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Ruigang Wu, Yi Wang, Ting Wu, Xuefeng Xu and Zhenhai Han

MYB (v-myb avian myeloblastosis viral oncogene homologs) transcription factors (TFs) are involved in diverse physiological processes, including cell shape determination, cell differentiation, and secondary metabolism, as well as abiotic stress response. In the present study, MdMYB4, an R2R3-MYB protein that is a homolog of Arabidopsis thaliana MYB4, was identified and characterized. Quantitative real-time polymerase chain reaction (qRT-PCR) expression analysis demonstrated that MdMYB4 is extensively expressed in various apple (Malus domestica) tissues and that its expression is induced by cold, osmotic, and salt stress. An MdMYB4-GFP fusion protein was localized in the nucleus of transformed onion (Allium cepa) epidermal cells and had a certain transcriptional activation activity by yeast one-hybrid assay. Overexpression of the MdMYB4 gene remarkably enhanced the tolerance of stably transgenic apple calli to severe salt and cold stress, and both the relative conductivity and malondialdehyde (MDA) accumulation of transgenic calli under salt and cold stress were significantly lower than in the wild type control. Taken together, these results suggest that MdMYB4 may play a positive regulatory role in both cold and salt stress responses.

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Yi Tan, Baisha Li, Yi Wang, Ting Wu, Zhenhai Han and Xinzhong Zhang

Agrobacterium-mediated genetic transformation is commonly used in dicotyledon plants such as apples. The regeneration ability of the recipient is an important factor in transformation efficiency. Here, the variations in bud regeneration rate (BRR) and the number of adventitious buds (NAB) formed per explant in Malus germplasm accessions with phenological stage were estimated. Both BRR and NAB of explants at the dormancy broken and spring sprouting stages were significantly higher than those at the autumn sprouting stage. The genetic diversity and inheritance of BRR and NAB were evaluated using 153 Malus germplasm accessions and 78 hybrid trees of Jonathan × Golden Delicious. Malus sieversii 31, Liberty, and Smoothee exhibited significantly high BRR (98.33%, 98.33%, and 93.33%, respectively) and a large NAB without vitrification. BRR and NAB linearly correlated with each other but not with callus formation rate. The broad sense heritability of the regeneration rate was 92.16%. The three Malus accessions that had high regeneration ability, and some of their sexual descendants, might be outstanding genetic resources for future genetic transformation.

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Rui Sun, Hui Li, Qiong Zhang, Dongmei Chen, Fengqiu Yang, Yongbo Zhao, Yi Wang, Yuepeng Han, Xinzhong Zhang and Zhenhai Han

Flesh browning is an important negative trait for quality preservation of fresh-cut fruits. To obtain a better understanding of the inheritance and genetic control of flesh browning in apple, the phenotype of a hybrid population derived from ‘Jonathan’ × ‘Golden Delicious’ was studied for 2 successive years. The inheritance of the flesh browning trait was analyzed by the frequency distribution of the phenotypes. Flesh browning-associated major genes were then mapped by screening genome-wide simple sequence repeat (SSR) markers. Flesh browning is inherited quantitatively and showed a clear bimodal frequency distribution, indicating that the segregation of major genes is involved in the variation. The segregation ratio of light and heavy browning was 7:1 in 2010, 2011, and 2010 + 2011, suggesting that the inheritance of the trait in apple involves three segregated loci of major genes. The heritability of the major gene effect was 72.14% and 72.76% in 2010 and 2011, respectively. SSR markers were screened from 600 pairs of SSR primers located on 17 apple linkage groups (LGs). The three major genes were mapped on LG10, 15, and 17 on the apple genome, respectively, by linkage analysis of flesh browning phenotypes and the genotypes of SSR markers. Two quantitative trait loci (QTLs) for flesh browning were mapped on LG15 of ‘Jonathan’ and LG17 of ‘Golden Delicious’, respectively, which are the same linkage groups that two major genes mapped on.

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Li Ma, Chang Wei Hou, Xin Zhong Zhang, Hong Li Li, De Guo Han, Yi Wang and Zhen Hai Han

Understanding of root growth patterns and architecture of apple (Malus ×domestica Borkh.) trees is very important for commercial apple production. Most commercial apple trees are usually a grafted complex consisting of the scion and the rootstock, each of which is a different genotype. Recently, rootstocks of dwarf tree species have been used extensively to meet the convenience in management; however, this practice appears to negatively impact root development. Using minirhizotrons, we investigated root dynamics, root spatial distribution, and shoot growth in ‘Red Fuji’ scion grown: 1) directly on dwarf and vigorous root stocks and 2) on a dwarf root stock placed in between the non-dwarf scion and non-dwarf rootstock (hereinafter referred to as an interstem). The results showed that: 1) one or two peaks in total root length density (TRLD) were observed in each scion/rootstock combinations every year; 2) the greatest TRLD peaks were always observed in between May and December. The peaks of shoot growth were always asynchronous with that of white root length density; 3) compared with scion/vigorous rootstock combinations, inserting a dwarfing interstem between the scion and vigorous seedling rootstock reduced the TRLD; 4) scion/vigorous rootstock combinations had a relatively deep, widespread and large root system. Scion/dwarfing rootstock combinations had a root system distributed in a small region; and the root systems of scion/dwarfing interstem/vigorous rootstock combinations tended to be intermediate between those of scion/vigorous rootstock and scion/dwarfing rootstock. This implies that the insertion of interstems altered the root architecture by not only the quantity of roots, but also the spatial distribution.

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Meiling Zhang, Ming Chen, Zhen Wang, Ting Wu, Yi Wang, Xinzhong Zhang and Zhenhai Han

Grafting has been widely used in orchard management because the rootstock can make the tree more tolerant to environmental stresses. Iron deficiency is one of the major limiting environmental factor in apple production worldwide. Systematic research has been made about iron-deficiency adaptive responses in the level of organs, cells, and subcells, whereas the interactions between Fe and other divalent cations in tissue level are little known. Synchrotron radiation X-ray fluorescence (SR-μXRF) was used to map the location of selected elements Fe, Zn, Mn, Ni, and Co in the longitudinal and latitudinal root samples of Malus xiaojinensis. Iron deficiency induced a significant increase in the relative contents of five micronutrients in epidermis and cortex. The ratio of element contents of roots under Fe-deficient condition and Fe-sufficient condition at same position increased obviously in the section of 1000- to 2000-μm distance from the root tip in xylem. Expression analysis of iron absorption- and transport-related genes in roots showed that MdNramp3 and MxCS1 increased significantly. These results indicated that iron deficiency promoted the long-distance transport of micronutrients in xylem, and MdNramp3 and MxCS1 might play an important role in this process. Importantly, this study directly provides visual divalent metals distribution in tissue level for an improved understanding of metal absorption process in apple rootstock.

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Haishan An, Feixiong Luo, Ting Wu, Yi Wang, Xuefeng Xu, Xinzhong Zhang and Zhenhai Han

Fine root (≤2 mm in diameter) systems play a pivotal role in water and mineral uptake in higher plants. However, the impact of fine root architecture on tree growth and development is not fully understood, especially in apple trees. Here, we summarize a 6-year-trial study using minirhizotrons to investigate the relationships between fine root production, mortality, and longevity in ‘Red Fuji’ trees grafted on five different rootstocks/interstems. Based on root length density (RLD), fine root production and mortality were markedly lower in ‘Red Fuji’ trees growing on dwarfing M.9 (M.9) and Shao series no. 40 (SH.40) rootstocks than in trees on standard Malus robusta ‘Baleng Crab’ (BC) rootstock. The use of M.9 and SH.40 as interstems led to an extensive reduction in fine root production and mortality in comparison with BC rootstock. Root number density (RND), but not average root length (ARL), showed similar patterns to RLD. About one-half of fine roots in ‘Red Fuji’ tree growing on M.9 were scattered within the top 0–20 cm of topsoil, indicating shallow root system in M.9, whereas in trees on BC, 55.15% of fine roots were distributed between 100- and 150-cm soil depth, indicating a deep root architecture. The addition of interstems did not alter fine root soil-depth distribution. For all rootstocks/interstems, fine roots with a life span of less than 80 days were generated in spring and summer, but fine roots which lived for more than 81 days were produced almost all the year round. In conclusion, lower fine root numbers were associated with the dwarfing effect in dwarfing rootstocks/interstems, but ARL and shallower rooting were not.

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Chenping Zhou, Ruiting Chen, Yaqiang Sun, He Wang, Yi Wang, Ting Wu, Xinzhong Zhang, Xuefeng Xu and Zhenhai Han

Bridge grafting is widely applied in trunk-wounded apple trees. In this study, we carried out semigirdling and ring girdling on the trunk of ‘Nagafu 2’/Malus baccata (L.) Borkh apple trees to simulate trunk injury. We then bridge grafted a M9 self-rooted rootstock on the injured trunks to study the effects of bridge grafting on flowering, fruit-set, tree vigor, and fruit characteristics in ‘Nagafu 2’ apple. The results showed that both semigirdling and ring girdling due to the large wounded area caused significant decrease in flowering, fruit-set, and tree vigor (estimated by measuring leaf area, leaf gas exchange, tree height, and shoot growth); in addition, ring girdling increased flesh and peel firmness. However, bridge grafting of M9 self-rooted rootstock on semigirdling and girdling apple trees resulted in partial recovery of tree vigor (leaf area and photosynthesis) and maintaining the reduction of vegetative growth, thereby increasing flowering, fruit-set, yield, fruit weight, and peel firmness.

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Shuai-Ping Gao, Kang-Di Hu, Lan-Ying Hu, Yan-Hong Li, Yi Han, Hui-Li Wang, Kai Lv, Yong-Sheng Liu and Hua Zhang

Hydrogen sulfide (H2S) was recently recognized as an endogenous gaseous molecule involved in seed germination, root organogenesis, abiotic stress tolerance, guard cell movement, and delay of senescence in plants. In the present study, we show that H2S participates in the regulation of postharvest ripening and senescence in fresh-cut kiwifruit, Actinidia deliciosa. Fumigation of fresh-cut kiwifruit with the H2S donor sodium hydrosulfide (NaHS) solution prolonged kiwifruit storage time and alleviated senescence and tissue softening in a dose-dependent manner at an optimal concentration of 1.0 mmol·L−1 NaHS. H2S treatment maintained higher levels of reducing sugars, soluble proteins, free amino acids, ascorbate, and chlorophyll and lowered carotenoid levels. H2S treatment also significantly decreased the contents of malondialdehyde (MDA), hydrogen peroxide (H2O2) and superoxide anion (O2 ) during fruit storage compared with water controls. Furthermore, the activities of guaiacol peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR) were increased by H2S treatment, whereas the activity of lipoxygenase (LOX) was decreased compared with untreated controls. Taken together, these results suggest that H2S is involved in prolonging postharvest shelf life and plays an antioxidative role in fresh-cut kiwifruit.

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Choun-Sea Lin, Huey-Ling Lin, Wann-Neng Jane, Han-Wen Hsiao, Chung-Chih Lin, Fang-Yi Jheng and Wei-Chin Chang

A xylem mutant (vse) was isolated from a Bambusa edulis (Odashima) Keng plantlet following vegetative micropropagation and subculture for 7 consecutive years and induced to proliferate in medium supplemented with 0.1 mg·L-1 (0.5 μm) thidiazuron (TDZ) and to develop roots in medium supplemented with 5 mg·L-1 (26.9 μm) α-naphthaleneacetic acid (NAA). Subsequent investigations comparing the growth habits of mutant plantlets with those of the wild type indicated that the growth of the former was retarded in a greenhouse. Several morphological abnormalities were observed in the vse mutant: it had thinner stems with fewer trichromes on the surface; the xylem vessels were smaller in diameter and contained crystal-like structures in the pith; the leaves were shorter and narrower with a sharp leaf blade angle; the roots were thinner and contained fewer xylem cells. The cation concentrations of both the mutant and wild type were similar in the in vitro analysis, except for those of iron and potassium, which were lower in mutant leaves in vivo. In 2-month-old mutant plants, iron chlorosis was observed on young leaves and a potassium deficiency was observed on older leaves. After 1 year of growth in the greenhouse, all of the wild-type plants had survived, but only 27% (16/60) of the mutant vse plants were alive.