1a. Objectives (from AD-416)
Utilize functional genomic/proteomic approaches to identify genes or proteins in fruit crops that confer resistance or susceptibility to freezing or water stress and fire blight. Develop and evaluate new transgenic technologies needed to control gene flow and expression in transgenic apple trees. Evaluate genetically engineered rootstocks as (1) a means of enhancing resistance to diseases and (2) a means of indirectly influencing gene expression in the scion for the improvement of performance or quality.
1b. Approach (from AD-416)
Transcript profiling will be used to identify genes associated with environmental stress, fire blight resistance, and susceptibility, while proteomic approaches will be used to better understand the relationship between gene expression and cognate protein levels. RNAi-induced gene silencing will be used to elucidate the role of specific candidate genes in resistance and adaptation. The ability to use genetically-engineered rootstocks of apple for scion trait modification will be explored by investigating graft-transmissible gene silencing. The use of floral-specific promoters to confine and regulate the expression of dehydrin genes (responsible for cold and dehydration tolerance) to flowers of fruit crops in order to improve frost tolerance will also be investigated.
3. Progress Report
Apple trees overexpressing a peach CBF transcription factor have been demonstrated to have increased cold hardiness and to enter dormancy in response to short days. Silencing lines directed against specific CBFs have also been established. Both silencing lines and an overexpressing line have been planted in the orchard to observe and record their phenotypic response to the environment and to record their growth characteristics. This study is being conducted to develop strategies for dealing with potential negative impacts of global climate change on fruit production. Wild Malus sieversii, previously collected by USDA-ARS in Kazakhstan, is an excellent source of disease resistance for apple. To identify genomic regions in M. sieversii associated with resistance to fire blight, over 725 molecular markers were screened in a mapping population (194 individuals) of 'Royal Gala' apple X M. sieversii PI631981. Replicate plants of the population were assessed for fire blight resistance in the greenhouse and in the field. Molecular markers associated with fire blight resistance were identified in two genomic regions. After validation, markers for the resistance genes will be made available to U.S. tree fruit breeding programs to facilitate the development of fire blight resistant apple and pear cultivars by marker assisted breeding. This research is being conducted to provide the necessary tools to develop germplasm that will reduce the use of chemicals, ensure the productivity of planted orchards, and provide resistance to any unforeseen introduction of foreign strains of the fire blight organism. Apple selections obtained from arid regions of Kazakhstan can be a source of genes for drought tolerance in commercial apples. One of these populations has been analyzed for water use efficiency. Within the population of 34 individuals, there was a range of water use values, some higher and some lower than a standard cultivar ('Royal Gala'). There was little correlation between water use efficiency and morphological traits traditionally associated with water use. Experiments to determine drought tolerance of individuals with high and low water use efficiency are currently in progress. Identification of drought resistant genes could lead to the development of fruit trees that reduce the need for irrigation. This study is being conducted to develop strategies to deal with potential negative impacts of global climate change on fruit production.
1. New sources of fire blight resistance in apple identified. Fire blight is a devastating disease of apple that can destroy entire young orchards and drastically limit productivity and longevity of mature orchards. Wild Malus sieversii, previously collected by USDA-ARS in Kazakhstan, is an excellent source of disease resistance for apple breeding because its large and palatable fruit facilitate rapid development of new varieties with superior eating quality. ARS researchers at Kearneysville, WV have created a genetic framework map for M. sieversii and identified molecular markers associated with fire blight resistance in two genomic regions. After validation, markers for the resistance genes will be made available to U.S. tree fruit breeding programs to facilitate the development of fire blight resistant apple and pear cultivars by marker assisted breeding.
Wisniewski, M.E., Norelli, J.L., Bassett, C.L., Artlip, T.S., Macarisin, D. 2011. Ecotopic expression of a novel peach (Prunus persica) CBF transcription factor in apple (Malus x domestica) results in short-day induced dormancy and increased cold hardiness. Planta. 233:921-983.