Application of Plant Defense Elicitors Fails to Enhance Herbivore Resistance or Mitigate Phytoplasma Infection in Cranberries

Synthetic elicitors of the salicylic acid (SA) and jasmonic acid (JA) plant defense pathways can be used to increase crop protection against herbivores and pathogens. In this study, we tested the hypothesis that elicitors of plant defenses interact with pathogen infection to influence crop resistance against vector and nonvector herbivores. To do so, we employed a trophic system comprising of cranberries (Vaccinium macrocarpon), the phytoplasma that causes false blossom disease, and two herbivores—the blunt-nosed leafhopper (Limotettix vaccinii), the vector of false blossom disease, and the nonvector gypsy moth (Lymantria dispar). We tested four commercial elicitors, including three that activate mainly SA-related plant defenses (Actigard, LifeGard, and Regalia) and one activator of JA-related defenses (Blush). A greenhouse experiment in which phytoplasma-infected and uninfected plants received repeated exposure to elicitors revealed that both phytoplasma infection and elicitor treatment individually improved L. vaccinii and L. dispar mass compared to uninfected, untreated controls; however, SA-based elicitor treatments reduced L. vaccinii mass on infected plants. Regalia also improved L. vaccinii survival. Phytoplasma infection reduced plant size and mass, increased levels of nitrogen (N) and SA, and lowered carbon/nitrogen (C/N) ratios compared to uninfected plants, irrespective of elicitor treatment. Although none of our elicitor treatments influenced transcript levels of a phytoplasma-specific marker gene, all of them increased N and reduced C/N levels; the three SA activators also reduced JA levels. Taken together, our findings reveal positive effects of both phytoplasma infection and elicitor treatment on the performance of L. vaccinii and L. dispar in cranberries, likely via enhancement of plant nutrition and changes in phytohormone profiles, specifically increases in SA levels and corresponding decreases in levels of JA. Thus, we found no evidence that the tested elicitors of plant defenses increase resistance to insect herbivores or reduce disease incidence in cranberries.

Elicitor Application in Strawberry Results in Long-Term Increase of Plant Resilience Without Yield Loss

For a first step integrating elicitor applications into the current IPM strategy increasing plant resilience against pests, we investigated repeated elicitor treatments in a strawberry everbearer nursery and cropping cycle under glass. During nursery methyl-jasmonate (MeJA), testing induction of defenses with plant bioassays was applied every 3 weeks. Thrips damage and reproduction by spider mites, whitefly and aphids were strongly reduced upon elicitor treatment. Subsequently, we applied MeJA every 3 weeks or based on scouting pests during a whole cropping cycle. Thrips leaf bioassays and LC-MS leaf metabolomics were applied to investigate the induction of defenses. Leaf damage by thrips was lower for both MeJA application schemes compared to the control except for the last weeks. While elicitor treatments after scouting also reduced damage, its effect did not last. Thrips damage decreased from vegetative to mature plants during the cropping cycle. At the end of the nursery phase, plants in the elicitor treatment were smaller. Surprisingly, growth during production was not affected by MeJA application, as were fruit yield and quality. LC-MS leaf metabolomics showed strong induction of vegetative plants decreasing during the maturation of plants toward the end of cultivation. Concurrently, no increase in the JA-inducible marker PPO was observed when measured toward the end of cultivation. Mostly flavonoid and phenolic glycosides known as plant defense compounds were induced upon MeJA application. While induced defense decreased with the maturation of plants, constitutive defense increased as measured in the leaf metabolome of control plants. Our data propose that young, relatively small plant stages lack constitutive defense necessitating an active JA defense response. As plants, mature constitutive defense metabolites seem to accumulate, providing a higher level of basal resistance. Our results have important implications for but are not limited to strawberry cultivation. We demonstrated that repeated elicitor application could be deployed as part of an integrated approach for sustainable crop protection by vertical integration with other management tactics and horizontal integration to control multiple pests concurrently. This approach forms a promising potential for long-term crop protection in greenhouses.

SUMO enables substrate selectivity by mitogen-activated protein kinases to regulate immunity in plants

The versatility of mitogen-activated protein kinases (MAPKs) in translating exogenous and endogenous stimuli into appropriate cellular responses depends on its substrate specificity. In animals, several mechanisms have been proposed about how MAPKs maintain specificity to regulate distinct functional pathways. However, little is known of mechanisms that enable substrate selectivity in plant MAPKs. Small ubiquitin-like modifier (SUMO), a posttranslational modification system, plays an important role in plant development and defense by rapid reprogramming of cellular events. In this study we identified a functional SUMO interaction motif (SIM) in Arabidopsis MPK3 and MPK6 that reveals a mechanism for selective interaction of MPK3/6 with SUMO-conjugated WRKY33, during defense. We show that WRKY33 is rapidly SUMOylated in response to Botrytis cinerea infection and flg22 elicitor treatment. SUMOylation mediates WRKY33 phosphorylation by MPKs and consequent transcription factor activity. Disruption of either WRKY33 SUMO or MPK3/6 SIM sites attenuates their interaction and inactivates WRKY33-mediated defense. However, MPK3/6 SIM mutants show normal interaction with a non-SUMOylated form of another transcription factor, SPEECHLESS, unraveling a role for SUMOylation in differential substrate selectivity by MPKs. We reveal that the SUMO proteases, SUMO PROTEASE RELATED TO FERTILITY1 (SPF1) and SPF2 control WRKY33 SUMOylation and demonstrate a role for these SUMO proteases in defense. Our data reveal a mechanism by which MPK3/6 prioritize molecular pathways by differentially selecting substrates using the SUMO–SIM module during defense responses.

Responses of inflammation signaling pathway by saucerneol D from elicitor-treated Saururus chinensis on pro-inflammatory responses in LPS-stimulated Raw 264.7 cell

This study confirmed the association with inflammation-related proteins, mediators, and cytokines using saucerneol D from Saururus chinensis leaf, a useful ingredient increased through elicitor treatment. To confirm the anti-inflammatory effect, saucerneol D were treated with lipopolysaccharide, which induces pro-inflammatory factors in Raw 264.7 cell. The pro-inflammatory influences were measured by dint of chemical assay and western blotting as well as ELISA. As a result, the content of saucerneol D was changed when eicitor was treated by various concentration (1.5, and 3 mg/mL) in S. chinensis leaves. In addition, the expression levels of hyaluronidase and pro-inflammatory-related factors [nitric oxide (NO), inducible NO synthase (iNOS), and cyclooxygenase-2 (COX-2)] were regulated according to the saucerneol D content in the elicitor-treated and non-treated groups. Therefore, after confirming that saucerneol D has an inhibitory effect on pro-inflammatory-related factors, saucerneol D was adjusted by concentration and compared with the control substance to verify the efficacy. Saucerneol D was adjusted to a concentration that did not toxic to macrophages through 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Saucerneol D controlled at various concentrations inhibited iNOS and COX-2 proteins. NO produced by iNOS activity, prostaglandin E2 (PGE2), an inflammatory mediator produced by COX-2 activity, and pro-inflammatory cytokines [interleukin-1β (IL-1β), IL-6, tumor necrosis factor-α (TNF-α)] were significantly suppressed. Therefore, it was confirmed that saucerneol D, an active ingredient increased by the elicitor treatment, could be used as a functional material that controls inflammatory factors.

Improvement of Phenylpropanoid Production with Elicitor Treatments in Pimpinella brachycarpa Nakai

Pimpinella brachycarpa Nakai, known as cham-na-mul in Korea, is a popular edible herb and vegetable. Phenolic compounds are recognized as a vital group of plant secondary metabolites that provide innumerable, valuable therapeutic properties. Elicitors are biofactors or chemicals from diverse sources that can trigger morphological and physiological responses in the target organism. This study examined the effect of methyl jasmonate (MeJA), salicylic acid (SA), and chitosan treatment on the accretion of phenolic compounds in P. brachycarpa Nakai. This plant was harvested under different concentration of elicitor treatment for time course. Eight phenolic compounds including were detected in response to elicitor using HPLC. While the untreated controls showed the lowest phenolic content, treatment with 0.3% chitosan, 0.1 mM SA, and 0.1 mM MeJA resulted in 1.43-, 1.39-, and 1.35-fold increase in the phenolic content, respectively. The highest content of most of the individual phenolic compounds followed a similar trend according to treatment type, with chitosan treatment showing the highest content, followed by SA and then MeJA treatments. Thus, we demonstrate that the treatment with optimal concentrations of these elicitors for an optimal period of time increases the production of phenolic compounds in P. brachycarpa Nakai.

Effects of spermine and putrescine polyamines on capsaicin accumulation in Capsicum annuum L. cell suspension cultures

This study examined the effects of different concentrations of spermine (Spm) and putrescine (Put) elicitors on capsaicin production at different times in cell suspension culture of peper (<em>Capsicum annuum </em>L‘Kahramanmaraş Hat-187’<em>.</em>), raised from pepper seeds. Callus was obtained from hypocotyl explants of pepper seedlings germinated <em>in vitro</em> conditions, and cell suspensions were prepared from calluses. Spm (0.1, 0.2 and 0.4 mg l^-1) and Put (0.1, 0.2 and 0.4 mg l^-1) elicitors were applied on cell suspensions, and control groups free from elicitor treatment were created. The amount of capsaicin in cells was found to be higher in the control groups and samples treated with Spm elicitors when compared to filtrates. The highest increase in the capsaicin amount in cells was determined on day 12 of elicitation with 0.2 mg l^-1Spm application. The highest capsaicin amount passing into the filtrate was determined as 0.1 mg l^-1Spm on day 8. The most effective Put concentration and time on capsaicin amount were found as 0.2 mg l^-1Put on day 12 in both cells and filtrates. The highest total capsaicin was also determined in the 0.2 mg l^-1Spm application on day 12with 312.747 ± 8.70 µg g^-1 of culture. Exogenous treatment of Spm and Put elicitors affected capsaicin accumulation.

Production of Prenylated Stilbenoids in Hairy Root Cultures of Peanut (Arachis hypogaea) and Its Wild Relatives A. ipaensis and A. duranensis via an Optimized Elicitation Procedure

Prenylated stilbenoids are phenolic compounds produced in a small number of plants such as peanut (Arachis hypogaea) to counteract biotic and abiotic stresses. In addition to their role in plant defense, they exhibit biological activities with potential application in human health. Whereas non-prenylated stilbenoids such as resveratrol are commercially available, the availability of prenylated stilbenoids is limited. To this end, hairy root cultures of peanut were developed as an elicitor-controlled bioproduction platform for prenylated stilbenoids. An orthogonal array design approach led to the elucidation of an optimized elicitation procedure consisting of co-treatment of the hairy root cultures with 18 g/L methyl-β-cyclodextrin, 125 µM methyl jasmonate, 3 mM hydrogen peroxide (H2O2) and medium supplementation with additional 1 mM magnesium chloride. After 168-h of elicitor treatment, the combined yield of the prenylated stilbenoids arachidin-1, arachidin-2, arachidin-3 and arachidin-5 reached approximately 750 mg/L (equivalent to 107 mg/g DW). Moreover, hairy root cultures from the wild Arachis species A. duranensis and A. ipaensis were developed and shown to produce prenylated stilbenoids upon elicitor treatment. These wild Arachis hairy root lines may provide a platform to elucidate the biosynthetic origin of prenylated stilbenoids in peanut.