Suppression and triggering of Arabidopsis immunity by Albugo species

• Main Author: Schultz-Larsen, T.
• Published: University of East Anglia 2012
• Subjects: 570
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.572103

Albugo species are obligate biotrophic phytopathogens. Like other biotrophs, they are anticipated to secrete effectors that can suppress or trigger plant defenses; the nature of Albugo effectors is currently unknown. Sequencing of A. laibachii isolate Nc14 (AlNc14) genome reveals 13032 genes encoded in a ~37 Mb genome. We analyze the effector complement of AlNc14 and find known effector classes but also classes unique to A. laibachii. Experiments reveal that CHXCs are a novel class of effectors that suppress host defense. We functionally characterize two predicted AlNc14 effectors in detail; CHXC1 a potential core effector conserved in other oomycete species, and SSP6, a fast-evolving effector specific to A. laibachii. CHXC1 encodes a nuclear localized HECT E3 ligase homolog, which suppresses host defenses dependent on cys651. We find 7 variants of SSP6 that are under diversifying selection. Two highly expressed variants SSP6-2c and SSP6-A are plasma membrane localized when expressed in planta. Interestingly, SSP6-2c but not SSP6-A, is able to enhance growth of P. infestans race blue 13 and suppress flg22-dependent ROS production. In Arabidopsis cells we find SSP6-2c localizes around AlNc14 haustoria. We propose that AlNc14 secretes the effectors SSP6-2c and CHXC1 into the plant cell to suppress defense and promote infection. Current methods to screen for virulence of effector candidates predominantly rely on measuring growth of bacterial pathogens. Quantitative assessment of resistance and susceptibility to eukaryotic pathogens is more difficult. We develop a semi-automated high-throughput system for assaying Hpa growth. We investigate the genetic basis of resistance to Albugo in Arabidopsis. We find that resistance to AlNc14 is linked to RAC1 and RAC3 in Ksk-1. In contrast, resistance to A. candida Nc2 (AcNc2) is linked to WRR4 in Col-0, Col-5 and Ksk-1. A second dominant locus, WRR5a/b in Col-5 also confers resistance to AlNc2. Thus, different R-genes and presumably different effectors govern resistance to AlNc14 and AcNc2.!