Constitutive exposure to the volatile methyl salicylate reduces per‐capita foraging efficiency of a generalist predator to learned prey associations

Understanding the factors that influence the ability of predators to find and kill herbivores is central to enhancing their impact on prey populations, but few studies have tested the impact of these factors on predation rates in realistic foraging environments. Using the tri‐trophic system consisting of tomato, Solanum lycopersicum L. (Solanaceae), hornworm caterpillars, Manduca sexta L. (Lepidoptera: Sphingidae), and the predaceous stink bug Podisus maculiventris (Say) (Hemiptera: Pentatomidae), we measured the effects of associative learning and plant volatile camouflage on predator behavior and foraging efficiency in field enclosures. To do so, we compared experienced vs. naive individuals under varying environmental contexts. Experienced predators were those with prior exposure to induced volatiles from the tomato–caterpillar association, whereas naive predators had not experienced tomato, only prey (caterpillars). We varied their environmental foraging matrix using either (1) tomato surrounded by basil (Ocimum basilicum L.; Lamiaceae) or (2) tomato exposed to the synthetic volatile, methyl salicylate (MeSA). We found that (1) experienced predators were more efficient than naive predators, capturing 28% more prey; (2) the tomato–basil combination did not affect predator–prey interactions; and (3) constitutive emission of synthetic MeSA caused a 22% reduction in P. maculiventris predation rate. These differences corresponded with distinct shifts in predator foraging; for example, experienced individuals were less stationary and exhibited unique behaviors such as stylet extension. Taken together, these results suggest that it is possible to improve the function of generalist predators in suppressing prey by coupling odors with food. However, constitutive emission of volatiles to attract natural enemies may ultimately camouflage neighboring plants, reducing the benefits of orientation to learned stimuli such as induced volatiles.     

Overexpression of salicylic acid carboxyl methyltransferase reduces salicylic acid-mediated pathogen resistance in Arabidopsis thaliana

We cloned a salicylic acid/benzoic acid carboxyl methyltransferase gene, OsBSMT1, from Oryza sativa. A recombinant OsBSMT1 protein obtained by expressing the gene in Escherichia coli exhibited carboxyl methyltransferase activity in reactions with salicylic acid (SA), benzoic acid (BA), and de-S-methyl benzo(1,2,3)thiadiazole-7-carbothioic acid (dSM-BTH), producing methyl salicylate (MeSA), methyl benzoate (MeBA), and methyl dSM-BTH (MeBTH), respectively. Compared to wild-type plants, transgenic Arabidopsis overexpressing OsBSMT1 accumulated considerably higher levels of MeSA and MeBA, some of which were vaporized into the environment. Upon infection with the bacterial pathogen Pseudomonas syringae or the fungal pathogen Golovinomyces orontii, transgenic plants failed to accumulate SA and its glucoside (SAG), becoming more susceptible to disease than wild-type plants. OsBSMT1-overexpressing Arabidopsis showed little induction of PR-1 when treated with SA or G. orontii. Notably, incubation with the transgenic plant was sufficient to trigger PR-1 induction in neighboring wild-type plants. Together, our results indicate that in the absence of SA, MeSA alone cannot induce a defense response, yet it serves as an airborne signal for plant-to-plant communication. We also found that jasmonic acid (JA) induced AtBSMT1, which may contribute to an antagonistic effect on SA signaling pathways by depleting the SA pool in plants. Electronic supplementary material The online version of this article (doi: ) contains supplementary material, which is available to authorized users.     

Overexpression of AtSGT1, an Arabidopsis salicylic acid glucosyltransferase, leads to increased susceptibility to Pseudomonas syringae

We reported previously that a recombinant salicylic acid (SA) glucosyltransferase1 (AtSGT1) from Arabidopsis thaliana catalyzes the formation of both SA 2-O-beta-D-glucoside (SAG) and the glucose ester of SA (SGE). Here, transgenic Arabidopsis plants overexpressing AtSGT1 have been constructed, and their phenotypes analyzed. Compared to wild-type plants, transgenic plants showed an increased susceptibility to Pseudomonas syringae and reduced the accumulation levels of both free SA and its glucosylated forms (SAG and SGE). On the other hand, the overexpression increased the levels of methyl salicylate (MeSA) and methyl salicylate 2-O-beta-D-glucoside (MeSAG), and also induced SA carboxyl methyltransferase1 (AtBSMT1) expression, whose products catalyze the conversion of SA to MeSA. Our data indicate that reduced resistance by AtSGT1 overexpression results from a reduction in SA content, which is at least in part caused by increases in MeSAG and MeSA levels at the expense of SA. Our study also suggests that genetic manipulation of AtSGT1 can be utilized as an important regulatory tool for pathogen control.     

Heavy metal stress-inducible early light-inducible gene CaELIP from hot pepper (Capsicum annuum) shows broad expression patterns under various abiotic stresses and circadian rhythmicity

An early light-inducible protein gene (CaELIP) was isolated from a cDNA library of hot pepper (Capsicum annuum) that showed heavy metal stress-inducible expressions. This gene contains an open reading frame (ORF) encoding a protein of 160 amino acids, and the protein has significant homology with reported early light-inducible proteins from other plant species. Topology analysis for CaELIP suggested three transmembrane domains. Genomic DNA blot analysis showed that CaELIP is a single copy gene in hot pepper. The treatment of seedling roots of hot pepper with Cu induced ROS generation in the root, and the level of ROS generation was paralleled to the concentration of Cu that again was matched to the increase in the CaELIP expression level. Results suggested that expression of CaELIP can be induced by the ROS generated by the excessive Cu in the plant. Exogenous SA treatment significantly alleviated Cu-induced expression of CaELIP, while exogenous JA treatment aggravated expression of CaELIP under Cu stress. CaELIP showed a transient expression when exposing the plant to light for 1 h. CaELIP also showed an endogenous circadian rhythmicity with high expression level in the morning and decreased expression level thereafter. The expression of CaELIP was also induced by high or low temperature, high salinity, drought, and stress hormone ABA. Taken together, the results suggest that CaELIP would function in responding to environmental signals and possibly regulating the response to the abiotic stresses that can be related to the abiotic stress tolerance in plants.