Adventitious root (AR) formation is indispensable for vegetative asexual propagation. Indole-3-butyric acid (IBA) functioned indirectly as precursor of IAA in regulating AR formation. Ethylene affects auxin synthesis, transport, and/or signaling processes. However, the interactions between auxin and ethylene that control AR formation in apple have not been elucidated. In this study, we investigated the effects of IBA and its interaction with ethylene on AR development in apple. The results revealed that IBA stimulated the formation of root primordia, increased the number of ARs, and upregulated expression of genes (MdWOX11, MdLBD16, and MdLBD29) involved in AR formation. Comparison of different periods of IBA application indicated that IBA was necessary for root primordium formation, while long time IBA treatment obviously inhibited root elongation. RNA-seq analysis revealed that many plant hormone metabolism and signal transduction related genes were differentially expressed. IBA stimulated the production of ethylene during AR formation. Auxin inhibiting ARs elongation depended on ethylene. Together, our results suggest that the inhibitory role of auxin on AR elongation in apples is partially mediated by stimulated ethylene production.
Apple is one of the most important fruit trees in temperate zones, and is cultivated widely throughout the world. Drought stress affects the normal growth of apple tree, and further affects fruit yield and quality. The present study examined the effects of drought on photosynthesis and water use efficiency (WUE) of two apple cultivars (Honeycrisp and Yanfu 3) that differ in drought tolerance. The results showed that the photosynthetic rate decreased in response to drought stress for both cultivars, with significant differences in intensity. Values for net photosynthetic rate (Pn) in stressed Yanfu 3 remained significantly lower than in the controls, while, for Honeycrisp, only a slight drop in photosynthesis. Similarly, stomatal conductance (Gs), intercellular CO 2 concentration (Ci), transpiration rate (Tr) were markedly reduced in Yanfu 3 under drought stress. However, Honeycrisp showed only minor changes. Under drought stress, the contents of Chl a, Chl b and Chl t in Yanfu 3 were all decreased significantly compared with the control. However, little difference in Honeycrisp was noted between stressed plants and controls. Values for WUE in stressed Yanfu 3 remained higher than in the controls from day 3 until the end of the experiment, while no significant difference was observed in Honeycrisp. Furthermore, Honeycrisp also exhibited superior physiological traits, as indicated by its anatomical and morphological characteristics. Therefore, we conclude that the superior drought tolerance of Honeycrisp was due to its anatomical and morphological characteristics, which possibly contributed to the maintenance of higher photosynthetic capacity than Yanfu 3.
Rootstocks play a vital role in regulating the environmental adaptability and controlling the growth and development of apple trees. M9T337, an excellent apple rootstock widely used in commercial orchards, could confer dwarf tree architectures, early fruiting and suitability for high-density planting. However, the rooting ability of M9T3337 is low when it is vegetatively propagated, and researchers have not yet established an efficient micropropagation system. The present study systematically evaluated the multiplication in adventitious shoots and the in vitro formation of adventitious roots to determine the effects of the culture media and plant growth regulators of M9T337 and a rapid micropropagation system was developed. For the shoot multiplication, the highest multiplication index of 3.93 was obtained on Murashige and Skoog (MS) media supplemented with 2.0 mg/L 6-BA, 0.1 mg/L NAA and 0.3 mg/L GA3 from 12 combinations of 6-BA and NAA. Stronger and taller adventitious shoots were grown on MS supplemented with 1.8 mg/L 6-BA and 0.5 mg/L NAA. The optimal media with 100% rooting was obtained using ½ MS supplemented with 0.3 mg/L IBA or MS supplemented with 0.6 mg/L IBA for the rooting induction, resulting in mean rooting numbers of 13.00 and 11.33, respectively. Additionally, the effect on rooting of adding 0.3 mg/L IBA or not on the 1/2 MS and MS media was compared; the results suggested that an appropriate IBA concentration was the key to successful rooting. The rooted plantlets were acclimatised in a shaded greenhouse with an 84% survival rate. The established micropropagation system could be used for the rapid propagation of M9T337 for commercial production.
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