1a. Objectives (from AD-416)
1. Improve temperate stone and pome fruit quality traits. 2. Improve host-plant resistance of temperate stone and pome fruit to plum pox virus, tomato ringspot virus, prunus necrotic virus, and prune dwarf virus. 3. Develop a model biotechnology risk mitigation system and efficient genetic transformation protocols for Rosaceae. PDRAM #R08 Program Increase Objective: 4. Initiate a FasTrack breeding project to develop, characterize, and select early and continual fruiting lines of plum, pear, and apple, and relevant rootstocks to accelerate fruit breeding programs.
1b. Approach (from AD-416)
This project proposes the development of genetic solutions to the major problems affecting temperate tree fruit production and consumer acceptance of tree fruits through a coordinated effort utilizing the most appropriate technologies. The improvement of fruit quality will be approached through traditional breeding of novel genetic material, and also through the isolation and transgenic manipulation of key genes that control fruit development, stone formation, and ripening. The use of these diverse technologies will be coordinated in order to develop tree fruit varieties with improved flavor and firmness that is maintained post-harvest and investigate the possibility of creating marketable stoneless varieties. Improvement of fire blight resistance in pear will be approached through hybridization of existing fire blight resistant germplasm to develop new resistant cultivars that have high fruit quality. Pathogen- and host-derived resistance to stone fruit viruses will be incorporated into commercial quality cultivars through genetic transformation. Improved tree form for high-density production systems in peach will be approached through hybridization of peach germplasm with desirable tree growth habits such as columnar and semi-columnar with the aid of molecular markers that improve breeding efficiency. Pear growth habits will be manipulated through genetic transformation. Foundational work in genetic engineering and risk mitigation will facilitate the development of enhanced technologies that will help alleviate industry concerns over marketing genetically engineered crops. Improved plant material generated from both the laboratory and field will be evaluated in collaboration with the industry and consumer groups to facilitate its utilization.
3. Progress Report
Towards the goal of developing stoneless plums, RNA expression of transcription factors that regulate endocarp formation was analyzed on normal and 'Stoneless' mutant plum fruit. To develop disease and insect resistant pears, selections were evaluated for fruit quality and resistance to fire blight, Fabraea leaf spot, and pear psylla. Crosses were made to introduce improved sweetness, acidity, and higher vitamin C content, and to study the inheritance of these traits, and fruit texture. Asian and European pear species were screened for effects on psylla larval feeding, development, and mortality. Pears were screened for susceptibility to European pear sawfly, and possible sources of genetic resistance were identified in Asian pears and hybrids. The interactions of light, photosynthesis, salicylic acid (SA), and defense responses were studied to understand the interactions of host plants and potyvirus. Changes in photosynthesis, susceptibility, and gene expression responses were measured following applications of SA under a range of light and stress conditions. SA appears to play a fundamental role in regulating photosynthetic stress and activates defense responses only when the plant is healthy and able to cope with the added stress that results from the activation of pathogen defense pathways. To find genes of interest for fruit quality traits and disease resistance, 'Improved French' plum, multiple seedlings of peach, and six apricots were whole genome sequenced. Transgene excision efficiency in sexual tissues to prevent unwanted transgene flow was analyzed in the Tissue-Specific Transgene Removal and Containment System (TRECS). Several transgenic lines demonstrated excision in flower buds. The plants were out-crossed, and in all of the seeds, the transgene had been completely excised indicating it was excised in the pollen. The TRECS excision system is more efficient in male tissues than female tissues. To improve the efficiency of excision in female tissues of TRECS constructs, two new floral meristem-specific promoters have been incorporated into TRECS constructs along with new insulators to prevent interference between promoters. To obtain transformation of plum cultivars rather than seedlings, regeneration rates have been increased in leaf tissue of 'Improved French'. Approximately 65% of the leaves had an average of two shoots regenerate per leaf. This regeneration rate is sufficient for transformation studies. To develop a fast breeding system, four early flowering plum lines were hybridized with 'HoneySweet' PPV resistant plum to produce the first generation of early flowering PPV resistant breeding lines. The four lines have also been crossed with elite California plum selections and varieties. These seedlings will be used as early flowering parents to develop new California-adapted plum cultivars with improved sugar profiles. Three cultivars of pear were transformed with the early flowering gene to develop resources for rapid introgression of traits into high quality pear germplasm.