Induced systemic resistance against Cucumber mosaic cucumovirus and promotion of cucumber growth by some plant growth-promoting rhizobacteria

During the investigation Cucumber mosaic cucumovirus (CMV) was isolated from cucumber plants showing virus like symptoms depending on indirect enzyme linked immunosorbant assay (I-ELISA) and Chenopodium quinoa as local lesion host. Three isolates from the predominant plant growth-promoting rhizobacteria (PGPR) were isolated from cucumber plants rhizosphere, and identified morphologically and physiologically to be related to Bacillus subtilis, Pseudomonas fluorescens and Azotobacter chroococcum species. The bacterial liquid crude cultures (72 h of age) and their supernatants were tested for their ability to induce systemic resistance within cucumber plants (Cucumis sativus L. cultiver Beit Alpha) against CMV infection. Two types of treatment were carried out: (1) spraying of healthy cucumber plants (carrying 4–5 leaves) and challenging by mechanical CMV inoculation at time intervals (5–10 days), (2) irrigation, as healthy cucumber seeds were irrigated with 200 ml from each bacterial culture or their supernatants and inoculated with CMV 15 days post treatment. Data proved that best results were obtained by treatment of irrigation with the Azotobacter crude culture, as the number of symptomless plants were 11 out of 30 plants inoculated, followed by Pseudomonas treated plants which gave eight asymptomatic plants. The induced resistance was tested using I-ELISA and immunocapture reverse transcription polymerase chain reaction (IC-RT-PCR) for the detection of CMV coat protein gene (cp), which proved that the mentioned symptomless plants were virus-free or with a low level of virus infection. Azotobacter treated plants giving virus-free results revealed the higher peroxidase and b-1,3-glucanase enzyme activities, 7 U/gm and 500 nktal/gm, respectively. Using gel electrophoresis and in comparison with control plants, a new protein band was detected in the protected cucumber plant extracts (molecular weight of about 30 KDa), assuming to be a plant pathogen related protein. Increase in growth measures was observed for Azotobacter and Pseudomonas treated cucumber plants, as the higher plant dry weights were 16.1 and 13.8 gm, respectively. Statistical lowest significant differences test (LSD) showed significant differences between Azotobacter and Pseudomonas results for biological data of plant dry weights.