Kari A. Peter

Discovery and Targeted LC-MS/MS of Purified Polerovirus Reveals Differences in the Virus-Host Interactome Associated with Altered Aphid Transmission

Circulative transmission of viruses in the Luteoviridae, such as cereal yellow dwarf virus (CYDV), requires a series of precisely orchestrated interactions between virus, plant, and aphid proteins. Natural selection has favored these viruses to be retained in the phloem to facilitate acquisition and transmission by aphids. We show that treatment of infected oat tissue homogenate with sodium sulfite reduces transmission of the purified virus by aphids. Transmission electron microscopy data indicated no gross change in virion morphology due to treatments. However, treated virions were not acquired by aphids through the hindgut epithelial cells and were not transmitted when injected directly into the hemocoel. Analysis of virus preparations using nanoflow liquid chromatography coupled to tandem mass spectrometry revealed a number of host plant proteins co-purifying with viruses, some of which were lost following sodium sulfite treatment. Using targeted mass spectrometry, we show data suggesting that several of the virus-associated host plant proteins accumulated to higher levels in aphids that were fed on CYDV-infected plants compared to healthy plants. We propose two hypotheses to explain these observations, and these are not mutually exclusive: (a) that sodium sulfite treatment disrupts critical virion-host protein interactions required for aphid transmission, or (b) that host infection with CYDV modulates phloem protein expression in a way that is favorable for virus uptake by aphids. Importantly, the genes coding for the plant proteins associated with virus may be examined as targets in breeding cereal crops for new modes of virus resistance that disrupt phloem-virus or aphid-virus interactions.

Mutation of the Erwinia amylovora argD Gene Causes Arginine Auxotrophy, Nonpathogenicity in Apples, and Reduced Virulence in Pears

ABSTRACT Fire blight is caused by Erwinia amylovora and is the most destructive bacterial disease of apples and pears worldwide. In this study, we found that E. amylovora argD(1000)::Tn5, an argD Tn5 transposon mutant that has the Tn5 transposon inserted after nucleotide 999 in the argD gene-coding region, was an arginine auxotroph that did not cause fire blight in apple and had reduced virulence in immature pear fruits. The E. amylovora argD gene encodes a predicted N-acetylornithine aminotransferase enzyme, which is involved in the production of the amino acid arginine. A plasmid-borne copy of the wild-type argD gene complemented both the nonpathogenic and the arginine auxotrophic phenotypes of the argD(1000)::Tn5 mutant. However, even when mixed with virulent E. amylovora cells and inoculated onto immature apple fruit, the argD(1000)::Tn5 mutant still failed to grow, while the virulent strain grew and caused disease. Furthermore, the pCR2.1-argD complementation plasmid was stably maintained in the argD(1000)::Tn5 mutant growing in host tissues without any antibiotic selection. Therefore, the pCR2.1-argD complementation plasmid could be useful for the expression of genes, markers, and reporters in E. amylovora growing in planta, without concern about losing the plasmid over time. The ArgD protein cannot be considered an E. amylovora virulence factor because the argD(1000)::Tn5 mutant was auxotrophic and had a primary metabolism defect. Nevertheless, these results are informative about the parasitic nature of the fire blight disease interaction, since they indicate that E. amylovora cannot obtain sufficient arginine from apple and pear fruit tissues or from apple vegetative tissues, either at the beginning of the infection process or after the infection has progressed to an advanced state.

First Report of Botryosphaeria dothidea Causing White Rot on Apple Fruit in Maryland

Botryosphaeria dothidea (Moug.: Fr.) Ces. & De Not has a worldwide distribution infecting species from over 80 genera of plants (1). Apart from being an important pathogen of apple trees in many countries, B. dothidea can cause pre- and postharvest decay on apple fruit (2). It has been known to cause canker and dieback of forest trees in Serbia (3), but has not been recorded either on apple trees or apple fruit. In December 2010, apple fruit cv. Idared (Malus × domestica Borkh.) with symptoms of white rot were collected from one storage in the area of Svilajnac in Serbia. The incidence of the disease was low but the symptoms were severe. Affected fruit were brown, soft, and almost completely decayed, while the internal decayed tissue appeared watery and brown. A fungus was isolated from symptomatic tissue of one fruit after surface sterilization with 70% ethanol (without rinsing) and aseptic removal of the skin. Small fragments of decayed tissue were placed on potato dextrose agar (PDA) and incubated in a chamber at 22°C under alternating light and dark conditions (12/12 h). Fungal colonies were initially whitish, but started turning dark gray to black after 5 to 6 days. Pycnidia were produced after 20 to 25 days of incubation at 22°C and contained one-celled, elliptical, hyaline conidia. Conidia were 17.19 to 23.74 μm (mean 18.93) × 3.72 to 4.93 μm (mean 4.45) (n = 50). These morphological characteristics are in accordance with those described for the fungus B. dothidea (4). Genomic DNA was isolated from the fungus and internal transcribed spacer (ITS) region of rDNA was amplified with the primers ITS1/ITS4 and sequenced. The nucleotide sequence has been assigned to GenBank Accession No. KC994640. BLAST analysis of the 528-bp segment showed a 100% similarity with several sequences of B. dothidea deposited in NCBI GenBank, which confirmed morphological identification. Pathogenicity was tested by wound inoculation of five surface-sterilized, mature apple fruit cv. Idared with mycelium plugs (5 mm in diameter) of the isolate grown on PDA. Five control fruit were inoculated with sterile PDA plugs. After 5 days of incubation in plastic containers, under high humidity (RH 90 to 95%) at 22°C, typical symptoms of white rot developed on inoculated fruit, while wounded, uninoculated, control fruit remained symptomless. The isolate recovered from symptomatic fruit showed the same morphological features as original isolate. To the best of our knowledge, this is the first report of B. dothidea on apple fruit in Serbia. Apple is widely grown in Serbia and it is important to further investigate the presence of this pathogen in apple storage, as well as in orchards since B. dothidea may cause rapid disease outbreaks that result in severe losses. References: (1) G. H. Hapting Agriculture Handbook 386, USDA, Forest Service, 1971. (2) A. L. Jones and H. S. Aldwinckle Compendium of Apple and Pear Diseases. APS Press, St. Paul, MN, 1990. (3) D. Karadžic et al. Glasnik Šumarskog Fakulteta 83:87, 2000. (4) B. Slippers et al. Mycologia 96:83, 2004.

Characterization of Postharvest Fungicide-Resistant Botrytis cinerea Isolates From Commercially Stored Apple Fruit

Botrytis cinerea causes gray mold and is an economically important postharvest pathogen of fruit, vegetables, and ornamentals. Fludioxonil-sensitive B. cinerea isolates were collected in 2011 and 2013 from commercial storage in Pennsylvania. Eight isolates had values for effective concentrations for inhibiting 50% of mycelial growth of 0.0004 to 0.0038 μg/ml for fludioxonil and were dual resistant to pyrimethanil and thiabendazole. Resistance was generated in vitro, following exposure to a sublethal dose of fludioxonil, in seven of eight dual-resistant B. cinerea isolates. Three vigorously growing B. cinerea isolates with multiresistance to postharvest fungicides were further characterized and found to be osmosensitive and retained resistance in the absence of selection pressure. A representative multiresistant B. cinerea strain caused decay on apple fruit treated with postharvest fungicides, which confirmed the in vitro results. The R632I mutation in the Mrr1 gene, associated with fludioxonil resistance in B. cinerea, was not detected in multipostharvest fungicide-resistant B. cinerea isolates, suggesting that the fungus may be using additional mechanisms to mediate resistance. Results from this study show for the first time that B. cinerea with dual resistance to pyrimethanil and thiabendazole can also rapidly develop resistance to fludioxonil, which may pose control challenges in the packinghouse environment and during long-term storage.

Erwinia amylovora pyrC mutant causes fire blight despite pyrimidine auxotrophy

Erwinia amylovora bacteria cause fire blight disease, which affects apple and pear production worldwide. The Erw. amylovora pyrC gene encodes a predicted dihydroorotase enzyme involved in pyrimidine biosynthesis. Here, we discovered that the Erw. amylovora pyrC244::Tn5 mutant was a uracil auxotroph. Unexpectedly, the Erw. amylovora pyrC244::Tn5 mutant grew as well as the wild‐type in detached immature apple and pear fruits. Fire blight symptoms caused by the pyrC244::Tn5 mutant in immature apple and pear fruits were attenuated compared to those caused by the wild‐type. The pyrC244::Tn5 mutant also caused severe fire blight symptoms in apple tree shoots. A plasmid‐borne copy of the wild‐type pyrC gene restored prototrophy and symptom induction in apple and pear fruit to the pyrC244::Tn5 mutant. These results suggest that Erw. amylovora can obtain sufficient pyrimidine from the host to support bacterial growth and fire blight disease development, although de novo pyrimidine synthesis by Erw. amylovora is required for full symptom development in fruits.

Characterization of a new apple luteovirus identified by high-throughput sequencing

Background‘Rapid Apple Decline’ (RAD) is a newly emerging problem of young, dwarf apple trees in the Northeastern USA. The affected trees show trunk necrosis, cracking and canker before collapse in summer. In this study, we discovered and characterized a new luteovirus from apple trees in RAD-affected orchards using high-throughput sequencing (HTS) technology and subsequent Sanger sequencing.MethodsIllumina NextSeq sequencing was applied to total RNAs prepared from three diseased apple trees. Sequence reads were de novo assembled, and contigs were annotated by BLASTx. RT-PCR and 5′/3’ RACE sequencing were used to obtain the complete genome of a new virus. RT-PCR was used to detect the virus.ResultsThree common apple viruses and a new luteovirus were identified from the diseased trees by HTS and RT-PCR. Sequence analyses of the complete genome of the new virus show that it is a new species of the genus Luteovirus in the family Luteoviridae. The virus is graft transmissible and detected by RT-PCR in apple trees in a couple of orchards.ConclusionsA new luteovirus and/or three known viruses were found to be associated with RAD. Molecular characterization of the new luteovirus provides important information for further investigation of its distribution and etiological role.

First Report of Apple (Malus domestica) as a Host of ‘Candidatus Phytoplasma pruni’ in the United States

In July - October of 2013-2015, more than forty blocks of Pennsylvania apple orchards from nine PA counties were surveyed for phytoplasma. Out of 218 apple samples submitted to Pennsylvania Department of Agriculture Plant Diagnostic Lab (PDA PDL) three mature trees from two farms in Adams County PA were found to be positive for Phytoplasma. Positive trees belonging to three different varieties showed abnormally small fruits, clumps of small leaves, leaf curl and premature reddening with the best symptoms showing in August-October. PDA PDL extracted total DNA from leaf mid-veins with a DNeasy Plant Mini Kit (Qiagen, Valencia, CA) according to manufacturers instructions. Positive results were obtained when screened for presence of phytoplasmal DNA using a real-time PCR assay (Hodgetts et al. 2009). For analysis of the 16S rDNA genomic region, nested PCR was carried out with primer pair P1/P7 followed by R16F2n/R16R2 or P1A/P7A (Lee et al. 2004). For analysis of ribosomal protein genes, DNA was amplified in...

Dominant Selectable Markers for Penicillium spp. Transformation and Gene Function Studies

Penicillium spp. has been genetically manipulated and gene function studies have utilized single gene deletion strains for phenotypic analysis. Fungal transformation experiments have relied on hygromycin and hygromycin phosphotransferase (hph) as the main dominant selectable marker (DSM) system in Penicillium spp. This poses a limitation on the number of loci that can be analyzed and complemented in reverse genetic studies. Additionally, many economically important Penicillium spp. have not been evaluated to determine the utility of additional chemicals that can serve as DSMs. Therefore, six compounds were examined for 15 blue mold strains and their Minimum Inhibitory Concentrations (MICs) determined. Phleomycin, neomycin and G418 were deemed ineffective, as Penicillium spp. growth was observed on media amended with 1000 μg/ml of each compound. The efficacy of bialophos to inhibit fungal growth was intermediate, with MICs ranging from 250 to 1000 μg/ml and was species-dependent. However, chlorimuron ethyl and benlate had the lowest MIC values and minimal variation in efficacy within and between species. Therefore, benlate and chlorimuron ethyl are good candidates for use as since corresponding fungal resistance genes have been cloned, characterized and are available from a variety of public and academic sources.