THE INVOLVEMENT OF THE TonB IRON UPTAKE SYSTEM AND POLYGALACTURONATE LYASE PRODUCTION OF XANTHOMONAS CAMPESTRIS PV. CAMPESTRIS IN THE INDUCTION OF THE HYPERSENSITIVE RESPONSE ON THE NON-HOST PLANT PEPPER (CAPSICUM ANNUUM)
H-G WIGGERICH, H KOSTER and A POHLER
Lehrstuhl for Genetik, Fakultit fur Biologie, Universitat Bielefeld, Postfach 1001 31, D-33501 Bielefeld, Germany
Background and objectives
Iron is considered to play a crucial role in plant-phytopathogenic interactions. Iron is one of the factors that limits bacterial growth in planta because micromolar concentrations of iron are necessary to support growth and multiplication. The common strategy of bacteria to overcome this problem is the utilization of siderophores with high affinity to ferric iron. Fe(III) is complexed by these chelators which are transported into the cell via an energy-coupled process mediated by the TonB system. In this work, we studied X. c. pv. campestris mutants defective in iron uptake and analysed their ability to induce a hypersensitive response of the non-host plant pepper (Capsicum annuum).
Results and conclusions
By mutational analysis the tonB, exbB and exbD1 genes were shown to be essential for ferric iron import in X. c. pv. campestris. In contrast, the exbD2 gene was not involved in the uptake of ferric iron. Mutational analysis also revealed that the tonB, exbB, exbD1 and exbD2 genes of X. c. pv. campestris are necessary for the induction of a hypersensitive response on the non-host plant pepper. Whereas the viable bacterial titre of a tonB, an exbB and an exbD1 mutant decreased after leaf infiltration of pepper, the exbD2 mutant multiplied in the pepper leaf tissue and grew even better than the wild-type strain. In further characterizations we found that all mutants mentioned did not produce the extracellular enzyme polygalacturonate lyase, whereas the activities of other extracelluar enzymes, such as amylase, protease and endoglucanase, remained unchanged. These findings indicated that the TonB system of X. c. pv. campestris is likely to be involved in the induction of polygalacturonate lyase production. Interestingly, an exbD2 mutant equipped with the constitutively expressed polygalacturonate lyase 1 gene of X. c. pv. campestris regained the ability to induce a hypersensitive response on pepper. Therefore we propose that polygalacturonate lyase plays an important role in the induction of a hypersensitive response in the interaction of X. c. pv. campestris and the non-host plant pepper.