A mycobacterium isolated from tissue cultures of mature Pinus sylvestris interferes with growth of Scots pine seedlings

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Abstract
We isolated a rapidly growing, pigment-producing mycobacterium from senescent tissue cultures derived from mature Scots pine (Pinus sylvestris L.). The bacterium was found in three senescent suspension cultures and in a senescent protoplast culture. Growth of Scots pine cells had ceased in all of these cultures. Exogenous contamination was eliminated by rigorous surface sterilization of the buds with hypochlorite before aseptic removal of the bud scales. Based on biochemical and physiological properties and DNA sequence comparisons, the isolated mycobacterium did not belong to any known species. Its sequence most closely resembled those of Mycobacterium obuense (97%) and M. aichiense (96%). Tissue browning was frequently observed in callus or suspension culture of Scots pine. Because the effect of the mycobacterium on growth of undifferentiated tissues that were browning was difficult to evaluate, we applied the bacterium to Scots pine seeds in aseptic conditions. Seedlings grown in the presence of the mycobacterium had shorter hypocotyls than control seedlings and seedlings cocultivated with a Pseudomonas strain known to be harmless to plants. However, hypocotyl growth of seedlings cocultivated with another mycobacterium, M. chlorophenolicum, was similar to that observed in the presence of the isolated mycobacterium. Phenylalanine ammonia-lyase activity of seedlings cocultivated with the mycobacterium isolate was significantly higher than that of control seedlings or seedlings cocultivated with M. chlorophenolicum or Pseudomonas fluorescens. We believe that this is the first report of the isolation of mycobacteria from tissue cultures of a tree. Our finding that the mycobacterium may interfere with the growth of Scots pine in vitro warrants further study.

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