Frontiers of torenia research: innovative ornamental traits and study of ecological interaction networks through genetic engineering

Advances in research in the past few years on the ornamental plant torenia (Torenia spps.) have made it notable as a model plant on the frontier of genetic engineering aimed at studying ornamental characteristics and pest control in horticultural ecosystems. The remarkable advantage of torenia over other ornamental plant species is the availability of an easy and high-efficiency transformation system for it. Unfortunately, most of the current torenia research is still not very widespread, because this species has not become prominent as an alternative to other successful model plants such as Arabidopsis, snapdragon and petunia. However, nowadays, a more global view using not only a few selected models but also several additional species are required for creating innovative ornamental traits and studying horticultural ecosystems. We therefore introduce and discuss recent research on torenia, the family Scrophulariaceae, for secondary metabolite bioengineering, in which global insights into horticulture, agriculture and ecology have been advanced. Floral traits, in torenia particularly floral color, have been extensively studied by manipulating the flavonoid biosynthetic pathways in flower organs. Plant aroma, including volatile terpenoids, has also been genetically modulated in order to understand the complicated nature of multi-trophic interactions that affect the behavior of predators and pollinators in the ecosystem. Torenia would accordingly be of great use for investigating both the variation in ornamental plants and the infochemical-mediated interactions with arthropods

Tetranins: new putative spider mite elicitors of host plant defense

Summary The two‐spotted spider mite (Tetranychus urticae) is a plant‐sucking arthropod herbivore that feeds on a wide array of cultivated plants. In contrast to the well‐characterized classical chewing herbivore salivary elicitors that promote plant defense responses, little is known about sucking herbivores' elicitors. To characterize the sucking herbivore elicitors, we explored putative salivary gland proteins of spider mites by using an Agrobacterium‐mediated transient expression system or protein infiltration in damaged bean leaves. Two candidate elicitors (designated as tetranin1 (Tet1) and tetranin2 (Tet2)) triggered early leaf responses (cytosolic calcium influx and membrane depolarization) and increased the transcript abundances of defense genes in the leaves, eventually resulting in reduced survivability of T. urticae on the host leaves as well as induction of indirect plant defenses by attracting predatory mites. Tet1 and/or Tet2 also induced jasmonate, salicylate and abscisic acid biosynthesis. Notably, Tet2‐induced signaling cascades were also activated via the generation of reactive oxygen species. The signaling cascades of these two structurally dissimilar elicitors are mostly overlapping but partially distinct and thus they would coordinate the direct and indirect defense responses in host plants under spider mite attack in both shared and distinct manners

The Composite Effect of Transgenic Plant Volatiles for Acquired Immunity to Herbivory Caused by Inter-Plant Communications

A blend of volatile organic compounds (VOCs) emitted from plants induced by herbivory enables the priming of defensive responses in neighboring plants. These effects may provide insights useful for pest control achieved with transgenic-plant-emitted volatiles. We therefore investigated, under both laboratory and greenhouse conditions, the priming of defense responses in plants (lima bean and corn) by exposing them to transgenic-plant-volatiles (VOCos) including (E)-β-ocimene, emitted from transgenic tobacco plants (NtOS2) that were constitutively overexpressing (E)-β-ocimene synthase. When lima bean plants that had previously been placed downwind of NtOS2 in an open-flow tunnel were infested by spider mites, they were more defensive to spider mites and more attractive to predatory mites, in comparison to the infested plants that had been placed downwind of wild-type tobacco plants. This was similarly observed when the NtOS2-downwind maize plants were infested with Mythimna separata larvae, resulting in reduced larval growth and greater attraction of parasitic wasps (Cotesia kariyai). In a greenhouse experiment, we also found that lima bean plants (VOCos-receiver plants) placed near NtOS2 were more attractive when damaged by spider mites, in comparison to the infested plants that had been placed near the wild-type plants. More intriguingly, VOCs emitted from infested VOCos-receiver plants affected their conspecific neighboring plants to prime indirect defenses in response to herbivory. Altogether, these data suggest that transgenic-plant-emitted volatiles can enhance the ability to prime indirect defenses via both plant-plant and plant-plant-plant communications

Herbivore-induced terpenoid emission in Medicago truncatula: concerted action of jasmonate, ethylene and calcium signaling

Plant volatiles emitted by Medicago truncatula in response to feeding larvae of Spodoptera exigua are composed of a complex blend of terpenoids. The cDNAs of three terpene synthases (TPSs), which contribute to the blend of terpenoids, were cloned from M. truncatula. Their functional characterization proved MtTPS1 to be a β-caryophyllene synthase and MtTPS5 to be a multi-product sesquiterpene synthase. MtTPS3 encodes a bifunctional enzyme producing (E)-nerolidol and geranyllinalool (precursors of C11 and C16 homoterpenes) from different prenyl diphosphates serving as substrates. The addition of jasmonic acid (JA) induced expression of the TPS genes, but terpenoid emission was higher from plants treated with JA and the ethylene precursor 1-amino-cyclopropyl-1-carboxylic acid. Compared to infested wild-type M. truncatula plants, lower amounts of various sesquiterpenes and a C11–homoterpene were released from an ethylene-insensitive mutant skl. This difference coincided with lower transcript levels of MtTPS5 and of 1-deoxy-d-xylulose-5-phosphate synthase (MtDXS2) in the damaged skl leaves. Moreover, ethephon, an ethylene-releasing compound, modified the extent and mode of the herbivore-stimulated Ca2+ variations in the cytoplasm that is necessary for both JA and terpene biosynthesis. Thus, ethylene contributes to the herbivory-induced terpenoid biosynthesis at least twice: by modulating both early signaling events such as cytoplasmic Ca2+-influx and the downstream JA-dependent biosynthesis of terpenoids

Recent Advances in Plant Early Signaling in Response to Herbivory

Plants are frequently attacked by herbivores and pathogens and therefore have acquired constitutive and induced defenses during the course of their evolution. Here we review recent progress in the study of the early signal transduction pathways in host plants in response to herbivory. The sophisticated signaling network for plant defense responses is elicited and driven by both herbivore-induced factors (e.g., elicitors, effectors, and wounding) and plant signaling (e.g., phytohormone and plant volatiles) in response to arthropod factors. We describe significant findings, illuminating the scenario by providing broad insights into plant signaling involved in several arthropod-host interactions

‘Hidden’ Terpenoids in Plants: Their Biosynthesis, Localization and Ecological Roles

Terpenoids are the largest group of plant specialized (secondary) metabolites. These naturally occurring chemical compounds are highly diverse in chemical structure. Although there have been many excellent studies of terpenoids, most have focused on compounds built solely of isoprene units. Plants, however, also contain many 'atypical' terpenoids, such as glycosylated volatile terpenes and composite-type terpenoids, the latter of which are synthesized by the coupling of isoprene units on aromatic compounds. This mini review describes these 'hidden' terpenoids, providing an overview of their biosynthesis, localization, and biological and ecological activities

Two arrays of defense strategies of Brassicaceae plants that eavesdrop on mint volatiles

Brassica rapa can eavesdrop on volatile compounds emitted from peppermint and consequently boost its defense responses against Plutella xylostella. In the current study, to assess how B. rapa receiver plants cultivated near peppermint plants resist P. xylostella, two biological assays were conducted. The data showed that the receiver plants cultivated with peppermint exhibited lower weight gain of P. xylostella larvae, compared to that exhibited by non-receiver plants. Moreover, a lower rate of oviposition of adult female P. xylostella was observed on B. rapa plants cultivated with peppermint plants compared to the rate on B. rapa plants cultivated with glass-enclosed peppermint plants, although receiver B. rapa plants that had been previously incubated with peppermint for 7 days and non-receiver B. rapa plants exhibited similar rates of oviposition. Taking all these findings together, we conclude that cultivation of B. rapa receiver plants with peppermint results in two arrays of defense strategies

Arabidopsis CPK3 plays extensive roles in various biological and environmental responses

Plant Ca2+-dependent protein kinase (CPK) signaling is involved in a wide array of intracellular signaling pathways involved in stomatal movement and plant adaptation to various environmental challenges including drought, salt and cold stress. Arabidopsis CPK3 appears to be extensively involved in such a wide range of aspects, and has been shown to function in mediating the signaling following Ca2+ influx after insect herbivory. The results reveal the involvement of CPK3 in the herbivory-induced signaling network through phosphorylating the substrate target HsfB2a (heat shock transcription factor) for transcriptional activation of the plant defensin gene PDF1.2. Proteomic studies based on the cell-free protein production system allowed us to mine CPK3 targets more extensively and clarify the nature of multifunctional CPK3