Genome-wide association study (GWAS) of leaf wax components of apple

The wax layer of apple leaves plays an important role in improving stress resistance, but relatively little is known about the mechanisms of wax synthesis and transport in apple leaves. In this study, 17 wax components, including alcohols, alkanes, fatty acids and terpenes, were analyzed by gas chromatography-tandem mass spectrometry (GC-MS) from the leaves of 123 apple germplasms. Whole-genome sequencing of these apple accessions yielded 5.9 million high-quality single nucleotide polymorphisms (SNPs). We performed a genome-wide association study (GWAS) on 17 wax components and identified several genes related to wax synthesis and transport, including MdSHN1 (SHINE1), MdLTP4 (LIPID TRANSFER PROTEIN4), MdWSD1 (WAX ESTER SYNTHASE/ACYL-COA DIAC-YLGLYCEROL ACYLTRANSFERASE1), MdRDR1 (RNA-DEPENDENT RNA POLYMERASE1), MdACBP6 (ACYL-COA-BINDING PROTEIN6), MdNLE (NOTCHLESS) and MdABCG21 (ATP-BINDING CASSETTE G21). Moreover, we identified some prominent SNPs that may affect gene expression and protein function. These results provide insights into mechanisms of wax synthesis and transport in apple leaves and broaden the genetic resources and basis for facilitating resistance breeding.