Transcriptome Analysis Reveals Potential Mechanisms for Ethylene-Inducible Pedicel-Fruit Abscission Zone Activation in Non-Climacteric Sweet Cherry (Prunus avium L.).

Seanna Hewitt,Jonathan Abarca,Matthew Whiting,Benjamin Kilian,Tyson Koepke,Amit Dhingra

doi:10.3390/horticulturae7090270

Abstract

The harvesting of sweet cherry (Prunus avium L.) fruit is a labor-intensive process. The mechanical harvesting of sweet cherry fruit is feasible; however, it is dependent on the formation of an abscission zone at the fruit–pedicel junction. The natural propensity for pedicel—fruit abscission zone (PFAZ) activation varies by cultivar, and the general molecular basis for PFAZ activation is not well characterized. In this study, ethylene-inducible change in pedicel fruit retention force (PFRF) was recorded in a developmental time-course with a concomitant analysis of the PFAZ transcriptome from three sweet cherry cultivars. In ‘Skeena’, mean PFRF for both control and treatment fruit dropped below the 0.40 kg-force (3.92 N) threshold for mechanical harvesting, indicating the activation of a discrete PFAZ. In ‘Bing’, mean PFRF for both control and treatment groups decreased over time. However, a mean PFRF conducive to mechanical harvesting was achieved only in the ethylene-treated fruit. While in ‘Chelan’ the mean PFRF of the control and treatment groups did not meet the threshold required for efficient mechanical harvesting. Transcriptome analysis of the PFAZ region followed by the functional annotation, differential expression analysis, and gene ontology (GO) enrichment analyses of the data facilitated the identification of phytohormone-responsive and abscission-related transcripts, as well as processes that exhibited differential expression and enrichment in a cultivar-dependent manner over the developmental time-course. Additionally, read alignment-based variant calling revealed several short variants in differentially expressed genes, associated with enriched gene ontologies and associated metabolic processes, lending potential insight into the genetic basis for different abscission responses between the cultivars. These results provide genetic targets for the induction or inhibition of PFAZ activation, depending on the desire to harvest the fruit with or without the stem attached. Understanding the genetic mechanisms underlying the development of the PFAZ will inform future cultivar development while laying a foundation for mechanized sweet cherry harvest.

Highlights

Sweet cherry (Prunus avium L.), a member of the Rosaceae family, is a commercially important tree fruit crop throughout the world, with approximately 2.3 million tons produced annually [1,2]
Sweet cherry phenotyping studies have shown that pedicel fruit retention force (PFRF) values remain consistent for these cultivars across multiple years, indicating that the abscission phenotypes are genetically stable and can perhaps be manipulated at the genetic level [6]. These findings suggest that the standardization of PFRF for mechanical harvesting across cultivars is possible if the ideal ethephon or other treatment regimens are determined for individual sweet cherry cultivars
This study investigated the changes in PFRF, gene expression changes, and enriched biological processes underlying pedicel—fruit abscission zone (PFAZ) activation in sweet cherry

Summary

Introduction

Sweet cherry (Prunus avium L.), a member of the Rosaceae family, is a commercially important tree fruit crop throughout the world, with approximately 2.3 million tons produced annually [1,2]. In addition to its worldwide economic value, production and consumption of sweet cherries has increased in recent years as consumers have become aware of Horticulturae 2021, 7, 270. The industry is evolving to address the growing market and the availability of labor during harvest and has begun adapting harvesting strategies to meet new consumer demand for stemless cherries [3,4]. Mechanical harvesting, on the other hand, is best achieved when the fruit abscises at the fruit–pedicel junction [4]. Increasing labor costs associated with traditional hand harvesting, in addition to the growing demand for stemless fruit, has made the adoption of mechanical harvesting strategies attractive if they can be made uniform across cultivars [6]. Unlike sour cherries (Prunus cerasus), which develop an anatomically and histologically distinct fruit–pedicel abscission zone (PFAZ)

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