Résumé
Xanthomonas albilineans is a systemic, xylem-invading pathogen that causes leaf scald in sugarcane. Symptoms vary from a single, white, narrow, sharply defined stripe to complete wilting and necrosis of infected leaves, leading to plant death. X. albilineans produces the toxin, albicidin, that blocks chloroplast differentiation, resulting in disease symptoms. Albicidin is the only previously known pathogenicity factor in X. albilineans, yet albicidin-deficient mutant strains are still able to efficiently colonise sugarcane. The complete genome of strain X. albilineans GPE PC73 from Guadeloupe was recently sequenced and annotated, providing a valuable tool to help identify new pathogenicity factors in this pathogen. In attempts to identify additional pathogenicity genes, site-directed deletions of candidate pathogenicity genes and also random Tn5 insertion mutagenesis were used, both followed by inoculation of sugarcane plants with new strains containing introduced mutations. Knockout mutagenesis of albXXI, which encodes a phosphopantetheinyl transferase that activates non ribosomal peptide synthetases (NRPSs), resulted in reduced capacity to colonise the sugarcane stalk by GPE PC73. Knockout mutagenesis of quorum sensing genes, rpfF and xanB2, did not affect albicidin production nor sugarcane colonisation by X. albilineans strain XaFL07-1 from Florida. However, four independent Tn5 insertion mutations of an OmpA family gene strongly affected both disease symptom development and sugarcane stalk colonisation without affecting albicidin production in strain XaFL07-1. Other candidate pathogenicity genes are currently being investigated. (Résumé d'auteur)