Ozone sensitivity in hybrid poplar is correlated with a lack of defense-gene activation

Ozone is a major gaseous pollutant thought to contribute to forest decline. Although the physiological and morphological responses of forest trees to ozone have been well characterized, little is known about the molecular bas is for these responses. Our studies compared the response to ozone of ozone -sensitive and ozone-tolerant clones of hybrid poplar (Populus maximorwizii x Populus trichocarpa) at the physiological and molecular levels. Gas-exch ange analyses demonstrated clear differences between the ozone-sensitive cl one 388 and the ozone-tolerant clone 245. Although ozone induced a decrease in photosynthetic rate and stomatal conductance in both clones, the magnit ude of the decrease in stomatal conductance was significantly greater in th e ozone-tolerant clone. RNA-blot analysis established that ozone-induced mR NA levels for phenylalanine ammonia-lyase, O-methyltransferase, a pathogene sis-related protein, and a wound-inducible gene were significantly higher i n the ozone-tolerant than in the ozone-sensitive plants. Wound- and pathoge n-induced levels of these mRNAs were also higher in the ozone-tolerant comp ared with the ozone-sensitive plants. The different physiological and molec ular responses to ozone exposure exhibited by clones 245 and 388 suggest th at ozone tolerance involves the activation of salicylic-acid- and jasmonic- acid-mediated signaling pathways, which may be important in triggering defe nse responses against oxidative stress.