Secreted in Xylem (SIX) Gene SIX9 Is Highly Conserved in Fusarium oxysporum f. sp. vasinfectum Race 4 Isolates from Cotton in the United States

Many Fusarium oxysporum formae speciales produce small, infection-dependent effector proteins called "secreted in xylem" (SIX) proteins. These proteins are secreted into the xylem of a plant during the infection process and are thought to promote virulence. In this study, a collection of Fusarium oxysporum isolates composed primarily of different races of Fusarium oxysporum f. sp. vasinfectum (FOV) was screened for the presence of 14 SIX effector genes (SIX1 to SIX14). Our results showed that some of the most virulent FOV races, FOV4 and FOV7, share a common SIX effector: SIX9. This effector is largely absent in other FOV races in North America, making SIX9 a potential target for rapid detection of these highly virulent FOV strains and enabling race-specific FOV quantification in infected host plants.