An overview of genetic rust resistance: From broad to specific mechanisms

Global agriculture is under threat due to the rapid evolution and spread of pathogenic fungi that cause rust diseases.For instance, the recently evolved races of wheat stem rust (Puccinia graminis f. sp.tritici) and stripe rust (P.striiformis f. sp.tritici) fungus in parts of Africa, Asia, and Europe are a menace to food security due to their ability to spread rapidly and overcome resistance in common wheat varieties [1].Similarly, new variants of Asian soybean rust (Phakopsora pachyrhizi) detected in Brazil and the United States pose a major constraint to soybean cultivation [2].Since genetic resistance can provide effective and chemical-free disease control, many efforts are directed towards isolating rust-resistance genes in crop plants and understanding how to best deploy them for durable resistance [3].In addition, related nonhost species are increasingly being utilised to identify new sources of resistance [4, 5].Here, we summarise current knowledge of rust resistance, focussing on race-specific, non-race-specific, and nonhost resistance mechanisms. Race-specific resistanceRace-specific resistance genes are effective against some but not all races of a rust pathogen and generally conform to the classical gene-for-gene model, where resistance depends on a specific genetic interaction between host-resistance (R) genes and pathogen avirulence (Avr) genes.R genes in plants predominantly encode nucleotide-binding and leucine-rich repeat (NLR) proteins, which act as immune receptors to recognise pathogen effector proteins delivered into host cells during infection [6].Much of our understanding of this mode of rust resistance comes from the flax rust pathosystem, in which 19 NLR-encoding R genes and 6 corresponding Avr gene families have been identified [7,8].The flax rust (Melampsora lini) Avr genes encode small secreted proteins that are expressed in haustoria and recognised inside the host cell (Fig 1C).For at least 2 cases, the recognition event leading to resistance occurs by direct interaction between the flax NLR protein and the M. lini Avr protein [8].For the L6 resistance protein, binding to its corresponding effector, AvrL567, favours the active signalling state of the protein in which the nucleotide-binding domain is bound to adenosine triphosphate (ATP) rather than adenosine diphosphate (ADP) [9].In this state, the N-terminal Tollinterleukin receptor (TIR) signalling domain is thought to be available for oligomerisation events necessary for signalling [10].Many race-specific rust-resistance genes have been defined genetically in wheat and other crops and are now being cloned in increasing numbers.