AvrACXcc8004, a Type III Effector with a Leucine-Rich Repeat Domain fromXanthomonas campestrisPathovar campestris Confers Avirulence in Vascular Tissues ofArabidopsis thalianaEcotype Col-0

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Abstract
Xanthomonas campestrispathovar campestris causes black rot, a vascular disease on cruciferous plants, includingArabidopsis thaliana. The gene XC1553 fromX. campestrispv. campestris strain 8004 encodes a protein containing leucine-rich repeats (LRRs) and appears to be restricted to strains ofX. campestrispv. campestris. LRRs are found in a number of type III-secreted effectors in plant and animal pathogens. These prompted us to investigate the role of the XC1553 gene in the interaction betweenX. campestrispv. campestris andA. thaliana. Translocation assays using the hypersensitive-reaction-inducing domain ofX. campestrispv. campestris AvrBs1 as a reporter revealed that XC1553 is a type III effector. Infiltration ofArabidopsisleaf mesophyll with bacterial suspensions showed no differences between the wild-type strain and an XC1553 gene mutant; both strains induced disease symptoms on Kashmir and Col-0 ecotypes. However, a clear difference was observed when bacteria were introduced into the vascular system by piercing the central vein of leaves. In this case, the wild-type strain 8004 caused disease on the Kashmir ecotype, but not on ecotype Col-0; the XC1553 gene mutant became virulent on the Col-0 ecotype and still induced disease on the Kashmir ecotype. Altogether, these data show that the XC1553 gene, which was renamedavrACXcc8004, functions as an avirulence gene whose product seems to be recognized in vascular tissues.