HrpB2 and HrpF from Xanthomonas are type III-secreted proteins and essential for pathogenicity and recognition by the host plant

Authors

  • Ombeline Rossier,

    1. Centre National de la Recherche Scientifique, Institut des Sciences Végétales, 91198 Gif-sur-Yvette, France.
    2. Institut für Genetik, Martin-Luther-Universität, 06120 Halle, Germany.
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  • Guido Van den Ackerveken,

    1. Institut für Genetik, Martin-Luther-Universität, 06120 Halle, Germany.
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  • Ulla Bonas

    Corresponding author
    1. Institut für Genetik, Martin-Luther-Universität, 06120 Halle, Germany.
    • *For correspondence. E-mail bonas@ genetik.uni-halle.de; Tel. (+49) 345 55 26 290; Fax (+49) 345 55 27 277. Present addresses: †Northwestern University, Department of Microbiology and Immunology, Chicago, IL 60611, USA. ‡University of Utrecht, Department of Molecular Cell Biology, 3508 TB Utrecht, The Netherlands.

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Abstract

The interaction between the plant pathogen Xanthomonas campestris pv. vesicatoria and its host plants is controlled by hrp genes (hypersensitive reaction and pathogenicity), which encode a type III protein secretion system. Among type III-secreted proteins are avirulence proteins, effectors involved in the induction of plant defence reactions. Using non-polar mutants, we investigated the role of 12 hrp genes in the secretion of the avirulence protein AvrBs3 from X. c. pv. vesicatoria and a heterologous protein, YopE, from Yersinia pseudotuberculosis. Genes conserved among type III secretion systems (hrcQ, hrcR, hrcS and hrcT) as well as non-conserved genes (hrpB1, hrpB2, hrpB4, hrpB5, hrpD5 and hrpD6) were shown to be required for secretion. Protein localization studies using specific antibodies showed that HrpB1 and HrpB4, as well as the putative ATPase HrcN, were mainly found in the soluble fraction of the bacterial cell. In contrast, HrpB2 and HrpF, which is related to NolX of Rhizobium fredii, are secreted into the culture medium in an hrp-dependent manner. As HrpB2, but not HrpF, is essential for type III protein secretion, there might be a hierarchy in the secretion process. We propose that HrpF, which is dispensable for protein secretion but required for AvrBs3 recognition in planta, functions as a translocator of effector proteins into the host cell.

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