Regulation of flavor and texture in apple fruit genetically modified for ethylene biosynthesis

1. Regulation of flavor and texture in apple fruit genetically modified for ethylene biosynthesis Bruno G. Defilippi, Gianni Teo, Sandra L. Uratsu, Adel A. Kader and Abhaya M. Dandekar Department of Pomology, University of California, Davis, CA 95616 INTRODUCTION A salient genetic attribute of tree fruits is the unique blend of sugar, acid and volatile components that determine their flavor, a hallmark of the quality of each kind of fruit (e.g., apple, peach, orange). This complex genetic trait is manifested in ripe fruit through a complex interaction of metabolic pathways and regulatory circuits that results in the unique fruit flavor composition (FFC) a key to fruit consumption. In spite of its significance, very little is known at the molecular genetic level of the genes and pathways that are responsible for the synthesis, accumulation and regulation of FFC. FFC is key to marketing fresh fruits and their products as it greatly affects consumer preferences, which in turn, impact on the livelihood of fruit growers, and indirectly on the nutritional quality of consumer diets. Fruit that are silenced for ethylene biosynthesis described here show a unique phenotype that can be used to investigate FFC as it affects the volatile flavor component of FFC without having an effect on the sugar-acid component. These fruit show a distinct phenotype in their texture and shelf-life. In the future ethylene suppressed apple fruit such as reported here can be used to study the channeling and regulation of metabolic pathways that lead to the manifestation of a complex trait like fruit quality. This new knowledge could lead to the development of more precise diagnostics for quality control leading to a more consistent and high quality fruit for the consumer. OBJECTIVES The goal this year was to evaluate the role of ethylene in regulating fruit quality (the complex relationship between sugar, acid, texture and volatile components) using transgenic apple fruit modified in their capacity to synthesize endogenous ethylene. EXPERIMENTAL STRATEGY RESULTS Figure 1. The Agrobacterium binary vectors pDU93.0114/pDU93.0128 for sense/antisense expression of apple ACC synthase (ACS) and pDU93.0412/pDU93.0402 for sense/antisense expression of apple ACC oxidase (ACO) respectively in apple. Methionine SAM ACC Ethylene Table 1. Ethylene biosynthesis of transgenic apples as a percentage of that observed for the untransformed lines* ACS Silencing (sense, antisense) ACO 2Values of control: ACS activity = 2.5 nmolmg-1(protein) h-1, ACC concentration = 1.6 nmoles ACC g-1 (tissue), ACO activity= 205nLC2H4 mg-1(protein)h-1, ethylene production=73 uL C2H4 kg-1h-1 nd – Not determined Figure 2. Northern analysis of transgenic lines suppressed for ethylene biosynthesis. Table 3. Quality attributes at harvest and after storage for 12 days at 20°C. SUMMARY 1Values are means  SE of 3 replicates of 5 fruits each. 2No differences in titratable acidity were detected. FUTURE WORK -Very low ethylene producing transgenic apple lines have been identified that are suppressed for either ACS (ACC Synthase) or ACO (ACC Oxidase) expression. -ACS/ACO suppressed fruits are firmer than the controls but show no significant differences In soluble solid content or acid accumulation. -ACS/ACO transgenic fruits that make very low ethylene are significantly suppressed in their capacity to make volatile esters. -Color changes in transgenic lines is delayed relative to control fruit. -Ethylene suppressed lines show a marked reduction in alfa-farnesene. -To determine the role of ethylene in the pattern of volatile components and biosynthesis in transgenic apple fruit silenced for ethylene biosynthesis. -To determine the role of ethylene in texture development and accumulation of carbohydrates and acids in transgenic apple fruit silenced for ethylene biosynthesis. -To understand the role of ethylene in overall flavor in apples, including sugars, organic acids and phenolic compounds. Figure 4. a) three year old transgenic apple tree of the line 80G. b) ACO silenced apple 1 month after storage at room temperature. c) wild type GS apple 1 month after storage. d) ACO silenced apple 3 months after storage at room temperature and e) wild type 3 months after storage at room temperature.