Diverse range of gene activity during Arabidopsis thaliana leaf senescenceincludes pathogen-independent induction of defense-related genes

To determine the range of gene activities associated with leaf senescence, we have identified genes that show preferential transcript accumulation dur ing this developmental stage. The mRNA levels of a diverse array of gene pr oducts increases during leaf senescence, including a protease, a ribosomal protein, two cinnamyl alcohol dehydrogenases, a nitrilase and glyoxalase II . Two of the genes identified are known to be pathogen-induced. The senesce nce specificity of each gene was determined by characterization of transcri pt accumulation during leaf development and in different tissues. The incre ased expression of nitrilase in senescent leaves is paralleled by an increa se in free indole-3-acetic acid (IAA) levels. Additionally, we have demonst rated that the induction of defense-related genes during leaf senescence is pathogen-independent and that salicylic acid accumulation is not essential for this induction. Our data indicate that the induction of certain genes involved in plant defense responses is a component of the leaf senescence p rogram.