Transcriptomic approach to uncover dynamic events in the development of mid-season sunburn in apple fruit

Authors: Waite, Jessica; KELLY, ELIZABETH - Pennsylvania State University; ZHANG, HUITING - Washington State University; Hargarten, Heidi; WALIULLAH, SUMYYA - Washington State University; ALTMAN, NAOMI - Pennsylvania State University; DEPAMPHILIS, CLAUDE - Pennsylvania State University; Honaas, Loren; KALCSITS, LEE - Washington State University

Submitted to: G3, Genes/Genomes/Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/2/2023
Publication Date: 6/1/2023
Citation: Waite, J.M., Kelly, E., Zhang, H., Hargarten, H.L., Waliullah, S., Altman, N.S., dePamphilis, C., Honaas, L.A., Kalcsits, L. 2023. Transcriptomic approach to uncover dynamic events in the development of mid-season sunburn in apple fruit. G3, Genes/Genomes/Genetics. 13(8). Article jkad120. https://doi.org/10.1093/g3journal/jkad120.
DOI: https://doi.org/10.1093/g3journal/jkad120

Interpretive Summary: Apple production in the Pacific Northwest, as well as other places in the world, takes place in a high-heat, high-light, low-humidity environment, which can lead to different types of sun injury on the fruit. Sun injury on apple fruit causes losses at harvest, as well as during months of postharvest storage. Thus, it is important to understand the causes and development of sun injury. To date, we know a lot about what happens with antioxidants and plant pigments in response to sun-related stresses, especially close to harvest time. However, less is known about stress responses earlier in the growing season. Here, using a technology called RNA-Seq that allows us to measure the activity of all ~40,000 apple genes at once, we surveyed the activity of genes in ‘Honeycrisp’ apple fruit. Our experiment monitored responses to high and medium heat stresses of apples on the tree; that is, how genes were turned on and off over time. This study helps us understand how apples respond to sunburn, giving us clues that might lead to new ways to mitigate and/or predict risk for sun-related apple fruit damage.

Technical Abstract: Apples grown in high heat, high light, and low humidity environments are at risk for sun injury disorders like sunburn and associated crop losses. Understanding the physiological and molecular mechanisms underlying sunburn will support improvement of mitigation strategies and breeding for more resilient varieties. Numerous studies have highlighted key biochemical processes involved in sun injury, such as the phenylpropanoid and reactive oxygen species (ROS) pathways, demonstrating both enzyme activities and expression of related genes in response to sunburn conditions. Most previous studies have focused on at-harvest activity of a small number of genes in response to heat stress. Thus, it remains unclear how stress events earlier in the season affect physiology and gene expression. Here, we applied heat stress to mid-season apples in the field, and collected tissue along a time course – 24, 48, and 72 hours following a heat stimulus – to investigate dynamic gene expression changes using a transcriptomic lens. We found a relatively small number of differentially expressed genes (DEGs) and enriched functional terms in response to heat treatments. Only a few of these belonged to pathways previously described to be involved in sunburn, such as the AsA-GSH pathway, while most DEGs had not yet been implicated in sunburn or heat stress in pome fruit.