Proceedings of the XLV Italian Society of Agricultural Genetics - SIGA Annual Congress

Salsomaggiore Terme, Italy - 26/29 September, 2001

ISBN 88-900622-1-5

Poster Abstract

BACTERIAL FLAVOHAEMOGLOBINS: PATHOGENICITY DETERMINANTS AFFECTING THE PLANT HYPERSENSITIVE DISEASE RESISTANCE RESPONSE

SEVERI E.*, BIONDANI A.*, ZEIER J.**, LAMB C.**, DELLEDONNE M.*

* Istituto di Genetica vegetale, Università Cattolica S.C., via Emilia Parmense 84, 29100 Piacenza, Italy

delledonne@pc.unicatt.it

** John Innes Centre, Norwich Research Park, Colney, Norwich NR4 7UH, UK

Flavohaemoglobins form a class of proteins found in a number of microrganisms, including yeasts and both animal and plant bacterial pathogens. They share two conserved regions, the first representing a haemoglobin domain, the second showing significant sequence homology to the catalytic domain of many flavin reductases. These proteins have been demonstrated to possess an intrinsic nitric oxide dioxigenase activity, able to convert nitric oxide (NO) to nitrate. Presence of flavohaemoglobins among plant pathogens suggests that they could play a role in plant-pathogen interaction. Nitric oxide is indeed recognized as the key molecule for the establishment of plant defense mechanisms leading to hypersensitive cell death as a response to pathogen attack. This work aims to assess the precise function of flavohaemoglobins in pathogenicity by focusing particularly on their effect on plant defense mechanisms modulated by NO, like the hypersensitive response (HR), in contrast to an unspecific NO detoxification mechanism as suggested for human/animal pathogens.

The hmpX gene from Erwinia chrysanthemi (identified as pathogenicity determinant, whose disruption in knocked-out strains leaded to loss of ability to fully macerate host plant leaves) has been cloned into the broad host, gram-negative vector pRK415 and introduced into the Arabidopsis thaliana pathogen Pseudomonas syringae pv. tomato. Arabidopsis leaves infiltrated with the avirulent strain Pst avrB expressing hmpX as a transgene exhibit reduced or no macroscopic HR symptoms, and lack to accumulate PAL transcript, while levels of GST and PR1 transcripts remain unaffected compared to leaves infiltrated with WT strain Pst avrB.

Work is in progress to understand whether this pathogenicity determinant can negatively modulate the establishment of the systemic acquired resistance, and to identify Pseudomonas syringae genome-encoded intrinsic flavohaemoglobin gene/s, in order to produce suitable knocked-out mutants.