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United States Department of Agriculture

Agricultural Research Service

Research Project: Identifying Genetic Relationships among Citrus and Citrus Relatives Using Molecular Markers

Location: National Clonal Germplasm Repository for Citrus & Dates

Project Number: 2036-21000-010-10
Project Type: Specific Cooperative Agreement

Start Date: May 01, 2013
End Date: Apr 30, 2015

Objective:
Provide expertise in the area of genetic analysis of germplasm of citrus, citrus relatives and date palm. This supports the mission to collect, evaluate, and distribute germplasm of citrus, citrus relatives, and date palm. Specific objectives: 1. Provide expertise to determine genetic relationships of cultivars of citrus and citrus relatives which appear to have tolerance to huanglongbing. 2. Provide expertise and aid to identify interactions among genes expressed by the bacterium associated with huanglongbing and genes expressed by the citrus host. 3. Provide expertise to determine effect of cryopreservation and/or cryotherapy on genetic trueness of type in cultivars and varieties which have undergone treatment. 4. Aid in identification of possible sources of redundancy in the Repository accessions by use of molecular markers.

Approach:
All research involving huanglongbing (HLB) will be conducted in Florida or other location where HLB is present and widespread. From a four year field screening of 87 genotypes of citrus and citrus relatives in Florida, some accessions appear to have some tolerance to HLB via differing mechanisms: one group became infected but survived and partially recovered; the second group expressed delayed susceptibility where after four years not all replications tested positive; and the third group would become HLB infected but was able to ward off the infection and tested negative. This research will determine the variability of the Candidatus Liberibacter asiaticus (Las) genome in plants that are in each group, and compared to the group which was extremely susceptible to HLB. In greenhouse tests, plants will be challenged using psyllids, and samples taken at time intervals for analyses of genes being expressed by the Las and by the plant host. Genome analysis of the host DNA of selected individual plants will be done by single nucleotide polymorphism (SNP) analysis of several nuclear genes and also by microsatellite or simple sequence repeat (SSR) analysis. Recently we have successfully adapted the methodology to preserve vegetative shoots of citrus in liquid nitrogen, and further adapted this methodology to eliminate graft transmissible pathogens by treatment in liquid nitrogen. A concern is that this treatment may induce genetic mutations in citrus. Citrus accessions which have been treated with liquid nitrogen will be recovered, allowed to grow, and planted in the field for horticultural evaluation. Samples will be collected and analyzed by SNP analyses of selected nuclear genes and/or by SSR markers to evaluate the genetic trueness of type of the source and recovered plants.

Last Modified: 11/27/2014
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