Constitutive activation of the jasmonate signaling pathway enhances the production of secondary metabolites in tomato

Priming of antimicrobial phenolics during induced resistance response towards Pectobacterium carotovorum in the ornamental monocot calla lily

Calla lilies are herbaceous monocotyledonous plants that are highly sensitive to Pectobacterium carotovorum, the causal agent of soft-rot disease. Results demonstrate that, in response to elicitation using plant defense activators, the calla lily produces elevated levels of antimicrobial phenolics and that these compounds contribute to increased resistance against P. carotovorum, as shown by reduced bacterial proliferation in elicited leaves. The polyphenolic nature of the induced compounds was supported by autofluorescence, absorbance spectra, and reaction with Folin-Ciocalteu reagent. Two plant defense activators, Bion and methyl jasmonate, differed in both their capacity to induce accumulation of polyphenols and their resistance against the pathogen. Methyl jasmonate elicitation brought about higher accumulation of free phenolics relative to Bion, suggesting priming of bioactive polyphenols as a principal factor in the calla lily defense against P. carotovorum. To further characterize the nature of induced compounds, two major compounds were collected and identified as swertisin and isovitexin by mass and nuclear magnetic resonance spectroscopies.

Jasmonates and its mimics differentially elicit systemic defence responses in Nicotiana attenuata

Coronalon (6-ethyl indanoyl isoleucine), a synthetic jasmonate mimic, is known to regulate levels of transcripts and secondary metabolites that are commonly elicited by methyl jasmonate (MeJA) in a variety of plants. The ability of coronalon and its derivative (In-L-Ile-Me) to elicit MeJA-activated transcriptional and defence responses [nicotine and trypsin proteinase inhibitors (TPIs)] was compared in treated and systemic untreated tissues of wild-type (WT) and NaLOX3-silenced Nicotiana attenuata plants which are unable to activate either local or systemic defence responses. Coronalon and its derivative significantly regulated 71% and 86% of genes up-regulated by MeJA and 53% and 66% of the genes down-regulated by MeJA in the treated leaves, but only 3% and 7% of all regulated genes in untreated, but phylotactically connected, leaves of WT plants. Consistent with their ability to elicit transcriptional responses in treated tissues, coronalon and In-L-Ile-Me increased nicotine and TPIs when applied to the tissues in which these metabolites are produced, namely roots and leaves. However, treating roots elicited TPI activity in leaves in both WT and NaLOX3-silenced plants, suggesting that mimics can be transported apoplastically from roots to leaves in the xylem. This response was lower in NaLOX3-silenced plants, suggesting that the ability of coronalon and In-L-Ile-Me to elicit TPI responses in leaves after root treatments requires intact jasmonic acid (JA) signalling. Treating leaves did not elicit detectable changes in endogenous JA levels but did decrease free salicylic acid contents. It is concluded that coronalon and In-L-Ile-Me elicit jasmonate responses in treated tissues and could be valuable tools for dissecting local and systemic jasmonate signalling networks in plants.

Tomato MAPKs LeMPK1, LeMPK2, and LeMPK3 function in the systemin-mediated defense response against herbivorous insects

Systemin is a wound-signaling peptide that mediates defenses of tomato plants against herbivorous insects. Perception of systemin by the membrane-bound receptor SR160 results in activation of MAPKs, synthesis of jasmonic acid (JA), and expression of defense genes. To test the function of MAPKs in the response to systemin, we used virus-induced gene silencing (VIGS) in plants that overexpress the systemin precursor prosystemin (35S::prosys plants). These transgenic plants accumulate high levels of defense proteins and exhibit increased resistance to herbivorous insects. Cosilencing of the MAPKs MPK1 and MPK2 reduced MPK1/2 kinase activity, JA biosynthesis, and expression of JA-dependent defense genes. Application of methyl-JA restored the full defense response. These data show that MPK1 and MPK2 are essential components of the systemin signaling pathway and most likely function upstream of JA biosynthesis. MPK1 and MPK2 are 95% identical at the amino acid level. Specific VIGS of only MPK1 or MPK2 resulted in the same reduction of defense gene expression as cosilencing of MPK1 and MPK2, indicating that gene dosage effects may be important for MPK signaling. In addition, VIGS of the closely related MPK3 also reduced systemin-induced defense responses. The function of MPK1/2 and orthologs in pathogen-induced defenses is well established. Here we show that cosilencing of MPK1 and MPK2 compromised prosystemin-mediated resistance to Manduca sexta (Lepidoptera) herbivory, demonstrating that MPK1 and MPK2 are also required for successful defenses against herbivorous insects.

Co(i)-ordinating defenses: NaCOI1 mediates herbivore- induced resistance in Nicotiana attenuata and reveals the role of herbivore movement in avoiding defenses

Arabidopsis and tomato plants mutated in the F-box protein COI1 mediating jasmonate (JA) responses are more susceptible to herbivores in laboratory trials, but the exact mechanisms of COI1-mediated resistance are not known. We silenced COI1 by transformation with an inverted repeat construct (ir-coi1) in Nicotiana attenuata, a plant the direct and indirect defenses of which against various herbivores have been well studied. ir-coi1 plants are male sterile and impaired in JA-elicited direct [nicotine, caffeoylputrescine and trypsin proteinase inhibitor (TPI) activity] and indirect (cis-alpha-bergamotene emission) defense responses; responses not elicited by JA treatment (ethylene production and flower TPI activity) were unaffected. Larvae of Manduca sexta, a common herbivore of N. attenuata, gained three times more mass feeding on ir-coi1 than on wild-type (WT) plants in glasshouse experiments. By regularly moving caterpillars to unattacked leaves of the same plant, we demonstrate that larvae on WT plants can grow and consume leaves as fast as those on ir-coi1 plants, a result that underscores the role of COI1 in mediating locally induced resistance in attacked leaves, and the importance of herbivore movement in avoiding the induced defenses of a plant. When transplanted into native habitats in the Great Basin Desert, ir-coi1 plants suffer greatly from damage by the local herbivore community, which includes herbivores not commonly found on N. attenuata WT plants. Choice assays with field-grown plants confirmed the increased attractiveness of ir-coi1 plants for both common and unusual herbivores. We conclude that NaCOI1 is essential for induced resistance in N. attenuata, and that ir-coi1 plants highlight the benefits of herbivore movement for avoiding induced defenses.

Systemin in Solanum nigrum. The tomato-homologous polypeptide does not mediate direct defense responses

We extend Ryan's seminal work on the 18-amino acid polypeptide systemin in tomato's (Solanum lycopersicum) systemic wound response to the closely related solanaceous species Solanum nigrum. We compared wild-type plants to plants transformed with an inverted repeat prosystemin construct (IRSys) to silence the expression of the endogenous S. nigrum prosystemin gene. In wild-type plants elicited with wounding + oral secretions from Manduca sexta larvae, trypsin-proteinase inhibitors (TPIs) accumulated even though prosystemin transcripts were down-regulated. Neither reducing the endogenous systemin levels by RNAi nor complementing the plants with systemin by exogenously supplying the polypeptide through excised stems significantly increased TPI activity, indicating that systemin and TPIs are not correlated in S. nigrum. The performance of two herbivore species from two feeding guilds, M. sexta larvae and Myzus persicae nicotianae, did not differ between wild-type and IRSys plants, demonstrating that varying endogenous systemin levels do not alter the direct defenses of S. nigrum. Field experiments with wild-type and IRSys plants and the flea beetle Epitrix pubescens supported these glasshouse data. That levels of oral secretion-elicited jasmonic acid did not differ between wild-type and IRSys plants suggests that systemin is unlikely to mediate jasmonate signaling in S. nigrum as it does in tomato. We conclude that the tomato-homologous polypeptide does not mediate direct defense responses in S. nigrum.