A genomic approach to understanding the relationship between apple colour and ethylene

Abstract

Apple (Malus x domestica) is an important export crop for the New Zealand economy. Apples contain a diverse set of secondary metabolites including anthocyanins, which are responsible for its distinct red colouration and beneficial dietary nutrition. Due to good postharvest storage qualities, apples are a common consumer choice worldwide. Recently, novel cultivars of red-fleshed apples have been produced by crossing wild red-fleshed apples with domesticated varieties. This phenotype is due to the misregulation of the transcription factor that controls anthocyanin pigmentation in apple, MdMYB10. This misregulation is caused by a mutation in the promoter of this key gene. The mutation results in high levels of anthocyanin in both the fruit skin and flesh. However, there are indications that this phenotype is also associated with elevated levels of the gaseous hormone ethylene and enzymatic browning during postharvest causing challenges for storage and quality control. The aim of this thesis is to understand how overexpression of the MdMYB10 gene may be responsible for causing an increase in ethylene production. Through a combination of RNASeq and gene ontology, nine candidate genes were selected for functional analysis. These included ACS, WRKY, ERF, and NAC genes. Transient overexpression of these genes were performed in Nicotiana benthamiana to identify their activity against target promoters, when co-infiltrated with MdMYB10. The promoter of MdERF106 was found to be up-regulated when co-infiltrated with MdMYB10 and MdMYB110a but down-regulated when infiltrated with the transcriptional repressors MdMYB15 and MDMYB16. In addition, the promoter of ethylene biosynthesis gene MdACS5a was tested for activity against a variety of apple MYB genes. These findings will help further understand how anthocyanin over-production affects ethylene levels in apples, and other highly anthocyanic fruits and vegetables.