Senescence-associated gene expression during ozone-induced leaf senescencein Arabidopsis

The expression patterns of senescence-related genes were determined during ozone (O-3) exposure in Arabidopsis. Rosettes were treated with 0.15 mu L L -1 O-3 for 6 h d(-1) for 14 d. O-3-treated leaves began to yellow after 10 d of exposure, whereas yellowing was not apparent in control leaves until d 14. Transcript levels for eight of 12 senescence related genes characteriz ed showed induction by O-3. SAG13 (senescence-associated gene), SAG21, ERD1 (early responsive to dehydration), and BCB (blue copper-binding protein) w ere induced within 2 to 4 d of O-3 treatment; SAG18, SAG20, and ACS6 (ACC s ynthase) were induced within 4 to 6 d; and CCH (copper chaperone) was induc ed within 6 to 8 d. In contrast, levels of photosynthetic gene transcripts, rbcS (small subunit of Rubisco) and cab (chlorophyll a/b-binding protein), declined after 6 d. Other markers of natural senescence, SAG12, SAG19, MT1 (metallothionein), and Atgsr2 (glutamine synthetase), did not show enhance d transcript accumulation. When SAG12 promoter-GUS (beta-glucuronidase) and SAG13 promoter-GUS transgenic plants were treated with O-3, GUS activity w as induced in SAG13-GUS plants after 2 d but was not detected in SAC12-GUS plants. SAG13 promoter-driven GUS activity was located throughout O-3-treat ed leaves, whereas control leaves generally showed activity along the margi ns. The acceleration of leaf senescence induced by O-3 is a regulated event involving many genes associated with natural senescence.