Defense enzymes induced in cucumber roots by treatment with plant growth-promoting rhizobacteria (PGPR) and Pythium aphanidermatum

Root and crown rot of cucumber caused by Pythium aphanidermatum can be suppressed by various rhizobacteria or PGPR (plant growth-promoting rhizobacteria). When cucumber roots were treated with Pseudomonas corrugata 13 or Pseudomonas aureofaciens 63-28, phenylalanine ammonia-lyase (PAL) activity was stimulated in root tissues in 2 days and this activated accumulation lasted for 16 days after bacterization. Peroxidase (PO) and polyphenol oxidase (PPO) activities were increased in roots 2-5 days after bacterization with P. corrugata strain 13. After bacterized cucumber roots were challenged with P. aphanidermatum, the enzyme activities of PAL, PO and PPO increased as the disease developed on the roots. These accumulations peaked 4-6 days after pathogen inoculation. A split root system demonstrated that the three enzymes could be systemically induced by the Pseudomonas strains 63-28 and 13, as well as P. aphanidermatum. Furthermore, isoperoxidase native PAGE (polyacrylamide gel electrophoresis) analysis indicated that the peroxidase isomer forms in cucumber roots induced by rhizobacteria were different from that in roots infected with P. aphanidermatum. These results suggest that the plant defense enzymes could be stimulated in cucumber roots which have been colonized by non-pathogenic rhizobacteria or in a compatible interaction between cucumber and P. aphanidermatum. The mechanisms of PO activation by the rhizobacteria may be different from those of pathogen infection. (C) 2000 Academic Press.