Spotlight on the Roles of Whitefly Effectors in Insect–Plant Interactions

The Bemisia tabaci species complex (whitefly) causes enormous agricultural losses. These phloem-feeding insects induce feeding damage and transmit a wide range of dangerous plant viruses. Whiteflies colonize a broad range of plant species that appear to be poorly defended against these insects. Substantial research has begun to unravel how phloem feeders modulate plant processes, such as defense pathways, and the central roles of effector proteins, which are deposited into the plant along with the saliva during feeding. Here, we review the current literature on whitefly effectors in light of what is known about the effectors of phloem-feeding insects in general. Further analysis of these effectors may improve our understanding of how these insects establish compatible interactions with plants, whereas the subsequent identification of plant defense processes could lead to improved crop resistance to insects. We focus on the core concepts that define the effectors of phloem-feeding insects, such as the criteria used to identify candidate effectors in sequence-mining pipelines and screens used to analyze the potential roles of these effectors and their targets in planta. We discuss aspects of whitefly effector research that require further exploration, including where effectors localize when injected into plant tissues, whether the effectors target plant processes beyond defense pathways, and the properties of effectors in other insect excretions such as honeydew. Finally, we provide an overview of open issues and how they might be addressed.

Cytological aspects of compatible and incompatible interactions between cashew (Anacardium occidentale L.) seedlings and isolates of Colletotrichum gloeosporioides (Penz.) complex

A strategy in the control anthracnose of cashew (Anacardium occidentale L.) is the management of crop phenology and defense mechanisms of this host. In previous studies, under controlled conditions, the seedling reactions of 5 cashew clones (CAP-14, CCP-06, CCP-09, CCP-76 and CCP-1001) to 36 isolates of Colletorichum gloeosporioides Penz. complex (LARS- 905 to 940) was evaluated. However, good field management requires information about the infection process. This research aimed to clarify cytophysiological aspects of three compatible interactions of this pathosystem (isolates LARS-905 and 910 × CCP-76; LARS-910 × CCP-1001) and an incompatible one (LARS-905 × CCP-1001), using infected leaves/stems and microscopy (light, scanning and transmission electron). No significant differences were found prior to penetration. In the susceptible combinations, 36-66 h after inoculation, a thin primary hypha (TPH) formed in the invaded epidermal cell, widening as a large primary hypha (LPH), which filled the cell lumen simultaneously with accumulation of of yellow-brown lignopolysaccharides. Then, a thin secondary hypha (TSH) developed from the LPH, penetrating adjacent cells before the first became necrotic. In the incompatible interaction, the response of the first invaded cell was faster and more intense, with formation of papilla and lignopolysaccharide-protein-silicon complex usually blocking the pathogen.

Diversity of Expression Types of Ht Genes Conferring Resistance in Maize to Exserohilum turcicum

Northern corn leaf blight (NCLB) is an important leaf disease in maize (Zea mays) worldwide and is spreading into new areas with expanding maize cultivation, like Germany. Exserohilum turcicum, causal agent of NCLB, infects and colonizes leaf tissue and induces elongated necrotic lesions. Disease control is based on fungicide application and resistant cultivars displaying monogenic resistance. Symptom expression and resistance mechanisms differ in plants carrying different resistance genes. Therefore, histological studies and DNA quantification were performed to compare the pathogenesis of E. turcicum races in maize lines exhibiting compatible or incompatible interactions. Maize plants from the differential line B37 with and without resistance genes Ht1, Ht2, Ht3, and Htn1 were inoculated with either incompatible or compatible races (race 0, race 1 and race 23N) of E. turcicum. Leaf segments from healthy and inoculated plants were collected at five different stages of infection and disease development from penetration (0–1 days post inoculation - dpi), until full symptom expression (14–18 dpi). Symptoms of resistance responses conveyed by the different Ht genes considerably differed between Ht1 (necrotic lesions with chlorosis), Ht2 (chlorosis and small lesions), Ht3 (chlorotic spots) and Htn1 (no lesions or wilt-type lesions). In incompatible interactions, fungal DNA was only detected in very low amounts. At 10 dpi, DNA content was elevated in all compatible interactions. Histological studies with Chlorazol Black E staining indicated that E. turcicum formed appressoria and penetrated the leaf surface directly in both types of interaction. In contrast to incompatible interactions, however, the pathogen was able to penetrate into xylem vessels at 6 dpi in compatible interactions and strongly colonized the mesophyll at 12 dpi, which is considered the crucial process differentiating susceptible from resistant interactions. Following the distinct symptom expressions, resistance mechanisms conferred by Ht1, Ht2, Ht3, and Htn1 genes apparently are different. Lower disease levels and a delayed progress of infection in compatible interactions with resistant lines imply that maize R genes to E. turcicum are associated with or confer additional quantitative resistance.

Highly conserved genes expressed in aphid saliva are candidates for host plant adaptation in Aphis gossypii

BackgroundAphids are major crop pests, most species attacking crops specialize on a narrow range of plant species from a single family. By contrast, Aphis gossypii is a highly polyphagous species, for which host races specializing on particular crops have been clearly described. Salivary components, which aphids inject into the phloem via their stylets, play a key role in establishing compatible interactions between plants and aphids, and are probably involved in specialization.ResultsWe used the extensive resources available for Myzus persicae and Acyrthosiphon pisum to identify putative salivary proteins expressed in Aphis gossypii, despite the lack of genomic resources for this species. In silico, we identified 51 putative salivary proteins; we focused on 17 genes with orthologs in at least one aphid species, assuming that some of the conserved genes expressed in salivary glands are involved in host specialization. We amplified and sequenced 10 coding sequences in full, from 17 clones of Aphis gossypii specialising on plants from Malvaceae, Cucurbitaceae or Solanaceae. We reconstructed the phylogenetic tree for these genes, on which we identified a clade corresponding to all clones specializing on cucurbits. Three of these genes were under positive selection.ConclusionsFull adaptation to a particular host plant may require a combination of alleles at quantitative trait loci in aphids. The three genes we identified could potentially be part of a cocktail of effectors manipulating the immune system of cucurbits and therefore responsible for A. gossypii specialization on that plant family.

Effect of SnTOX effectors on the virulence degree of Stagonospora nodorum isolates

Stagonospora nodorum Berk. is the causal agent of Septoria nodorum blotch (SNB) of wheat (Triticum aestivum L.). It synthesizes host-specific necrotrophic effectors (NEs), which facilitate infection process and ensure virulence of pathogen on host plant with a dominant susceptibility gene. The interaction of virulence genes products of the NEs pathogen (SnTox) with susceptibility genes products of the host plant (Snn) in the S. nodorum - wheat pathosystem is carried out in inverted gene-for-gene system and causes the development of disease. In this study, we tested three main NEs SnToxA, SnTox1, SnTox3, which have already been identified in S. nodorum at the gene level. The NEs role in the development of SNB has already been proven; however, the overall host response to SNB does not always strictly follow the inverted gene-for-gene system, as multiple SnTox-Snn interactions can be additive or epistatic. In this regard, the aim of the work was to identify the NE genes in three S. nodorum isolates and to study effect of NEs genes transcriptional activity on the isolate virulence. We have shown that all three NEs SnToxA, SnTox3 and SnTox1 played an important role in the development of the disease in compatible interactions. Effectors SnTox3 and SnTox1 exhibited epistatic interaction that was removed by a triple compatible interaction (SnTox3-Snn3, SnToxA-Tsn1 and SnTox1-Snn1). This effect was shown by us for the first time. The mechanisms of epistatic and additive interactions, as well as the virulence of the isolate were associated with the regulation of the NEs genes transcriptional activity. The avirulent isolate Sn4VD lacked transcription of all three NEs genes, and the virulent isolate Sn9MH was characterized by a high level of mRNA accumulation of all three NEs genes during infection on susceptible cultivar. We also showed that SnTox expression depended both on the host genotype in SnToxA and SnTox3 and on the number of compatible interactions exhibiting additive or epistatic interactions in SnTox1 and SnTox3. Finally, the virulence of the S. nodorum isolate depended on the qualitative and quantitative composition of NEs.

Investigations into a putative role for the novel BRASSIKIN pseudokinases in compatible pollen-stigma interactions in Arabidopsis thaliana

Background In the Brassicaceae, the early stages of compatible pollen-stigma interactions are tightly controlled with early checkpoints regulating pollen adhesion, hydration and germination, and pollen tube entry into the stigmatic surface. However, the early signalling events in the stigma which trigger these compatible interactions remain unknown. Results A set of stigma-expressed pseudokinase genes, termed BRASSIKINs (BKNs), were identified and found to be present in only core Brassicaceae genomes. In Arabidopsis thaliana Col-0, BKN1 displayed stigma-specific expression while the BKN2 gene was expressed in other tissues as well. CRISPR deletion mutations were generated for the two tandemly linked BKNs, and very mild hydration defects were observed for wild-type Col-0 pollen when placed on the bkn1/2 mutant stigmas. In further analyses, the predominant transcript for the stigma-specific BKN1 was found to have a premature stop codon in the Col-0 ecotype, but a survey of the 1001 Arabidopsis genomes uncovered three ecotypes that encoded a full-length BKN1 protein. Furthermore, phylogenetic analyses identified intact BKN1 orthologues in the closely related outcrossing Arabidopsis species, A. lyrata and A. halleri. Finally, the BKN pseudokinases were found to be plasma-membrane localized through the dual lipid modification of myristoylation and palmitoylation, and this localization would be consistent with a role in signaling complexes. Conclusion In this study, we have characterized the novel Brassicaceae-specific family of BKN pseudokinase genes, and examined the function of BKN1 and BKN2 in the context of pollen-stigma interactions in A. thaliana Col-0. Additionally, premature stop codons were identified in the predicted stigma specific BKN1 gene in a number of the 1001 A. thaliana ecotype genomes, and this was in contrast to the out-crossing Arabidopsis species which carried intact copies of BKN1. Thus, understanding the function of BKN1 in other Brassicaceae species will be a key direction for future studies.

A Fruitful Journey: Pollen Tube Navigation from Germination to Fertilization

In flowering plants, pollen tubes undergo tip growth to deliver two nonmotile sperm to the ovule where they fuse with an egg and central cell to achieve double fertilization. This extended journey involves rapid growth and changes in gene activity that manage compatible interactions with at least seven different cell types. Nearly half of the genome is expressed in haploid pollen, which facilitates genetic analysis, even of essential genes. These unique attributes make pollen an ideal system with which to study plant cell–cell interactions, tip growth, cell migration, the modulation of cell wall integrity, and gene expression networks. We highlight the signaling systems required for pollen tube navigation and the potential roles of Ca2+signals. The dynamics of pollen development make sexual reproduction highly sensitive to heat stress. Understanding this vulnerability may generate strategies to improve seed crop yields that are under threat from climate change.