No abstract

The tobacco (Nicotiana tabacum) cultivar Xanthi-nc (genotype NN) produces high levels of salicylic acid (SA) after inoculation with the tobacco mosaic virus (TMV). Gaseous methyl salicylate (MeSA), a major volatile produced in TMV-inoculated tobacco plants, was recently shown to be an airborne defense signal. Using an assay developed to measure the MeSA present in tissue, we have shown that in TMV-inoculated tobacco plants the level of MeSA increases dramatically, paralleling increases in SA. MeSA accumulation was also observed in upper, noninoculated leaves. In TMVinoculated tobacco shifted from 32 to 24°C, the MeSA concentration increased from nondetectable levels to 2318 ng/g fresh weight 12 h after the temperature shift, but subsequently decreased with the onset of the hypersensitive response. Similar results were observed in plants inoculated with Pseudomonas syringae pathovar phaseolicola, in which MeSA levels were highest just before the hypersensitive response-induced tissue desiccation. Transgenic NahG plants unable to accumulate SA also did not accumulate MeSA after TMV inoculation, and did not show increased resistance to TMV following MeSA treatment. Based on the spatial and temporal kinetics of its accumulation, we conclude that tissue MeSA may play a role similar to that of volatile MeSA in the pathogeninduced defense response.Tobacco (Nicotiana tobaccum L. cv Xanthi-nc) plants carrying the N gene produce high levels of SA after inoculation with TMV. An increase in the tissue content of SA follows the appearance of the virus-induced HR (Malamy et al., 1990). The correlation between SA accumulation and pathogen resistance has also been shown in cucumber (Metraux et al., 1990; Rasmussen et al., 1991), tomato (Hammond-Kosack et al., 1996), and Arabidopsis (Delaney et al., 1995). Following the primary infection, the inoculated plant becomes resistant to subsequent pathogen attack, both locally and systemically, in pathogen-free leaves. This phenomenon, called SAR, has attracted scientific interest for more than 60 years (Chester, 1933). The dependence of SAR on SA accumulation was shown in transgenic NahG tobacco plants expressing the salicylate hydroxylase, which accumulate little or no SA and do not exhibit SAR (Gaffney et al., 1993).While investigating the movement and distribution of SA in TMV-inoculated tobacco, we discovered that the plants evolve large amounts of gaseous MeSA (Shulaev et al., 1997). In contrast, healthy or mechanically wounded plants did not evolve any detectable amounts of MeSA. Application of gaseous MeSA to healthy tobacco plants increased the expression of the PR-1 gene and TMV resistance (measured as the size reduction of the TMV-induced lesions). The amount of MeSA produced by TMVinoculated tobacco plants was sufficient to induce PR-1 gene expression and resistance in healthy plants receiving air from the headspace of enclosed TMV-inoculated plants.The accumulation of SA in tissue exposed to MeSA gas and labeling studies using [14 C]SA and MeSA suggest that MeSA is synt...

The expression and secretion of acid phosphatase (APase) was investigated in Indian mustard (Brassica juncea L. Czern.) plants using sensitive in vitro and activity gel assays. Phosphorus (P) starvation induced two APases in Indian mustard roots, only one of which was secreted. Northern-blot analysis indicated transcriptional regulation of APase expression. Polymerase chain reaction and Southern-blot analyses revealed two APase homologs in Indian mustard, whereas in Arabidopsis, only one APase homolog was detected. The Arabidopsis APase promoter region was cloned and fused to the ␤-glucuronidase (GUS) and green fluorescent protein (GFP) reporter genes. GUS expression was first evident in leaves of the P-starved Arabidopsis plants. In P-starved roots, the expression of GUS initiated in lateral root meristems followed by generalized expression throughout the root. GUS expression diminished with the addition of P to the medium. Expression of GFP in P-starved roots also initiated in the lateral root meristems and the recombinant GFP with the APase signal peptide was secreted by the roots into the medium. The APase promoter was specifically activated by low P levels. The removal of other essential elements or the addition of salicylic or jasmonic acids, known inducers of gene expression, did not activate the APase promoter. This novel APase promoter may be used as a plant-inducible gene expression system for the production of recombinant proteins and as a tool to study P metabolism in plants.

(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.

Jersey 08903-0231 (P. Silverman, M.S., I.R.)The possible role of the octadecanoid signaling pathway with jasmonic acid (JA) as the central component i n defense-gene regulation of pathogen-attacked rice was studied. Rice (Oryza sativa 1.) seedlings were treated with JA or inoculated with the rice blast fungus Magnaporthe grisea (Hebert) Barr., and gene-expression patterns were compared between the two treatments. JA application induced the accumulation of a number of pathogenesis-related (PR) gene products at the mRNA and protein levels, but pathogen attack did not enhance the levels of (-)-JA during the time required for PR gene expression. Pathogen-induced accumulation of PR1 -like proteins was reduced in plants treated with tetcyclacis, a nove1 inhibitor of jasmonate biosynthesis. There was an additive and negative interaction between JA and an elicitor from M. grisea with respect to induction of PR1-like proteins and of an abundant JA-and wound-induced protein of 26 kD, respectively. Finally, activation of the octadecanoid signaling pathway and induction of a number of PR genes by exogenous application of JA did not confer local acquired resistance t o rice. The data suggest that accumulation of nonconjugated (-)-JA is not necessary for induction of PR genes and that JA does not orchestrate localized defense responses in pathogen-attacked rice. Instead, JA appears to be embedded in a signaling network with another pathogen-induced pathway(s) and may be required at a certain minimal leve1 for induction of some PR genes.The plant-growth regulator JA, first described as the methyl ester MeJA in the essential oils of Jasminum (Demole et al., 1962), has been attributed to a number of regulating functions in plant development (for review, see Sembdner and Parthier 119931). Only recently has attention been paid to jasmonates as the key molecules of the octadecanoid signaling pathway mediating the activation of defense responses in herbivore-or pathogen-attacked plants (for review, see Sembdner and Parthier [1993], Farmer [1994], and Creelman and Mullet [1995]). The wound and pathogen