Paul Silverman

Salicylic Acid in Rice (Biosynthesis, Conjugation, and Possible Role)

Salicylic acid (SA) is a natural inducer of disease resistance in some dicotyledonous plants. Rice seedlings (Oryza sativa L.) had the highest levels of SA among all plants tested for SA content (between 0.01 and 37.19 [mu]g/g fresh weight). The second leaf of rice seedlings had slightly lower SA levels than any younger leaves. To investigate the role of SA in rice disease resistance, we examined the levels of SA in rice (cv M-201) after inoculation with bacterial and fungal pathogens. SA levels did not increase after inoculation with either the avirulent pathogen Pseudomonas syringae D20 or with the rice pathogens Magnaporthe grisea, the causal agent of rice blast, and Rhizoctonia solani, the causal agent of sheath blight. However, leaf SA levels in 28 rice varieties showed a correlation with generalized blast resistance, indicating that SA may play a role as a constitutive defense compound. Biosynthesis and metabolism of SA in rice was studied and compared to that of tobacco. Rice shoots converted [14C]cinnamic acid to SA and the lignin precursors p-coumaric and ferulic acids, whereas [14C]benzoic acid was readily converted to SA. The data suggest that in rice, as in tobacco, SA is synthesized from cinnamic acid via benzoic acid. In rice shoots, SA is largely present as a free acid; however, exogenously supplied SA was converted to [beta]-O-D-glucosylSA by an SA-inducible glucosyltransferase (SA-GTase). A 7-fold induction of SA-GTase activity was observed after 6 h of feeding 1 mM SA. Both rice roots and shoots showed similar patterns of SA-GTase induction by SA, with maximal induction after feeding with 1 mM SA.

Signal molecules in systemic plant resistance to pathogens and pests

Alexander J. Enyedi, Nasser Yaipani, Paul Silverman, and iiya Raskin AgBiotech Center Cook College Rutgers University New Brunswick, New Jersey 08903-0231 introduction Vertebrate animals possess an inducible antigen-anti- body immune system that acts as a defense against dis- ease. it is a lesser known fact that plants can also be immunized against disease-causing pathogens or feeding pests (Kuc, 1982). This phenomenon is the result of the development of systemic acquired resistance (SAR), a term coined by Ross (1981 b) to describe an increase in resistance to subsequent pathogen attack in inoculated and uninoculated parts of the plant. SAR in plants was documented almost 60 years ago (Chester, 1933) and fur- ther characterized in 1952 for Sweet William (Dianthus barbatus L.) infected by a carnation mosaic virus (Gilpa- trick and Weintraub, 1952). in this review, we extend the definition of SAR to include any form of resistance that develops after localized attack by viral, bacterial, or fungai pathogens or by invertebrate pests. SAR to pathogens usually develops after the appear- ance of a necrotic lesion around the inoculation site. This localized ceil suicide is called the hypersensitive response (HR). While the HR effectively traps pathogens in and around lesions, it makes the whole plant more resistant to a wide range of disease-causing microorganisms (Mad- amanchi and Kuc, 1991; White and Antoniw, 1991). For example, SAR induced by necrotizing viruses develops in uninfected tissues two to three days after inoculation, lasts for several weeks, and may provide protection against pathogenic bacteria, fungi, and other viruses. increased resistance to insect feeding was also observed in plants previously damaged by chewing pests or mechanical wounding. Although not as extensively documented as SAR to pathogens, SAR to pests was shown in some cases to be an effective deterrent to continued herbivory (re- viewed in Tailamy and Raupp, 1991). SAR provides the third and final line of plant defense against pathogens and pests. The first line consists of genetically inherited resistance mechanisms that make plants constitutiveiy resistant to the majority of pathogenic organisms and pests present in the environment. The sec- ond line of defense is activated in the immediate vicinity of the infected or wounded sites in an attempt to prevent the spread of the pathogen throughout the plant or to deter insect feeding. These local resistance responses develop more rapidly than SAR and involve ceil wall and cuticle strengthening, synthesis of toxins (phytoaiexins), antifeed- ants, and the production of defense-related proteins, which include the pathogenesis-related (PR) proteins de- scribed below. In addition to long-distance signal moie-

Jasmonate-Inducible Genes Are Activated in Rice by Pathogen Attack without a Concomitant Increase in Endogenous Jasmonic Acid Levels.

The possible role of the octadecanoid signaling pathway with jasmonic acid (JA) as the central component in defense-gene regulation of pathogen-attacked rice was studied. Rice (Oryza sativa L.) 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 novel 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 to 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 level for induction of some PR genes.

A study of soft and hard breakdown - Part I: Analysis of statistical percolation conductance

A theory of the statistical origin of soft and hard breakdown, that can explain a wide range of experimental data, is proposed. The theory is based on the simple premise that the severity of breakdown depends on the magnitude of the power dissipation through the sample-specific, statistically distributed percolation conductance, rather than on any physical difference between the traps involved. This model (a) establishes the connection between the statistical distribution of the theoretically predicted percolation conductance and the distribution of experimentally measured conductances after soft breakdown (Part I), and (b) explains the thickness, voltage, stress, and circuit configuration dependence of soft and hard breakdown (Part II). Connections to previous theories are made explicit, and contradictions to alternate models are resolved.

A study of soft and hard breakdown - Part II: Principles of area, thickness, and voltage scaling

For Part I see ibid., vol.49, no.2, pp.232-8 (2002). Based on the theory of soft and hard breakdown established in Part I of this paper, we now study the principles of area, thickness, voltage, and circuit configuration dependence of hard and soft breakdown. These scaling principles allow us to conclude that breakdown in ultrathin oxides stressed at operating voltages (1.0-1.5 V) can never be hard, which should allow a more relaxed reliability specification for these oxides.

The Role of Salicylic Acid as a Plant Signal Molecule

The salicylates are a widely distributed class of aromatic compounds. Modifications of the free acid result in a diverse array of compounds such as salicin, the salicyl alcohol glucoside, and methylsalicylate (oil of wintergreen). First isolated from willow (Salix), salicylates have been used as folk remedies since the 4th century B.C. (Weissman, 1991). Salicylates are currently used as antimicrobials, food preservatives and analgesics.