Comparative ultrastructure of cucumbers pretreated with plant growth-promoting rhizobacteria, DL-3-aminobutyric acid or amino salicylic acid after inoculation with Colletotrichum orbiculare

The infection behaviour of Colletotrichum orbiculare was observed using a transmission electron microscope (TEM) in the leaves of cucumber plants preinoculated with plant growth-promoting rhizobacteria (PGPR) Serratia marcescens (90-166) or Pseudomonas fluorescens (89B61) after inoculation with the anthracnose pathogen. Moreover, the structural defence mechanisms mediated by pretreating dl-3-amino butyric acid or amino salicylic acid was compared with those mediated by these two PGPR. In untreated plants broadly spread intercellular/intracellular hyphae were found in leaf tissues. The cytoplasm of fungal hyphae appeared intact and the electron densities of intercellular and intracellular hyphae were not high even at 5 days after fungal inoculation, indicating lack of defence responses. However, in the leaves of plants preinoculated with both PGPR active defence responses were observed, i.e. sheath formation at penetration sites, and accumulations of endoplasmic reticula or of numerous vesicles around intracellular hyphae. Furthermore, the electron densities of most intracellular and intercellular hyphae were higher than those of untreated control plants. In the leaves of plants pretreated with both chemicals, the hyphae of the pathogen were rarely observed at 5 days after challenge inoculation. Similar to the case of PGPR-pretreated plants, the most intercellular and intracellular hyphae in plant tissues were electron dense. However, no active defence responses were observed in leaf tissues. Moreover, in some cases, hyphal growth was restricted to the epidermal cell layer, and in particular, was not observed to develop in mesophyll cells of the plants. These results suggest that the mechanisms of systemically induced resistance are differently expressed by preinoculating with PGPR or by pretreating with chemicals.