Air pollutant stress changes the steady-state transcript levels of three photosynthesis genes

Six-week-old tobacco plants (Nicotiana tabacum L., cv. SR-1) were fumigated with different, but defined and realistic combinations of NO(2), automobile exhaust and ozone for two days. To determine the effect of an additional environmental stress during exposure, plants were either wounded mechanically or treated with salicylic acid to simulate pathogen attack. The steady-state mRNA levels of genes encoding the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcS), chlorophyll a/b-binding protein (cab) and a 10 kDa protein of the water-evolving complex of photosystem II (ST-LS1) were probed in Northern analysis. Our results clearly show a decline of the mRNA levels for all three photosynthesis genes under automobile exhaust and/or ozone fumigation. Wounding and also elicitor treatment enhance this effect. In contrast, exposure to NO(2) either increased the transcription level of all three genes, or counteracted the negative effect of automobile exhaust and ozone on their expression. At this time no major changes in the concentrations of the corresponding proteins could be detected by Western blot analyses. These results are discussed in the present paper.

Transcriptional activation of plant defence genes by short-term air pollutant stress

The expression of defence genes was monitored by RNA blot analyses in tobacco plants (Nicotiana tabacum cv. SR-1) treated with various air pollutants at realistic concentrations that prevail in urban areas. Six-week-old plants responded with an increase in the steady-state mRNA levels of phenylalanine ammonia-lyase (PAL), chalcone synthase (CHS), chitinase and beta-1,3-glucanase, when exposed to defined and subnecrotic concentrations of automobile exhaust and/or ozone over a period of 48 h. An enhanced expression of genes encoding mitochondrial and cytosolic superoxide dismutases suggested that air pollutants induced considerable oxidative stress. Moreover, wounding or elicitor treatment of plants already exposed to automobile exhaust and/or ozone additionally increased the expression of the above defence genes, but not so in NO(2). Since the main difference between NO(2) and exhaust gas is the absence of the hydrocarbon compounds in the former, we regard hydrocarbons as favourite candidates for the toxic effect of exhaust gas, and they possibly act by generating an enhanced oxidative stress.