Avirulence Gene D from Pseudomonas Syringae pv. Tomato and Its Interaction with Resistance Gene Rpg4 in Soybean

Abstract

Certain pathogen avirulence genes have been associated with production by the pathogen of specific elicitors, chemicals that initiate hypersensitive defense responses (HR) only in those plant cultivars which carry complementary disease resistance genes. We have cloned and characterized one of these avirulence genes, avrD, from Pseudomonas syringae pv. tomato. Various Gram-negative bacteria containing this cloned avirulence gene produce a low molecular weight compound(s) that causes the HR exclusively in soybean cultivars carrying the disease resistance gene, Rpg4. We isolated two major avrD elicitor-active molecules and have partially characterized them as homologous C13 and C15 hydrocarbons that are heavily substituted with oxygen atoms and cyclized at one end. The physiologic role of the avrD elicitor molecules in P.s. pv. tomato remains unclear since mutants deficient in avrD retained virulence in tomato plants. However, the fact that expression of avrD was greatly stimulated when P.s. pv. tomato was grown on plant leaves or in cell suspension cultures suggests that the gene may be important for the survival of the bacteria on or in plant leaves. The purified elicitor-active molecules specifically induced PAL and CHS mRNAs in the soybean cultivar Norchief, which carries the Rpg4 resistance gene, but not in Acme which lacks this gene. Thus, gene-for-gene specificity mediated by the elicitor was reflected at the level of defense gene expression. The avrD specific elicitor preparations also elicited necrosis and phytoalexin production in soybean callus and cotyledons. The evidence therefore indicates that the avrD elicitor molecules are the signal which elicits the HR in the avrD-Rpg4 gene-for-gene interaction.