceptors are located at the island domain, the last LRR, and the juxtamembrane domain, all close to the transmembrane domain. However, in FLS2 the BAK1-interacting residues in the crystal structure are located 108–300 amino acids from the predicted transmembrane domain, at repeats #18 to 26 of the LRR domain. This predicts a relatively recumbent orientation for the FLS2 ectodomain, bent down toward the plasma membrane. FLS2 mediates perception of bacterial flagellin protein, an abundant MAMP, and FLS2 recognizes in particular a,20 amino acid region that is relatively conserved across flagellins from diverse Gram-negative bacteria. Many aspects of FLS2 structure and function have been characterized. There is a third surprising feature of the FLS2-flg22-BAK1 ECD co-crystal structure. Most research regarding FLS2 utilizes as ligand, in place of flagellin protein, a 22 amino acid “flg22” peptide whose sequence matches the recognized domain of Pseudomonas aeruginosa flagellin, or utilizes other small peptides based on similar sequences from various bacteria. The FLS2-flg22-BAK1 ECD co-crystal structure predicts a tight pocket for the flg22 peptide, which may not be compatible with analogous binding of flg22 domains embedded within full-length flagellin proteins. In this study we first explored the possibility that a relatively universal SERK interaction site has evolved in the LRR domains of different SERK-interacting LRR-RLKs. The overall goal of this study was to furnish a more clear understanding of the requirements for formation of a signaling-competent plant basal immune system MAMP receptor – an understanding that may be essential to allow future engineering of PRRs with broadened or otherwise improved performance. Surprisingly, in vivo FLS2mediated signaling persisted and was only minimally reduced in plants expressing most single-mutant forms of FLS2, including mutants that exhibited no detectable flg22-induced co-immunoprecipitation with BAK1. As a general trend across the multiple independent transgenic lines tested for each FLS2 construct, mutations to alanine allowed stronger FLS2 signaling than mutations to tryptophan. The results suggest that reduced-affinity or more transient interactions of FLS2 and BAK1 occur with many of the FLS2 mutants described in Figures 5 and 6, and that those interactions are sufficient for flg22-stimulated FLS2 signaling even if the stability of FLS2-BAK1 interactions is reduced below levels detectable in standard coimmunoprecipitation experiments. Although some FLS2 signaling capacity was still conferred by FLS2 R428 abmole constructs mutated at single predicted FLS2 BAK1-interaction sites, with the double mutation Q530A+Q627A FLS2-mediated signaling was significantly impaired, supporting current models of FLS2 structure and function. Alternative hypotheses, other than reduced-affinity or more transient interactions of FLS2 and BAK1.