Induction of defence responses against Erwinia soft rot by an endogenous pectate lyase in potatoes

Pectate lyase (PL) enzymes are major virulence factors of Erwinia carotovora (Ec) bacteria. They degrade plant cell wall pectin into unsaturated oligogalacturonates (OG) known to elicit plant defence responses. Therefore, a gene encoding the isoenzyme PL3 of Ec ssp. atroseptica was transformed by means of Agrobacterium into potatoes of cv. Désirée. Four PL-transgenic potato plant lines selected on the basis of greenhouse experiments were grown over a period of 4 years (1997–2000) in the field. It is shown that the heterologous PL enzyme mediated an enhanced resistance to Erwinia soft rot in field grown tubers. Thus compared to the non-transgenic counterpart extension of rotting was significantly reduced (P < 0.001) on the wound surface of PL-expressing tubers. Moreover, the threshold density ofEc -bacteria causing a progressive soft rot was up to 19-fold higher on tuber tissue containing the PL enzyme. An induction of plant defence responses in PL-transgenic potatoes may be indicated by an enhanced resistance of tuber tissue cell walls to Erwinia -derived enzymes, an increased PPO- and PAL-activity in tuber tissue as well as by a strengthened formation of necrosis on the wound surface of tubers after infection with Ec -bacteria.