Abstract
Apples (Malus domestica Borkh) are prone to preharvest fruit drop, which is more pronounced in ‘Honeycrisp’. Hexanal is known to improve fruit retention in several economically important crops. The effects of hexanal on the fruit retention of ‘Honeycrisp’ apples were assessed using physiological, biochemical, and transcriptomic approaches. Fruit retention and fruit firmness were significantly improved by hexanal, while sugars and fresh weight did not show a significant change in response to hexanal treatment. At commercial maturity, abscisic acid and melatonin levels were significantly lower in the treated fruit abscission zone (FAZ) compared to control. At this stage, a total of 726 differentially expressed genes (DEGs) were identified between treated and control FAZ. Functional classification of the DEGs showed that hexanal downregulated ethylene biosynthesis genes, such as S-adenosylmethionine synthase (SAM2) and 1-aminocyclopropane-1-carboxylic acid oxidases (ACO3, ACO4, and ACO4-like), while it upregulated the receptor genes ETR2 and ERS1. Genes related to ABA biosynthesis (FDPS and CLE25) were also downregulated. On the contrary, key genes involved in gibberellic acid biosynthesis (GA20OX-like and KO) were upregulated. Further, hexanal downregulated the expression of genes related to cell wall degrading enzymes, such as polygalacturonase (PG1), glucanases (endo-β-1,4-glucanase), and expansins (EXPA1-like, EXPA6, EXPA8, EXPA10-like, EXPA16-like). Our findings reveal that hexanal reduced the sensitivity of FAZ cells to ethylene and ABA. Simultaneously, hexanal maintained the cell wall integrity of FAZ cells by regulating genes involved in cell wall modifications. Thus, delayed fruit abscission by hexanal is most likely achieved by minimizing ABA through an ethylene-dependent mechanism.
Highlights
Not meet above requirements considered as non-significant (NS); (c) heat map of the 726 If any gene did not meet the above requirements are considered as non-significant (NS); (c) heat map of the 726 differentially expressed genes (DEGs) shows the variation across the four samples harvested from two sites.H1 and H2 represent the hexanal treated samples harvested from site A and B, respectively
Various pathways could be identified under each Gene Ontology (GO) term class, we focused on various pathways could be identified under each GO term class, we focused on enriched specific pathways that could contribute to fruit retention at commercial maturity
Genes related to auxin biosynthesis, tryptophan aminotransfer7 of 19 ase-related protein 3 (TAR3), and response (CHS, MYB113-like, and CYP75B1) were downregulated by hexanal in the fruit abscission zone (FAZ), while genes involved in auxin transports, such as protein big grain 1-like (BG1), protein walls are thin-1 (WAT1), and thermospermine synthase-5, cytokinin, and BR as a result of hexanal treatment (Table S4)
Summary
Previous studies on fruit abscission have reported that the cell-separation within the FAZ is the result of a cascade of physiological events due to the coordinated expression of PGR related genes resulting in abrupt changes in endogenous levels of plant hormones [7,15,16]. We previously reported that the preharvest hexanal spray improved postharvest storage attributes of ‘Honeycrisp’ apples [9] Based on those studies, we hypothesize that hexanal improves fruit retention either by regulating the abscission through an ethylene. Based on those studies, we hypothesize that hexanal improves fruit retention either by regulating the abscission through an ethylene dependent mechanism or directly manipulating the ABA/GA mechanism.
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