Verlyn K. Stromberg

Physiological and molecular adaptations to drought in Andean potato genotypes

The drought stress tolerance of two Solanum tuberosum subsp. andigena landraces, one hybrid (adg×tbr) and Atlantic (S. tuberosum subsp. tuberosum) has been evaluated. Photosynthesis in the Andigena landraces during prolonged drought was maintained significantly longer than in the Tuberosum (Atlantic) line. Among the Andigena landraces, ‘Sullu’ (SUL) was more drought resistant than ‘Negra Ojosa’ (NOJ). Microarray analysis and metabolite data from leaf samples taken at the point of maximum stress suggested higher mitochondrial metabolic activity in SUL than in NOJ. A greater induction of chloroplast-localized antioxidant and chaperone genes in SUL compared with NOJ was evident. ABA-responsive TFs were more induced in NOJ compared with SUL, including WRKY1, mediating a response in SA signalling that may give rise to increased ROS. NOJ may be experiencing higher ROS levels than SUL. Metabolite profiles of NOJ were characterized by compounds indicative of stress, for example, proline, trehalose, and GABA, which accumulated to a higher degree than in SUL. The differences between the Andigena lines were not explained by protective roles of compatible solutes; hexoses and complex sugars were similar in both landraces. Instead, lower levels of ROS accumulation, greater mitochondrial activity and active chloroplast defences contributed to a lower stress load in SUL than in NOJ during drought.

Detection and Characterization of a New Strain of Citrus Canker Bacteria from Key/Mexican Lime and Alemow in South Florida

In the Wellington and Lake Worth areas of Palm Beach County, FL, citrus canker appeared on Key/Mexican lime (Citrus aurantiifolia) and alemow (C. macrophylla) trees over a period of about 6 to 7 years before detection, but nearby canker-susceptible citrus, such as grapefruit (C. × paradisi) and sweet orange (C. sinensis), were unaffected. Colonies of the causal bacterium, isolated from leaf, stem, and fruit lesions, appeared similar to the Asiatic group of strains of Xanthomonas axonopodis pv. citri (Xac-A) on the nutrient agar plate, but the growth on lima bean agar slants was less mucoid. The bacterium produced erumpent, pustule-like lesions of typical Asiatic citrus canker syndrome after inoculation into Key/Mexican lime, but brownish, flat, and necrotic lesions on the leaves of Duncan grapefruit, Madame Vinous sweet orange, sour orange (C. aurantium), citron (C. medica), Orlando tangelo (C. reticulata × C. × paradisi), and trifoliate orange (Poncirus trifoliata). The bacterium did not react with the Xac-A specific monoclonal antibody A1 using enzyme-linked immunosorbent assay (ELISA) and could not be detected by polymerase chain reaction (PCR)-based assays using primers selected for Xac-A. DNA reassociation analysis confirmed that the pathogen, designated as Xac-AW, was more closely related to Xac-A and Xac-A* strains than X. axonopodis pv. aurantifolii or the citrus bacterial spot pathogen (X. axonopodis pv. citrumelo). The strain can be easily differentiated from Xac-A and Xac-A* using ELISA, PCR-based tests, fatty acid analysis, pulsed-field gel electrophoresis of genomic DNA, and host specificity.

RNA translocation between parasitic plants and their hosts

Recent research indicates that RNA translocation occurs between certain parasitic plant species and their hosts. The movement of at least 27 mRNAs has been demonstrated between hosts and Cuscuta pentagona Engelm., with the largest proportion of these being regulatory genes. Movement of RNAi signals has been documented from hosts to the parasites Triphysaria versicolor (Frisch & CA Mey) and Orobanche aegyptiaca (Pers.), demonstrating that the regulation of genes in one species can be influenced by transfer of RNA signals through a parasitic association. This review considers the implications of these findings in light of present understanding of host-parasite connections and the growing body of evidence that RNAs are able to act as signal molecules that convey regulatory information in a cell- and tissue-specific manner. Together, this suggests that parasitic plants can exchange RNAs with their hosts, and that this may be part of the coordinated growth and development that occurs during the process of parasitism. This phenomenon offers promise for new insights into parasitic plants, and new opportunities for the control of parasitic weeds.

Relatedness of Chromosomal and Plasmid DNAs of Erwinia pyrifoliae and Erwinia amylovora

ABSTRACT The plant pathogen Erwinia pyrifoliae has been classified as a separate species from Erwinia amylovora based in part on differences in molecular properties. In this study, these and other molecular properties were examined for E. pyrifoliae and for additional strains of E. amylovora, including strains from brambles (Rubus spp.). The nucleotide composition of the internal transcribed spacer (ITS) region was determined for six of the seven 16S-23S rRNA operons detected in these species with a 16S rRNA gene probe. Each species contained four operons with a tRNAGlu gene and two with tRNAIle and tRNAAla genes, and analysis of the operons from five strains of E. amylovora indicated a high degree of ITS variability among them. One tRNAGlu-containing operon from E. pyrifoliae Ep1/96 was identical to one in E. amylovora Ea110, but three tRNAGlu operons and two tRNAIle and tRNAAla operons from E. pyrifoliae contained unique nucleotide changes. When groEL sequences were used for species-specific identification, E. pyrifoliae and E. amylovora were the closest phylogenetic relatives among a set of 12 bacterial species. The placement of E. pyrifoliae distinct from E. amylovora corroborated molecular hybridization data indicating low DNA-DNA similarity between them. Determination of the nucleotide sequence of plasmid pEP36 from E. pyrifoliae Ep1/96 revealed a number of presumptive genes that matched genes previously found in pEA29 from E. amylovora and similar organization for the genes and origins of replication. Also, pEP36 and pEA29 were incompatible with clones containing the reciprocal origin regions. Finally, the ColE1-like plasmid pEP2.6 from strain Ep1/96 contained sequences found in small plasmids in E. amylovora strains IL-5 and IH3-1.

Complementation of an Erwinia carotovora subsp. carotovora protease mutant with a protease-encoding cosmid

SummaryWe report the complementation of a genetic lesion in the genome of Erwinia carotovora subsp. carotovora (Ecc), a pathogenic bacterium that incites soft rot of plants. A Sau3AI genomic library of Ecc was constructed using the conjugal cosmid pLAFR-3 as a vector. Sixteen cosmid clones encoding various plant tissue-degrading enzymes were identified, including a proteolytic clone, five cellulolytic clones, and ten pectolytic clones. We detected a mutant of Ecc with no proteolytic activity following transposon mutagenesis with an unstable Tn5-carrying plasmid. Conjugal transfer of the protease-encoding cosmid to this mutant restored near-wildtype extracellular protease production. Further manipulation and study of genes encoding pathogenic determinants in Ecc will be possible using this system.

Erwinia Carotovora: Molecular Cloning of a 3.4 Kilobase DNA Fragment Mediating Production of Pectate Lyases

(1986). Erwinia Carotovora: Molecular Cloning of a 3.4 Kilobase DNA Fragment Mediating Production of Pectate Lyases. Canadian Journal of Plant Pathology: Vol. 8, No. 1, pp. 17-27.