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 supp ressed by various rhizobacteria or PGPR (plant growth-promoting rhizobacter ia). When cucumber roots were treated with Pseudomonas corrugata 13 or Pseu domonas 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 (P PO) 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 th e disease developed on the roots. These accumulations peaked 4-6 days after pathogen inoculation. A split root system demonstrated that the three enzy mes could be systemically induced by the Pseudomonas strains 63-28 and 13, as well as P. aphanidermatum. Furthermore, isoperoxidase native PAGE (polya crylamide gel electrophoresis) analysis indicated that the peroxidase isome r forms in cucumber roots induced by rhizobacteria were different from that in roots infected with P. aphanidermatum. These results suggest that the p lant defense enzymes could be stimulated in cucumber roots which have been colonized by non-pathogenic rhizobacteria or in a compatible interaction be tween cucumber and P. aphanidermatum. The mechanisms of PO activation by th e rhizobacteria may be different from those of pathogen infection. (C) 2000 Academic Press.