Secreted in xylem 6 (SIX6) mediates Fusarium oxysporum f.sp. fragariae race 1 avirulence on FW1-resistant strawberry cultivars

Authors: DILLA-ERMITA, CHRISTINE - University Of California; Goldman, Polly; Anchieta, Amy; FELDMANN, MITCHELL - University Of California; PINCOT, DOMINIQUE - University Of California; FAMULA, RANDY - University Of California; VACHEV, MISHI - University Of California; COLE, GLENN - University Of California; KNAPP, STEVE - University Of California; Klosterman, Steven; Henry, Peter

Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/27/2024
Publication Date: 6/24/2024
Citation: Dilla-Ermita, C.J., Goldman, P.H., Anchieta, A.G., Feldmann, M.J., Pincot, D.D., Famula, R.A., Vachev, M., Cole, G.S., Knapp, S.J., Klosterman, S.J., Henry, P.M. 2024. Secreted in xylem 6 (SIX6) mediates Fusarium oxysporum f. sp. fragariae race 1 avirulence on FW1-resistant strawberry cultivars. Molecular Plant-Microbe Interactions. 37(6):530-541. https://doi.org/10.1094/MPMI-02-24-0012-R.
DOI: https://doi.org/10.1094/MPMI-02-24-0012-R

Interpretive Summary: Growing strawberry cultivars with resistance to Fusarium wilt is the most economically and environmentally sound strategy to control this disease. Many strawberry cultivars have a single genetic locus called “FW1” that confers resistance to Fusarium oxysporum f. sp. fragariae (Fof) race 1, the cause of Fusarium wilt. These cultivars are susceptible to Fof race 2. Based on examples from similar pathosystems, we hypothesized that a gene at the strawberry locus FW1 interacted with a gene present in all Fof race 1 isolates to trigger resistance. This Fof race 1 specific gene would confer avirulence on FW1-resistant cultivars and was called AvrFW1. We compared the genomes of Fof race 1 and 2 isolates and discovered a single strong candidate for AvrFW1. We mutated Fof race 1 isolates to disrupt this gene and found transformants lacking this gene gained the ability to cause disease on cultivars with the FW1 locus, confirming the function of this gene as AvrFW1. However, most strains in which AvrFW1 was knocked out were less virulent on the susceptible cultivar than the original strain or other strains that went through the transformation procedure but retained AvrFW1. This additional result indicates AvrFW1 can promote virulence during infection of susceptible strawberry cultivars. This work informs our understanding of the durability of resistance conferred by FW1 and enables future research investigating the molecular interactions Fof and strawberry during infection.

Technical Abstract: Fusarium oxysporum f.sp. fragariae (Fof) race 1 is avirulent on cultivars with the dominant resistance locus, FW1, while Fof race 2 is virulent on FW1-resistant cultivars. We hypothesized that FW1 had a gene-for-gene interaction with an avirulence gene (AvrFW1) in Fof race 1. To identify a candidate AvrFW1, we compared genomes of twenty-four Fof race 1 and three Fof race 2 isolates. We found one candidate gene that was present in race 1, absent in race 2, highly expressed in planta, and homologous to a known effector, secreted in xylem 6 (SIX6). We knocked out SIX6 in two Fof race 1 isolates by homologous recombination. All SIX6 knockout transformants ('SIX6) gained virulence on FW1/fw1 cultivars, whereas ectopic transformants and the wildtype isolates remained avirulent. Most 'SIX6 isolates displayed reduced virulence on the susceptible cultivar, Monterey, suggesting this gene can also act as a virulence factor. 'SIX6 isolates were quantitatively less virulent on FW1/fw1 cultivars Fronteras and San Andreas than fw1/fw1 cultivars. Seedlings from an FW1/fw1 × fw1/fw1 population were genotyped for FW1 and tested for susceptibility to a SIX6 knockout isolate. Results suggested that additional minor-effect quantitative resistance genes could be present at FW1. This work demonstrates that SIX6 can act as an avirulence factor interacting with a resistance gene at FW1 and as a virulence factor during infection of susceptible cultivars. The identification of AvrFW1 enables surveillance for Fof race 2 and provides insight into the durability of FW1-mediated resistance.