Ultrastructural study of anther parasitism of Ficus laevigata by Ficophagus laevigatus (Aphelenchoididae).

Transmission electron microscopy (TEM) was used to compare the ultrastructural differences between healthy male florets (anthers) and one floret parasitized by Ficophagus laevigatus in late phase C syconia of Ficus laevigata from southern Florida. Previous light-microscopic examination of paraffin-sectioned material showed that F. laevigatus-infested anthers of F. laevigata manifested as malformed, often with aberrant pollen and hypertrophied epidermal cells closest to regions containing propagating nematodes. Female florets or fig wasp-parasitized female florets were not observed to be parasitized by nematodes. Considering that plant-feeding in the Aphelenchoididae is purportedly much less specialized than in certain groups of the Tylenchomorpha, where specialized hypertrophied feeder cells are produced in response to nematode feeding, we examined the putative induced response in this unusual aphelenchoidid system with the higher resolution afforded by TEM. TEM confirmed the expression of significant epidermal cell hypertrophy of the anther and anther filament in the presence of propagating nematodes, which was expressed as cell enlargement (2-5X), fractionation of large electron-dense stores into smaller aggregates, irregularly shaped nuclei enclosed by an elongated nuclear envelope, nucleolus enlargement, increased organelle production, and apparent metabolism with increased numbers of mitochondria, pro-plastids, and endoplasmic reticulum, as well as increased thickening of the cell walls. Pathological effects were observed in adjacent cells/tissue (e.g., anther and anther filament parenchymal cells, pollen tubes, pollen, and endothecium) with apparent diminishment as the distance from propagating nematodes increased (which was also probably affected by number of nematodes). Some TEM sections captured previously undocumented ultrastructural highlights of propagating individuals of F. laevigatus.

The Aphelenchoides genomes reveal substantial horizontal gene transfers in the last common ancestor of free-living and major plant-parasitic nematodes.

Aphelenchoides besseyi is a plant-parasitic nematode (PPN) in the family Aphelenchoididae capable of infecting more than 200 plant species. A. besseyi is also a species complex with strains exhibiting varying pathogenicity to plants. We present the genome and annotations of six Aphelenchoides species, four of which belonged to the A. besseyi species complex. Most Aphelenchoides genomes have a size of 44.7-47.4 Mb and are among the smallest in clade IV, with the exception of A. fujianensis, which has a size of 143.8 Mb and is one of the largest. Phylogenomic analysis successfully delimited the species complex into A. oryzae and A. pseudobesseyi and revealed a reduction of transposon elements in the last common ancestor of Aphelenchoides. Synteny analyses between reference genomes indicated that three chromosomes in A. besseyi were derived from fission and fusion events. A systematic identification of horizontal gene transfer (HGT) genes across 27 representative nematodes allowed us to identify two major episodes of acquisition corresponding to the last common ancestor of clade IV or major PPNs, respectively. These genes were mostly lost and differentially retained between clades or strains. Most HGT events were acquired from bacteria, followed by fungi, and also from plants; plant HGT was especially prevalent in Bursaphelenchus mucronatus. Our results comprehensively improve the understanding of HGT in nematodes.

Sedentary Plant-Parasitic Nematodes Alter Auxin Homeostasis via Multiple Strategies.

Sedentary endoparasites such as cyst and root-knot nematodes infect many important food crops and are major agro-economical pests worldwide. These plant-parasitic nematodes exploit endogenous molecular and physiological pathways in the roots of their host to establish unique feeding structures. These structures function as highly active transfer cells and metabolic sinks and are essential for the parasites' growth and reproduction. Plant hormones like indole-3-acetic acid (IAA) are a fundamental component in the formation of these feeding complexes. However, their underlying molecular and biochemical mechanisms are still elusive despite recent advances in the field. This review presents a comprehensive overview of known functions of various auxins in plant-parasitic nematode infection sites, based on a systematic analysis of current literature. We evaluate multiple aspects involved in auxin homeostasis in plants, including anabolism, catabolism, transport, and signalling. From these analyses, a picture emerges that plant-parasitic nematodes have evolved multiple strategies to manipulate auxin homeostasis to establish a successful parasitic relationship with their host. Additionally, there appears to be a potential role for auxins other than IAA in plant-parasitic nematode infections that might be of interest to be further elucidated.

Salinity effect on Cuscuta campestris Yunck. Parasitism on Arabidopsis thaliana L.

Stem holoparasitic flowering plants of the genus Cuscuta are globally distributed invasive species and agricultural pests. The present research represents the combined effect of salt stress (e.g. abiotic stress) and Cuscuta campestris infection (e.g. biotic stress) on the model host plant Arabidopsis thaliana and the response of the parasite to salinity. The response of these parasites to abiotic stress conditions including salinity is poorly studied. Arabidopsis plants were continuously irrigated with 0, 50 and 150 mM NaCl and subjected to C. campestris infection. The influence of both abiotic and biotic stresses on the major osmoprotectant L-proline and three antioxidant enzymes - catalase, superoxide dismutase and guaiacol peroxidase, was assessed in both the parasite and the host plant. All four biochemical markers were differentially affected by stress, showing that the influence of C. campestris parasitism and its interaction with salinity is mostly in the site of infection (direct response) and also in roots (indirect vertical response) rather than on non-infected leaves of infected plants (indirect horizontal response). Despite its absence of soil contact, C. campestris was also significantly affected by salinity (indirect response). The mutual adaptation of the parasite-host pair to salinity slightly altered the regular response to abiotic stress of A. thaliana, but no detrimental additive effect of biotic and abiotic stress was observed.

Mixotrophy in Land Plants: Why To Stay Green?

Mixotrophic plants combine photosynthesis and heterotrophic nutrition. Recent research suggests mechanisms explaining why mixotrophy is so common in terrestrial ecosystems. First, mixotrophy overcomes nutrient limitation and/or seedling establishment constraints. Second, although genetic drift may push mixotrophs to full heterotrophy, the role of photosynthesis in reproduction stabilizes mixotrophy.

Genome-wide expert annotation of the epigenetic machinery of the plant-parasitic nematodes Meloidogyne spp., with a focus on the asexually reproducing species.

BACKGROUND: The renewed interest in epigenetics has led to the understanding that both the environment and individual lifestyle can directly interact with the epigenome to influence its dynamics. Epigenetic phenomena are mediated by DNA methylation, stable chromatin modifications and non-coding RNA-associated gene silencing involving specific proteins called epigenetic factors. Multiple organisms, ranging from plants to yeast and mammals, have been used as model systems to study epigenetics. The interactions between parasites and their hosts are models of choice to study these mechanisms because the selective pressures are strong and the evolution is fast. The asexually reproducing root-knot nematodes (RKN) offer different advantages to study the processes and mechanisms involved in epigenetic regulation. RKN genomes sequencing and annotation have identified numerous genes, however, which of those are involved in the adaption to an environment and potentially relevant to the evolution of plant-parasitism is yet to be discovered. RESULTS: Here, we used a functional comparative annotation strategy combining orthology data, mining of curated genomics as well as protein domain databases and phylogenetic reconstructions. Overall, we show that (i) neither RKN, nor the model nematode Caenorhabditis elegans possess any DNA methyltransferases (DNMT) (ii) RKN do not possess the complete machinery for DNA methylation on the 6th position of adenine (6mA) (iii) histone (de)acetylation and (de)methylation pathways are conserved between C. elegans and RKN, and the corresponding genes are amplified in asexually reproducing RKN (iv) some specific non-coding RNA families found in plant-parasitic nematodes are dissimilar from those in C. elegans. In the asexually reproducing RKN Meloidogyne incognita, expression data from various developmental stages supported the putative role of these proteins in epigenetic regulations. CONCLUSIONS: Our results refine previous predictions on the epigenetic machinery of model species and constitute the most comprehensive description of epigenetic factors relevant to the plant-parasitic lifestyle and/or asexual mode of reproduction of RKN. Providing an atlas of epigenetic factors in RKN is an informative resource that will enable researchers to explore their potential role in adaptation of these parasites to their environment.

The Transcriptomes of Xiphinema index and Longidorus elongatus Suggest Independent Acquisition of Some Plant Parasitism Genes by Horizontal Gene Transfer in Early-Branching Nematodes.

Nematodes have evolved the ability to parasitize plants on at least four independent occasions, with plant parasites present in Clades 1, 2, 10 and 12 of the phylum. In the case of Clades 10 and 12, horizontal gene transfer of plant cell wall degrading enzymes from bacteria and fungi has been implicated in the evolution of plant parasitism. We have used ribonucleic acid sequencing (RNAseq) to generate reference transcriptomes for two economically important nematode species, Xiphinema index and Longidorus elongatus, representative of two genera within the early-branching Clade 2 of the phylum Nematoda. We used a transcriptome-wide analysis to identify putative horizontal gene transfer events. This represents the first in-depth transcriptome analysis from any plant-parasitic nematode of this clade. For each species, we assembled ~30 million Illumina reads into a reference transcriptome. We identified 62 and 104 transcripts, from X. index and L. elongatus, respectively, that were putatively acquired via horizontal gene transfer. By cross-referencing horizontal gene transfer prediction with a phylum-wide analysis of Pfam domains, we identified Clade 2-specific events. Of these, a GH12 cellulase from X. index was analysed phylogenetically and biochemically, revealing a likely bacterial origin and canonical enzymatic function. Horizontal gene transfer was previously shown to be a phenomenon that has contributed to the evolution of plant parasitism among nematodes. Our findings underline the importance and the extensiveness of this phenomenon in the evolution of plant-parasitic life styles in this speciose and widespread animal phylum.

Genome Evolution of Plant-Parasitic Nematodes.

Plant parasitism has evolved independently on at least four separate occasions in the phylum Nematoda. The application of next-generation sequencing (NGS) to plant-parasitic nematodes has allowed a wide range of genome- or transcriptome-level comparisons, and these have identified genome adaptations that enable parasitism of plants. Current genome data suggest that horizontal gene transfer, gene family expansions, evolution of new genes that mediate interactions with the host, and parasitism-specific gene regulation are important adaptations that allow nematodes to parasitize plants. Sequencing of a larger number of nematode genomes, including plant parasites that show different modes of parasitism or that have evolved in currently unsampled clades, and using free-living taxa as comparators would allow more detailed analysis and a better understanding of the organization of key genes within the genomes. This would facilitate a more complete understanding of the way in which parasitism has shaped the genomes of plant-parasitic nematodes.

From Fossil Parasitoids to Vectors: Insects as Parasites and Hosts.

Within Metazoa, it has been proposed that as many as two-thirds of all species are parasitic. This propensity towards parasitism is also reflected within insects, where several lineages independently evolved a parasitic lifestyle. Parasitic behaviour ranges from parasitic habits in the strict sense, but also includes parasitoid, phoretic or kleptoparasitic behaviour. Numerous insects are also the host for other parasitic insects or metazoans. Insects can also serve as vectors for numerous metazoan, protistan, bacterial and viral diseases. The fossil record can report this behaviour with direct (parasite associated with its host) or indirect evidence (insect with parasitic larva, isolated parasitic insect, pathological changes of host). The high abundance of parasitism in the fossil record of insects can reveal important aspects of parasitic lifestyles in various evolutionary lineages. For a comprehensive view on fossil parasitic insects, we discuss here different aspects, including phylogenetic systematics, functional morphology and a direct comparison of fossil and extant species.