Genome investigation suggests MdSHN3, an APETALA2-domain transcription factor gene, to be a positive regulator of apple fruit cuticle formation and an inhibitor of russet development.

Justin Lashbrooke,Fabrizio Costa,Asaph Aharoni

doi:10.1093/jxb/erv366

Abstract

The outer epidermal layer of apple fruit is covered by a protective cuticle. Composed of a polymerized cutin matrix embedded with waxes, the cuticle is a natural waterproof barrier and protects against several abiotic and biotic stresses. In terms of apple production, the cuticle is essential to maintain long post-harvest storage, while severe failure of the cuticle can result in the formation of a disorder known as russet. Apple russet results from micro-cracking of the cuticle and the formation of a corky suberized layer. This is typically an undesirable consumer trait, and negatively impacts the post-harvest storage of apples. In order to identify genetic factors controlling cuticle biosynthesis (and thus preventing russet) in apple, a quantitative trait locus (QTL) mapping survey was performed on a full-sib population. Two genomic regions located on chromosomes 2 and 15 that could be associated with russeting were identified. Apples with compromised cuticles were identified through a novel and high-throughput tensile analysis of the skin, while histological analysis confirmed cuticle failure in a subset of the progeny. Additional genomic investigation of the determined QTL regions identified a set of underlying genes involved in cuticle biosynthesis. Candidate gene expression profiling by quantitative real-time PCR on a subset of the progeny highlighted the specific expression pattern of a SHN1/WIN1 transcription factor gene (termed MdSHN3) on chromosome 15. Orthologues of SHN1/WIN1 have been previously shown to regulate cuticle formation in Arabidopsis, tomato, and barley. The MdSHN3 transcription factor gene displayed extremely low expression in lines with improper cuticle formation, suggesting it to be a fundamental regulator of cuticle biosynthesis in apple fruit.

Highlights

The plant cuticle is a continuous lipophilic layer covering the role is probably the interaction point between the plant and above-ground epidermal layer of the plant
The development of russeting on fruit collected from each seedling of the ‘Golden Delicious’ and ‘Braeburn’ (G×B)’ progeny was evaluated over 2 years
The simultaneous investigation of the tensile performance of apple cuticles and the occurrence of russeting demonstrated the relationship between these two properties, i.e. cuticle failure is a prerequisite for apple russet formation

Summary

Introduction

The plant cuticle is a continuous lipophilic layer covering the role is probably the interaction point between the plant and above-ground epidermal layer of the plant. While mild cuticle failure may lead to excess water loss or an increase in fungal infection rates (Shi et al, 2013), severe cuticle failure in fruit results in a disorder known as russeting (Lara et al, 2014). This disorder results from the development of a series of microscopic cracks in the cuticle and the subsequent formation of a waterproofing periderm layer consisting largely of suberin (Khanal et al, 2013). The development of cuticular cracks accelerates the development of flesh browning due to an enhanced oxidative process (Lara et al, 2014), and may result in softer internal tissue due to the loss of an external mechanical support (Saladié et al, 2005., 2007)

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Discussion

Conclusion