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 plant senescence was investigated by evaluating the presence of specific transcripts in various leaves of tobacco senescing plants. Expression of most genes examined was found to be induced shortly after flowering; however, each gene had its own characteristic timing of expression and level of RNA accumulation. Studies of the symptoms developed in senescing leaves responding to bacterial inoculation suggest that the accumulation of defence-related transcripts in these tissues might be related with the mechanism of senescence rather than with protection of the plant against pathogen infection. We observed that the high level of GUS expression directed by the beta -1,3-glucanase gn1 promoter, in senescing leaves of transgenic tobacco plants, decreased after bacterial inoculation, in correlation with the formation of symptoms. Reduction of gene expression was likely to be the reflection of the additional damage caused by the bacteria in the senescent tissues inoculated.