Ozone induced gene responses in tolerant and sensitive plants

Among the major gaseous pollutants, tropospheric ozone surely possesses the higher phyto-toxic potential: ozone has been demonstrated to alter basic metabolic processes of plants, and the alterations can affect crop productivity and contribute to forest decline. Environmental concentrations of ozone increased from 1% to 2% per year during the past 20 years in Europe and United States, and in the next decades the critical threshold concentration of 40 nl l-1 will be reached more often in rural regions than in urban areas. The response of plant to ozone exposure includes a number of physiological and biochemical changes that are the direct result of selective increase or decreases in gene expression. Although the physiological and morphological responses of plant to ozone have been well characterised, little is known about the molecular basis for these responses. To elucidate the molecular mechanisms of ozone responses in arboreous and herbaceous species, we have chosen hybrid poplars (Populus deltoides x maximowiczii Eridano clone, O3-sensitive, e P. x euramericana I-214 clone, O3-tolerant) and bean (Phaseolus vulgaris var. Groffi, O 3-tolerant, e Phaseolus vulgaris var. Pinto, O3-sensitive). With the aim to characterise the gene expression induced by ozone exposure and the molecular basis of ozone tolerance we have applied an improved techniques for finding differentially regulated genes, such as Suppressive Subtractive Hybridisation (SSH): this methods is based on the construction of subtracted cDNA libraries that allow the identification and isolation of differentially expressed transcript. At present we have isolated some interesting clones. The sequences, coming from automated sequencing of the clones, were analysed using the FASTA, BLAST, ProDom, BLOCK software for their identification. For example, in the cDNA library obtained from Phaseolus vulgaris var. Pinto, O3-sensitive, proteins were recognised that are required in the defence response for different type of biotic or abiotic stress (components of 26S proteasome; proteins that catalyse the covalent attachment of ubiquitin to other proteins; glutathione S-transferases that participate in the detoxification of reactive electrophilic compounds by catalysing their conjugation to glutathione). This may suggest that the mechanisms involving ozone-induced proteins are quite different in sensitive or tolerant varieties and that the roles of the above mentioned proteins are not sufficient in conferring tolerance.