Acquired Resistance in Barley (The Resistance Mechanism Induced by 2,6-Dichloroisonicotinic Acid Is a Phenocopy of a Genetically Based Mechanism Governing Race-Specific Powdery Mildew Resistance)

Expression of beta-1,3-glucanase and chitinase in healthy, stem-rust-affected and elicitor-treated near-isogenic wheat lines showing Sr5-or Sr24-specified race-specific rust resistance

Pathogenesis-related expression of the two antifungal hydrolases beta-1,3-glucanase (EC 3.2.1.39) and chitinase (EC 3.2.1.14) was studied in wheat (Triticum aestivum L.) as part of the defence response to stem rust (Puccinia graminis f.sp. tritici, Pgt), mediated by the semi-dominantly acting resistance genes Sr5 and Sr24. Complete resistance (infection type 0), mediated by the Sr5 gene in cultivar Pre-Sr5, closely correlates with the hypersensitive response of penetrated cells at early stage of the interaction, when the first haustorium is formed. In contrast, cultivar Pre-Sr24 shows intermediate resistance (infection type 2-3) which is not directly linked to cell death. In both cases, the plant response included a rapid increase in beta-1,3-glucanase activity between 24 and 48 h after inoculation. One main extracellular 30-kDa isform of beta-1,3-glucanase was present in both lines, as shown by polyacrylamide-gel electrophoresis. Two additional minor isoforms (32 and 23 kDa) were detected only in Pre-Sr24, and only at later time points. Increased enzme activity and the appearance of new isoforms in the resistance lines was preceded by accumulation of mRNAs encoding beta-1,3-glucanase and chitinases. However, there were no changes in chitinase activity or isoforms. A high constitutive level of chitinase activity was observed in all wheat genotypes. Serological studies indicated the presence of a class II chitinase of 26 kDa. Accumulation of beta-1,3-glucanase and chitinase transcripts was detected before the pathogen penetrated the leaves through stomata and approximately 16 h before the typical hypersensitive response was observed, indicating that signal(s) for defense gene activation were recognised by the host plant long before a tight contact between the pathogen and a host cell is established. A glycoprotein (Pgt elicitor) derived from hyphal walls, strongly induced beta-1,3-glucanase. We discuss the possible role of the elicitor in the early signalling mediating Sr5- and Sr24-specified resistance in wheat.

Systemic acquired resistance

This paper examines induced resistance (SAR) in plants against various insect and pathogenic invaders. SAR confers quantitative protection against a broad spectrum of microorganisms in a manner comparable to immunization in mammals, although the underlying mechanisms differ. Discussed here are the molecular events underlying SAR: the mechanisms involved in SAR, including lignification and other structural barriers, pathogenesis-related proteins and their expression, and the signals for SAR including salicylic acid. Recent findings on the biological role of systemin, ethylene, jasmonates, and electrical signals are reviewed. Chemical activators of SAR comprise inorganic compounds, natural compounds, and synthetic compounds. Plants known to exhibit SAR and induced systemic resistance are listed.

Influence of Salicylic Acid on the Induction of Competence for H2O2 Elicitation (Comparison of Ergosterol with Other Elicitors)

Hypocotyls from etiolated cucumber (Cucumis sativus L.) seedlings were gently abraded at their epidermal surface, and cut segments were used to study the rapid and transient elicitation of H2O2 by ergosterol, chitosan, mastoparan, and a polymeric fungal elicitor. Freshly abraded segments were only barely competent for any H2O2 production, but they developed this competence subsequent to abrasion. This process was enhanced by 2,6-dichloroisonicotinic acid and salicylic acid, which induced acquired resistance to fungal penetration in the epidermal cells. Enhancement of competence induction by salicylic acid was also evident for spontaneous H2O2 production and differed in degree for the various elicitors, indicating that mainly the enzyme complex producing H2O2, but also other components of the elicitation system, improved. Ergosterol, chitosan, and fungal elicitor also rendered the segments refractory to a second stimulation by the same compound, whereas mastoparan was inactive in this respect. The four elicitors also differed markedly in their ability to diminish or enhance H2O2 production by a second treatment with a different elicitor, indicating that several sites of the H2O2 elicitation system are subject to short-term regulation.

Benzothiadiazole, a novel class of inducers of systemic acquired resistance, activates gene expression and disease resistance in wheat

Systemic acquired resistance is an important component of the disease resistance repertoire of plants. In this study, a novel synthetic chemical, benzo(1,2,3)thiadiazole-7-carbothioic acid S-methyl ester (BTH), was shown to induce acquired resistance in wheat. BTH protected wheat systemically against powdery mildew infection by affecting multiple steps in the life cycle of the pathogen. The onset of resistance was accompanied by the induction of a number of newly described wheat chemically induced (WCI) genes, including genes encoding a lipoxygenase and a sulfur-rich protein. With respect to both timing and effectiveness, a tight correlation existed between the onset of resistance and the induction of the WCI genes. Compared with other plant activators, such as 2,6-dichloroisonicotinic acid and salicylic acid, BTH was the most potent inducer of both resistance and gene induction. BTH is being developed commercially as a novel type of plant protection compound that works by inducing the plant's inherent disease resistance mechanisms.

Pretreatment of Parsley Suspension Cultures with Salicylic Acid Enhances Spontaneous and Elicited Production of H2O2

Suspension-cultured cells of parsley (Petroselinum crispum L.) were used to study the regulation of extracellular H2O2. After resuspension, the washed cells regulated the H2O2 concentration spontaneously to a constant level that was greatly increased when the cultures were pretreated for 1 d with salicylic acid (SA). The H2O2 level was further increased on addition of a fungal elicitor preparation, macromolecular chitosan, the sterol-binding polyene macrolide amphotericin B, the G protein-activating peptide mastoparan, or La3+. In all cases, this induced H2O2 burst was also greatly enhanced in cell suspensions pretreated with SA. Both the spontaneous and the induced H2O2 production were decreased by the protein kinase inhibitor K-252a. It is suggested that production of extracellular H2O2 occurs by an endogenously controlled plasma membrane enzyme complex that requires continuous phosphorylation for function and whose activity is increased by pretreatment of the cells with SA. This system can also receive various external stimuli, including those resulting from binding of fungal elicitor. SA can induce acquired resistance against pathogens. The conditioning of the parsley suspension culture by SA represents, therefore, a model for the long-term regulation of apoplastic H2O2 concentration by this signal substance, as suggested previously for the wound hormone methyl jasmonate.