For example, small sub-populations of FLS2 receptors may exist in the cell that, because of different localization or post-translational modifications, continue to exhibit robust flg22-dependent interaction with BAK1 despite presence of mutations that disrupt interaction between most of the cellular FLS2 and BAK1. As another possibility, the single mutations that transition FLS2 away from high-affinity flg22-dependent binding with BAK1 may have allowed or even enhanced interaction with other SERK proteins, to an extent that allows defense signaling. LRRs are a protein structure evolved to display widely varying surface amino acid combinations on a relatively invariant scaffold. A previous study of over 1200 FLS2 LRR mutations of predicted LRR solvent-exposed residues at and adjacent to flg22 binding sites, carrying changes to all possible amino acids, found that the vast majority of LRR surface mutations do not disrupt FLS2 function. Thus the structural alterations caused by the FLS2 mutations of the present study are PR-171 likely to be highly local. Their disruption of FLS2-BAK1 interactions detected via co-immunoprecipitation supports the relevance of the FLS2-flg22-BAK1 configuration in the published co-crystal structure. Mutation of FLS2 residues D557 and S559, which reside close to but outside of the BAK1-interaction residues in the solved crystal structure, did not disrupt flg22-stimulated FLS2-BAK1 co-immunoprecipitation. Hence the functional disruption of signaling caused by the presumably additive effect of two alanine substitutions in FLS2 Q530A+Q627A provides further in vivo functional evidence indicating the requirement for this site both for FLS2-BAK1 interaction and for flg22 induction of FLS2-dependent immune signaling. Our results also indicate that, if SERK proteins other than BAK1 make residual contributions to FLS2 activation, the FLS2 Q530A+Q627A mutations are sufficient to disrupt functional signaling mediated by those interactions as well. However, numerous aspects of the published FLS2-flg22-BAK1 co-crystal structure are substantiated by experimental evidence. Hence we consider it equally likely that the published FLS2-flg22BAK1 co-crystal is essentially correct in representing in vivo configurations, and predict that flagellin proteins within plants must be fragmented rather than intact in order to form the FLS2flagellin-BAK1 complexes that elicit plant innate immune system activation. The success of scleractinian corals in oligotrophic tropical waters is based on the endosymbiosis between the coral host and single-celled microalgae, i.e., dinoflagellates in the genus Symbiodinium that reside within the host’s endodermal cells. The algal symbionts translocate up to 95% of their photosynthetically fixed carbon to the coral host under optimal conditions, whilst the algal symbionts receive nutrients and shelter from the host. There is considerable genotypic variation within the Symbiodinium genus.