Molecular analysis of leaf senescence in Arabidopsis thaliana

Several new leaf senescence-associated genes (SAGs) from Arabidopsis thaliana were identified. Sequence analysis of these new SAGs predicted a diverse range of gene products, reflecting the complexity of the leaf senescence program. Among the SAGs identified were five defense-related genes. These genes were expressed in senescent leaves of plants that were grown in axenic conditions, indicating that their senescence-associated expression was not due to an inconspicuous pathogen infection. These results imply that the expression of defense-related genes is an integral part of the senescence program. Analysis of gene expression during leaf development showed that the expression profiles of the defense-related genes were similar. These genes were also found to be induced by salicylic acid. However, their expression during leaf senescence does not seem to be dependent upon salicylic acid because they still showed expression in senescent leaves of transgenic plants unable to accumulate salicylic acid. It was concluded that there is overlap of gene expression between the leaf senescence and pathogen-response programs. Another SAG that was identified, SFT1, was similar to monosaccharide transporters of the Major Facilitator Superfamily. Genomic sequence analysis led to the identification of SFT2, a closely linked gene that is highly similar to SFT1. It is possible that these two genes may have originated from a duplication event. Despite their similarity and common chromosomal location, analysis of gene expression showed that SFT1 and SFT2 are differentially regulated, and only SFT1 is induced during leaf senescence. Although different monosaccharides were found to accumulate during leaf senescence, studies of sft1-1, a loss-of-function mutant, did not identify a reproducible difference in the levels of sugars in mutant versus wild type or a difference in the sensitivity to exogenous sugars.