Xanthomonas PROGRAMMED CELL DEATH HAS SIMILARITIES WITH EUKARYOTIC APOPTOSIS

Xanthomonas bacterium exhibited post-exponential programmed cell death (PCD) under certain nutritional conditions especially in a protein rich medium. PCD was not displayed by Xanthomonas cells while growing in minimal medium like starch minimal medium or M9 medium. While undergoing PCD, the organism was found to transform to spherical membranous apoptotic body like structures. The transformation was confirmed by scanning electron microscopy. The membranous bodies were found to contain DNA, which was devoid of the indigenous plasmids of the organism. These bodies were also found to bind annexin V indicative of the externalization of membrane phosphatidyl serine. Nicking of DNA was also detected using a Tunnel assay in Xanthomonas campestris pv. glycines (Xcg) cultures undergoing PCD in LB medium. The PCD in Xanthomonas was found to be associated with the synthesis of an endogenous enzyme similar to human caspase-3, a known marker of apoptosis in eukaryotes. Caspase-3 like protein was detected only in the cells growing in proteinous medium and not in those growing in minimal medium. It was found that starch or its hydrolytic product acted as the repressor of biosynthesis for the Xanthomonas caspase, thereby, preventing the organism from undergoing PCD, upon its addition. Caspase negative mutants of Xcg strain AM2 did not display PCD. The importance of PCD in Xanthomonas life-cycle needs to be elucidated as the system appears to have significant similarities with the eukaryotic apoptosis.