Ozone Sensitivity in Hybrid Poplar Correlates with Insensitivity to Both Salicylic Acid and Jasmonic Acid. The Role of Programmed Cell Death in Lesion Formation

Abstract

(M.S.) Our earlier studies demonstrated that the ozone-sensitive hybrid poplar clone NE-388 displays an attenuated level of ozone-, wound-, and phytopathogen-induced defense gene expression. To determine if this reduced gene activation involves signal transduction pathways dependent on salicylic acid (SA) and/or jasmonic acid (JA), we compared the responses of NE-388 and an ozone-tolerant clone, NE-245, to these signal molecules. JA levels increased in both clones in response to ozone, but only minimal increases in SA levels were measured for either clone. Treatment with SA and methyl jasmonate induced defense gene expression only in NE-245, indicating that NE-388 is insensitive to these signal molecules. DNA fragmentation, an indicator of programmed cell death (PCD), was detected in NE-245 treated with either ozone or an avirulent phytopathogen, but was not detected in NE-388. We conclude that these clones undergo two distinct mechanisms of ozoneinduced lesion formation. In NE-388, lesions appear to be due to toxic cell death resulting from a limited ability to perceive and subsequently activate SA-and/or JA-mediated antioxidant defense responses. In NE-245, SA-dependent PCD precedes lesion formation via a process related to the PCD pathway activated by phytopathogenic bacteria. These results support the hypothesis that ozone triggers a hypersensitive response.

The physiological, transcriptional and genetic responses of an ozone- sensitive and an ozone tolerant poplar and selected extremes of their F 2 progeny

“…This suggests that at the global scale, induced gene responses are conserved across species and that there appears to be a conserved general abiotic stress-induced remodelling of the transcriptome, in response to a variety of stresses. Genes encoding enzymes involved in the phenylpropanoid pathway were up-regulated as a result of ozone exposure, as was found in similar studies in a range of species (Koch et al, 2000;Ludwikow et al, 2004;Matsuyama et al, 2002;Puckette et al, 2008). Re-modelling of secondary metabolism is apparently an important and conserved mechanism of response to ozone exposure, and is a response that will divert energy from primary metabolism.…”

Salicylic Acid Biosynthesis and Metabolism

“…In other plants, such as rice and Populus species, although SA (and SA-derived compounds) play an important role in disease and herbivory resistance, SA synthesis is not dramatically induced by (a)biotic stress. Instead, basal SA plus SA glucoside levels are high, typically greater than those found in pathogen-induced Arabidopsis leaves (Silverman et al, 1995;Koch et al 2000). Precursor feeding and PAL inhibition studies have implicated the PAL pathway for SA synthesis in rice (Silverman et al, 1995) and poplar (Ruuhola and Julkunen-Tiito, 2003).…”

Salicylate Accumulation Inhibits Growth at Chilling Temperature in Arabidopsis

“…Detailed evidence implicates SA in PR gene expression, systemic acquired resistance, and the hypersensitive response (Kunkel and Brooks, 2002). SA also seems to be involved in responses to abiotic stresses, such as ozone (Sharma et al, 1996;Rao and Davis, 1999;Koch et al, 2000), salt and osmotic stress (Borsani et al, 2001;Molina et al, 2002;Shim et al, 2003), UV-B (Surplus et al, 1998), drought (Senaratna et al, 2000;Nemeth et al, 2002), paraquat , and heat (Dat et al, 1998a(Dat et al, , 1998b(Dat et al, , 2000Lopez-Delgado et al, 1998a;Senaratna et al, 2000;Larkindale and Knight, 2002;Clarke et al, 2004). Stress-influenced developmental transitions, including flowering (Hatayama and Takeno, 2003;Martinez et al, 2004), tuberization (Lopez-Delgado and Scott, 1997), and senescence (Morris et al, 2000), may also involve SA.…”