Xanthomonas campestris pv. campestris (Xcc) is an important plant pathogen which causes black rot disease in cruciferous plants. The ppk gene encodes a polyphosphate kinase (PPK) responsible for the synthesis of inorganic polyphosphate (poly P). Poly P is a polymer of tens to hundreds of orthophosphate (Pi) residues linked by high-energy phosphoanhydride bonds , as in ATP. Poly P is ubiquitous , having been found in all organisms (archaea, bacteria, fungi, plants, insects, and animal cell). It is well known that poly P plays a role in the physiological adaptation of microbial cells during growth and development and is required for the adaptation to stress. Xcc ppk mutant is motile and is normal in the secretion of virulence factors, such as exopolysaccharide and exocellular enzymes which are essential for Xcc to invade and establish infections in host cells , production. Neither ppk mutants nor their parental strains can survive after a prolonged incubation at stationary phase. Both ppk mutants and wild-type strains are sensitive to acidified media (pH below 4.0). No homolog of rpoS is present in Xcc genome , that might explain the fragility of Xcc in stringency. In rich medium, ppk mutants grow as rapid as their parental strains. Nevertheless, ppk mutants present a serious delay in growth in poor medium especially after a downshift of nutrition. This suggests that ppk mutants need more time to adapt to an environmental change.
