campestris pv. campestris with its host plants, the missing pectate lyase activity could be a reason for the absence of HR in the X. campestris pv. campestris mutants defective in tonB1, exbB1, exbD1, or exbD2. This hypothesis was checked in a complementation experiment.

The pglI gene coding for pectate lyase isoform I had been functionally characterized based on X. campestris pv. campestris wild-type strain 8004 [38, 39]. This gene, which is orthologous to the X. campestris pv. campestris B100 gene termed pel1, was cloned from cosmid pIJ3051 [39] to finally obtain the plasmid pHGW267, where pglI was constitutively expressed under the control of the aacC1 Pout promoter (see methods section for details). This plasmid, which could not replicate see more in X. campestris pv. campestris, was integrated Rapamycin solubility dmso into the chromosomes of the X. campestris pv. campestris wild-type strain B100 and of the exbD2 mutant, which was not affected in iron uptake [64]. The pectate lyase of the resulting complemented strains was also active in the absence of pectate, although the activity was decreased by about 50% when compared to the pectate-induced wild-type (Additional file 3: Table S2). So these strains did not require induction for pectate lyase activity. Both X. campestris pv. campestris strains carrying the constitutively expressed pglI gene, the wild-type as well as the exbD2 mutant, were then infiltrated into C. annuum leafs. Here, the

complemented exbD2 mutant induced an HR with symptoms similar to the wild-type, although with a delay of one day (Figure 3). Hence, the intensity of the HR correlated well with pectate lyase activity. The results show that X. campestris pv. campestris pectate lyase activity is required to invoke an HR on pepper. Figure 3 Complementation of an X. campestris pv. campestris exbD2 mutant by a constitutively expressed CHIR-99021 pglI gene from X. campestris pv. campestris 8004. When compared to the X. campestris pv. campestris

wild-type strain B100, it becomes obvious that the mutant strain defective in exbD2, B100-11.03, which had been demonstrated before to induce no symptoms like necrotic lesions [66], could be functionally complemented with a constitutively expressed pglI gene on plasmid pHGW267 that was integrated into the chromosome. (A) The complemented mutant strain regained its pectate lyase activity, although not to the full extent of the wild-type strain. (B) This correlates well with the reconstituted but attenuated hypersensitive response that this complemented mutant evoked on C. annuum. Elicitor-activity upon co-incubation of X. campestris pv. campestris with C. annuum cell wall material The successful complementation of an exbD2 mutant with a pectate lyase gene indicated an important role of this gene in the recognition of X. campestris pv. campestris pathogens by non-host plants. However, the molecular characteristics of the elicitor that caused the HR were still unknown. The pectate lyase itself could act as a MAMP.