Ozone and aging up-regulate type II metacaspase gene expression and global metacaspase activity in the leaves of field-grown maize (Zea mays L.) plants

Maize plants (Zea mays L. cv. NK Perform) were exposed to O3-enriched air, using a new field fumigation system. Transcriptional changes for three type II-metacaspase genes were studied in the leaves (ranks 10 and 12), using quantitative real-time PCR. Global metacaspase activity was measured using metacaspase-specific synthetic tripeptide Boc–GRR–AMC. Aging had little effect on mRNA accumulation whereas four to six-fold increases were observed for the most O3-responsive type II metacaspase genes, in the older leaves 10. Global metacaspase activity increased by 257% and 333% in leaves 12 and 10, respectively, in response to the highest cumulated concentration. In non-fumigated plants, metacaspase activity progressively increased over the course of the experiment and always was higher in the older leaves 10. Together, these results suggest that metacaspase-mediated proteolysis is a crucial step in leaf responses to both O3 and age-mediated senescence.

► A new field fumigation system was used to expose maize plants to O3-enriched air. ► We examined leaf type II-metacaspases gene expression and global MC activity. ► Both parameters were influenced by O3 exposure, leaf rank and age. ► Global MC activity increased dramatically in response to elevated ozone exposures. ► Age-mediated senescence also stimulated global leaf MC activity.