Selection of potential inducing bacteria by using a classical hypersensitive reaction

In general, a certain beneficial bacterium selected from rhizosphere or rhizoplane of the plant has ability to be a good inducing agent. However, a screening of potential inducing bacteria is processed with a long duration in the greenhouse assay. The hypersensitive reaction (HR) has been reported to support the elicitor/receptor model of induced systemic resistance in which the first step can be recognized by cell death. The objective of this study is to use the classical HR for selection of potential inducing bacteria from vegetable crops grown in Phitsanulok areas located approximately 15 km far from Naresuan University. Bacteria were isolated from rhizoshere, rhizoplane, and internal roots of vegetables. Then, all isolated bacteria, with the concentration of 10**(11) cells/ml, were tested for their inducing potential using the classical HR method by infiltration into tobacco leaf-panel grown in the greenhouse. The plain phosphate buffer (pH 7.0) was used as a control. Only the bacteria causing visible HR using the above concentration were selected for testing classical HR with lower concentrations. The results showed that there were 29 isolates and 30 isolates of bacteria isolated from rhizosphere and rhizoplane of sampling vegetable crops, respectively. There was no bacterium isolated from internal roots of vegetables. Four rhizosphere bacteria and five rhizoplane bacteria gave a necrosis patch on tobacco leaf-panel when infiltrated with 10**(11) cells/ml of bacterial suspension. All those bacteria caused nothing when infiltrated with the minimum requirement for classical HR test. Leaf-panels infiltrated with a plain phosphate buffer showed nothing. Since the death of the cells has been considered as a trigger of defense mechanisms in plants, this could suggest that nine isolates of bacteria causing a necrosis in tobacco leaf panel could have ability to be good inducing agents. Therefore, the classical HR could be used as the initial method for screening potential inducing bacteria in a short period of time.