Activation of defence-related genes during senescence: a correlation between gene expression and cellular damage

The correlation between activation of defence-related gene expression and p lant senescence was investigated by evaluating the presence of specific tra nscripts in various leaves of tobacco senescing plants. Expression of most genes examined was found to be induced shortly after flowering; however, ea ch gene had its own characteristic timing of expression and level of RNA ac cumulation. Studies of the symptoms developed in senescing leaves respondin g to bacterial inoculation suggest that the accumulation of defence-related transcripts in these tissues might be related with the mechanism of senesc ence rather than with protection of the plant against pathogen infection. W e observed that the high level of GUS expression directed by the beta -1,3- glucanase gn1 promoter, in senescing leaves of transgenic tobacco plants, d ecreased after bacterial inoculation, in correlation with the formation of symptoms. Reduction of gene expression was likely to be the reflection of t he additional damage caused by the bacteria in the senescent tissues inocul ated.