Constitutively active Pto induces a Prf-dependent hypersensitive response in the absence of avrPto

Resistance in tomato to Pseudomonas syringae pv tomato (avrPto) is conferred by the gene Pto in a gene-for-gene relationship. A hypersensitive disease resistance response (HR) is elicited when Pto and avrPto are expressed experimentally within the same plant cell. The kinase capability of Pto was required for AvrPto-dependent HR induction. Systematic mutagenesis of the activation segment of Pto kinase confirmed the homologous P+1 loop as an AvrPto-binding determinant. Specific amino acid substitutions in this region led to constitutive induction of HR upon expression in the plant cell in the absence of AvrPto. Constitutively active Pto mutants required kinase capability for activity, and were unable to interact with proteins previously shown to bind to wild-type Pto. The constitutive gain-of-function phenotype was dependent on a functional Prf gene, demonstrating activation of the cognate disease resistance pathway and precluding a role for Prf upstream of Pto.