Old fungus, new trick

Where Does the Peanut Smut Pathogen, Thecaphora frezii, Fit in the Spectrum of Smut Diseases?

Smut fungi, such as Ustilago maydis, have been studied extensively as a model for plant-pathogenic basidiomycetes. However, little attention has been paid to smut diseases of agronomic importance that are caused by species of the genus Thecaphora, probably due to their more localized distribution. Peanut smut incited by Thecaphora frezii has been reported only in South America, and Argentina is the only country where this disease has been noted in commercial peanut production. In this work, important advances in deciphering T. frezii specific biology/pathobiology in relation to potato (T. solani), wheat (U. tritici), and barley (U. nuda) smuts are presented. We summarize the state of knowledge of fungal effectors, functionally characterized to date in U. maydis and most recently in T. thlaspeos, as well as the potential to be present in other Thecaphora species involved in dicot-host interactions like T. frezii-peanut. We also discuss applicability and limitations of currently available methods for identification of smut fungi in different situations and management strategies to reduce their impact on agri-food quality. We conclude by describing some of the challenges in elucidating T. frezii strategies that allow it to infect the host and tolerate or evade plant immune defense mechanisms, and assessing other aspects related to pest control and their implications for human health.

Two NLR immune receptors acquired high-affinity binding to a fungal effector through convergent evolution of their integrated domain

A subset of plant NLR immune receptors carry unconventional integrated domains in addition to their canonical domain architecture. One example is rice Pik-1 that comprises an integrated heavy metal-associated (HMA) domain. Here, we reconstructed the evolutionary history of Pik-1 and its NLR partner, Pik-2, and tested hypotheses about adaptive evolution of the HMA domain. Phylogenetic analyses revealed that the HMA domain integrated into Pik-1 before Oryzinae speciation over 15 million years ago and has been under diversifying selection. Ancestral sequence reconstruction coupled with functional studies showed that two Pik-1 allelic variants independently evolved from a weakly binding ancestral state to high-affinity binding of the blast fungus effector AVR-PikD. We conclude that for most of its evolutionary history the Pik-1 HMA domain did not sense AVR-PikD, and that different Pik-1 receptors have recently evolved through distinct biochemical paths to produce similar phenotypic outcomes. These findings highlight the dynamic nature of the evolutionary mechanisms underpinning NLR adaptation to plant pathogens.

Two NLR immune receptors acquired high-affinity binding to a fungal effector through convergent evolution of their integrated domain

A subset of plant NLR immune receptors carry unconventional integrated domains in addition to their canonical domain architecture. One example is rice Pik-1 that comprises an integrated heavy metal-associated (HMA) domain. Here, we reconstructed the evolutionary history of Pik-1 and its NLR partner, Pik-2, and tested hypotheses about adaptive evolution of the HMA domain. Phylogenetic analyses revealed that the HMA domain integrated into Pik-1 before Oryzinae speciation over 15 million years ago and has been under diversifying selection. Ancestral sequence reconstruction coupled with functional studies showed that two Pik-1 allelic variants independently evolved from a weakly binding ancestral state to high-affinity binding of the blast fungus effector AVR-PikD. We conclude that for most of its evolutionary history the Pik-1 HMA domain did not sense AVR-PikD, and that different Pik-1 receptors have recently evolved through distinct biochemical paths to produce similar phenotypic outcomes. These findings highlight the dynamic nature of the evolutionary mechanisms underpinning NLR adaptation to plant pathogens.