Priming of Immune System in Tomato by Treatment with Low Concentration of L-Methionine.

Various metabolites, including phytohormones, phytoalexins, and amino acids, take part in the plant immune system. Herein, we analyzed the effects of L-methionine (Met), a sulfur-containing amino acid, on the plant immune system in tomato. Treatment with low concentrations of Met enhanced the resistance of tomato to a broad range of diseases caused by the hemi-biotrophic bacterial pathogen Pseudomonas syringae pv. tomato (Pst) and the necrotrophic fungal pathogen Botrytis cinerea (Bc), although it did not induce the production of any antimicrobial substances against these pathogens in tomato leaf tissues. Analyses of gene expression and phytohormone accumulation indicated that Met treatment alone did not activate the defense signals mediated by salicylic acid, jasmonic acid, and ethylene. However, the salicylic acid-responsive defense gene and the jasmonic acid-responsive gene were induced more rapidly in Met-treated plants after infection with Pst and Bc, respectively. These findings suggest that low concentrations of Met have a priming effect on the phytohormone-mediated immune system in tomato.

Characterization of Disease Resistance Induced by a Pyrazolecarboxylic Acid Derivative in Arabidopsis thaliana.

Systemic acquired resistance (SAR) is a potent innate immunity system in plants that is induced through the salicylic acid (SA)-mediated signaling pathway. Here, we characterized 3-chloro-1-methyl-1H-pyrazole-5-carboxylic acid (CMPA) as an effective SAR inducer in Arabidopsis. The soil drench application of CMPA enhanced a broad range of disease resistance against the bacterial pathogen Pseudomonas syringae and fungal pathogens Colletotrichum higginsianum and Botrytis cinerea in Arabidopsis, whereas CMPA did not show antibacterial activity. Foliar spraying with CMPA induced the expression of SA-responsible genes such as PR1, PR2 and PR5. The effects of CMPA on resistance against the bacterial pathogen and the expression of PR genes were observed in the SA biosynthesis mutant, however, while they were not observed in the SA-receptor-deficient npr1 mutant. Thus, these findings indicate that CMPA induces SAR by triggering the downstream signaling of SA biosynthesis in the SA-mediated signaling pathway.

Strigolactones Modulate Salicylic Acid-Mediated Disease Resistance in Arabidopsis thaliana.

Strigolactones are low-molecular-weight phytohormones that play several roles in plants, such as regulation of shoot branching and interactions with arbuscular mycorrhizal fungi and parasitic weeds. Recently, strigolactones have been shown to be involved in plant responses to abiotic and biotic stress conditions. Herein, we analyzed the effects of strigolactones on systemic acquired resistance induced through salicylic acid-mediated signaling. We observed that the systemic acquired resistance inducer enhanced disease resistance in strigolactone-signaling and biosynthesis-deficient mutants. However, the amount of endogenous salicylic acid and the expression levels of salicylic acid-responsive genes were lower in strigolactone signaling-deficient max2 mutants than in wildtype plants. In both the wildtype and strigolactone biosynthesis-deficient mutants, the strigolactone analog GR24 enhanced disease resistance, whereas treatment with a strigolactone biosynthesis inhibitor suppressed disease resistance in the wildtype. Before inoculation of wildtype plants with pathogenic bacteria, treatment with GR24 did not induce defense-related genes; however, salicylic acid-responsive defense genes were rapidly induced after pathogenic infection. These findings suggest that strigolactones have a priming effect on Arabidopsis thaliana by inducing salicylic acid-mediated disease resistance.

Response of tomatoes primed by mycorrhizal colonization to virulent and avirulent bacterial pathogens.

Most plants interact with arbuscular mycorrhizal fungi, which enhance disease resistance in the host plant. Because the effects of resistance against bacterial pathogens are poorly understood, we investigated the effects of mycorrhizal colonization on virulent and avirulent pathogens using phytopathological and molecular biology techniques. Tomato plants colonized by Gigaspora margarita acquired resistance not only against the fungal pathogen, Botrytis cinerea, but also against a virulent bacterial pathogen, Pseudomonas syringae pv. tomato DC3000 (Pst). In G. margarita-colonized tomato, salicylic acid (SA)- and jasmonic acid (JA)-related defense genes were expressed more rapidly and strongly compared to those in the control plants when challenged by Pst, indicating that the plant immunity system was primed by mycorrhizal colonization. Gene expression analysis indicated that primed tomato plants responded to the avirulent pathogen, Pseudomonas syringae pv. oryzae, more rapidly and strongly compared to the control plant, where the effect on the JA-mediated signals was stronger than in the case with Pst. We found that the resistance induced by mycorrhizal colonization was effective against both fungal and bacterial pathogens including virulent and avirulent pathogens. Moreover, the activation of both SA- and JA-mediated signaling pathways can be enhanced in the primed plant by mycorrhizal colonization.

OsGGC2, Gγ Subunit of Heterotrimeric G Protein, Regulates Plant Height by Functionally Overlapping with DEP1 in Rice.

Plant heterotrimeric G proteins have been shown to regulate the size of various organs. There are three types of Gγ subunits in plants: type A, consisting of a canonical Gγ domain; type B, possessing a plant-specific domain at the N-terminus of the Gγ domain; and type C, possessing a plant-specific domain at the C-terminal of the Gγ domain. There is one type A, one type B, and three type C of the five γ-subunits in the rice genome. In type C Gγ subunits, GS3, which controls grain size; DEP1, which controls plant height and panicle branching; and their homolog OsGGC2, which affects grain size, have been reported; however, the function of each gene, their interactions, and molecular mechanisms for the control of plant height have not yet been clarified. In this study, we generated loss-of-function mutants of DEP1 and OsGGC2, which have high homology and similar expression, and investigated their phenotypes. Since both dep1 and osggc2 mutants were dwarfed and the double mutants showed a synergistic phenotype, we concluded that both DEP1 and OsGGC2 are positive regulators of plant height and that their functions are redundant.

Induction of tocopherol biosynthesis through heat shock treatment in Arabidopsis.

Plants have developed various self-defense systems to survive many types of unfavorable conditions. Heat shock (HS) treatment, an abiotic stress, activates salicylic acid (SA) biosynthesis to enhance resistance to biotic stresses in some plant species. Since SA is produced from the shikimate pathway, other related metabolic pathways were expected to be upregulated by HS treatment. We speculated that tocopherol biosynthesis utilizing chorismic acid would be activated by HS treatment. In Arabidopsis, expression analysis of tocopherol biosynthetic genes, HPPD, VTE2, VTE3, VTE1, and VTE4, in combination with measurement of metabolites, indicated that HS treatment enhanced the biosynthesis and accumulation of tocopherols. Analyses using an SA biosynthesis-deficient mutant indicated that the upregulation of tocopherol biosynthesis was independent of the SA-mediated signaling pathway.

Characterization of plant immunity-activating mechanism by a pyrazole derivative.

A newly identified chemical, 4-{3-[(3,5-dichloro-2-hydroxybenzylidene)amino]propyl}-4,5-dihydro-1H-pyrazol-5-one (BAPP) was characterized as a plant immunity activator. BAPP enhanced disease resistance in rice against rice blast disease and expression of a defense-related gene without growth inhibition. Moreover, BAPP was able to enhance disease resistance in dicotyledonous tomato and Arabidopsis plants against bacterial pathogen without growth inhibition, suggesting that BAPP could be a candidate as an effective plant activator. Analysis using Arabidopsis sid2-1 and npr1-2 mutants suggested that BAPP induced systemic acquired resistance (SAR) by stimulating between salicylic acid biosynthesis and NPR1, the SA receptor protein, in the SAR signaling pathway.

Activation of cell proliferation in Arabidopsis root meristem by isoprothiolane.

A plant growth regulating agent "Fuji-one" has been used to control non-parasitic damping-off (Murenae disease) of rice seedlings. Its active ingredient, isoprothiolane (diisopropyl 1,3-dithiolan-2-ylidenemalonate, IPT), enhances root elongation of rice and Arabidopsis. To understand the mechanisms of IPT's effect on root development, its effect on Arabidopsis root cells was investigated histologically. IPT at a lower concentration (12.5 µg/mL) had no effect on root cell elongation, whereas it enhanced cell division in the root meristem. Histological analysis using phytohormone-related mutants indicated that jasmonic acid and ethylene were involved in the enhanced cell division. In contrast, IPT at a higher concentration (75 µg/mL) suppressed both cell elongation and cell division, in which jasmonic acid and ethylene were not involved. In addition, root hair formation was suppressed by treatment with IPT. These analyses demonstrated that IPT (12.5 µg/mL) enhanced root elongation by activating cell division in a jasmonic acid- and ethylene-dependent manner.

Involvement of phytohormones in root elongation activity of isoprothiolane in Arabidopsis.

Isoprothiolane (diisopropyl 1,3-dithiolan-2-ylidenemalonate, IPT), an active ingredient of "Fuji-one," has been used as a plant growth regulating agent to control non-parasitic damping-off (MURENAE disease) of rice seedlings. To understant plant growth regulating activity of IPT, its effect on root development was investigated in Arabidopsis. IPT enhanced root elongation at a lower concentration (12.5 µg/mL) but suppressed it at a higher concentration (75 µg/mL). Analysis using phytohormone-related mutants and chemical inhibitors revealed that the enhancement of root elongation by IPT required auxin, jasmonic acid, and ethylene signal transduction. Activation of the signal transduction mediated by these three phytohormones was confirmed by gene expression analysis. More detailed mechanisms of IPT's effect on root development were demonstrated via investigation using Arabidopsis and chemical inhibitors.

Involvement of ethylene signaling in Azospirillum sp. B510-induced disease resistance in rice.

A bacterial endophyte Azospirillum sp. B510 induces systemic disease resistance in the host without accompanying defense-related gene expression. To elucidate molecular mechanism of this induced systemic resistance (ISR), involvement of ethylene (ET) was examined using OsEIN2-knockdown mutant rice. Rice blast inoculation assay and gene expression analysis indicated that ET signaling is required for endophyte-mediated ISR in rice. ABBREVIATIONS: ACC: 1-aminocyclopropane-1-carboxylic acid; EIN2: ethylene-insensitive protein 2; ET: ethylene; ISR: induced systemic resistance; JA: jasmonic acid; RNAi: RNA interference; SA: salicylic acid; SAR: systemic acquired resistance.

Abscisic acid modulates salicylic acid biosynthesis for systemic acquired resistance in tomato.

Among the regulatory mechanisms of systemic acquired resistance (SAR) in tomato, antagonistic interaction between salicylic acid (SA) and abscisic acid (ABA) signaling pathways was investigated. Treatment with 1,2-benzisothiazol-3(2H)-one1,1-dioxide (BIT) induced SAR in tomato thorough SA biosynthesis. Pretreatment of ABA suppressed BIT-induced SAR including SA accumulation, suggesting that ABA suppressed SAR by inhibiting SA biosynthesis.

Effects of colonization of a bacterial endophyte, Azospirillum sp. B510, on disease resistance in tomato.

A plant growth-promoting bacteria, Azospirillum sp. B510, isolated from rice, can enhance growth and yield and induce disease resistance against various types of diseases in rice. Because little is known about the interaction between other plant species and this strain, we have investigated the effect of its colonization on disease resistance in tomato plants. Treatment with this strain by soil-drenching method established endophytic colonization in root tissues in tomato plant. The endophytic colonization with this strain-induced disease resistance in tomato plant against bacterial leaf spot caused by Pseudomonas syringae pv. tomato and gray mold caused by Botrytis cinerea. In Azospirillum-treated plants, neither the accumulation of SA nor the expression of defense-related genes was observed. These indicate that endophytic colonization with Azospirillum sp. B510 is able to activate the innate immune system also in tomato, which does not seem to be systemic acquired resistance.