Apple is one of the most widely cultivated fruit crops in the world. Tree size is critical for early, high yielding, and efficient apple crops. Dwarf trees require reduced spray volumes, produce higher yields per unit of land through higher planting densities (Atkinson and Else, 2001), and allow mechanical harvesting. Development of compact apples through conventional breeding may control growth (Talwara et al., 2013), but it is a costly and time-consuming approach with a high probability of adverse changes in fruit characteristics. Chemical growth retardants are used as a supplementary treatment to control excessive vegetative growth, but decrease profitability (Wiesman and Lavee, 1994). Apple trees become smaller in size when scions are grafted on dwarfing rootstocks or interstocks. Dwarfing rootstocks impart characteristics to the tree as early flowering from planting, more efficient stable yield, and reduced vegetative growth.
Most growth retardants act by inhibiting the biosynthesis of GA, a class of plant hormones. GA plays important roles in plant development, including stem growth, leaf expansion, flower development, fruit set, and seed germination (Yamaguchi, 2008). Studies have revealed that one of the most obvious functions of GA is to promote vegetative growth, including the elongation of stems and the expansion of leaves (Olszewski et al., 2002). GA-deficient mutants tend to have small and dark green leaves and reduced stem length (Wang et al., 2012a). Application of exogenous active GA on wild plants or increased GA levels in mutants results in tall and spindly phenotypes (Fagoaga et al., 2007). Most of GA-deficient mutants have a dwarf phenotype with significantly decreased GA content and can be restored to normal phenotype by spraying with GA3 solution (Wang et al., 2012b). The GA biosynthetic pathway has been well characterized and can be divided into three major stages according to the localization and the enzymes involved (Lee and Zeevaart, 2007). The final stages of GA biosynthesis require the action of 2-oxoglutarate-dependent dioxygenases, including the GA20-ox that catalyzes the penultimate step in the formation of bioactive GA, and has been shown to be an important regulator in GA biosynthesis pathway (Zhao et al., 2010).
The aim of this study was to develop dwarf apple lines by silencing of the MdGA20-ox gene.
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