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

Summary Leaf senescence is a complex process that is controlled by multiple developmental and environmental signals and is manifested by induced expression of a large number of different genes. In this paper we describe experiments that show, for the first time, that the salicylic acid (SA)‐signalling pathway has a role in the control of gene expression during developmental senescence. Arabidopsis plants defective in the SA‐signalling pathway (npr1 and pad4 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 patterns were accompanied by a delayed yellowing and reduced necrosis in the mutant 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 signalling factors is required for the optimum expression of many genes during senescence.