Insect eggs trigger systemic acquired resistance against a fungal and an oomycete pathogen

The Key Role of Plant Hormone Signaling Transduction and Flavonoid Biosynthesis Pathways in the Response of Chinese Pine (Pinus tabuliformis) to Feeding Stimulation by Pine Caterpillar (Dendrolimus tabulaeformis)

In nature, plants have developed a series of resistance mechanisms to face various external stresses. As understanding of the molecular mechanisms underlying plant resistance continues to deepen, exploring endogenous resistance in plants has become a hot topic in this field. Despite the multitude of studies on plant-induced resistance, how plants respond to stress under natural conditions remains relatively unclear. To address this gap, we investigated Chinese pine (Pinus tabuliformis) using pine caterpillar (Dendrolimus tabulaeformis) under natural conditions. Healthy Chinese pine trees, approximately 10 years old, were selected for studying induced resistance in Huangtuliangzi Forestry, Pingquan City, Chengde City, Hebei Province, China. Pine needles were collected at 2 h and 8 h after feeding stimulation (FS) via 10 pine caterpillars and leaf clipping control (LCC), to simulate mechanical damage caused by insect chewing for the quantification of plant hormones and transcriptome and metabolome assays. The results show that the different modes of treatments significantly influence the contents of JA and SA in time following treatment. Three types of differentially accumulated metabolites (DAMs) were found to be involved in the initial response, namely phenolic acids, lipids, and flavonoids. Weighted gene co-expression network analysis indicated that 722 differentially expressed genes (DEGs) are positively related to feeding stimulation and the specific enriched pathways are plant hormone signal transduction and flavonoid biosynthesis, among others. Two TIFY transcription factors (PtTIFY54 and PtTIFY22) and a MYB transcription factor (PtMYB26) were found to be involved in the interaction between plant hormones, mainly in the context of JA signal transduction and flavonoid biosynthesis. The results of this study provide an insight into how JA activates, serving as a reference for understanding the molecular mechanisms of resistance formation in conifers responding to mandibulate insects.

Plant defensive responses to insect eggs are inducible by general egg-associated elicitors

Egg deposition by herbivorous insects is well known to elicit defensive plant responses. Our study aimed to elucidate the insect and plant species specificity of these responses. To study the insect species specificity, we treated Arabidopsis thaliana with egg extracts and egg-associated secretions of a sawfly (Diprion pini), a beetle (Xanthogaleruca luteola) and a butterfly (Pieris brassicae). All egg extracts elicited salicylic acid (SA) accumulation in the plant, and all secretions induced expression of plant genes known to be responsive to the butterfly eggs, among them Pathogenesis-Related (PR) genes. All secretions contained phosphatidylcholine derivatives, known elicitors of SA accumulation and PR gene expression in Arabidopsis. The sawfly egg extract did not induce plant camalexin levels, while the other extracts did. Our studies on the plant species specificity revealed that Solanum dulcamara and Ulmus minor responded with SA accumulation and cell death to P. brassicae eggs, i.e. responses also known for A. thaliana. However, the butterfly eggs induced neoplasms only in S. dulcamara. Our results provide evidence for general, phosphatidylcholine-based, egg-associated elicitors of plant responses and for conserved plant core responses to eggs, but also point to plant and insect species-specific traits in plant-insect egg interactions.

Metabolites and Plant Hormones Related to the Resistance Response to Feeding Stimulation and Leaf Clipping Control in Chinese Pine (Pinus tabuliformis Carr.)

This experiment was conducted to define changes in metabolic pathways in response to mandibulate insect feeding and to provide a reference for further investigation of the molecular mechanisms underlying the development of conifer resistance. Chinese pine (Pinus tabuliformis Carr.) in good growth status in natural condition was chosen for stimulation by 10 pine caterpillars (Dendrolimus tabulaefomis Tsai et Liu) as feeding stimulation (FS), leaf clipping control (LCC) as mechanical damage, and CK group (with no treatment) (recorded as 0 h). The metabolome and total flavonoid content were measured in the needles at 0, 2, and 8 h after treatment. Plant hormones were measured with needles at 0, 0.5, 1, 1.5, 2, 4, 6, and 8 h after different treatments. The results show that a total of 30.8% flavonoids are identified by metabolomics analysis. Compared with leaf clipping control, feeding stimulation of Chinese pine caterpillars significantly induced the upregulation of metabolites in the flavonoid pathway in Chinese pine, and the plant hormones JA and IAA showed expression trends consistent with those of the metabolome. According to the biological processes of the four plant hormones involved, JA and SA are mostly involved in resistance formation, and in this study, both of them also have fluctuating expressions influenced by feeding stimulation, while the expressions of the growth-related hormones IAA and ABA have no significant changes at other time points except for 1 h after treatment. Thus, the flavonoid pathway is one of the main pathways involved in resistance formation in conifers, and JA and IAA are involved in the formation of resistance.

Adaptive Plasticity of Insect Eggs in Response to Environmental Challenges

Insect eggs are exposed to a plethora of abiotic and biotic threats. Their survival depends on both an innate developmental program and genetically determined protective traits provided by the parents. In addition, there is increasing evidence that (a) parents adjust the egg phenotype to the actual needs, (b) eggs themselves respond to environmental challenges, and (c) egg-associated microbes actively shape the egg phenotype. This review focuses on the phenotypic plasticity of insect eggs and their capability to adjust themselves to their environment. We outline the ways in which the interaction between egg and environment is two-way, with the environment shaping the egg phenotype but also with insect eggs affecting their environment. Specifically, insect eggs affect plant defenses, host biology (in the case of parasitoid eggs), and insect oviposition behavior. We aim to emphasize that the insect egg, although it is a sessile life stage, actively responds to and interacts with its environment.

Insect egg-induced innate immunity: Who benefits?

Plants perceive the presence of insect eggs deposited on leaves as a cue of imminent herbivore attack. Consequential plant signaling events include the accumulation of salicylic acid and reactive oxygen species, transcriptional reprogramming, and cell death. Interestingly, egg-induced innate immunity shows similarities with immune responses triggered upon recognition of microbial pathogens, and in recent years, it became apparent that egg perception affects plant-microbe interactions. Here, we highlight recent findings on insect egg-induced innate immunity and how egg-mediated signaling impacts plant-microbe interactions. Ecological considerations beg the question: Who benefits from egg perception in these complex interactions?

The peptide SCOOP12 acts on reactive oxygen species homeostasis to modulate cell division and elongation in Arabidopsis primary root

Small secreted peptides have been described as key contributors to complex signalling networks that control plant development and stress responses. The Brassicaceae-specific PROSCOOP family encodes precursors of Serine riCh endOgenOus Peptides (SCOOPs). In Arabidopsis SCOOP12 has been shown to promote the defence response against pathogens and to be involved in root development. Here, we explore its role as a moderator of Arabidopsis primary root development. We show that the PROSCOOP12 null mutation leads to longer primary roots through the development of longer differentiated cells while PROSCOOP12 overexpression induces dramatic plant growth impairments. In comparison, the exogenous application of synthetic SCOOP12 peptide shortens roots through meristem size and cell length reductions. Moreover, superoxide anion (O2·-) and hydrogen peroxide (H2O2) production in root tips vary according to SCOOP12 abundance. By using reactive oxygen species scavengers that suppress the proscoop12 phenotype, we showed that root growth regulation by SCOOP12 is associated with reactive oxygen species metabolism. Furthermore, our results suggest that peroxidases act as potential SCOOP12 downstream targets to regulate H2O2 production, which in turn triggers cell wall modifications in root. Finally, a massive transcriptional reprogramming, including the induction of genes from numerous other pathways, including ethylene, salicylic acid, and glucosinolates biosynthesis, was observed, emphasizing its dual role in defence and development.

Priming of Arabidopsis resistance to herbivory by insect egg deposition depends on the plant's developmental stage

While traits of plant resistance to herbivory often change during ontogeny, it is unknown whether the primability of this resistance depends on the plant's developmental stage. Resistance in non-flowering Arabidopsis thaliana against Pieris brassicae larvae is known to be primable by prior egg deposition on leaves. We investigated whether this priming effect is maintained in plants at the flowering stage. Larval performance assays revealed that flowering plants' resistance to herbivory was not primable by egg deposition. Accordingly, transcriptomes of flowering plants showed almost no response to eggs. In contrast, egg deposition on non-flowering plants enhanced the expression of genes induced by subsequent larval feeding. Strikingly, flowering plants showed constitutively high expression levels of these genes. Larvae performed generally worse on flowering than on non-flowering plants, indicating that flowering plants constitutively resist herbivory. Furthermore, we determined the seed weight in regrown plants that had been exposed to eggs and larvae during the non-flowering or flowering stage. Non-flowering plants benefitted from egg priming with a smaller loss in seed yield. The seed yield of flowering plants was unaffected by the treatments, indicating tolerance towards the larvae. Our results show that the primability of anti-herbivore defences in Arabidopsis depends on the plant's developmental stage.

The MIK2/SCOOP Signaling System Contributes to Arabidopsis Resistance Against Herbivory by Modulating Jasmonate and Indole Glucosinolate Biosynthesis

Initiation of plant immune signaling requires recognition of conserved molecular patterns from microbes and herbivores by plasma membrane-localized pattern recognition receptors. Additionally, plants produce and secrete numerous small peptide hormones, termed phytocytokines, which act as secondary danger signals to modulate immunity. In Arabidopsis, the Brassicae-specific SERINE RICH ENDOGENOUS PEPTIDE (SCOOP) family consists of 14 members that are perceived by the leucine-rich repeat receptor kinase MALE DISCOVERER 1-INTERACTING RECEPTOR LIKE KINASE 2 (MIK2). Recognition of SCOOP peptides elicits generic early signaling responses but knowledge on how and if SCOOPs modulate specific downstream immune defenses is limited. We report here that depletion of MIK2 or the single PROSCOOP12 precursor results in decreased Arabidopsis resistance against the generalist herbivore Spodoptera littoralis but not the specialist Pieris brassicae. Increased performance of S. littoralis on mik2-1 and proscoop12 is accompanied by a diminished accumulation of jasmonic acid, jasmonate-isoleucine and indolic glucosinolates. Additionally, we show transcriptional activation of the PROSCOOP gene family in response to insect herbivory. Our data therefore indicate that perception of endogenous SCOOP peptides by MIK2 modulates the jasmonate pathway and thereby contributes to enhanced defense against a generalist herbivore.