Regulation of developmental senescence is conserved between Arabidopsis and Brassica napus

SAG12 is a developmentally controlled, senescence-specific gene from Arabidopsis which encodes a cysteine protease. Using SAG12 as a probe, we isolated two SAG12 homologues (BnSAG12-1 and BnSAG12-2) from Brassica napus. Structural comparisons and expression studies indicate that these two genes are orthologues of SAG12. The expression patterns of BnSAG12-1 and BnSAG12-2 in Arabidopsis demonstrate that the senescence-specific regulation of this class of cysteine proteases is conserved across these species. Gel-shift assays using the essential promoter regions of SAG12, BnSAG12-1, and BnSAG12-2 show that the extent of binding of a senescence-specific, DNA-binding protein from Arabidopsis is proportional to the expression levels of these genes in Arabidopsis. Therefore, the expression levels of these genes may reflect the affinities of the senescence-specific DNA-binding protein for the promoter element.