Publications

4859

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.

Mung bean (Vigna radiata (L.) R. Wilczek) is an important edible legume grown in Asia, particularly in the Indian subcontinent, where it is used for human and animal consumption. In September 2013, 10% of a group of 90 mung bean breeding lines in experimental plots of S. V. Agricultural College, Tirupati, Andhra Pradesh, India, exhibited symptoms typical of a phytoplasma infection, including stunting, extensive proliferation of branches, reduction in leaf size, phyllody, and longitudinal splitting of green pods followed by germination of green seeds producing small plants. These symptoms have been associated with mung bean phyllody (1,3). While the severity of infection varied within each line, on average, 20% of symptomatic plants did not produce seeds at all. Leaf samples from two symptomatic plants and one asymptomatic plant were collected and DNA was extracted from leaves following a CTAB DNA extraction procedure (2). Direct PCR and nested PCR assay was performed using phytoplasma 16S rRNA universal primer pairs P1/P7 and R16F2n/R16R2 (4). Both of the symptomatic samples produced 1.8 kb and 1.2 kb size amplicons after direct and nested PCR cycles, respectively. No amplicons were produced with DNA from the asymptomatic sample. Nested PCR products (1.2 kb) from the two symptomatic samples corresponding to the F2nR2 region of 16S rDNA were directly sequenced on an ABI 3730 XL automated sequencer at McLab sequencing services (McLab, CA). Both samples were 100% identical and the representative sequence was designated as APMBP and deposited in GenBank with the accession number KF811205. BLAST analysis revealed a 100% sequence identity with 16SrII group ‘Candidatus Phytoplasma aurantifolia’ phytoplasmas that include sesame phyllody phytoplasmas isolated from sesame and tomato in India (KF429485 and JX104335, respectively). Subgroup identification was performed using the iPhyClassifier online tool (5). The samples were identified as 16SrII-D subgroup phytoplasma based on their 100% identity to the reference strain (GenBank Accession No. Y10097) and virtual RFLP profiles. Phylogenetic analysis based on the F2nR2 sequences with the representative sequences were placed the APMBP in a single distinct cluster with the 16SrII-D reference strain Y10097. Although occurrence of phyllody on mung bean in India was first reported in 1988 (3), the report was based on the appearance of symptoms. This pathogen was recently reported as associated with mung bean phyllody in Pakistan (1). To our knowledge, this is the first report of ‘Ca. P. aurantifolia’ strain infecting mung bean in India. Phytoplasmas belonging to subgroup 16SrII-D are known to have a wide host range, including chickpea, peanuts, sesame, and tomato, which are commonly grown in this region. Mung bean plants infected early failed to produce normal seeds indicating of the potential of this 16SrII-D phytoplasma to become a production constraint for mung bean and other host crops in the area. References: (1) K. P. Akhtar et al. Plant Pathol. 59:399, 2010. (2) J. J. Doyle and J. L. Doyle. Phytochem. Bull. 19:11, 1987. (3) P. Lakshmanan et al. Curr. Sci. 57:809, 1988. (4) I. M. Lee et al. Int. J. Syst. Bacteriol. 48:1153, 1998. (5) Y. Zhao et al. Int. J. Syst. Evol. Microbiol. 59:2582, 2009.

ABSTRACT “ Candidatus Liberibacter asiaticus” is an uncultured alphaproteobacterium that systemically colonizes its insect host both inter- and intracellularly and also causes a severe, crop-destroying disease of citrus called huanglongbing, or citrus “greening.” In planta , “ Ca . Liberibacter asiaticus” is also systemic but phloem limited. “ Ca . Liberibacter asiaticus” strain UF506 carries two predicted prophages, SC1 and SC2. Bacteriophage particles have been observed in experimentally “ Ca . Liberibacter asiaticus”-infected periwinkle but not in any other host. Comparative gene expression analysis of predicted SC1 late genes showed a much higher level of late gene expression, including holin transcripts (SC1_gp110), in “ Ca . Liberibacter asiaticus”-infected periwinkle relative to “ Ca . Liberibacter asiaticus”-infected citrus. To functionally characterize predicted holin and endolysin activity, SC1_gp110 and two predicted endolysins, one within SC1 (SC1_gp035) and another well outside the predicted prophage region (CLIBASIA_04790), were cloned and expressed in Escherichia coli . Both SC1 genes inhibited bacterial growth consistent with holin and endolysin function. The holin (SC1_gp110) promoter region was fused with a uidA reporter on pUFR071, a wide bacterial host range (repW) replicon, and used to transform Liberibacter crescens strain BT-1 by electroporation. BT-1 is the only liberibacter strain cultured to date and was used as a proxy for “ Ca . Liberibacter asiaticus.” pUFR071 was >95% stable without selection in BT-1 for over 20 generations. The reporter construct exhibited strong constitutive glucuronidase (GUS) activity in culture-grown BT-1 cells. However, GUS reporter activity in BT-1 was suppressed in a dose-dependent manner by crude aqueous extracts from psyllids. Taken together with plant expression data, these observations indicate that “ Ca . Liberibacter asiaticus” prophage activation may limit “ Ca . Liberibacter asiaticus” host range and culturability.