Auxin response and transport during induction of pedicel abscission in tomato

Auxin plays a central role in control of organ abscission, and it is thought that changes in the auxin gradient across the abscission zone are the primary determinant of the onset of abscission. The nature of this gradient, whether in concentration, flow, or perhaps in the response system has not conclusively been determined. We employed a DR5::GUS auxin response reporter system to examine the temporal and spatial distribution of the auxin response activity in response to developmental and environmental cues during pedicel abscission in tomato. In pedicels of young and fully open flowers, auxin response, as indicated by GUS activity, was predominantly detected in the vascular tissues and was almost entirely confined to the abscission zone (AZ) and to the distal portion of the pedicel, with a striking reduction in the proximal tissues below the AZ—a ‘step’, rather than a gradient. Following pollination and during early fruit development, auxin response increased substantially throughout the pedicel. Changes in GUS activity following treatments that caused pedicel abscission (flower removal, high temperature, darkness, ethylene, or N-1-naphthylphthalamic acid (NPA) treatment) were relatively minor, with reduced auxin response in the AZ and some reduction above and below it. Expression of genes encoding some auxin efflux carriers (PIN) and influx carriers (AUX⁄LAX) was substantially reduced in the abscission zone of NPA-treated pedicels, and in pedicels stimulated to abscise by flower removal. Our results suggest that changes in auxin flow distribution through the abscission zone are likely more important than the auxin response system in the regulation of abscission.

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

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.

Transcriptomics of Improved Fruit Retention by Hexanal in ‘Honeycrisp’ Reveals Hormonal Crosstalk and Reduced Cell Wall Degradation in the Fruit Abscission Zone

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.

Preliminary Study Mechanism of Ethylene Induced Abscission Zone Formation and Function of GhArfGAP in Upland Cotton Boll Stalk

Background: With the continuous growth of population, the demand for fiber is also rising sharply. As one of the main fiber plants in the world, cotton fiber yield of upland cotton is affected by boll abscission, which is related to the formation of abscission zone. Therefore, we explored the formation of the abscission zone of upland cotton.Result: The formation of abscission layer of cotton boll stalk was promoted by exogenous ethylene. It was found that both the number of Golgi apparatus and the number of stacking layers increased in the dissociated cells. The GhArfGAP gene family in upland cotton was screened by bioinformatics method, and the species and evolutionary relationship of GhArfGAP gene family were analyzed. qRT-PCR showed that the expression patterns of GhArfGAP13,GhArfGAP15, GhArfGAP25 and GhArfGAP34 in cotton were spatiotemporal specific. Subcellular localization suggested that GhArfGAP25 played a role in Golgi apparatus . The expression of GhArfGAP25 in transgenic Arabidopsis increased in the root, stem and leaf.Conclusions: Ethylene could induce the formation of abscission zone in upland cotton. GhArfGAP13,GhArfGAP15,GhArfGAP25,GhArfGAP34 might regulate the changes of Golgi apparatus in abscisson zone.Taken together the findings provide new ideas for the study of cotton abscission formation.

Transcriptomics of improved fruit retention by hexanal in 'Honeycrisp' reveals hormonal crosstalk and reduced cell-wall remodelling in the fruit-abscission zone

Apples (Malus domestica Borkh) are prone to pre-harvest fruit drop which is more pronounced in 'Honeycrisp'. Using a transcriptomic approach, we analyzed the molecular mechanisms of fruit retention in 'Honeycrisp'. A total of 726 differentially expressed genes (DEGs) were identified in the abscission zone of hexanal-treated and untreated fruit (FAZ). Hexanal down-regulated the genes involved in ethylene biosynthesis, such as S-adenosylmethionine synthase (SAM2) and 1-aminocyclopropane-1carboxylic acid oxidases (ACO3, ACO4 and ACO4-like). Genes related to ABA biosynthesis (FDPS and CLE25) were also down-regulated. On the contrary, gibberellic acid (GA) biosynthesis genes, gibberellin 20 oxidase1-like (GA20OX-like) and ent-kaurene oxidase (KO) were up-regulated. Further, hexanal down-regulated the expression of genes related to cell-wall remodelling enzymes such as polygalacturonase (PG1), glucanases (endo-β-1,4-glucanase; EG) and expansins (EXPA1-like, EXPA6, EXPA8, EXPA10-like, EXPA16-like). Hexanal also reduced ethylene, and abscisic acid (ABA) production at commercial harvest stage. Hexanal reduced ethylene production in fruits and thus 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. Our findings show that fruit abscission is delayed by hexanal, by down regulating ABA through an ethylene-dependent mechanism.

A tomato prolyl-4-hydroxylase causes relocation of abscission zone and alters abscission kinetics

The detachment of organs is controlled by highly regulated molecular mechanisms. The position of the tomato abscission zone (AZ) is defined by the ratio of the proximal to distal part of the pedicel. In this study, the ratio was altered due to a shift in the position of the AZ which was attributed to shorter and longer pedicels of SlP4H3 RNAi and OEX lines due to changes on cell division and expansion in AZ and distal part. This might be associated with LM2- and JIM8-AGPs which increased in OEX and decreased in RNAi lines throughout the pedicel. The JIM13 AGPs were downregulated in the flower AZ of OEX lines, pointing to a role on abscission regulation. In addition, Co-IP in flower AZ with SlP4H3-GFP fusion proteins showed interaction with LM2-, JIM13- and JIM8-epitopes suggesting proline hydroxylation by SlP4H3. The lower content of methyl-esterified HGs and higher of demethyl-esterified HGs in the AZs of RNAi lines might be responsible for increased rigidity of the AZ cell walls, accounting for the higher force required for AZ tissue detachment to occur. Moreover, ethylene-induced flower abscission was accelerated in the RNAi lines and delayed in OEX lines, while exactly the opposite response was observed in the red ripe fruit AZs. This was partly attributed to alterations in the expression of cell wall hydrolases. Overall, these results indicate that P4Hs might regulate molecular and structural features of cell walls in the AZ as well as abscission progression by regulating the structure and function of AGPs.