Adaptive evolution of rice immune receptors

Bialas, Aleksandra (2020) Adaptive evolution of rice immune receptors. Doctoral thesis, University of East Anglia.

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Plants have evolved intracellular immune receptors, known as nucleotide-binding domain leucine-rich repeat-containing (NLR) proteins, that trigger effective immune responses upon perception of pathogen-derived effectors. Although typical NLRs share a conserved multidomain architecture, some of them carry unconventional so-called integrated domains that appear to have evolved from host targets of pathogen effectors. One example of such NLR is Pik-1—a rice immune receptor that confers disease resistance to the blast fungus Magnaporthe oryzae. Pik-1 carries a heavy metal–associated (HMA) domain that directly binds the AVR-Pik effector from M. oryzae by mimicking the host targets of this effector. In this thesis, I aimed to understand the evolutionary history of the Pik-1 receptor and its partner, Pik-2, and test hypotheses about adaptive evolution of the Pik-1–integrated HMA domain. Phylogenetic analyses of Pik orthologues revealed that the HMA domain integrated into Pik-1 before Oryzinae speciation and has been subject to the strong diversifying selection. Ancestral HMA sequence reconstruction coupled with functional studies showed that different allelic variants of Pik-1, Pikp-1 and Pikm-1, convergently evolved to recognise AVR-PikD. Using biochemical and biophysical approaches, I functionally characterised two regions in the Pik-1 HMA that independently evolved towards high-affinity AVR-PikD binding from the weaker ancestral state. In both cases the HMA domain only recently acquired the capacity to bind the AVR-PikD effector with high affinity, indicating that for most of its evolutionary history the HMA had not been subject to selection pressure imposed by this blast effector. In addition, although Pikp-1 and Pikm-1 receptors evolved to produce similar phenotypic outcomes, they underwent different evolutionary trajectories to do so. These findings paint a complex picture of the mechanisms and evolutionary dynamics of NLR adaptation and provide a robust evolutionary framework that can contribute a more comprehensive understanding of plant–microbe systems.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Biological Sciences
Depositing User: Jackie Webb
Date Deposited: 21 Oct 2021 13:52
Last Modified: 12 Nov 2021 01:41

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