Arabidopsis, tobacco, nightshade and elm take insect eggs as herbivore alarm and show similar transcriptomic alarm responses

Plants respond to insect eggs with transcriptional changes, resulting in enhanced defence against hatching larvae. However, it is unknown whether phylogenetically distant plant species show conserved transcriptomic responses to insect eggs and subsequent larval feeding. We used Generally Applicable Gene set Enrichment (GAGE) on gene ontology terms to answer this question and analysed transcriptome data from Arabidopsis thaliana, wild tobacco (Nicotiana attenuata), bittersweet nightshade (Solanum dulcamara) and elm trees (Ulmus minor) infested by different insect species. The different plant–insect species combinations showed considerable overlap in their transcriptomic responses to both eggs and larval feeding. Within these conformable responses across the plant–insect combinations, the responses to eggs and feeding were largely analogous, and about one-fifth of these analogous responses were further enhanced when egg deposition preceded larval feeding. This conserved transcriptomic response to eggs and larval feeding comprised gene sets related to several phytohormones and to the phenylpropanoid biosynthesis pathway, of which specific branches were activated in different plant–insect combinations. Since insect eggs and larval feeding activate conserved sets of biological processes in different plant species, we conclude that plants with different lifestyles share common transcriptomic alarm responses to insect eggs, which likely enhance their defence against hatching larvae.

Systemic Colonization and Expression of Disease Symptoms on Bittersweet Nightshade (Solanum dulcamara) Infected with a GFP-Tagged Dickeya solani IPO2222 (IPO2254)

Colonization of Solanum dulcamara (bittersweet nightshade) plants by a GFP-tagged Dickeya solani type strain IPO2222 (IPO2254) was investigated by selective plating and epifluorescence stereomicroscopy (ESM), using in vitro plants and plants grown in compost soil. Replicated experiments were carried out in a growth chamber and the progress of infection and disease symptoms on tissue of the cultured plants, following leaf- and stem-base inoculations with bacteria, was evaluated. Microscopy observations were confirmed by spread-plating dilutions of plant extracts onto agar medium directly after the harvest. In experiments where the stem base of in vitro plants inoculated with a range of inocula of D. solani (104 to 108 colony forming units [cfu] ml−1) was examined at 14 days post infection (dpi), blackleg-like symptoms developed in more than 80% plants together with a reduction of the plant fitness (disease symptoms, weight, height, and appearance). In leaf-inoculated plants at 14 dpi, 15% of the plants exhibited severe blackleg-like symptoms. In detached S. dulcamara leaf assays, IPO2254 survived on the adaxial surface for 14 days at populations of 106 cfu per leaf. Thirty days after stem inoculation of plants grown in compost soil in pots, up to 104 cfu g−1 of GFP-tagged D. solani were found inside the stems. D. solani were detected inside the vascular tissue (xylem vessels) of stems, in the pith tissue in roots, and on the internal surface of the stem hollow. The implications of S. dulcamara infection by D. solani for the long-distance dispersal of the bacterial inoculum are discussed.

Inferring local movement of pathogen vectors among hosts

Herbivores often move among spatially interspersed host plants, tracking high-quality resources through space and time. This dispersal is of particular interest for vectors of plant pathogens. Existing molecular tools to track such movement have yielded important insights, but often provide insufficient genetic resolution to infer spread at finer spatiotemporal scales. Here, we explore the use of Nextera-tagmented reductively-amplified DNA (NextRAD) sequencing to infer movement of a highly-mobile winged insect, the potato psyllid (Bactericera cockerelli), among host plants. The psyllid vectors the pathogen that causes zebra chip disease in potato (Solanum tuberosum), but understanding and managing the spread of this pathogen is limited by uncertainty about the insect’s host plant(s) outside of the growing season. We identified 8,443 polymorphic loci among psyllids separated spatiotemporally on potato or in patches of bittersweet nightshade (S. dulcumara), a weedy plant proposed to be the source of potato-colonizing psyllids. A subset of the psyllids on potato exhibited close genetic similarity to insects on nightshade, consistent with regular movement between these two host plants. However, a second subset of potato-collected psyllids was genetically distinct from those collected on bittersweet nightshade; this suggests that a currently unrecognized host-plant species could be contributing to psyllid populations in potato. Oftentimes, dispersal of vectors of plant or animal pathogens must be tracked at a relatively fine scale in order to understand, predict, and manage disease spread. We demonstrate that emerging sequencing technologies that detect SNPs across a vector’s entire genome can be used to infer such localized movement.

First Report of Natural Infection by ‘Candidatus Liberibacter solanacearum’ in Bittersweet Nightshade (Solanum dulcamara) in the Columbia Basin of Eastern Oregon

Potatoes are a major crop in the Columbia Basin of Oregon and Washington, representing an annual farm gate value of almost $750 million. Zebra chip disease (ZC), a new and economically important disease of potato, was first reported in Oregon and Washington in 2011 (1). The disease is caused by the bacterium ‘Candidatus Liberibacter solanacearum’ (Lso, also referred to as ‘Ca. L. psyllaurous’), which is vectored by the potato psyllid (Bactericera cockerelli Sulc) (1,2). Identifying alternative hosts for Lso may facilitate management of ZC disease, which has increased potato production costs in the region. The perennial weed, bittersweet nightshade (Solanum dulcamara L.), is a year-round host of the potato psyllid (3) and is also a suspected host of Lso. However, little is known about the role of this weed in ZC epidemiology. Naturally occurring bittersweet nightshade plants (n = 21) were sampled at six different locations near Hermiston, Oregon, between May and October in 2012. These plants exhibited several symptoms associated with Lso, ranging from asymptomatic to slight purpling, chlorosis, or scorching of the foliage. However, S. dulcamara exhibits similar symptoms under a variety of environmental conditions (drought stress, etc.); therefore, it was difficult to identify potentially infected plants based solely on symptomology. Leaf and stem tissue (n = 21) was analyzed with high-fidelity PCR using species-specific primers for the 16S rDNA gene, CLipoF, and OI2c (2,4). Approximately 27.3% of the plants tested positive for Lso using these primers, including plants from the following locations on 16 April, 16 May, and 24 May, respectively: Hat Rock, OR (45°55.033′ N, 119°10.495′ W), Irrigon, OR (45°54.560′ N, 119°24.857′ W), and Stanfield, OR (45°46.971′ N, 119°13.203′ W). Three plants were selected for further PCR analysis with primers for the outer membrane protein gene, 1482f and 2086r (1). Amplicons obtained with both sets of PCR primers were directly sequenced. A BLAST analysis showed that the 16S rDNA gene sequence (993 to 1,000 bp) shared 99 to 100% identity with several Lso accessions, including JN848751.1 (from Washington) and JN848753.1 (from Oregon). Likewise, the outer membrane protein gene sequence (600 to 601 bp) shared 99 to 100% identity with ‘Ca. L. solanacearum’ accession KC768330.1 (from Honduras). All six sequences were deposited in GenBank (Accession Nos. KJ854199 to KJ854204). According to these findings, bittersweet nightshade may be an important annual source of Lso in the region, particularly since it serves as a host for the potato psyllid. Potato psyllids were also detected at two of the locations with infected S. dulcamara: Irrigon, OR, and Stanfield, OR. A subsample of the psyllids collected in 2012 were analyzed with PCR and Lso was detected in a sample from Stanfield, OR (5). Identifying perennial hosts of Lso promotes a better understanding of both ZC disease epidemiology and management. To our knowledge, this is the first report of Lso causing natural infections in S. dulcamara in the United States. References: (1) J. M. Crosslin et al. Plant Dis. 96:452, 2012. (2) S. Jagoueix et al. Mol. Cell. Probes 10:43, 1996. (3) A. F. Murphy et al. Am. J. Pot. Res. 90:294, 2013. (4) G. A. Secor et al. Plant Dis. 93:574, 2009. (5) K. D. Swisher et al. Am. J. Pot. Res. 90:570, 2013.

Investigating the Host Range of the US-22, US-23, and US-24 Clonal Lineages of Phytophthora infestans on Solanaceous Cultivated Plants and Weeds

Phytophthora infestans causes late blight, one of the most important diseases of potato and tomato worldwide. Recently in the United States, three newly identified clonal lineages, US-22, US-23, and US-24, have become widespread. While potato and tomato are the most commonly infected solanaceous hosts for P. infestans, new lineages may have a broader or different host range. Under controlled conditions, we determined the host range of isolates representing US-22, US-23, and US-24 genotypes of P. infestans on detached tissues of cultivated solanaceous plants and solanaceous weeds common to the upper midwestern production region. None of the isolates representing the clonal lineages produced late blight symptoms or signs on foliage of selected cultivars of eggplant, pepper, tomatillo, or ground cherry in a detached leaf assay. Symptoms and signs were evident on the potato and tomato cultivars tested, although with the US-24 isolate, infection on tomato was limited. None of the isolates sporulated on the common weed black nightshade, but some sporulation and necrosis was observed with all representatives of the lineages on bittersweet nightshade and petunia. Hairy nightshade supported abundant sporulation and symptoms, and sporangial production was not significantly different than that on tomato for each of the isolates representing the three lineages, indicating the potential for this weed to be a source of inoculum and contribute substantially to late blight epidemics. Interestingly, black nightshade had the highest incidence of sporulation on berries, but the lowest on leaves, suggesting the importance of testing multiple plant organs when determining susceptibility of a species. Our results update knowledge of the host range of the ever-changing P. infestans populations and will help to improve late blight management strategies by targeting these additional hosts.

Comparative Behavior of Ralstonia solanacearum Biovar 2 in Diverse Plant Species

Ralstonia solanacearum causes bacterial wilt in numerous plant species worldwide. Although biovar 2 mostly affects solanaceous crops, identification of new hosts remains a matter of concern since there is still no clear-cut distinction between host and nonhost plants. In this work we provide data based on histological studies on the status of 20 plant species, most of them of potential interest in crop rotation. Plants were watered with a β-glucuronidase-expressing derivative of R. solanacearum biovar 2, and after a month of incubation, sections of roots and stems were analyzed to localize the pathogen on surface, in cortex and/or xylem. Depending on whether the xylem was colonized or not, plants were classified as hosts or nonhosts, respectively. Hosts generally affected in a few xylem vessels or occasionally in all xylem bundles were classified as tolerant. These included some cabbage, kidney bean, and rutabaga cultivars, and the weed bittersweet nightshade (Solanum dulcamara). Nonhosts were the cultivars tested of alfalfa, barley, black radish, carrot, celery, colocynth, fennel, fiber flax, field bean, field pea, horseradish, maize, and zucchini. However, barley and maize, though nonhosts, may act as reservoirs for the pathogen. The present work constitutes a basis for further studies on cropping systems in fields where R. solanacearum has been detected.