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

Resistance in tomato to Psendomonas syringae pv tomato (avrPto) is conferre d by the gene Pto in a gene-for-gene relationship. A hypersensitive disease resistance response (HR) is elicited when Pto and avrPto are expressed exp erimentally within the same plant cell. The kinase capability of Pto was re quired for AvrPto-dependent HR induction. Systematic mutagenesis of the act ivation segment of Pto kinase confirmed the homologous P + 1 loop as an Avr Pto-binding determinant. Specific amino acid substitutions in this region l ed to constitutive induction of HR upon expression in the plant cell in the absence of AvrPto. Constitutively active Pto mutants required kinase capab ility for activity, and were unable to interact with proteins previously sh own to bind to wild-type Pto, The constitutive gain-of-function phenotype w as dependent on a functional Prf gene, demonstrating activation of the cogn ate disease resistance pathway and precluding a role for Prf upstream of Pt o.