SIEVE ELEMENT-LINING CHAPERONE1 Restricts Aphid Feeding on Arabidopsis during Heat Stress

Loss-of-function of LIGULELESS1 activates the jasmonate pathway and promotes maize resistance to corn leaf aphids

Corn leaf aphids (Rhopalosiphum maidis) are highly destructive pests of maize (Zea mays) that threaten growth and seed yield, but resources for aphid resistance are scarce. Here, we identified an aphid-resistant maize mutant, resistance to aphids 1 (rta1), which is allelic to LIGULELESS1 (LG1). We confirmed LG1's role in aphid resistance using the independent allele lg1-2, allelism tests and LG1 overexpression lines. LG1 interacts with, and increases the stability of ZINC-FINGER PROTEIN EXPRESSED IN INFLORESCENCE MERISTEM (ZIM1), a central component of the jasmonic acid (JA) signalling pathway, by disturbing its interaction with the F-box protein CORONATINE INSENSITIVE 1a (COI1a). Natural variation in the LG1 promoter was associated with aphid resistance among inbred lines. Moreover, a loss-of-function mutant in the LG1-related gene SPL8 in the dicot Arabidopsis thaliana conferred aphid resistance. This study revealed the aphid resistance mechanism of lg1, providing a theoretical basis and germplasm for breeding aphid-resistant crops.

Evaluation of aphid resistance on different rose cultivars and transcriptome analysis in response to aphid infestation

BACKGROUND

The rose is one of the most important ornamental flowers in the world for its aesthetic beauty but can be attacked by many pests such as aphids. Aphid infestation causes tremendous damage on plant tissues leading to harmed petals and leaves. Rose cultivars express different levels of resistance to aphid infestation yet the information remains unclear. Not only that, studies about the transcriptional analysis on defending mechanisms against aphids in rose are limited so far.

RESULTS

In this study, the aphid resistance of 20 rose cultivars was evaluated, and they could be sorted into six levels based on the number ratio of aphids. And then, a transcriptome analysis was conducted after aphid infestation in one high resistance (R, Harmonie) and one highly susceptibility (S, Carefree Wonder) rose cultivar. In open environment the majority of rose cultivars had the highest aphid number at May 6th or May 15th in 2020 and the resistance to infestation could be classified into six levels. Differential expression analysis revealed that there were 1,626 upregulated and 767 downregulated genes in the R cultivar and 481 upregulated and 63 downregulated genes in the S cultivar after aphid infestation. Pathway enrichment analysis of the differentially expressed genes revealed that upregulated genes in R and S cultivars were both enriched in defense response, biosynthesis of secondary metabolites (phenylpropanoid, alkaloid, and flavonoid), carbohydrate metabolism (galactose, starch, and sucrose metabolism) and lipid processing (alpha-linolenic acid and linolenic acid metabolism) pathways. In the jasmonic acid metabolic pathway, linoleate 13S-lipoxygenase was specifically upregulated in the R cultivar, while genes encoding other crucial enzymes, allene oxide synthase, allene oxide cyclase, and 12-oxophytodienoate reductase were upregulated in both cultivars. Transcription factor analysis and transcription factor binding search showed that WRKY transcription factors play a pivotal role during aphid infestation in the R cultivar.

CONCLUSIONS

Our study indicated the potential roles of jasmonic acid metabolism and WRKY transcription factors during aphid resistance in rose, providing clues for future research.

Temporal and species-specific resistance of sugar beet to green peach aphid and black bean aphid: mechanisms and implications for breeding

BACKGROUND

Sugar beet (Beta vulgaris ssp. vulgaris), a key crop for sugar production, faces significant yield losses caused by the black bean aphid Aphis fabae (Scop.) and the green peach aphid Myzus persicae (Sulzer), which also transmits viruses. The restriction on neonicotinoid usage in Europe has intensified this problem, emphasizing the urgent need for breeding resistant crop varieties. This study evaluated 26 sugar beet germplasms for resistance against both aphid species by using performance and feeding behavior assays. Additionally, whole plant bioassays and semi-field experiments were carried out with Myzus persicae.

RESULTS

Our findings demonstrate the presence of temporal resistance against both aphid species in the primary sugar beet gene pool. Beet yellows virus (BYV) carrying aphids showed enhanced performance. Different levels of plant defense mechanisms were involved including resistance against Myzus persicae before reaching the phloem, particularly in sugar beet line G3. In contrast, resistance against Aphis fabae turned out to be predominately phloem-located. Furthermore, a high incidence of black inclusion bodies inside the stomach of Myzus persicae was observed for approximately 85% of the plant genotypes tested, indicating a general and strong incompatibility between sugar beet and Myzus persicae in an initial phase of interaction.

CONCLUSION

Sugar beet resistance against aphids involved different mechanisms and is species-specific. The identification of these mechanisms and interactions represents a crucial milestone in advancing the breeding of sugar beet varieties with improved resistance. © 2023 Julius Kühn-Institut and The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Molecular Mechanisms Underlying Host Plant Specificity in Aphids

Aphids are serious pests of agricultural and ornamental plants and important model systems for hemipteran-plant interactions. The long evolutionary history of aphids with their host plants has resulted in a variety of systems that provide insight into the different adaptation strategies of aphids to plants and vice versa. In the past, various plant-aphid interactions have been documented, but lack of functional tools has limited molecular studies on the mechanisms of plant-aphid interactions. Recent technological advances have begun to reveal plant-aphid interactions at the molecular level and to increase our knowledge of the mechanisms of aphid adaptation or specialization to different host plants. In this article, we compile and analyze available information on plant-aphid interactions, discuss the limitations of current knowledge, and argue for new research directions. We advocate for more work that takes advantage of natural systems and recently established molecular techniques to obtain a comprehensive view of plant-aphid interaction mechanisms.

Transcriptome analysis of aphid-resistant and susceptible near isogenic lines reveals candidate resistance genes in cowpea (Vigna unguiculata)

BACKGROUND

Cowpea (Vigna unguiculata) is a crucial crop for regions of the world that are prone to both heat and drought; however, the phytotoxic cowpea aphid (Aphis craccivora) impairs plant physiology at low population levels. Both antibiotic and antixenotic forms of resistance to the aphid have been mapped to two quantitative trait loci (QTLs) and near isogenic lines (NILs). The molecular mechanism for this resistance response remains unknown.

RESULTS

To understand the genes underlying susceptibility and resistance, two cowpea lines with shared heritage were infested along a time course and characterized for transcriptome variation. Aphids remodeled cowpea development and signaling relative to host plant resistance and the duration of feeding, with resource acquisition and mobilization determining, in part, susceptibility to aphid attack. Major differences between the susceptible and resistant cowpea were identified including two regions of interest housing the most genetic differences between the lines. Candidate genes enabling aphid resistance include both conventional resistance genes (e.g., leucine rich repeat protein kinases) as well as multiple novel genes with no known orthologues.

CONCLUSIONS

Our results demonstrate that feeding by the cowpea aphid globally remodels the transcriptome of cowpea, but how this occurs depends on both the duration of feeding and host-plant resistance. Constitutive expression profiles of the resistant genotype link aphid resistance to a finely-tuned resource management strategy that ultimately reduces damage (e.g., chlorosis) and delays cell turnover, while impeding aphid performance. Thus, aphid resistance in cowpea is a complex, multigene response that involves crosstalk between primary and secondary metabolism.

Common resistance mechanisms are deployed by plants against sap-feeding herbivorous insects: insights from a meta-analysis and systematic review

Despite their abundance and economic importance, the mechanism of plant resistance to sap-feeding insects remains poorly understood. Here we deploy meta-analysis and data synthesis methods to evaluate the results from electrophysiological studies describing feeding behaviour experiments where resistance mechanisms were identified, focussing on studies describing host-plant resistance and non-host resistance mechanisms. Data were extracted from 108 studies, comprising 41 insect species across eight insect taxa and 12 host-plant families representing over 30 species. Results demonstrate that mechanisms deployed by resistant plants have common consequences on the feeding behaviour of diverse insect groups. We show that insects feeding on resistant plants take longer to establish a feeding site and have their feeding duration suppressed two-fold compared with insects feeding on susceptible plants. Our results reveal that traits contributing towards resistant phenotypes are conserved across plant families, deployed against taxonomically diverse insect groups, and that the underlying resistance mechanisms are conserved. These findings provide a new insight into plant-insect interaction and highlight the need for further mechanistic studies across diverse taxa.

Plant metabolism and defence strategies in the flowering stage: Time-dependent responses of leaves and flowers under attack

Plants developing into the flowering stage undergo major physiological changes. Because flowers are reproductive tissues and resource sinks, strategies to defend them may differ from those for leaves. Thus, this study investigates the defences of flowering plants by assessing processes that sustain resistance (constitutive and induced) and tolerance to attack. We exposed the annual plant Brassica nigra to three distinct floral attackers (caterpillar, aphid and bacterial pathogen) and measured whole-plant responses at 4, 8 and 12 days after the attack. We simultaneously analysed profiles of primary and secondary metabolites in leaves and inflorescences and measured dry biomass of roots, leaves and inflorescences as proxies of resource allocation and regrowth. Regardless of treatments, inflorescences contained 1.2 to 4 times higher concentrations of primary metabolites than leaves, and up to 7 times higher concentrations of glucosinolates, which highlights the plant's high investment of resources into inflorescences. No induction of glucosinolates was detected in inflorescences, but the attack transiently affected the total concentration of soluble sugars in both leaves and inflorescences. We conclude that B. nigra evolved high constitutive rather than inducible resistance to protect their flowers; plants additionally compensated for damage by attackers via the regrowth of reproductive parts. This strategy may be typical of annual plants.

Rapid systemic responses to herbivory

Rapid systemic signals travel within the first seconds and minutes after herbivore infestation to mount defense responses in distal tissues. Recent studies have revealed that wound-induced hydraulic pressure changes play an important role in systemic electrical signaling and subsequent calcium and reactive oxygen species waves. These insights raise new questions about signal specificity, the role of insect feeding guild and feeding style and the impact on longer term plant defenses. Here, we integrate the current molecular understanding of wound-induced rapid systemic signaling in the framework of insect-plant interactions.

Phloem: At the center of action in plant defense against aphids

The location of the phloem deep inside the plant, the high hydrostatic pressure in the phloem, and the composition of phloem sap, which is rich in sugar with a high C:N ratio, allows phloem sap feeding insects to occupy a unique ecological niche. The anatomy and physiology of aphids, a large group of phytophagous insects that use their mouthparts, which are modified into stylets, to consume large amounts of phloem sap, has allowed aphids to successfully exploit this niche, however, to the detriment of agriculture and horticulture. The ability to reproduce asexually, a short generation time, the development of resistance to commonly used insecticides, and their ability to vector viral diseases makes aphids among the most damaging pests of plants. Here we review how plants utilize their ability to occlude sieve elements and accumulate antibiotic and antinutritive factors in the phloem sap to limit aphid infestation. In addition, we summarize progress on understanding how plants perceive aphids to activate defenses in the phloem.

Genetic analysis reveals three novel QTLs underpinning a butterfly egg-induced hypersensitive response-like cell death in Brassica rapa

BACKGROUND

Cabbage white butterflies (Pieris spp.) can be severe pests of Brassica crops such as Chinese cabbage, Pak choi (Brassica rapa) or cabbages (B. oleracea). Eggs of Pieris spp. can induce a hypersensitive response-like (HR-like) cell death which reduces egg survival in the wild black mustard (B. nigra). Unravelling the genetic basis of this egg-killing trait in Brassica crops could improve crop resistance to herbivory, reducing major crop losses and pesticides use. Here we investigated the genetic architecture of a HR-like cell death induced by P. brassicae eggs in B. rapa.

RESULTS

A germplasm screening of 56 B. rapa accessions, representing the genetic and geographical diversity of a B. rapa core collection, showed phenotypic variation for cell death. An image-based phenotyping protocol was developed to accurately measure size of HR-like cell death and was then used to identify two accessions that consistently showed weak (R-o-18) or strong cell death response (L58). Screening of 160 RILs derived from these two accessions resulted in three novel QTLs for Pieris brassicae-induced cell death on chromosomes A02 (Pbc1), A03 (Pbc2), and A06 (Pbc3). The three QTLs Pbc1-3 contain cell surface receptors, intracellular receptors and other genes involved in plant immunity processes, such as ROS accumulation and cell death formation. Synteny analysis with A. thaliana suggested that Pbc1 and Pbc2 are novel QTLs associated with this trait, while Pbc3 also contains an ortholog of LecRK-I.1, a gene of A. thaliana previously associated with cell death induced by a P. brassicae egg extract.

CONCLUSIONS

This study provides the first genomic regions associated with the Pieris egg-induced HR-like cell death in a Brassica crop species. It is a step closer towards unravelling the genetic basis of an egg-killing crop resistance trait, paving the way for breeders to further fine-map and validate candidate genes.

Identification of new proteins in mature sieve elements

The phloem enables vascular plants to transport photoassimilates from source tissues to heterotrophic sink tissues. In the phloem, unbroken strings of enucleated sieve elements, which lose the majority of their cellular contents upon maturation, provide a low resistance path for mass flow. The protein machinery in mature sieve elements performs vital functions to maintain the flow, transmit systemic signals and defend the sugar stream against pests. However, our knowledge of this particular protein population is very limited since mature sieve elements are difficult to isolate and not amenable to transcriptomic analysis due to their enucleate nature. Here, we used co-expression analysis and published gene clusters from transcriptomic studies to generate a list of sieve element proteins that potentially survive the enucleation process to reside in mature sieve elements. We selected seven candidates and show that they all localize in sieve elements in Arabidopsis roots and six of them in bolting stems. Our results support the idea that nascent sieve elements prior to enucleation translate part of the protein machinery found in mature sieve elements. Our co-expression list and the publicly available gene clusters expressed in late proto- and meta-phloem sieve elements are valuable resources for uncharacterized genes that may function in mature sieve elements.

Stationary sieve element proteins

Vascular plants use the phloem to move sugars and other molecules from source leaves to sink organs such as roots and fruits. Within the phloem, enucleate sieve elements provide the low-resistance pipe system that enable bulk flow of sap. In this review, we provide an overview of the highly specific protein machinery that localize to mature sieve elements without entering the phloem translocation stream. Generally, the proteins either maintain the flow, protect the sieve element against pathogens or transmit system wide signals. A notable exception is found in poppy, where part of the opium biosynthesis is compartmentalized in sieve elements. Biosynthesis of sieve element proteins happens either continuously in companion cell or transiently in immature sieve elements before nuclear disintegration. The latter population is translated during differentiation and stays functional without turnover during the entire lifespan of sieve elements. We discuss how protein longevity imposes some interesting restrictions on plants, especially in arborescent monocots with long living sieve elements.

SLI1 confers broad-spectrum resistance to phloem-feeding insects

Resistance (R) genes usually compete in a coevolutionary arms race with reciprocal effectors to confer strain-specific resistance to pathogens or herbivorous insects. Here, we investigate the specificity of SLI1, a recently identified R gene in Arabidopsis that encodes a small heat shock-like protein involved in resistance to Myzus persicae aphids. In a panel with several aphid and whitefly species, SLI1 compromised reproductive rates of three species: the tobacco aphid M. persicae nicotianae, the cabbage aphid Brevicoryne brassicae and the cabbage whitefly Aleyrodes proletella. Electrical penetration graph recording of aphid behaviour, revealed shorter salivations and a 3-to-5-fold increase in phloem feeding on sli1 loss-of-function plants. The mustard aphid Lipaphis erysimi and Bemisia tabaci whitefly were not affected by SLI1. Unlike the other two aphid species, L. erysimi exhibited repetitive salivations preceding successful phloem feeding, indicating a role of salivary effectors in overcoming SLI1-mediated resistance. Microscopic characterization showed that SLI1 proteins localize in the sieve tubes of virtually all above- and below-ground tissues and co-localize with the aphid stylet tip after penetration of the sieve element plasma membrane. These observations reveal an unconventional R gene that escapes the paradigm of strain specificity and confers broad-spectrum quantitative resistance to phloem-feeding insects.

Impact of Temperature on Survival Rate, Fecundity, and Feeding Behavior of Two Aphids, Aphis gossypii and Acyrthosiphon gossypii, When Reared on Cotton

Aphid performance is sensitive to temperature changes. Previous studies found that Acyrthosiphon gossypii (Mordviiko) was more sensitive to high temperature than Aphis gossypii (Glover). However, the effects of high temperatures on the survival, fecundity, and feeding behavior of these two aphid adults are not clear. This study examined the effect of different temperatures (29 °C, 32 °C, and 35 °C) on the adult survival rate, fecundity, and feeding behavior of these two aphid species. Our results showed that the adverse effects of high temperatures (32 °C and 35 °C) on aphid adult survival and fecundity were greater for Ac. gossypii than Ap. gossypii. The electrical penetration graph (EPG) data showed that Ac. gossypii spent more time feeding on xylem than phloem under all temperature treatments, which contrasted with Ap. gossypii. The time of phloem ingestion by Ap. gossypii at 32 °C was significantly higher than at 29 °C, while for Ac. gossypii, this value significantly decreased when temperature increased. These feeding patterns indicate that Ac. gossypii obtains less nutrition from phloem in support of its development and fecundity. Data generated in this study will serve as the basis for predicting the effects of increased temperature on these two cotton aphids.

Plant Defense Networks against Insect-Borne Pathogens

Upon infection with insect-borne microbial pathogens, plants are exposed to two types of damage simultaneously. Over the past decade, numerous molecular studies have been conducted to understand how plants respond to pathogens or herbivores. However, investigations of host responses typically focus on a single stress and are performed under static laboratory conditions. In this review, we highlight research that sheds light on how plants deploy broad-spectrum mechanisms against both vector-borne pathogens and insect vectors. Among the host genes involved in multistress resistance, many are involved in innate immunity and phytohormone signaling (especially jasmonate and salicylic acid). The potential for genome editing or chemical modulators to fine-tune crop defensive signaling, to develop sustainable methods to control insect-borne diseases, is discussed.

Ento(o)mics: the intersection of 'omic' approaches to decipher plant defense against sap-sucking insect pests

Plants are constantly challenged by insect pests that can dramatically decrease yields. Insects with piercing-sucking mouthparts, for example, aphids, whiteflies, and leaf hoppers, seemingly cause less physical damage to tissues, however, they feed on the plant's sap by piercing plant tissue and extracting plant fluids, thereby transmitting several plant-pathogenic viruses as well. As a counter-defense, plants activate an array of dynamic defense machineries against insect pests including the rapid reprogramming of the host cell processes. For a holistic understanding of plant-sap-sucking insect interactions, there is a need to call for techniques with the capacity to concomitantly capture these dynamic changes. Recent progress with various 'omic' technologies possess this capacity. In this review, we will provide a concise summary of application of 'omic' technologies and their utilization in plant and sap-sucking insect interaction studies. Finally, we will provide a perspective on the integration of 'omics' data in uncovering novel plant defense mechanisms against sap-sucking insect pests.

Defense Response in Chickpea Pod Wall due to Simulated Herbivory Unfolds Differential Proteome Profile

The pod wall of legumes is known to protect the developing seeds from pests and pathogens. However, the mechanism of conferring defense against insects has not yet been deciphered. Here, we have utilized 2-dimensional gel electrophoresis (2D-GE) coupled with mass spectrometry (MS/MS) to identify over expressed proteins in the pod wall of two different cultivars (commercial cultivar: JG 11 and tolerant cultivar: ICC 506-EB) of chickpea after 12 h of application of Helicoverpa armigera oral secretions (simulated herbivory). The assays were performed with a view that larvae are a voracious feeder and cause substantial damage to the pod within 12 h. A total of 600 reproducible protein spots were detected on gels, and the comparative analysis helped identify 35 (12 up-regulated, 23 down-regulated) and 20 (10 up-regulated, 10 down-regulated) differentially expressed proteins in JG 11 and ICC 506-EB, respectively. Functional classification of protein spots of each cultivar after MS/MS indicated that the differentially expressed proteins were associated with various metabolic activities. Also, stress-related proteins such as mannitol dehydrogenase (MADH), disease resistance-like protein-CSA1, serine/threonine kinase (D6PKL2), endoglucanase-19 etc. were up-regulated due to simulated herbivory. The proteins identified with a possible role in defense were further analyzed using the STRING database to advance our knowledge on their interacting partners. It decoded the involvement of several reactive oxygen species (ROS) scavengers and other proteins involved in cell wall reinforcement. The biochemical analysis also confirmed the active role of ROS scavengers during simulated herbivory. Thus, our study provides valuable new insights on chickpea-H.armigera interactions at the protein level.

Defenses against Virus and Vector: A Phloem-Biological Perspective on RTM- and SLI1-Mediated Resistance to Potyviruses and Aphids

Combining plant resistance against virus and vector presents an attractive approach to reduce virus transmission and virus proliferation in crops. RestrictedTobacco-etch virus Movement (RTM) genes confer resistance to potyviruses by limiting their long-distance transport. Recently, a close homologue of one of the RTM genes, SLI1, has been discovered but this gene instead confers resistance to Myzus persicae aphids, a vector of potyviruses. The functional connection between resistance to potyviruses and aphids, raises the question whether plants have a basic defense system in the phloem against biotic intruders. This paper provides an overview on restricted potyvirus phloem transport and restricted aphid phloem feeding and their possible interplay, followed by a discussion on various ways in which viruses and aphids gain access to the phloem sap. From a phloem-biological perspective, hypotheses are proposed on the underlying mechanisms of RTM- and SLI1-mediated resistance, and their possible efficacy to defend against systemic viruses and phloem-feeding vectors.

Aphid resistance in Capsicum maps to a locus containing LRR-RLK gene analogues

A QTL for aphid resistance on pepper chromosome 2 was identified and validated. This QTL affects aphid survival and reproduction, and was fine mapped to a locus containing LRR-RLK analogues. Myzus persicae is one of the most threatening insect pests that adversely affects pepper (Capsicum) cultivation. Resistance to aphids was previously identified in Capsicum baccatum. This study aimed at elucidating the genetics of aphid resistance in C. baccatum. A QTL analysis was carried out for M. persicae resistance in an F₂ population derived from an intraspecific cross between a highly resistant plant and a susceptible plant. Survival and reproduction were used as resistance parameters. Interval mapping detected two QTLs affecting aphid survival (Rmpas-1) and reproduction (Rmprp-1), respectively, both localized in the same area and sharing the same top marker on chromosome 2. Use of this marker as co-factor in multiple-QTL mapping analysis revealed a second, minor QTL (Rmprp-2) only affecting aphid reproduction, on chromosome 4. Fine mapping confirmed the effects of Rmpas-1 and Rmprp-1 and narrowed the major QTL Rmprp-1 down to a genomic region of 96 kb which is predicted to encode four analogues of resistance genes of the receptor-like kinase family containing a leucine-rich repeat domain (LRR-RLKs). This work provides not only initial information for breeding aphid-resistant pepper varieties, but also forms the basis for future molecular analysis of gene(s) involved in aphid resistance.

PECTIN ACETYLESTERASE9 Affects the Transcriptome and Metabolome and Delays Aphid Feeding

The plant cell wall plays an important role in damage-associated molecular pattern-induced resistance to pathogens and herbivorous insects. Our current understanding of cell wall-mediated resistance is largely based on the degree of pectin methylesterification. However, little is known about the role of pectin acetylesterification in plant immunity. This study describes how one pectin-modifying enzyme, PECTIN ACETYLESTERASE 9 (PAE9), affects the Arabidopsis (Arabidopsis thaliana) transcriptome, secondary metabolome, and aphid performance. Electro-penetration graphs showed that Myzus persicae aphids established phloem feeding earlier on pae9 mutants. Whole-genome transcriptome analysis revealed a set of 56 differentially expressed genes (DEGs) between uninfested pae9-2 mutants and wild-type plants. The majority of the DEGs were enriched for biotic stress responses and down-regulated in the pae9-2 mutant, including PAD3 and IGMT2, involved in camalexin and indole glucosinolate biosynthesis, respectively. Relative quantification of more than 100 secondary metabolites revealed decreased levels of several compounds, including camalexin and oxylipins, in two independent pae9 mutants. In addition, absolute quantification of phytohormones showed that jasmonic acid (JA), jasmonoyl-Ile, salicylic acid, abscisic acid, and indole-3-acetic acid were compromised due to PAE9 loss of function. After aphid infestation, however, pae9 mutants increased their levels of camalexin, glucosinolates, and JA, and no long-term effects were observed on aphid fitness. Overall, these data show that PAE9 is required for constitutive up-regulation of defense-related compounds, but that it is not required for aphid-induced defenses. The signatures of phenolic antioxidants, phytoprostanes, and oxidative stress-related transcripts indicate that the processes underlying PAE9 activity involve oxidation-reduction reactions.

Challenging battles of plants with phloem-feeding insects and prokaryotic pathogens

For the past 4 decades, intensive molecular studies of mostly leaf mesophyll cell-infecting pathogens and chewing insects have led to compelling models of plant-pathogen and plant-insect interactions. Yet, some of the most devastating pathogens and insect pests live in or feed on the phloem, a systemic tissue belonging to the plant vascular system. Phloem tissues are difficult to study, and phloem-inhabiting pathogens are often impossible to culture, thus limiting our understanding of phloem-insect/pathogen interactions at a molecular level. In this Perspective, we highlight recent literature that reports significant advances in the understanding of phloem interactions with insects and prokaryotic pathogens and attempt to identify critical questions that need attention for future research. It is clear that study of phloem-insect/pathogen interactions represents an exciting frontier of plant science, and influx of new scientific expertise and funding is crucial to achieve faster progress in this important area of research that is integral to global food security.

Performance of Chaetosiphon fragaefolii (Hemiptera: Aphididae) in Different Strawberry Cultivars

The green aphid Chaetosiphon fragaefolii (Cockerell) (Hemiptera: Aphididae) is one of the most important pests of strawberry production systems worldwide. One of the fundamental points for developing management strategies for this aphid is the understanding of its population growth since this allows predictions about future population peaks and which are the most appropriate hosts for its survival and reproduction. Thus, the goal of this study was to evaluate the biological and population growth parameters of C. fragaefolii in four strawberry cultivars (Albion, Aromas, Camarosa, and San Andreas). The highest survival of the nymph stage was observed in 'San Andreas' (87%) and the lowest in 'Camarosa' (43%). 'Albion' had the highest net reproductive rate (R₀ = 6.39) and the mean time for the population to double in number (TD = 5.61), thus presenting the best reproductive parameters for C. fragaefolii. 'Camarosa' and 'Aromas' had the lowest R₀ values (2.65 and 2.70, respectively) and the highest TD values (10.89 and 10.34, respectively). We conclude that antibiosis mechanisms are involved in the resistance process of 'Camarosa' to C. fragaefolii. The characterization and the use of resistant cultivars can be an essential strategy to assist aphid management.

Molecular Interactions Between Plants and Insect Herbivores

Diverse molecular processes regulate the interactions between plants and insect herbivores. Here, we review genes and proteins that are involved in plant-herbivore interactions and discuss how their discovery has structured the current standard model of plant-herbivore interactions. Plants perceive damage-associated and, possibly, herbivore-associated molecular patterns via receptors that activate early signaling components such as Ca²⁺, reactive oxygen species, and MAP kinases. Specific defense reprogramming proceeds via signaling networks that include phytohormones, secondary metabolites, and transcription factors. Local and systemic regulation of toxins, defense proteins, physical barriers, and tolerance traits protect plants against herbivores. Herbivores counteract plant defenses through biochemical defense deactivation, effector-mediated suppression of defense signaling, and chemically controlled behavioral changes. The molecular basis of plant-herbivore interactions is now well established for model systems. Expanding molecular approaches to unexplored dimensions of plant-insect interactions should be a future priority.

Plant defense against aphids, the pest extraordinaire

Aphids are amongst the most damaging pests of plants that use their stylets to penetrate the plant tissue to consume large amounts of phloem sap and thus deprive the plant of photoassimilates. In addition, some aphids vector important viral diseases of plants. Plant defenses targeting aphids are broadly classified as antibiosis, which interferes with aphid growth, survival and fecundity, and antixenosis, which influences aphid behavior, including plant choice and feeding from the sieve elements. Here we review the multitude of steps in the infestation process where these defenses can be exerted and highlight the progress made on identifying molecular factors and mechanisms that contribute to host defense, including plant resistance genes and signaling components, as well as aphid-derived effectors that elicit or attenuate host defenses. Also discussed is the impact of aphid-vectored plant viruses on plant-aphid interaction and the concept of tolerance, which allows plant to withstand or recover from damage resulting from the infestation.

An Integrated System for the Automated Recording and Analysis of Insect Behavior in T-maze Arrays

Host-plant resistance to insects like thrips and aphids is a complex trait that is difficult to phenotype quickly and reliably. Here, we introduce novel hardware and software to facilitate insect choice assays and automate the acquisition and analysis of movement tracks. The hardware consists of an array of individual T-mazes allowing simultaneous release of up to 90 insect individuals from their individual cage below each T-maze with choice of two leaf disks under a video camera. Insect movement tracks are acquired with computer vision software (EthoVision) and analyzed with EthoAnalysis, a novel software package that allows for automated reporting of highly detailed behavior parameters and statistical analysis. To validate the benefits of the system we contrasted two Arabidopsis accessions that were previously analyzed for differential resistance to western flower thrips. Results of two trials with 40 T-mazes are reported and we show how we arrived at optimized settings for the different filters and statistics. The statistics are reported in terms of frequency, duration, distance and speed of behavior events, both as sum totals and event averages, and both for the total trial period and in time bins of 1 h. Also included are higher level analyses with subcategories like short-medium-long events and slow-medium-fast events. The time bins showed how some behavior elements are more descriptive of differences between the genotypes during the first hours, whereas others are constant or become more relevant at the end of an 8 h recording. The three overarching behavior categories, i.e., choice, movement, and halting, were automatically corrected for the percentage of time thrips were detected and 24 out of 38 statistics of behavior parameters differed by a factor 2-6 between the accessions. The analysis resulted in much larger contrasts in behavior traits than reported previously. Compared to leaf damage assays on whole plants or detached leaves that take a week or more to complete, results were obtained in 8 h, with more detail, fewer individuals and higher significance. The potential value of the new integrated system, named EntoLab, for discovery of genetic traits in plants and insects by high throughput screening of large populations is discussed.

Plant Genes Benefitting Aphids-Potential for Exploitation in Resistance Breeding

Aphids are phloem sap-feeding insects common as pests in various crops. Here we review 62 omics studies of aphid/plant interactions to search for indications of how aphids may manipulate the plants to make them more suitable as hosts, i.e. more susceptible. Our aim is to try to reveal host plant susceptibility (S) genes, knowledge which can be exploited for making a plant more resistant to its pest by using new plant breeding techniques to knock out or down such S genes. S genes may be of two types, those that are involved in reducing functional plant defense and those involved in further increasing plant factors that are positive to the aphid, such as facilitated access to food or improved nutritional quality. Approximately 40% of the omics studies we have reviewed indicate how aphids may modify their host to their advantage. To exploit knowledge obtained so far, we suggest knocking out/down candidate aphid S genes using CRISPR/Cas9 or RNAi techniques in crops to evaluate if this will be sufficient to keep the aphid pest at economically viable levels without severe pleiotropic effects. As a complement, we also propose functional studies of recessively inherited resistance previously discovered in some aphid-crop combinations, to potentially identify new types of S genes that later could be knocked out or down also in other crops to improve their resistance to aphids.

Live-Cell Imaging of Fluorescently Tagged Phloem Proteins with Confocal Microscopy

Confocal laser scanning microscopy can enable observation of phloem cells in living tissues. Here we describe live imaging of phloem cells in the leaves and roots of Arabidopsis thaliana using fluorescently tagged proteins, either expressed in the vasculature using phloem specific promoters or constitutively expressed reference marker lines. Now, the majority of phloem cell types can be identified, allowing a precise cellular and subcellular localization of phloem proteins.

The rendez-vous of mobile sieve-element and abundant companion-cell proteins

Thousands of sieve tube exudate proteins (STEP) have now been identified and predicted to fulfill a diversity of functions. However, most STEPs should be considered putative, since methods to collect sieve tube exudates have many technical drawbacks, and advanced functional characterization will be required to distinguish contaminant from bonafide proteins, and determine the latter's location and activity in sieve elements (SE). One major challenge is to develop new approaches to elucidate the function of these SE proteins, which in turn, is expected to shed light on intriguing aspects of SE cell biology.