Transcriptome sequencing identified wax-related genes controlling the glossy phenotype formation of Ganqi 3, a bud mutant derived from wild-type Newhall navel orange

Plant aerial surfaces are covered with cuticular waxes, a barrier structure protecting plant organs from biotic and abiotic stresses. Although the wax ultrastructure and composition in navel orange (Citrus sinensis [L.] Osbeck) have been studied in various reports, the molecular mechanisms of wax biosynthesis and export are still unknown. In this report, two cDNA libraries from the fruit peels of Newhall navel orange (WT) and its glossy mutant Ganqi 3 (MT) were constructed and sequenced using the Illumina sequencing platform to identify differentially expressed genes (DEGs) involved in wax biosynthesis and export. A total of 72 wax-related DEGs were identified from WT and MT fruits. Of these, 48 DEGs encoded functional proteins, 3 DEGs encoded transcription factors (TFs), 4 DEGs were involved in post-transcriptional processes, 2 DEGs with post-translational functions were involved in cuticular wax biosynthesis, and 15 DEGs encoding ABCG transporters were related to wax transport. Most wax-related DEGs, including 52 functional DEGs, 3 DEGs encoding TFs, 3 post-transcriptional DEGs, and 2 post-translational DEGs exhibited decreased expression levels in MT fruits, suggesting that cuticular wax biosynthesis in navel orange was regulated at the transcriptional, post-transcriptional, and post-translational levels. Moreover, many DEGs were related to the stress response, hormone-mediated signaling pathways, and developmental processes, suggesting that cuticular wax biosynthesis in navel orange was affected by various stresses, hormone levels, and developmental processes. Combined with the results from our previous studies investigating the wax structure and chemical composition differences between WT and MT fruits, we suggest that the decrease in most wax-related DEG expressions in MT fruit peels resulted in the decrease of aliphatic wax on MT fruit surfaces, leading to a reduced number of epicuticular crystals and finally resulting in the glossy surface of MT fruits.