Infection of plants by necrotizing pathogens or colonization of plant roots with certain beneficial microbes causes the induction of a unique physiological state called "priming." The primed state can also be induced by treatment of plants with various natural and synthetic compounds. Primed plants display either faster, stronger, or both activation of the various cellular defense responses that are induced following attack by either pathogens or insects or in response to abiotic stress. Although the phenomenon has been known for decades, most progress in our understanding of priming has been made over the past few years. Here, we summarize the current knowledge of priming in various induced-resistance phenomena in plants.

The nonprotein amino acids ␥-aminobutyric acid (GABA) and ␤-aminobutyric acid (BABA) have known biological effects in animals and plants. Their mode of action has been the object of thorough research in animals but remains unclear in plants. Our objective was to study the mode of action of BABA in the protection of Arabidopis plants against virulent pathogens. BABA protected Arabidopsis against the oomycete pathogen Peronospora parasitica through activation of natural defense mechanisms of the plant such as callose deposition, the hypersensitive response, and the formation of trailing necroses. BABA was still fully protective against P. parasitica in transgenic plants or mutants impaired in the salicylic acid, jasmonic acid, and ethylene signaling pathways. Treatment with BABA did not induce the accumulation of mRNA of the systemic acquired resistance (SAR)-associated PR-1 and the ethylene-and jasmonic acid-dependent PDF1.2 genes. However, BABA potentiated the accumulation of PR-1 mRNA after attack by virulent pathogenic bacteria. As a result, BABA-treated Arabidopsis plants were less diseased compared with the untreated control. In the case of bacteria, BABA protected mutants insensitive to jasmonic acid and ethylene but was not active in plants impaired in the SAR transduction pathway. Thus, BABA protects Arabidopsis against different virulent pathogens by potentiating pathogenspecific plant resistance mechanisms. In addition, we provide evidence that BABA-mediated papilla formation after P. parasitica infection is independent of the SAR signaling pathway.

SummaryPlants counteract attack by herbivorous insects using a variety of inducible defence mechanisms. The production of toxic proteins and metabolites that instantly affect the herbivore's development are examples of direct induced defence. In addition, plants may release mixtures of volatile organic compounds (VOCs) that indirectly protect the plant by attracting natural enemies of the herbivore. Recent studies suggest that these VOCs can also prime nearby plants for enhanced induction of defence upon future insect attack. However, evidence that this defence priming causes reduced vulnerability to insects is sparse. Here we present molecular, chemical and behavioural evidence that VOC-induced priming leads to improved direct and indirect resistance in maize. A differential hybridization screen for inducible genes upon attack by Spodoptera littoralis caterpillars identified 10 defence-related genes that are responsive to wounding, jasmonic acid (JA), or caterpillar regurgitant. Exposure to VOCs from caterpillar-infested plants did not activate these genes directly, but primed a subset of them for earlier and/or stronger induction upon subsequent defence elicitation. This priming for defence-related gene expression correlated with reduced caterpillar feeding and development. Furthermore, exposure to caterpillar-induced VOCs primed for enhanced emissions of aromatic and terpenoid compounds. At the peak of this VOC emission, primed plants were significantly more attractive to parasitic Cotesia marginiventris wasps. This study shows that VOC-induced priming targets a specific subset of JAinducible genes, and links these responses at the molecular level to enhanced levels of direct and indirect resistance against insect attack.

Plants treated with the nonprotein amino acid β-aminobutyric acid (BABA) develop an enhanced capacity to resist biotic and abiotic stresses. This BABA-induced resistance (BABA-IR) is associated with an augmented capacity to express basal defense responses, a phenomenon known as priming. Based on the observation that high amounts of BABA induce sterility in Arabidopsis thaliana, a mutagenesis screen was performed to select mutants impaired in BABA-induced sterility (ibs). Here, we report the isolation and subsequent characterization of three T-DNA–tagged ibs mutants. Mutant ibs1 is affected in a cyclin-dependent kinase–like protein, and ibs2 is defective in AtSAC1b encoding a polyphosphoinositide phosphatase. Mutant ibs3 is affected in the regulation of the ABA1 gene encoding the abscisic acid (ABA) biosynthetic enzyme zeaxanthin epoxidase. To elucidate the function of the three IBS genes in plant resistance, the mutants were tested for BABA-IR against the bacterium Pseudomonas syringae pv tomato, the oomycete Hyaloperonospora parasitica, and BABA-induced tolerance to salt. All three ibs mutants were compromised in BABA-IR against H. parasitica, although to a different extent. Whereas ibs1 was reduced in priming for salicylate (SA)-dependent trailing necrosis, mutants ibs2 and ibs3 were affected in the priming for callose deposition. Only ibs1 failed to express BABA-IR against P. syringae, which coincided with a defect in priming for SA-inducible PR-1 gene expression. By contrast, ibs2 and ibs3 showed reduced BABA-induced tolerance to salt, which correlated with an affected priming for ABA-inducible gene expression. For all three ibs alleles, the defects in BABA-induced sterility and BABA-induced protection against P. syringae, H. parasitica, and salt could be confirmed in independent mutants. The data presented here introduce three novel regulatory genes involved in priming for different defense responses.

Drought and salt stress tolerance of Arabidopsis (Arabidopsis thaliana) plants increased following treatment with the nonprotein amino acid beta-aminobutyric acid (BABA), known as an inducer of resistance against infection of plants by numerous pathogens. BABA-pretreated plants showed earlier and higher expression of the salicylic acid-dependent PR-1 and PR-5 and the abscisic acid (ABA)-dependent RAB-18 and RD-29A genes following salt and drought stress. However, non-expressor of pathogenesis-related genes 1 and constitutive expressor of pathogenesis-related genes 1 mutants as well as transgenic NahG plants, all affected in the salicylic acid signal transduction pathway, still showed increased salt and drought tolerance after BABA treatment. On the contrary, the ABA deficient 1 and ABA insensitive 4 mutants, both impaired in the ABA-signaling pathway, could not be protected by BABA application. Our data demonstrate that BABA-induced water stress tolerance is based on enhanced ABA accumulation resulting in accelerated stress gene expression and stomatal closure. Here, we show a possibility to increase plant tolerance for these abiotic stresses through effective priming of the preexisting defense pathways without resorting to genetic alterations.

SummaryWe have examined the role of the callose synthase PMR4 in basal resistance and b-aminobutyric acid-induced resistance (BABA-IR) of Arabidopsis thaliana against the hemi-biotrophic pathogen Pseudomonas syringae and the necrotrophic pathogen Alternaria brassicicola. Compared to wild-type plants, the pmr4-1 mutant displayed enhanced basal resistance against P. syringae, which correlated with constitutive expression of the PR-1 gene. Treating the pmr4-1 mutant with BABA boosted the already elevated levels of PR-1 gene expression, and further increased the level of resistance. Hence, BABA-IR against P. syringae does not require PMR4-derived callose. Conversely, pmr4-1 plants showed enhanced susceptibility to A. brassicicola, and failed to show BABA-IR. Wild-type plants showing BABA-IR against A. brassicicola produced increased levels of JA. The pmr4-1 mutant produced less JA upon A. brassicicola infection than the wild-type. Blocking SA accumulation in pmr4-1 restored basal resistance, but not BABA-IR against A. brassicicola. This suggests that the mutant's enhanced susceptibility to A. brassicicola is caused by SA-mediated suppression of JA, whereas the lack of BABA-IR is caused by its inability to produce callose. A. brassicicola infection suppressed ABA accumulation. Pre-treatment with BABA antagonized this ABA accumulation, and concurrently potentiated expression of the ABA-responsive ABI1 gene. Hence, BABA prevents pathogen-induced suppression of ABA accumulation, and sensitizes the tissue to ABA, causing augmented deposition of PMR4-derived callose.

beta-Aminobutyric acid (BABA) was used to induce resistance in grapevine (Vitis vinifera) against downy mildew (Plasmopara viticola). This led to a strong reduction of mycelial growth and sporulation in the susceptible cv. Chasselas. Comparing different inducers, the best protection was achieved with BABA followed by jasmonic acid (JA), whereas benzo (1,2,3)-thiadiazole-7-carbothionic acid-S-methyl ester (a salicylic acid [SA] analog) and abscisic acid (ABA) treatment did not increase the resistance significantly. Marker genes for the SA and JA pathways showed potentiated expression patterns in BABA-treated plants following infection. The callose synthesis inhibitor 2-deoxy-D-glucose partially suppressed BABA- and JA-induced resistance against P viticola in Chasselas. Application of the phenylalanine ammonia lyase inhibitor 2-aminoindan-2-phosphonic acid and the lipoxygenase (LOX) inhibitor 5, 8, 11, 14-eicosatetraynoic acid (ETYA) also led to a reduction of BABA-induced resistance (BABA-IR), suggesting that callose deposition as well as defense mechanisms depending on phenylpropanoids and the JA pathways all contribute to BABA-IR. The similar phenotype of BABA- and JA-induced resistance, the potentiated expression pattern of JA-regulated genes (LOX-9 and PR-4) following BABA treatment, and the suppression of BABA-IR with ETYA suggest an involvement of the JA pathway in BABA-IR of grapevine leading to a primed deposition of callose and lignin around the infection sites.

Protein kinase C (PKC) is a multigene family of serine/threonine kinases that are central to many signal transduction pathways. Among the PKC isozymes, only PKCe has been reported to exhibit full oncogenic potential. PKCe also displays unique substrate specificity and intracellular localization. To examine the interrelationship between the biological effects and domain structure of PKCe, NIH 3T3 cells were stably transfected to overexpress different epitopetagged fragments of PKCe. The overexpressed proteins each contain the E-tag peptide at the C terminus to allow ready detection with an antibody specific for the tag. The holo-PKCe was found to localize with the Golgi network and other compartments, whereas the zinc-finger domain localized exclusively at the Golgi. Golgi-specific glycosaminoglycan sulfation was strongly inhibited in cells overexpressing either holo-PKCE or its zinc-finger domain, while the secretion of sulfated glycosaminoglycans into the medium was impaired in cells expressing the PKCE zinc-finger domain. Thus, these results suggest that PKCE may be involved in specifically regulating Golgi-related processes. Further, the results indicate that PKCE domains other than the kinase domain may also have biological activity and that the zinc-finger domain may function as a subcellular localization signal.Protein kinase C (PKC) consists of a family of more than 10 closely related phospholipid-dependent protein phosphotransferase isozymes (1, 2). The various PKC isozymes show considerable diversity in their domain structure, regulatory properties, and biological effects (1, 2). Although overexpression of most of the PKC isozymes has some effect on the morphology and growth characteristics of cells, only PKCs has been reported to exhibit full oncogenic potential (3, 4). PKCs also has been implicated in regulating other biological processes, such as antiviral resistance (5), neuropeptide signal transduction (6), and transporter regulation (7). PKCS has unique substrate specificity (8), and a portion of PKCs can always be detected in a membrane-associated state (3, 4, 7). NIH 3T3 cells have been reported to contain PKCs (2, 9). To study the interrelationship of the function, subcellular localization, and domain organization of PKCs, we used NIH 3T3 cell lines overexpressing holo-PKCs and various truncated derivatives of PKCs. For purposes of uniform detection, the C-terminal 12 amino acids of PKCS were added to the C termini of all constructs as an antibody epitope tag. The zinc-finger domain of PKCs was found to contain all the information necessary for exclusive localization to the Golgi. Further, sulfate uptake and Golgi-specific sulfation of glycosaminoglycan (GAG) chains were inhibited in cell lines overexpressing either PKCs or its zinc-finger domain, indicating that PKCs is involved in modulating Golgi function.The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 s...