Effects of Pseudomonas syringae pv. morsprunorum Strain C28, Outer Membrane and LPS Preparations on Cherry Callus

Abstract

Callus cultures were established from cherry (Prunus avium) cvs. Napoleon and Colt, respectively susceptible and resistant to race-1 strains of Pseudomonas syringae pv. morsprunorum, by growth on Schenk–Hildebrandt medium. On Napoleon callus, necrosis began earlier and proceeded more rapidly when inoculated with the virulent race-1 cherry isolate strain C28, than with mutants of diminished virulence derived from it, or with the virulent plum isolates D10 and D17. Colt tissue displayed poorer viability and showed susceptibility to strain C28 and the plum isolates. Callus from both sources was somewhat susceptible to the saprophytes P. aeruginosa NCIMB 8295 and P. fluorescens NCIMB 3756. Strain C28 grew on suspended Napoleon callus cells over a period of 3–4 days, causing leakage of UV-absorbing compounds and K+, with a concomitant rise in extracellular pH. P. fluorescens NCIMB 3756 showed no growth on suspended callus for 6 days. EDTA-extracted outer membrane (OM) from strain C28 caused leakage of UV absorbing material and K+, which was later reabsorbed, with little change in pH. The presence of OM suppressed the growth of a subsequent inoculum of strain C28, possibly due to complexation of the available Ca2+ and/or Mg2+ in the surrounding medium, by the component lipopolysaccharide (LPS). OM from the rough avirulent mutant strain C28-2 induced leakage of K+, but not of UV-absorbing material, and did not prevent the growth of subsequently inoculated strain C28. Smooth LPS from strain C28 did not cause leakage of K+ or of UV-absorbing material and did not prevent growth of C28. The relevance of these findings is discussed in relation to disease.