Human selection has reshaped crop genomes. Here we report an apple genome variation map generated through genome sequencing of 117 diverse accessions. A comprehensive model of apple speciation and domestication along the Silk Road is proposed based on evidence from diverse genomic analyses. Cultivated apples likely originate from Malus sieversii in Kazakhstan, followed by intensive introgressions from M. sylvestris. M. sieversii in Xinjiang of China turns out to be an “ancient” isolated ecotype not directly contributing to apple domestication. We have identified selective sweeps underlying quantitative trait loci/genes of important fruit quality traits including fruit texture and flavor, and provide evidences supporting a model of apple fruit size evolution comprising two major events with one occurring prior to domestication and the other during domestication. This study outlines the genetic basis of apple domestication and evolution, and provides valuable information for facilitating marker-assisted breeding and apple improvement.

Nitrogen (N) is one of the most required mineral elements for plant growth, and potassium (K) plays a vital role in nitrogen metabolism, both elements being widely applied as fertilizers in agricultural production. However, the exact relationship between K and nitrogen use efficiency (NUE) remains unclear. Apple dwarf rootstock seedlings (M9T337) were used to study the impacts of different K levels on plant growth, nitrogen metabolism, and carbon (C) assimilation in water culture experiments for 2 years. The results showed that both deficiency and excess K inhibited the growth and root development of M9T337 seedlings. When the K supply concentration was 0 mM and 12 mM, the biomass of each organ, root-shoot ratio, root activity and NO 3 − ion flow rate decreased significantly, net photosynthetic rate (P n) and photochemical efficiency (F v /F m) being lower. Meanwhile, seedlings treated with 6 mM K + had higher N and C metabolizing enzyme activities and higher nitrate transporter gene expression levels (NRT1.1; NRT2.1). 13 C and 15 N labeling results showed that deficiency and excess K could not only reduce 15 N absorption and 13 C assimilation accumulation of M9T337 seedlings, but also reduced the 15 N distribution ratio in leaves and 13 C distribution ratio in roots. These results suggest that appropriate K supply (6 mM) was optimal as it enhanced photoassimilate transport from leaves to roots and increased NUE by influencing photosynthesis, C and N metabolizing enzyme activities, nitrate assimilation gene activities, and nitrate transport.

We examined the effects of three fertilization treatments (CK, NPK, and NPKM) on soil pH, soil organic carbon (SOC), total nitrogen (TN), C/N ratio, and available nutrients [alkali extractable-N (AN), available phosphorus (AP), exchangeable K (EK)] in the plow layer (0-40 cm) in an apple orchard from 1988 to 2016 in North China, as well as the mechanisms of nitrogenous fertilizer action on soil acidification through a three-year experiment.

Excessive application of nitrogen (N) fertilizer is common in Chinese apple production. High N reduced the contents of soluble sugar and total flavonoids by 16.05 and 19.01%, respectively, resulting in poor fruit quality. Moreover, high N increased the total N and decreased the total C and C/N ratio of apple fruits. On the basis of the transcriptomic, proteomic, and metabolomic analyses, the global network was revealed. High N inhibited the accumulation of carbohydrates (sucrose, glucose, and trehalose) and flavonoids (rhamnetin-3-O-rutinoside, rutin, and trihydroxyisoflavone-7-O-galactoside) in fruits, and more C skeletons were used to synthesize amino acids and their derivatives (especially low C/N ratio, e.g., arginine) to be transferred to N metabolism. This study revealed new insights into the decline in soluble sugar and flavonoids caused by high N, and hub genes (MD07G1172700, MD05G1222800, MD16G1227200, MD01G1174400, and MD02G1207200) and hub proteins (PFK, gapN, and HK) were obtained.

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