Influence of ozone on ribonuclease activity in wheat (Triticum aestivum) leaves.

Ribonucleases (RNases) degrade RNA and exert a major influence on gene expression during development and in response to biotic and abiotic stresses. RNase activity typically increases in response to pathogen attack, wounding and phosphate (Pi) deficiency. Activity also increases during senescence and other programmed cell death processes. The air pollutant ozone (O3) often induces injury and accelerated senescence in many plants, but the biochemical mechanisms involved in these responses remain unclear. The objective of this study was to determine whether RNase activity and isozyme expression was stimulated in wheat ( Triticum aestivum L.) flag leaves following treatment with O3. Plants were treated in open-top chambers with charcoal-filtered air (27 nmol O3 mol−1) (control) or non-filtered air plus O3 (90 nmol O3 mol−1) (O3) from seedling to reproductive stage. After exposure for 56 days, RNase activity was 2.1 times higher in flag leaf tissues from an O3-sensitive cultivar in the O3 treatment compared with the control, which generally coincided with foliar injury and lower soluble protein concentration, but not soluble leaf [Pi]. Soluble [Pi] in leaf tissue extracts from the O3 and control treatments was not significantly different. RNase activity gels indicated the presence of three major RNases and two nucleases, and their expression was enhanced by the O3 treatment. Isozymes stimulated in the O3 treatment were also stimulated in naturally senescent flag leaf tissues from plants in the control. However, soluble [Pi] in extracts from naturally senescent flag leaves was 50% lower than that found in green flag leaves in the control treatment. Thus, senescence-like pathological responses induced by O3 were accompanied by increased RNase and nuclease activities that also were observed in naturally senescent leaves. However, [Pi] in the leaf tissue samples suggested that O3-induced injury and accelerated senescence was atypical of normal senescence processes in that Pi export was not observed in O3-treated plants. [ABSTRACT FROM AUTHOR]