Abstract
The innate immune system of plants consists of two layers. The first layer, called basal resistance, governs recognition of conserved microbial molecules and fends off most attempted invasions. The second layer is based on Resistance (R) genes that mediate recognition of effectors, proteins secreted by pathogens to suppress or evade basal resistance. Here, we show that a plant-pathogenic fungus secretes an effector that can both trigger and suppress R gene-based immunity. This effector, Avr1, is secreted by the xylem-invading fungus Fusarium oxysporum f.sp. lycopersici (Fol) and triggers disease resistance when the host plant, tomato, carries a matching R gene (I or I-1). At the same time, Avr1 suppresses the protective effect of two other R genes, I-2 and I-3. Based on these observations, we tentatively reconstruct the evolutionary arms race that has taken place between tomato R genes and effectors of Fol. This molecular analysis has revealed a hitherto unpredicted strategy for durable disease control based on resistance gene combinations.
Highlights
Long periods of co-evolution of plants and microorganisms have led to complex mechanisms of attack and defence, involving the innate immune system of plants and virulence factors of pathogens [1]
Races of Fol are named historically according to the R gene that is effective against them: the I gene and the I-1 gene are effective against race 1, race 2 overcomes I and I-1, but is stopped by I-2, while race 3 overcomes I, I-1 and I-2 but is blocked by I-3 [9]
The most environmentally friendly way to combat plant diseases is to make use of the innate immune system of plants, for instance by crossing into crop varieties polymorphic resistance genes that occur in natural populations of the crop plant or its close relatives
Summary
Long periods of co-evolution of plants and microorganisms have led to complex mechanisms of attack and defence, involving the innate immune system of plants and virulence factors of pathogens [1]. Many plant-pathogenic fungi secrete proteins that are recognized by R-genes [4,5] One of these fungi is Fusarium oxysporum, a common soil inhabitant. We describe here the identification and analysis of a second avirulence factor of Fol, Avr1 This protein does act as an avirulence factor in conjunction with the I gene, and suppresses disease resistance mediated by I-2 and I-3. Identification of Avr In an initial analysis of the xylem sap proteome of tomato plants infected with Fol race 1 using 2-D gel electrophoresis and mass spectrometry, three small secreted proteins of Fol were identified in addition to Avr (Six1), named Six, Six and Six, and their genes cloned [18].
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