Apomictic Dwarf Apple Germplasm Innovation

Rootstocks are applied to control growth vigor and enhance tolerance to adverse environmental stresses, diseases and insect pests. In China, apple yield accounts for about 1/4 of total fruit yield. Apple plays an important role in national economy, especially in North-West China. There is a great disparity in apple production between China and developed countries. About 90% apple trees are grafted to dwarfing rootstocks in developed countries, while only 1% in China because of the lackness of dwarfing rootstocks adaptable to native growth and cultivation conditions in China. Most M series dwarfing apple rootstocks imported from East Malling Research Station, England, do not adapt well in China. It is important to breed apple dwarfing rookstocks performing well in native conditions.'Pingyitiancha'(PYTC) (Malus hupehensis) is originally found in Mt. Meng, Shandong Province in China. Characterized by producing apomitic seeds, PYTC is widely used as apple rootstock in China. As a vigorous rootstock, however, PYTC is unsuitable to intensive production. To introduce dwarfism into apomixis or vice versa, several cross combination were carried out between PYTC and dwarfing rootstocks such as M9, B9, Maypole and so on.In addition, PYTC seeds were treated withγ-ray radiation for genetically screening apomictic dwarf mutants. The main results were shown as follows.1. Several hybrids of apomictic dwarf rootstock were selected from the crosses of PYTC×B9, PYTC×M9 and PYTC×Maypole. Hybrid 99PMA108 from cross PYTC×Maypole produced seeds of which over 90% were apomictic ones. The average height, trunk diameter and internode length of 1-year-old seedlings were 27.88cm, 0.5cm and 1.11cm, respectively, for hybrid 99PMA108, while those were 51.80cm, 0.66cm and 1.70cm, respectively, for PYTC. 99PCO211 produced both apomictic seeds and pollen. So it could be used as male parent. Both of 04A1ZH-1 and 04A1ZH-2 had a short juvenile phase of two years, and could be used as female parent to shorten breeding peroid.2. Diploid(2x), triploid(3x), tetraploid(4x), and aneuploid(2x~3x, 3x~4x) were found in the hybrid populations of cross between PYTC(3x) and Maypole(2x). So female gametes were 1x, 2x and 3x (2n female gamete).3. Co-60 induced dwarf mutants of Malus hupehensis which produce apomictic seeds and dwarf seedlings. The apomictic rates of these mutants ranged from 80% to 90%.4. These mutants were GA-insensitive, indicating that the signling pathway were blocked.5. MhGAI1 and MhGAI2 were isolated from Malus hupehensis, registered as EF026101 and EF495201 at GenBank. Their predicted amino acid sequences contained DELLA and VHYNP domains.6. A one nuclitide deletion was detected in mutant C2 which thus predictively produce a shortened peptide lacking 47 amino acids due to frameshift.7. Compared with PYTC, GA20ox was up-regulated and LUE1 was down-regulated in mutant A1.8. Growth cession and bud set, internode length, shoot length of Gala grafted on dwarf mutants were different a little, while tree heights of Gala/B1,Gala/B2,Gala/C3 were similar , Gala/A1 was the smallest, distributing between Gala/M26/PYTC and Gala/PYTC.