Biological Role of Xanthomonadin Pigments in Xanthomonas campestris pv. Campestris

Abstract

Previous studies have indicated that the yellow pigments (xanthomonadins) produced by phytopathogenicXanthomonas bacteria are unimportant during pathogenesis but may be important for protection against photobiological damage. We used a Xanthomonas campestris pv. campestris parent strain, single-site transposon insertion mutant strains, and chromosomally restored mutant strains to define the biological role of xanthomonadins. Although xanthomonadin mutant strains were comparable to the parent strain for survival when exposed to UV light; after their exposure to the photosensitizer toluidine blue and visible light, survival was greatly reduced. Chromosomally restored mutant strains were completely restored for survival in these conditions. Likewise, epiphytic survival of a xanthomonadin mutant strain was greatly reduced in conditions of high light intensity, whereas a chromosomally restored mutant strain was comparable to the parent strain for epiphytic survival. These results are discussed with respect to previous results, and a model for epiphytic survival of X. campestris pv. campestris is presented.Xanthomonas bacteria are the causal agents of disease on at least 124 monocot and 268 dicot plant hosts (12), and many of them can survive and multiply as epiphytes (8,24). Most Xanthomonas bacteria produce yellow, membrane-bound, brominated aryl-polyene pigments referred to as xanthomonadins (24). Xanthomonadins are unique to Xanthomonas bacteria and serve as useful chemotaxonomic (2, 25) and diagnostic (22) markers. With methods of artificial infection, xanthomonadin-deficient strains were not affected in pathogenicity, symptomatology, or in planta growth (15). Thus, the xanthomonadins apparently are not important to the pathogen after infection of the host plant.Xanthomonas campestris pv. campestris, the causal agent of black rot of crucifers and one of the most serious disease problems in crucifer production, naturally infects its host via hydathodes or wounds in the leaves (28). A cluster of seven transcriptional units required for xanthomonadin production (pigA to pigG) was previously identified in X. campestris pv. campestris (13,15). In addition to a loss of xanthomonadin production, pigB mutant strains were also greatly impaired in the production of extracellular polysaccharide (EPS) and a pheromone (DF). Mutations in the other pig transcriptional units did not appear to have pleiotropic affects. When tested on the host plant, pigB mutants were significantly reduced in epiphytic survival and natural host infection via hydathodes (16). DF extracellularly restored all of these traits to a pigB mutant strain (4, 15, 16), indicating that DF is needed for xanthomonadin and EPS production, as well as for epiphytic survival and host infection. These results suggest that DF acts as a signal for the initiation of xanthomonadin and EPS production and that xanthomonadins and EPS may play a role in host infection and/or epiphytic survival.The results of other studies suggest an association of xanthomonadins with protec...

Biosynthesis of the immunosuppressants FK506, FK520, and rapamycin involves a previously undescribed family of enzymes acting on chorismate

“…5) remains to be determined, but on the available evidence they are also likely to act as chorismatases/ 3HBA synthases. For example, the FkbO/RapK homolog in Xanthomonas species (the causative agent of black rot in plants) resides in a gene cluster (44) for the biosynthesis of xanthomonadins, yellow brominated aryl pigments apparently derived via the polyketide pathway from a 3HBA starter unit (45). A block in aroE in the early part of the shikimate pathway abolishes xanthomonadin biosynthesis (46).…”

Genetic Locus Encoding Functions Involved in Biosynthesis and Outer Membrane Localization of Xanthomonadin in Xanthomonas oryzae pv. oryzae

“…oryzae (37), and Xanthomonas campestris pv. campestris (33) are more susceptible to photooxidative damage than are wild-type strains. In vitro, xanthomonadin protects membrane lipids in egg-phosphatidylcholine liposomes against peroxidation (37).…”

Antioxidant, antibacterial, and ultraviolet-protective properties of carotenoids isolated from Micrococcus spp.

“…[2] Several researchers have reported that some pigmented bacteria, which are rich in carotenoids have been resistant to radiation when subjected to sub-lethal and lethal doses of ionizing radiations, due to the accumulation of the radio-protective pigments in the outer membrane. [3][4][5][6] Carotenoids are the terpenoid pigments produced by a wide variety of plants and microbes, which are reported to have radio-protective property. [7] Carotenoid pigments of bacterial origin have been reported to have radio-protective and antioxidant properties, and as natural coloring agents.…”