Ethylene Signaling Modulates Herbivore-Induced Defense Responses in the Model Legume Medicago truncatula

Proteomic Analysis of Salivary Secretions from the Tea Green Leafhopper, Empoasca flavescens Fabrecius

Saliva plays a crucial role in shaping the compatibility of piercing-sucking insects with their host plants. Understanding the complex composition of leafhopper saliva is important for developing effective and eco-friendly control strategies for the tea green leafhopper, Empoasca flavescens Fabrecius, a major piercing-sucking pest in Chinese tea plantations. This study explored the saliva proteins of tea green leafhopper adults using a custom collection device, consisting of two layers of Parafilm stretched over a sucrose diet. A total of 152 proteins were identified using liquid chromatography-tandem mass spectrometry (LC-MS/MS) following the filter-aided sample preparation (FASP). These proteins were categorized into six groups based on their functions, including enzymes, transport proteins, regulatory proteins, cell structure proteins, other proteins, and unknown proteins. Bioinformatics analyses predicted 16 secreted proteins, which were successfully cloned and transcriptionally analyzed across various tissues and developmental stages. Genes encoding putative salivary secretory proteins, including Efmucin1, EfOBP1, EfOBP2, EfOBP3, Efmucin2, low-density lipoprotein receptor-related protein (EfLRP), EFVg1, and EFVg2, exhibited high expressions in salivary gland (SG) tissues and feeding-associated expressions at different developmental stages. These findings shed light on the potential elicitors or effectors mediating the leafhopper feeding and defense responses in tea plants, providing insights into the coevolution of tea plants and leafhoppers. The study's conclusions open avenues for the development of innovative leafhopper control technologies that reduce the reliance on pesticides in the tea industry.

Multigenic regulation in the ethylene biosynthesis pathway during coffee flowering

Ethylene regulates different aspects of the plant's life cycle, such as flowering, and acts as a defense signal in response to environmental stresses. Changes induced by water deficit (WD) in gene expression of the main enzymes involved in ethylene biosynthesis, 1-aminocyclopropane-1-carboxylic acid synthase (ACS) and oxidase (ACO), are frequently reported in plants. In this study, coffee (Coffea arabica) ACS and ACO family genes were characterized and their expression profiles were analyzed in leaves, roots, flower buds, and open flowers from plants under well-watered (WW) and water deficit (WD) conditions. Three new ACS genes were identified. Water deficit did not affect ACS expression in roots, however soil drying strongly downregulated ACO expression, indicating a transcriptional constraint in the biosynthesis pathway during the drought that can suppress ethylene production in roots. In floral buds, ACO expression is water-independent, suggesting a higher mechanism of control in reproductive organs during the final flowering stages. Leaves may be the main sites for ethylene precursor (1-aminocyclopropane-1-carboxylic acid, ACC) production in the shoot under well-watered conditions, contributing to an increase in the ethylene levels required for anthesis. Given these results, we suggest a possible regulatory mechanism for the ethylene biosynthesis pathway associated with coffee flowering with gene regulation in leaves being a key point in ethylene production and ACO genes play a major regulatory role in roots and the shoots. This mechanism may constitute a regulatory model for flowering in other woody species.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-022-01235-y.

Dual Transcriptional Profiling of Carrot and 'Candidatus Liberibacter solanacearum' at Different Stages of Infection Suggests Complex Host-Pathogen Interaction

The interactions between the phloem-limited pathogen 'Candidatus Liberibacter solanacearum' haplotype C and carrot (Daucus carota subsp. sativus) were studied at 4, 5, and 9 weeks postinoculation (wpi), by combining dual RNA-Seq results with data on bacterial colonization and observations of the plant phenotype. In the infected plants, genes involved in jasmonate biosynthesis, salicylate signaling, pathogen-associated molecular pattern- and effector-triggered immunity, and production of pathogenesis-related proteins were up-regulated. At 4 wpi, terpenoid synthesis-related genes were up-regulated, presumably as a response to the psyllid feeding, whereas at 5 and 9 wpi, genes involved in both the terpenoid and flavonoid production were down-regulated and phenylpropanoid genes were up-regulated. Chloroplast-related gene expression was down-regulated, in concordance with the observed yellowing of the infected plant leaves. Both the RNA-Seq data and electron microscopy suggested callose accumulation in the infected phloem vessels, likely to impair the transport of photosynthates, while phloem regeneration was suggested by the formation of new sieve cells and the upregulation of cell wall-related gene expression. The 'Ca. L. solanacearum' genes involved in replication, transcription, and translation were expressed at high levels at 4 and 5 wpi, whereas, at 9 wpi, the Flp pilus genes were highly expressed, suggesting adherence and reduced mobility of the bacteria. The 'Ca. L. solanacearum' genes encoding ATP and C4-dicarboxylate uptake were differentially expressed between the early and late infection stages, suggesting a change in the dependence on different host-derived energy sources. HPE1 effector and salicylate hydroxylase were expressed, presumably to suppress host cell death and salicylic acid-dependent defenses during the infection.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Specific tissue proteins 1 and 6 are involved in root biology during normal development and under symbiotic and pathogenic interactions in Medicago truncatula

ST1 and ST6 are possibly involved in primary and lateral root and symbiotic nodule development, but only ST6 participates in the interaction with hemibiotrophic fungi. Specific tissue (ST) proteins have been shown to be involved in several processes related to plant nutritional status, development, and responses to biotic agents. In particular, ST1 and ST6 are mainly expressed in roots throughout plant development. Here, we analyze where and how the expression of the genes encoding both proteins are modulated in the legume model plant Medicago truncatula in response to the plant developmental program, nodulation induced by a beneficial nitrogen-fixing bacterium (Sinorhizobium meliloti) and the defense response triggered by a pathogenic hemibiotrophic fungus (Fusarium oxysporum). Gene expression results show that ST1 and ST6 participate in the vasculature development of both primary and lateral roots, although only ST6 is related to meristem activity. ST1 and ST6 clearly display different roles in the biotic interactions analyzed, where ST1 is activated in response to a N₂-fixing bacterium and ST6 is up-regulated after inoculation with F. oxysporum. The role of ST1 and ST6 in the nodulation process may be related to nodule organogenesis rather than to the establishment of the interaction itself, and an increase in ST6 correlates with the activation of the salicylic acid signaling pathway during the infection and colonization processes. These results further support the role of ST6 in response to hemibiotrophic fungi. This research contributes to the understanding of the complex network that controls root biology and strengthens the idea that ST proteins are involved in several processes such as primary and lateral root development, nodule organogenesis, and the plant-microbe interaction.

Ethylene functions as a suppressor of volatile production in rice

We examined the role of ethylene in the production of rice (Oryza sativa) volatile organic compounds (VOCs), which act as indirect defense signals against herbivores in tritrophic interactions. Rice plants were exposed to exogenous ethylene (1 ppm) after simulated herbivory, which consisted of mechanical wounding supplemented with oral secretions (WOS) from the generalist herbivore larva Mythimna loreyi. Ethylene treatment highly suppressed VOCs in WOS-treated rice leaves, which was further corroborated by the reduced transcript levels of major VOC biosynthesis genes in ethylene-treated rice. In contrast, the accumulation of jasmonates (JA), known to control VOCs in higher plants, and transcript levels of primary JA response genes, including OsMYC2, were not largely affected by ethylene application. At the functional level, flooding is known to promote internode elongation in young rice via ethylene signaling. Consistent with the negative role of ethylene on VOC genes, the accumulation of VOCs in water-submerged rice leaves was suppressed. Furthermore, in mature rice plants, which naturally produce less volatiles, VOCs could be rescued by the application of the ethylene perception inhibitor 1-methylcyclopropene. Our data suggest that ethylene acts as an endogenous suppressor of VOCs in rice plants during development and under stress.

Evolving together, evolving apart: measuring the fitness of rhizobial bacteria in and out of symbiosis with leguminous plants

Most plant-microbe interactions are facultative, with microbes experiencing temporally and spatially variable selection. How this variation affects microbial evolution is poorly understood. Given its tractability and ecological and agricultural importance, the legume-rhizobia nitrogen-fixing symbiosis is a powerful model for identifying traits and genes underlying bacterial fitness. New technologies allow high-throughput measurement of the relative fitness of bacterial mutants, strains and species in mixed inocula in the host, rhizosphere and soil environments. I consider how host genetic variation (G × G), other environmental factors (G × E), and host life-cycle variation may contribute to the maintenance of genetic variation and adaptive trajectories of rhizobia - and, potentially, other facultative symbionts. Lastly, I place these findings in the context of developing beneficial inoculants in a changing climate.

Legumes Protease Inhibitors as Biopesticides and Their Defense Mechanisms against Biotic Factors

Legumes are affected by biotic factors such as insects, molds, bacteria, and viruses. These plants can produce many different molecules in response to the attack of phytopathogens. Protease inhibitors (PIs) are proteins produced by legumes that inhibit the protease activity of phytopathogens. PIs are known to reduce nutrient availability, which diminishes pathogen growth and can lead to the death of the pathogen. PIs are classified according to the specificity of the mechanistic activity of the proteolytic enzymes, with serine and cysteine protease inhibitors being studied the most. Previous investigations have reported the efficacy of these highly stable proteins against diverse biotic factors and the concomitant protective effects in crops, representing a possible replacement of toxic agrochemicals that harm the environment.

Local and systemic hormonal responses in pepper (Capsicum annuum L.) leaves under green peach aphid (Myzus persicae Sulzer) infestation

This study examined the temporal changes in the leaf content of defence-involved phytohormones in pepper (Capsicum annuum L.) plants responding to the green peach aphid (Myzus persicae Sulzer) infestation, at both local and systemic level. Aphid infestation did not alter the content of cis-12-oxo-phytodienoic acid, the jasmonic acid (JA) precursor, even though endogenous levels of JA and its bioactive isoleucine-conjugated form (JA-Ile) significantly increased from 8 to 96 h in local infested leaves. Systemic effects in jasmonates were only showed at 48 h for JA, and 8 and 48 h in the case of JA-Ile. SA accumulated only in local infested leaves after 96 h of infestation, when the level of JA-Ile decreased in these leaves. This suggests a possible antagonistic interaction between JA and SA pathways, although other pathways may be also involved. Endogenous level of indole-3-acetic acid was higher in systemic relative to local infested leaves at 3 and 24 h, although no significant changes in its content were found compared to control leaves. Abscisic acid content was lower in local infested relative to control leaves at 24 h, but was higher at 48 h when it also increased systemically. The possible roles of the studied phytohormones in plant defence responses against aphids are discussed.

Hyperspectral imaging to characterize plant-plant communication in response to insect herbivory

BACKGROUND

In studies of plant stress signaling, a major challenge is the lack of non-invasive methods to detect physiological plant responses and to characterize plant-plant communication over time and space.

RESULTS

We acquired time series of phytocompound and hyperspectral imaging data from maize plants from the following treatments: (1) individual non-infested plants, (2) individual plants experimentally subjected to herbivory by green belly stink bug (no visible symptoms of insect herbivory), (3) one plant subjected to insect herbivory and one control plant in a separate pot but inside the same cage, and (4) one plant subjected to insect herbivory and one control plant together in the same pot. Individual phytocompounds (except indole-3acetic acid) or spectral bands were not reliable indicators of neither insect herbivory nor plant-plant communication. However, using a linear discrimination classification method based on combinations of either phytocompounds or spectral bands, we found clear evidence of maize plant responses.

CONCLUSIONS

We have provided initial evidence of how hyperspectral imaging may be considered a powerful non-invasive method to increase our current understanding of both direct plant responses to biotic stressors but also to the multiple ways plant communities are able to communicate. We are unaware of any published studies, in which comprehensive phytocompound data have been shown to correlate with leaf reflectance. In addition, we are unaware of published studies, in which plant-plant communication was studied based on leaf reflectance.

Altering Plant Defenses: Herbivore-Associated Molecular Patterns and Effector Arsenal of Chewing Herbivores

Chewing herbivores, such as caterpillars and beetles, while feeding on the host plant, cause extensive tissue damage and release a wide array of cues to alter plant defenses. Consequently, the cues can have both beneficial and detrimental impacts on the chewing herbivores. Herbivore-associated molecular patterns (HAMPs) are molecules produced by herbivorous insects that aid them to elicit plant defenses leading to impairment of insect growth, while effectors suppress plant defenses and contribute to increased susceptibility to subsequent feeding by chewing herbivores. Besides secretions that originate from glands (e.g., saliva) and fore- and midgut regions (e.g., oral secretions) of chewing herbivores, recent studies have shown that insect frass and herbivore-associated endosymbionts also play a critical role in modulating plant defenses. In this review, we provide an update on a growing body of literature that discusses the chewing insect HAMPs and effectors and the mechanisms by which they modulate host defenses. Novel "omic" approaches and availability of new tools will help researchers to move forward this discipline by identifying and characterizing novel insect HAMPs and effectors and how these herbivore-associated cues are perceived by host plant receptors.

Medicago truncatula Oleanolic-Derived Saponins Are Correlated with Caterpillar Deterrence

Plant resistance mechanisms to insect herbivory can potentially be bred into crops as an important strategy for integrated pest management. Medicago truncatula ecotypes inoculated with the rhizobium Ensifer medicae (Sinorhizobium medica) WSM419 were screened for resistance to herbivory by caterpillars of the beet armyworm, Spodoptera exigua, through leaf and whole plant choice studies; TN1.11 and F83005.5 are identified as the least and most deterrent ecotypes, respectively. In response to caterpillar herbivory, both ecotypes mount a robust burst of plant defensive jasmonate phytohormones. Restriction of caterpillars to either of these ecotypes does not adversely affect pest performance. This argues for an antixenosis (deterrence) resistance mechanism associated with the F83005.5 ecotype. Unbiased metabolomic profiling identified strong ecotype-specific differences in metabolite profile, particularly in the content of oleanolic-derived saponins that may act as antifeedants. Compared to the more susceptible ecotype, F83005.5 has higher levels of oleanolic-type zanhic acid- and medicagenic acid-derived compounds. Together, these data support saponin-mediated deterrence as a resistance mechanism of the F83005.5 ecotype and implicates these compounds as potential antifeedants that could be used in agricultural sustainable pest management strategies.

Salt-tolerant and -sensitive alfalfa (Medicago sativa) cultivars have large variations in defense responses to the lepidopteran insect Spodoptera litura under normal and salt stress condition

In nature, plants are often exposed to multiple stress factors at the same time. Yet, little is known about how plants modulate their physiology to counteract simultaneous abiotic and biotic stresses, such as soil salinity and insect herbivory. In this study, insect performance bioassays, phytohormone measurements, quantification of transcripts, and protein determination were employed to study the phenotypic variations of two alfalfa (Medicago sativa) cultivars in response to insect Spodoptera litura feeding under normal and salt stress condition. When being cultivated in normal soil, the salt-tolerant alfalfa cultivar Zhongmu-1 exhibited lower insect resistance than did the salt-sensitive cultivar Xinjiang Daye. Under salinity stress, the defense responses of Xinjiang Daye were repressed, whereas Zhongmu-1 did not show changes in resistance levels. It is likely that salinity influenced the resistance of Xinjiang Daye through suppressing the accumulation of jasmonic acid-isoleucine (JA-Ile), which is the bioactive hormone inducing herbivore defense responses, leading to attenuated trypsin proteinase inhibitor (TPI) activity. Furthermore, exogenous ABA supplementation suppressed the insect herbivory-induced JA/JA-Ile accumulation and levels of JAR1 (jasmonate resistant 1) and TPI, and further decreased the resistance of Xinjiang Daye, whereas Zhongmu-1 showed very little response to the increased ABA level. We propose a mechanism, in which high levels of abscisic acid induced by salt treatment may affect the expression levels of JAR1 and consequently decrease JA-Ile accumulation and thus partly suppress the defense of Xinjiang Daye against insects under salt stress. This study provides new insight into the mechanism by which alfalfa responds to concurrent abiotic and biotic stresses.

A comparison of plants and animals in their responses to risk of consumption

Both plants and animals reduce their risk of being eaten by detecting and responding to herbivore and predator cues. Plants tend to be less mobile and rely on more local information perceived with widely dispersed and redundant tissues. As such, plants can more easily multi-task. Plants are more tolerant of damage and use damage to their own tissues as reliable cues of risk; plants have a higher threshold before responding to the threat of herbivory. Plants also use diverse cues that include fragments of plant tissue and molecular patterns from herbivores, herbivore feeding, or microbial associates of herbivores. Instead of fleeing from attackers, plants reallocate valuable resources to organs at less risk. They minimize unnecessary defenses against unrealized risks and costs of failing to defend against actual risk. Plants can remember and learn, although these abilities are poorly understood.

Cues from chewing insects - the intersection of DAMPs, HAMPs, MAMPs and effectors

Chewing herbivores cause massive damage when crushing plant tissues with their mandibles, thus releasing a vast array of cues that may be perceived by the plant to mobilize defenses. Besides releasing damage cues in wounded tissues, herbivores deposit abundant cues from their saliva, regurgitant and feces that trigger herbivore specific responses in plants. Herbivores can manipulate the perception mechanisms and defense signals to suppress plant defenses by secreting effectors and/or by exploiting their associated oral microbes. Recent studies indicate that both the composition of herbivore cues and the plant's ability to recognize them are highly dependent upon the specific plant-herbivore system. There is a growing amount of work on identifying herbivore elicitors and effectors, but the most significant bottleneck in the discipline is the identification and characterization of plant receptors that perceive these herbivore-specific cues.

Impacts of insect oral secretions on defoliation-induced plant defense

Plant responses to biotic stress involve non-self perception, signaling, and altered defense phenotypes. During attack, defoliating insects deposit gland secretions (GS) and complex foregut derived oral secretions (OS) that include GS and combined products of plant, insect, and microbial interactions. GS-derived and OS-derived biochemicals that trigger defense are termed Herbivore Associated Molecular Patterns (HAMPs) while those that promote susceptibility are termed effectors. These functions are highly context and species specific. The magnitude and direction of plant responses are orchestrated by the interaction of damage, OS/GS components, predicted receptor-ligand interactions, ion fluxes, protein kinase signaling cascades, phytohormone interactions, transcription factor activation, altered translation, and defense biosynthesis. Unlike plant-pathogen recognition, a remaining challenge is the discovery of plant receptors for defoliator-derived HAMPs.