Salicylic acid has a role in regulating gene expression during leaf senescence

Leaf senescence is a complex process that is controlled by multiple develop mental and environmental signals and is manifested by induced expression of a large number of different genes. In this paper we describe experiments t hat show, for the first time, that the salicylic acid (SA)-signalling pathw ay has a role in the control of gene expression during developmental senesc ence. Arabidopsis plants defective in the SA-signalling pathway (npr1 and p ad4 mutants and NahG transgenic plants) were used to investigate senescence -enhanced gene expression, and a number of genes showed altered expression patterns. Senescence-induced expression of the cysteine protease gene SAG12 , for example, was conditional on the presence of SA, together with another unidentified senescence-specific factor. Changes in gene expression patter ns were accompanied by a delayed yellowing and reduced necrosis in the muta nt plants defective in SA-signalling, suggesting a role for SA in the cell death that occurs at the final stage of senescence. We propose the presence of a minimum of three senescence-enhanced signalling factors in senescing leaves, one of which is SA. We also suggest that a combination of signallin g factors is required for the optimum expression of many genes during senes cence.