The Magnaporthe oryzae MAX effector AVR-Pia binds a novel group of rice HMA domain-containing proteins

Phytopathogenic fungi secrete effector proteins to promote virulence. MAX ( <underline>M</underline>agnaporthe <underline>A</underline>vrs and <underline>T</underline>oxB-like) effectors form a sequence-diverse family sharing a conserved protein structure. AVR-Pia, a MAX effector from the rice blast fungus Magnaporthe oryzae, is recognised by the paired rice nucleotide-binding leucine-rich repeat immune receptors OsRGA4/OsRGA5 through direct binding to an integrated heavy metal-associated (HMA) domain in OsRGA5. Here, we identify previously unknown host targets of AVR-Pia: four HMA domain-containing rice proteins, belonging to the <underline>H</underline>MA Isoprenylated <underline>P</underline>lant <underline>P</underline>rotein (HIPP) and <underline>H</underline>MA <underline>P</underline>lant <underline>P</underline>rotein (HPP) families. AVR-Pia interacts with all four proteins both in vitro and in planta and binds their HMA domains with varying affinities. The crystal structure of AVR-Pia bound to the HMA domain of OsHPP09 revealed the molecular details of the binding interface. Structure-guided mutagenesis of OsHPP09 identified a single point mutation which prevents AVR-Pia binding, providing a foundation for targeted engineering of HMA domains to evade effector binding.