So, also a DAMP could not be ruled out as a possible cause of the HR. This hypothesis was tested experimentally by co-incubating the bacteria with isolated cell wall material. Plant cell HDAC inhibitor mechanism walls were prepared from C. annuum leafs detached from 6 week old plants grown in the greenhouse. The plant material
was homogenized and extracted with aqueous and organic solvent systems, resulting in a crude cell wall preparation. This cell wall material was incubated for 12 h with X. campestris pv. campestris B100-Bac2 cells. The incubation was carried out in phosphate buffer to avoid interference by any additional nutrient source for the bacteria. After removing cell wall fragments and bacteria by centrifugation, the supernatant
was boiled to inactivate all enzyme activity (5 min, 100°C), centrifuged again and dialyzed with a molecular-weight cut-off of 1000 Da. These samples were tested for elicitor activity in tobacco suspension cell cultures by measuring H2O2 generation, the so-called oxidative burst. While the supernatant of incubated cell walls induced no activity, the cell walls co-incubated with X. campestris pv. campestris gave rise to an oxidative burst that indicated the presence of a soluble elicitor (Figure 4A). All experiments performed to characterize the elicitor were initially carried out with plant HSP990 mw suspension cell cultures from the non-host N. tabacum, since they are easier to handle and more responsive to elicitors than pepper cell cultures. Parallel controls containing just X. campestris pv. campestris Galeterone bacteria or just cell wall material, respectively, were prepared. Unexpectedly, the X. campestris pv. campestris control caused an oxidative
burst reaction with an amplitude that indicated nearly half of the activity JQ-EZ-05 cell line observed in the measurement with the X. campestris pv. campestris-cell wall co-incubation. A possible explanation could be derived from previous experiments. It was shown that X. campestris pv. campestris lipopolysaccharides (LPSs) are MAMPs that induce pronounced elicitor activity [26, 69]. Since LPS is also released to the supernatant and would not be removed or inactivated by the heat treatment, these molecules could be responsible for the oxidative burst caused by the X. campestris pv. campestris supernatant. To purify the supernatants from LPS, polymyxin B agarose was employed, which binds LPS with high affinity. By this method, essentially all elicitor activity could be removed from the X. campestris pv. campestris supernatant (Figure 4B). In contrast, for the X. campestris pv. campestris cell wall co-incubation, the polymyxin B agarose treatment reduced the elicitor activity only by about 50%. Obviously, a heat-stable elicitor, differing from bacterial LPS, had remained within this sample. Figure 4 Oxidative burst reactions in heterologous N.