Apomictic Malus plants exhibit abnormal pollen development.

Apomixis is the asexual reproduction through seeds that leads to the production of genetically uniform progeny. It has become an important tool in plant breeding because it facilitates the retention of genotypes with desirable traits and allows seeds to be obtained directly from mother plants. Apomixis is rare in most economically important crops, but it occurs in some Malus species. Here, the apomictic characteristics of Malus were examined using four apomictic and two sexually reproducing Malus plants. Results from transcriptome analysis showed that plant hormone signal transduction was the main factor affecting apomictic reproductive development. Four of the apomictic Malus plants examined were triploid, and pollen was either absent or present in very low densities in the stamen. Variation in the presence of pollen was associated with variation in the apomictic percentage; specifically, pollen was absent in the stamens of tea crabapple plants with the highest apomictic percentage. Furthermore, pollen mother cells failed to progress normally into meiosis and pollen mitosis, a trait mostly observed in apomictic Malus plants. The expression levels of meiosis-related genes were upregulated in apomictic plants. Our findings indicate that our simple method of detecting pollen abortion could be used to identify apple plants that are capable of apomictic reproduction.

Apple SUMO E3 ligase MdSIZ1 is involved in the response to phosphate deficiency.

In plants, SIZ1 regulates abiotic and biotic stress responses by promoting the SUMOylation of proteins. The apple MdSIZ1 protein has conserved domains similar to those of Arabidopsis AtSIZ1. Real-time fluorescent quantitative analysis showed that MdSIZ1 gene expression was induced by phosphate-deficient conditions. In addition, the level of SUMOylation was also significantly increased under these conditions. The MYB transcription factor MdPHR1 might be a target for the SUMO protein, which is a phosphorus starvation-dependent protein. Subsequently, an MdSIZ1 expression vector was constructed and transformed in Arabidopsis mutant siz1-2 and apple callus. The MdSIZ1 transgenic Arabidopsis partially complemented the defect phenotype of siz1-2 under phosphate-deficient conditions. The survival rate, length of primary root, and number or density of lateral roots were similar between the transgenic lines and wild type (WT). Under phosphate-deficient conditions, the SUMO conjugate and fresh weight of the MdSIZ1 transgenic apple callus were improved compared with WT. The MdSIZ1 transgenic apple callus grew under phosphate-deficient conditions, whereas the MdSIZ1 sense apple callus did not. Therefore, MdSIZ1 is involved in the regulation of the phosphate-deficiency response in apple.

[Evaluation of zinc deficiency tolerance in different kinds of apple rootstocks].

The objective of this study was to screen and evaluate the zinc deficiency tolerance among eight apple rootstocks, i.e., Malus baccata, M. rockii, M. xiaojinensis, M. sikkimensis, M. sieversii, M. robusta, M. hupehensis and Malus 'Flame'. The experiment took these 8 kinds of root-stocks as the research materials to observe and analyze the index of the rootstock's height, dry biomass, root architecture and zinc concentration, and with help of the fuzzy membership function to work out a comprehensive evaluation on their zinc deficiency tolerance. The result showed that several obvious zinc deficiency symptoms were observed in three kinds of rootstocks (M. rockii, M. sikkimensis and M. sieversii), such as dwarfed plant and newborn small leaves, while such symptoms were not obvious in M. xiaojinensis and M. 'Flame'. The plant biomass, height and zinc accumulation of aerial part greatly decreased under zinc deficiency stress, while smaller reduction was observed in M. xiaojinensis than in other rootstocks. M. xiaojinensis and M. baccata had higher zinc concentrations in leaves than others. According to the fuzzy membership function and cluster analysis, the resistance of the eight apple rootstocks to zinc deficiency was ranked: M. xiaojinensis was the highest one; M. 'Flame' was the second; M. baccata, M. sikkimensis, M. robusta and M. hupehensis were rather weaker; M. rockii and M. sieversii demonstrated the highest sensitivity to zinc deficiency.