Cytological observations of cucumber plants during induced resistance elicited by rhizobacteria

Expression of induced resistance was cytologically compared between cucumber plants induced with either plant growth-promoting rhizobacteria (PGPR) or chemicals. Inoculation with PGPR strains Serratia marcescens (90–166) and Pseudomonas fluorescens (89B61) induced systemic protection in the aerial part of cucumber plants against the anthracnose pathogen Colletotrichum orbiculare. Disease development was significantly reduced in these plants compared to control plants that were not inoculated with the PGPR strains. Inoculation with the PGPR strains caused no visible toxicity, necrosis, or other morphological changes. Induction with image-3-aminobutyric acid (BABA) or amino salicylic acid (ASA) also significantly reduced disease development. Soil drench with 10 mM BABA and 1.0 mM ASA-induced resistance in cucumber leaves without any toxicity to the plants. Higher concentrations of ASA (up to 10 mM) were phytotoxic, resulting in plant stunting and blighted appearance of leaves. Cytological studies using fluorescent microscopy revealed a higher frequency of autofluorescent epidermal cells, which are related to accumulation of phenolic compounds, at the sites of fungal penetration in plants induced with PGPR and challenged by the pathogen. Neither spore-germination rate nor formation of appressoria was affected by PGPR treatments. In contrast, both BABA and ASA significantly reduced spore-germination rate and appressoria formation, while there were no differences from controls in the frequency of autofluorescent epidermal cells at the sites of fungal penetration. Our findings suggest that PGPR and chemical inducers cause different plant responses during induced resistance.