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Gloria A. Moore

We have produced a number of transgenic citrus plants via Agrobacterium-mediated transformation of seedling stem segments with a vector plasmid containing a β-glucuronidase (GUS) gene. All regenerated green shoots produced in our experiments are assayed histochemically for expression of GUS by cutting a section from the base of the shoot. Many of the shoots express GUS only in sectors, which vary in size from shoot to shoot. Analyses suggest that sectored regenerated shoots are chimeric, consisting of nontransformed cells as well as transformed cells. However, plants derived from shoots with large GUS+ sectors in the original assays do not necessarily contain the GUS gene; conversely, some plants derived from shoots with small sectors appear solidly transformed. Plants that appear solidly transformed have maintained gene expression for up to 5 years. None of the transgenic plants have obviously altered morphologies. It has not been possible to analyze progeny plants because of the long juvenile periods and polyembryony of the primary transformants. However, because citrus is clonally propagated, long-term phenotypic stability of primary transformants is the most important factor in producing useful transgenic plants.

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Diane Luth and Gloria A. Moore

Many Citrus species accumulate large amounts of flavonoids, specifically flavanone glycosides, that impart an intensely bitter flavor to the fruit. In grapefruit, this bitterness decreases the acceptability of fresh fruit and juice; in other species, these compounds entirely prevent fruit consumption. No physiological purpose for the accumulation of these compounds has been determined; they do not function in color production or, as far as is known, in defense responses. As has been found in other plants, the accumulation of specific flavonoids in citrus appears to be under genetic control, but no definitive genetic analyses have been done. The long-term objective of this research is to determine whether the production of bitter-tasting flavanone glycosides (neohesperidosides) in citrus can be manipulated using molecular genetic techniques. As a first step, cDNAs for chalcone synthase and chalcone isomerase, the first two biosynthetic enzymes specific to the flavonoid pathway, were isolated from a grapefruit leaf cDNA library using heterologous probes. Southern analyses showed that both genes appear to be part of multigene families, as expected. Northern analyses are underway to determine steady state mRNA levels in various grapefruit tissues, and Western blots to characterize protein expression are also being attempted.

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Courtney A. Weber and Gloria A. Moore

A greater saturation of the previously constructed genetic linkage map of Citrus is important in the long term goal of mapping quantitative trait loci (QTL) such as those controlling cold and salt tolerance. Segregation for cold tolerance appears to be greatly enhanced in the intergeneric F1 population of Citrus grandis × Poncirus trifoliata as compared to the BC1 population previously used for mapping due to the higher percentage of P. trifoliata genes present. This is not unexpected since P. trifoliata is the source of cold tolerance in this cross and is a highly heterozygous species. An integration of the maps of the two populations using about 50 random amplified polymorphic DNA (RAPD) markers common to the two populations is possible using the JoinMap computer program. This will allow the placing of approximately 100 new polymorphic RAPD markers from the F1 population identified by screening from 42 random oligonucleotide primers onto the Citrus map. This saturated map will be used to locate QTL following bulk segregation analysis of cold tolerance in the F1 population.

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Pan-chi Liou, Fred G. Gmitter Jr., and Gloria A. Moore

Citrus genetic studies and cultivar improvement have been difficult with conventional techniques. Alternative approaches are needed to enhance efficiency of such studies. Our objectives were to characterize the Citrus genome and to initiate development of a linkage map using RFLP and isozyme analysis. Methods of Citrus DNA extraction were developed to allow the isolation of chromosomal DNA of acceptable quality for recombinant' DNA manipulations. A PstI Citrus genomic library was constructed to create DNA clones for the RFLP survey. A rapid, reliable procedure was developed to facilitate screening of the library for useful clones. The methods used and strategy followed minimized contamination with organelle DNA, increased the frequency of single copy clones, and allowed rapid screening of the newly–constructed library. Linkage relationships of 49. markers, including 36 RFLP and 6 isozyme loci, were analyzed and a map comprised of 8 linkage groups was constructed. Insertions or deletions were responsible for at least 30% of the RFLPs identified. A hypothesis of transposon activity in Citrus was proposed based on our observations.

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Ilhami Tozlu, Charles L. Guy, Ouinvin Cai, and Gloria A. Moore

There is wide variation in Citrus and related genera in tolerance to cold and salt stress. While Poncirus trifoliata (L.) Raf. is an important rootstock for cold regions, it is salt sensitive. C. grandis (L.) Osb., on the other hand, is cold sensitive, but is relatively salt hardy. We are attempting to map genes (quantitative trait loci, QTLs) influencing salt and cold tolerance in Cirrus, using a BC1 population from [C. grandis × (C. grandis × P. trifoliata)]. As a first step, 2 year old containerized replicates of individual BC1 progeny plants have been salinized with 30 mM NaCl over a 9 month period under greenhouse conditions. Growth response under saline conditions, as evaluated by plant height and node number, varied significantly between individual progeny. Concentrations of 11 macro- and micro-elements, including Na and Cl, in leaf and root tissues were also determined. Ultimately, this data will be analyzed in conjunction with our current linkage map of this population, which consists of more than 200 marker genes, in order to map QTLs for salt tolerance.

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Catalina M. Anderson, William S. Castle, and Gloria A. Moore

Isozyme analysis was the basis for determining the frequency of occurrence and the characteristics of zygotic plants in Swingle citrumelo seedling populations from various sources of open-pollinated seeds, in a commercial nursery of Swingle citrumelo before and after roguing, and in commercial orchards and rootstock trials where this rootstock was used. Most zygotic seedlings identified by isozyme analysis could be distinguished by careful examination of morphological characteristics. Frequencies of zygotic seedlings varied among seedling populations, but were in the range (≈5% to 10%) found in previous studies. Roguing based primarily on size and growth habit of seedlings was effective in removing some, but not all, zygotic seedlings. Most of the remaining zygotic plants in the rogued population were found among the smaller seedlings. Trees budded on zygotic rootstock seedlings were found in two of the three groves studied, and in some instances an apparent incompatibility was developing in young trees.

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Courtney A. Weber, Gloria A. Moore, Z. Deng, F. Gmitter, and Courtney A. Weber

Specific primers were designed for 61 cloned RAPD fragments and from 10 Citrus EST sequences for the production of SCAR, CAPS, and STS markers for a Citrus grandis `DPI 6-4' × Poncirus trifoliata `Rubideaux' F1 pseudo-testcross population. Fifteen SCAR, three CAPS, and one EST/STS markers were developed. An additional 17 SCAR and CAPS primer pairs developed at the Citrus Research and Education Center for a Citrus grandis `Thong Dee' × (Citrus grandis `Thong Dee' × Poncirus trifoliata `Pomeroy') BC1 population were screened in the pseudo-testcross population. A total of 27 markers were identified and scored in the pseudo-testcross population in which 24 were mapped; 13 in the Citrus parental linkage map on seven linkage groups and 11 in the Poncirus parental map on five linkage groups. In the BC1 population, 20 of 27 markers tested were found to be polymorphic and 13 mapped to seven of nine linkage groups. Of these, 11 were mapped in both populations and could be used for aligning presumed homologous regions on the three linkage maps.

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Courtney A. Weber, Gloria A. Moore, Zhanao Deng, and Fred G. Gmitter Jr.

Mapping quantitative trait loci (QTL) associated with freeze tolerance was accomplished using a Citrus grandis (L.) Osb. × Poncirus trifoliata (L.) Raf. F1 pseudo-testcross population. A progeny population of 442 plants was acclimated and exposed to temperatures of -9 °C and -15 °C in two separate freeze tests. A subpopulation of 99 progeny was genotyped for random amplified polymorphic DNA (RAPD), cleaved amplified polymorphic sequence (CAPS), sequence characterized amplified region (SCAR), and sequence tagged site (STS) markers to produce a linkage map for each parent. Potential QTL were identified by interval mapping, and their validity was corroborated with results from means comparison (t test), one-way analysis of variance (F test), and bulked segregant analysis (BSA). Multiple analytical methods provided evidence supporting putative QTL and decreased the probability of missing significant QTL associated with freeze tolerance. QTL with a large effect on freeze tolerance were located on both the Citrus and Poncirus linkage maps. In addition, clusters of markers with significantly different means between marker present and absent classes indicating minor QTL that contribute smaller effects on the level of tolerance were found on the linkage maps of both species.

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Ed Stover, Robert G. Shatters Jr., Barrett Gruber, Prem Kumar, and Gloria A. Moore

Plants inoculated with the huanglongbing (HLB)-associated bacterium, Candidatus Liberibacter asiaticus (CLas) typically must be monitored for 8–10 months to identify differences in susceptibility between genotypes. Continuous light is reported to accelerate development of HLB symptoms and field observations suggest that trees girdled by tags or tree ties showed greater symptoms. Therefore, an experiment was conducted assessing HLB susceptibility as influenced by light/dark periods of 12 hours: 12 hours and 24 hours: 0 hours, in combination with scoring tree trunks to disrupt phloem. Sixty trees of each of three citrus genotypes (‘Kuharske’, previously shown to be HLB resistant; rough lemon, previously shown to be HLB tolerant; and ‘Valencia’, highly HLB susceptible) were bud grafted using two CLas-infected buds (rough lemon and citron) per tree on 26 Mar. 2012, and were placed in controlled growth rooms (one 12 hour light: 12 hour dark and one constant light) on 4 June 2012. Ten trees of each genotype in each growth room were scored 10 cm above the soil (cutting through the bark but not the wood) with a knife on 18 July 2012 and the scoring was repeated at the same scoring wounds on 30 Aug. 2012. Trees were removed from growth rooms on 12 Dec. 2012 and subsequently maintained in a greenhouse. At two to three month intervals between June 2012 and May 2013, HLB symptoms and stem diameter at 5 cm above the soil were assessed, and three leaves per tree were collected for quantitative polymerase chain reaction (qPCR) determination of CLas titer. Six months after inoculation and 3 months following imposition of treatments, the ‘Valencia’ scored in the 12 hour light: 12 hour dark regime, the ‘Valencia’ non scored trees in 24 hours of light and the ‘Kuharske’ scored trees in 24 hours of light displayed higher CLas titers than most other trees. After an additional two months, both scored and non-scored trees of all three genotypes in 24 hours of light had significantly elevated CLas titers compared with trees in 12 hour light: 12 hour dark regime, but within most treatments all three genotypes had titers which were not statistically different from each other. Growth of ‘Kuharske’ and rough lemon was enhanced; whereas ‘Valencia’ growth was reduced when graft-inoculated plants were maintained in continuous light. Scoring enhanced early CLas development in ‘Kuharske’ when combined with continuous light, had no effect in rough lemon, and showed inconsistent effects in ‘Valencia’. Although continuous lighting enhanced disease progression, it did not reveal differences in HLB susceptibility.

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Ed Stover, David G. Hall, Robert G. Shatters Jr., and Gloria A. Moore

Assessments of the resistance of citrus germplasm to huanglongbing (HLB) can be expedited by inoculating plants under laboratory or greenhouse settings with the HLB bacterium, Candidatus Liberibacter asiaticus (CLas). Consistent rapid screening is critical to efficiently assess disease resistance among plant materials; however, a number of factors may govern the efficacy of such inoculations. Despite the rapidity at which HLB can spread in a grove, it often takes 8 to 10 months for high levels of CLas and HLB symptoms to develop even in highly susceptible sweet orange. Therefore, two experiments were conducted to assess factors that might influence efficiency in screening for HLB resistance. In one experiment, three test citrus genotypes (‘Kuharske’, previously shown to be HLB resistant; rough lemon, previously shown to be HLB tolerant; and ‘Valencia’, HLB susceptible) were bud grafted using CLas-infected buds from four different source genotypes. All bud source genotypes had similar levels of CLas titer, but citron, rough lemon, and Volkamer lemon were hypothesized to be better bud inoculum sources as they are more tolerant of HLB than ‘Valencia’. Among the three test genotypes over all sources of infected buds, inoculations of ‘Kuharske’ resulted in lower CLas titers and fewer HLB symptoms than inoculations of rough lemon or ‘Valencia’. Inoculations of rough lemon resulted in higher CLas titers and more pronounced HLB symptoms when it was inoculated using infected buds from rough lemon or ‘Valencia’. Grafting ‘Valencia’ with infected buds from Volkamer lemon resulted in less disease than when ‘Valencia’ was grafted with infected citron, rough lemon, or ‘Valencia’ buds. Overall, these results suggest that the source of CLas-infected buds used to graft-inoculate some genotypes will influence disease development. Trunk cross-sectional area increase for the year following infection was 3× higher in ‘Kuharske’ and rough lemon, compared with ‘Valencia’. ‘Kuharske’ had very low levels of CLas (30 CLas/µg DNA), whereas ‘Valencia’ (43,000 CLas/µg DNA) and rough lemon (6700 CLas/ µg DNA) had relatively high levels. As an alternative to graft-inoculating plants with CLas-infected buds, plants can be subjected to infestations of CLas-infected Asian citrus psyllid (ACP) as occurs naturally. Of interest is if transmission rates of CLas and the development of HLB in a genotype are greater when the ACP have been feeding on the same host genotype. An experiment was therefore conducted to assess transmission of CLas by ACP reared on CLas-infected rough lemon to five different genotypes (‘Carrizo’, ‘Flame’ grapefruit, rough lemon, ‘Temple’, and ‘Valencia’). These assessments were made using a detached leaf assay recognized as a faster method of gauging transmission rates of CLas than using whole plants. Higher percentages of ACP died when they were transferred from infected rough lemon to healthy ‘Carrizo’, and lower percentages died when they were transferred to rough lemon or ‘Flame’. However, CLas transmission by infected ACP occurred to at least some leaves of each genotype in each of the five different assays, with 70% or more leaves of each genotype becoming infected in at least one assay. Over all assays, there was relatively little variation among genotypes in the percentage of leaves becoming CLas infected and in the titer of CLas developing in infected leaves. However, there were relatively large differences in transmission rates among individual assays unrelated to differences among test genotypes. Because of the rapidity of the detached leaf assay, efforts are merited to improve consistency of this inoculation method.