Fruitlet Abscission Research Articles

Effects of reflective material deployed at bloom for 1, 2 or 5 months on ‘WA 38’ apple fruit set, quality and yield

The self-thinning nature of the apple variety ‘WA 38’ was previously characterized in Washington State; the self-regulating abscission of early-stage fruitlets lasts up to 8 weeks after full bloom and can result in the natural drop of 80 to 90% of the original flowers. Several approaches can be pursued to increase the fruit set, but in the present study, the focus was centered on A) alleviating the competition between young fruitlets during the early weeks of fruit development to reduce the loss by natural abscission and B) increasing the number of flower buds the following year. The study was set up in a 2018 planting of ‘WA 38’/G.935 orchard without hail-nets trained to V-trellis-system (4,975 trees/ha) in Central Washington (USA). The treatment consisted of the deployment, at early king bloom, of 80% reflective white material (RM, Extenday®) in the orchard inter-rows. The hypothesis relied upon the fact that enhanced light (mainly diffuse) could increase leaf C assimilation to support and nourish a higher proportion of fruitlets and decrease the natural abscission. The RM treatments had three different durations: reflective cover removed one month after installation (“RM for 1 M”), two months after (“RM for 2 M”) and until harvest (“RM until harvest”). They were compared against untreated control (“No RM (CTRL)”) in a 2-year trial. RM treatments did not result in a significant positive impact on fruit retention, net photosynthetic rate, yield, or return bloom. Trees subjected to whole-season RM had altered canopy and soil microclimates with drier and warmer canopies and cooler soil with higher moisture compared to untreated control. In the second season, a greater proportion of large apples (264-339 g or 72-64 apples/40-lb-box) was graded in RM until harvest treatment, indicating a positive influence of the enhanced light treatment in C allocation to fruit.

The Biological and Genetic Mechanisms of Fruit Drop in Apple Tree (Malus × domestica Borkh.)

The apple tree (Malus × domestica Borkh.) belongs to the Rosaceae. Due to its adaptability and tolerance to different soil and climatic conditions, it is cultivated worldwide for fresh consumption. The priorities of apple growers are high-quality fruits and stable yield for high production. About 90 to 95 percent of fruits should fall or be eliminated from apple trees to avoid overcropping and poor-quality fruits. Apple trees engage in a complex biological process known as yield self-regulation, which is influenced by several internal and external factors. Apple buds develop in different stages along the branches, and they can potentially give rise to new shoots, leaves, flowers, or fruit clusters. The apple genotype determines how many buds will develop into fruit-bearing structures and the capacity for yield self-regulation. Plant hormones such as ethylene, cytokinins, auxins, and gibberellins play a crucial role in regulating the fruit set, growth, and development, and the balance of these hormones influences the flowering intensity, fruit size, and fruit number on the apple tree. Apple growers often interfere in the self-regulation process by manually thinning fruit clusters. Different thinning methods, such as by hand, mechanical thinning, or applying chemical substances, are used for flower and fruit thinning. The most profitable in commercial orchards is the use of chemicals for elimination, but more environmentally sustainable solutions are required due to the European Green Deal. This review focuses on the biological factors and genetic mechanisms in apple yield self-regulation for a better understanding of the regulatory mechanism of fruitlet abscission for future breeding programs targeted at self-regulating yield apple varieties.

LcMPK3 and LcMPK6 positively regulate fruitlet abscission in litchi

Mitogen-activated protein kinase (MAPK) cascades have been discovered to play a fundamental role in regulating organ abscission. However, the identity of protein substrates targeted by MAPK cascades, as well as whether the role of MAPK protein cascades in the abscission process is conserved across different plant species, remain unknown. Here, the role of homologs of MPK3 and MPK6 in regulating fruit abscission were characterized in litchi. Ectopic expression of LcMPK3 or LcMPK6 in Arabidopsis mpk3 mpk6 mutant rescued the deficiency in floral organ abscission, while silencing of LcMPK3 or LcMPK6 in litchi significantly decreased fruitlet abscission. Importantly, a total of 49 proteins interacting with LcMPK3 were identified through yeast two-hybrid screening, including two components of the MAPK signaling cascade, five transcription factors, and two aquaporins. Furthermore, the interaction between LcMPK3/6 with LcBZR1/2, core components in brassinosteroids signaling that suppress litchi fruitlet abscission, was confirmed using in vitro and in vivo assays. Moreover, phos-tag assays demonstrated that LcMPK3/6 could phosphorylate LcBZR1/2, with several phosphorylation residues identified. Together, our findings suggest that LcMPK3 and LcMPK6 play a positive regulatory role in fruitlet abscission in litchi, and offer crucial information for the investigation of mechanisms underlying MPK3/6-mediated organ abscission in plants.

Selective Retention of Cross-Fertilised Fruitlets during Premature Fruit Drop of Hass Avocado

The productivity of many tree crops is limited by low yield, partly due to high rates of fruitlet abscission during early fruit development. Early studies suggested that cross-pollinated fruitlets may be selectively retained during fruit development, although paternity testing of fruitlets to test this hypothesis was technically challenging. We used MassARRAY genotyping to determine the effects of pollen parentage on fruitlet retention and fruit quality of Hass avocado. We identified the paternity of abscised and retained fruitlets at 6 and 10 weeks, and mature fruit at 36 weeks, after peak anthesis. We measured the embryo mass, pericarp mass, total mass and nutrient concentrations of fruitlets, and the seed mass, flesh mass, total mass, diameter, length, nutrient concentrations and fatty-acid composition of mature fruit. The percentages of progeny on the tree that were cross-fertilised increased from 4.6% at 6 weeks after peak anthesis to 10.7% at fruit maturity. Only 1.0% of freshly abscised fruitlets on the ground at 10 weeks after peak anthesis were cross-pollinated even though 6.5% of retained fruitlets on the tree were cross-pollinated. At this stage, cross-pollinated fruitlets had similar nutrient concentrations to self-pollinated fruitlets, but they had higher total contents of P, K, Al, Ca, Fe, Mn and Zn due to having greater fruitlet mass. At maturity, cross-pollinated fruit were 6% heavier and had 2% greater diameter than self-pollinated fruit, without significant differences in flesh nutrient concentrations or fatty acid composition. The results demonstrate that Hass avocado trees selectively retain cross-pollinated fruitlets, which are larger than self-pollinated fruitlets and ultimately produce larger mature fruit. Avocado growers can increase fruit size and yield by improving the opportunities for cross-pollination, possibly by closely interplanting type A and type B cultivars and introducing more beehives into orchards.

An ethylene response factor AcERF116 identified from A. catechu is involved in fruitlet abscission

Procedural abscission of outer reproductive organs during flower and fruit development occurs in most plant lineages. Undesired abscission, such as fruitlet shedding causes considerable yield loss in many fruit-producing species. Ethylene is one of the key factors regulating organ abscission. However, the participants involved in the ethylene-mediated abscission pathway remains largely unidentified. In this study, we focused on the ethylene response transcription factors (ERFs) regulating fruitlet abscission in an industrial tree species, A. catechu. A total of 165 ERF genes have been found in the A. catechu genome and eight of these showed distinct expression between the “about-to-abscise” and “non-abscised” samples. An AcERF116 gene with high expression level in the fruit abscission zone (FAZ) was selected for further study. Overexpression of the AcERF116 gene accelerated cell separation in the abscission zone (AZ) and promoted pedicel abscission in transgenic tomato lines. The PG (ploygalacturonase) activity was enhanced in the FAZs of A. catechu fruitlets during ethylene-induced fruitlet abscission, while the PME (pectin methylesterase) activity was suppressed. In addition, cytosolic alkalization was observed in the AZs during abscission in both tomato and A. catechu. Our results suggest that AcERF116 plays a critical role in the crosstalk of ethylene and fruitlet abscission in A. catechu.

The transcriptional control of LcIDL1-LcHSL2 complex by LcARF5 integrates auxin and ethylene signaling for litchi fruitlet abscission.

At the physiological level, the interplay between auxin and ethylene has long been recognized as crucial for the regulation of organ abscission in plants. However, the underlying molecular mechanisms remain unknown. Here, we identified transcription factors involved in indoleacetic acid (IAA) and ethylene (ET) signaling that directly regulate the expression of INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) and its receptor HAESA (HAE), which are key components initiating abscission. Specifically, litchi IDA-like 1 (LcIDL1) interacts with the receptor HAESA-like 2 (LcHSL2). Through in vitro and in vivo experiments, we determined that the auxin response factor LcARF5 directly binds and activates both LcIDL1 and LcHSL2. Furthermore, we found that the ETHYLENE INSENSITIVE 3-like transcription factor LcEIL3 directly binds and activates LcIDL1. The expression of IDA and HSL2 homologs was enhanced in LcARF5 and LcEIL3 transgenic Arabidopsis plants, but reduced in ein3 eil1 mutants. Consistently, the expressions of LcIDL1 and LcHSL2 were significantly decreased in LcARF5- and LcEIL3-silenced fruitlet abscission zones (FAZ), which correlated with a lower rate of fruitlet abscission. Depletion of auxin led to an increase in 1-aminocyclopropane-1-carboxylic acid (the precursor of ethylene) levels in the litchi FAZ, followed by abscission activation. Throughout this process, LcARF5 and LcEIL3 were induced in the FAZ. Collectively, our findings suggest that the molecular interactions between litchi AUXIN RESPONSE FACTOR 5 (LcARF5)-LcIDL1/LcHSL2 and LcEIL3-LcIDL1 signaling modules play a role in regulating fruitlet abscission in litchi and provide a long-sought mechanistic explanation for how the interplay between auxin and ethylene is translated into the molecular events that initiate abscission.

Phytohormones and candidate genes synergistically regulate fruitlet abscission in Areca catechu L.

BackgroundThe fruit population of most plants is under the control of a process named “physiological drop” to selectively abort some developing fruitlets. However, frequent fruitlet abscission severely restricts the yield of Areca catechu. To reveal the physiological and molecular variations in this process, we detected the variation of phytohormone levels in abscised and non-abscised fruitlets in A. catechu.ResultsThe levels of gibberellin acid, jasmonic acid, salicylic acid, abscisic acid and zeatin were elevated, while the indole-3-acetic acid and indole-3-carboxaldehyde levels were declined in the “about-to-abscise” part (AB) of abscission zone (AZ) compared to the “non-abscised” part (CK). Then the differentially expressed genes (DEGs) between AB and CK were screened based on transcriptome data. DEGs involved in phytohormone synthesis, response and transportation were identified as key genes. Genes related to cell wall biosynthesis, degradation, loosening and modification, and critical processes during fruit abscission were identified as role players. In addition, genes encoding transcription factors, such as NAC, ERF, WRKY, MADS and Zinc Finger proteins, showed differentially expressed patterns between AB and CK, were also identified as candidates.ConclusionsThese results unraveled a phytohormone signaling cross talk and key genes involved in the fruitlet abscission process in A. catechu. This study not only provides a theoretical basis for fruitlet abscission in A. catechu, but also identified many candidate genes or potential molecular markers for further breeding of fruit trees.

Apple Fruitlet Abscission Prediction. I. Development and Evaluation of Reflectance Spectroscopy Models

Chemical thinning, the most common and cost-effective thinning method, is conducted during early apple fruit development over a 3- to 4-week period using multiple applications of plant growth regulators. It is critical to provide apple growers with tools to assess the efficacy of chemical thinners quickly and accurately because visible responses are not apparent for up to 2 weeks after application. The objective of this study was to build a model to predict apple fruitlet abscission following a chemical thinner application with in situ reflectance data obtained with a portable visible and near infrared (Vis/NIR) spectrophotometer. Developed models were compared with the currently available fruitlet growth model (FGM). ‘Honeycrisp’ fruitlet diameter and reflectance were measured on dates around a chemical thinner application across a 2-year period. After June drop, measured fruitlets were determined to have either persisted or abscised. Random forest, partial least squares regression, and XGBoost classification models were used to predict fruitlet abscission from reflectance data. Each classification model was developed with 2021, 2022, and combined 2021 + 2022 data. For each dataset, 5-fold cross validation was used to assess three model performance metrics: 1) overall accuracy, 2) recall, and 3) specificity. Datasets tested were either unbalanced, majority class down-sampled, or minority class up-sampled with synthetic minority oversampling technique. In both years, the FGM reliably estimated chemical thinner efficacy 9 days after application. Before this time point, the FGM had low prediction accuracy of the minority class in both years—persisting fruitlets in 2021 and abscising fruitlets in 2022. For reflectance spectroscopy, the developed random forest models that were balanced with synthetic minority over-sampling technique were found to be the best combination in predicting chemical thinner efficacy. The combined 2021 + 2022 dataset overall model accuracy ranged from 84% the day before to 93% at 9 days after thinner application. These results show that Vis/NIR is a promising tool to predict chemical thinner efficacy. This technology had high prediction accuracies over a range of fruitlet abscission potential and two growing seasons. Further development and testing of the model over cultivars, chemical thinner timings, and growing regions would facilitate commercialization of the technology.

Apple Fruitlet Abscission Prediction. II. Characteristics of Fruitlets Predicted to Persist or Abscise by Reflectance Spectroscopy Models

Apple (Malus ×domestica L. Borkh.) growers need tools to predict the efficacy of chemical thinners that are applied to induce fruitlet abscission to aid in crop load management decisions. Recently, reflectance spectroscopy-based models to predict fruitlet abscission rates were developed. Using spectroscopy, persisting fruitlets had lower reflectance in the red-light (∼600 nm) and near infrared (∼950 nm) regions than abscising fruitlets. The goal of this study was to better understand how reflectance models distinguished between fruitlets that ultimately persisted or abscised. Individual models for the difference and ratio of each combination of wavelengths were developed to identify key wavelengths for abscission prediction from reflectance models. Accuracy for wavelength difference and ratio models was improved for all model prediction dates when reflectance (R) from R640–675 was subtracted from or divided by R675–696. This spectra region indicates differences in chlorophyll content between persisting and abscising fruitlets. Calculation of the chlorophyll concentration index (R522–579:R640–700) from nondestructively measured spectra supported this result. Chlorophyll concentration index was higher for fruitlets that ultimately persisted than abscised fruitlets (P < 0.01) for all measurement dates –1 to 9 days after thinner (DAT) in both years, except –1 DAT in 2021 (P = 0.468). Plant water index (R950–970:R890–900) was lower for persisting than abscising fruitlets for 3 to 9 DAT in 2021 (P < 0.001) and on –1 (P < 0.01) and 9 DAT (P < 0.001) in 2022. The relationship of fruit size and plant pigment (anthocyanins or chlorophyll) content in fruitlets to reflectance spectra between persisting and abscising fruitlets was also followed. Fruitlet persistence or abscission was predicted from developed models for destructively sampled fruitlets using measured reflectance spectra. Whole-fruit chlorophyll content was numerically higher in fruitlets predicted to persist than abscise for all collection dates. Higher total chlorophyll was correlated to a larger fruit size in persisting than abscising fruitlets. This higher chlorophyll content led to a lower reflectance of red light and was a key factor in model development. These results indicate that chlorophyll and water content can distinguish physiological parameters between persisting and abscising fruitlets.

LcERF10 functions as a positive regulator of litchi fruitlet abscission

Phytohormone ethylene is well-known in positive modulation of plant organ abscission. However, the molecular mechanism underlying ethylene-induced abscission remains largely unknown. Here, we identified an ethylene-responsive factor, LcERF10, as a key regulatory gene in litchi fruitlet abscission. LcERF10 was strongly induced in the fruitlet abscission zone (FAZ) during the ethylene-activated abscission. Silencing of LcERF10 in litchi weakened the cytosolic alkalization of the FAZ and reduced fruitlet abscission. Moreover, LcERF10 directly bound the promoter and repressed the expression of LcNHX7, a Na+/H+ exchanger that was down-regulated in FAZ following the ethylene-activated abscission and up-regulated after LcERF10 silencing. Additionally, ectopic expression of LcERF10 in Arabidopsis promoted the cytosolic alkalization of the floral organ AZ and accelerated the floral organ abscission. Collectively, our results suggest that the transcription factor LcERF10 plays a positive role in litchi fruitlet abscission.

Ectopic Expression of AGAMOUS-like 18 from Litchi (Litchi chinensis Sonn.) Delayed the Floral Organ Abscission in Arabidopsis

The regulation of abscission has a significant impact on fruit yield and quality. Thus, understanding the mechanisms underlying abscission, particularly identifying key genes, is critical for improving fruit crop breeding and cultivation practices. Here, to explore the key genes involved in litchi fruitlet abscission, the two closest homologs of AGAMOUS-like 15/18 (LcAGL15 and LcAGL18) were identified. During the litchi fruitlet abscission process, LcAGL15 expression was reduced, whereas LcAGL18 expression was increased at the abscission zone. The abscission of floral organs was unaffected by ectopic expression of LcAGL15 in Arabidopsis. Moreover, high expression of LcAGL18 significantly delayed the abscission process of floral organs, particularly the sepals. Overexpression of LcAGL18 in Arabidopsis consistently repressed the expression of abscission-related genes, including HAESA (HAE) and HAESA-LIKE2 (HSL2), and cell wall remodeling genes at the abscission zone. Furthermore, LcAGL18 was localized in the nucleus and acted as a transcriptional inhibitor. Collectively, these results suggest that AGL18 homologs have conserved functions in Arabidopsis and litchi, and that LcAGL18 might function as a key regulator in litchi fruitlet abscission.

Molecular characterization of the SAUR gene family in sweet cherry and functional analysis of PavSAUR55 in the process of abscission

Small auxin up RNA (SAUR) is a large gene family that is widely distributed among land plants. In this study, a comprehensive analysis of the SAUR family was performed in sweet cherry, and the potential biological functions of PavSAUR55 were identified using the method of genetic transformation. The sweet cherry genome encodes 86 SAUR members, the majority of which are intron-less. These genes appear to be divided into seven subfamilies through evolution. Gene duplication events indicate that fragment duplication and tandem duplication events occurred in the sweet cherry. Most of the members mainly underwent purification selection pressure during evolution. During fruit development, the expression levels of PavSAUR16/45/56/63 were up-regulated, and conversely, those of PavSAUR12/61 were down-regulated. Due to the significantly differential expressions of PavSAUR13/16/55/61 during the fruitlet abscission process, they might be the candidate genes involved in the regulation of physiological fruit abscission in sweet cherry. Overexpression of PavSAUR55 in Arabidopsis produced earlier reproductive growth, root elongation, and delayed petal abscission. In addition, this gene did not cause any change in the germination time of seeds and was able to increase the number of lateral roots under abscisic acid (ABA) treatment. The identified SAURs of sweet cherry play a crucial role in fruitlet abscission and will facilitate future insights into the mechanism underlying the heavy fruitlet abscission that can occur in this fruit crop.

Studies on fruit pedicel concerning fruitlet abscission in kinnow mandarin

Fruit drop is a major constraint for the production of kinnow mandarin. The present investigation comparedthe physical, biochemical and anatomical dynamics taking place in abscised and non-abscised fruit pedicelsof Kinnow mandarin at physiological and pathological fruit drop stages. The pedicel parameters (length, girthand weight) were higher in non-abscised than the abscised fruits. The activities of polygalacturonase (4.01μg D-glucose released g-1 FW min-1) and cellulase (4.34 μg D-glucose released g-1 FW min-1) enzymes weresignificantly low in non-abscised as compared to abscised fruit pedicels. The transverse sections of nonabscisedhealthy fruit pedicels revealed intact phloem and long xylem vessels in the vascular tissue. However,pedicels of abscised fruits showed degradation of cells and formation of the abscission zone. Fruit pedicelparameters had positive correlation with mineral nutrients, total soluble sugars and total soluble proteins,however, non-abscised fruit pedicels were negatively correlated with cell wall degrading enzymes. The fruitpedicel health has been found important and positively associated with the diminution in physiological andpathological fruit abscission in kinnow mandarin.

Quantification and Prediction with Near Infrared Spectroscopy of Carbohydrates throughout Apple Fruit Development

Carbohydrates play a key role in apple fruit growth and development. Carbohydrates are needed for cell division/expansion, regulate fruitlet abscission, and influence fruit maturation and quality. Current methods to quantify fruit carbohydrates are labor intensive and expensive. We quantified carbohydrates throughout a growing season in two cultivars and evaluated the use of near infrared spectroscopy (NIR) to predict apple carbohydrate content throughout changes in fruit development. Carbohydrates were quantified with high performance liquid chromatography (HPLC) at five timepoints between early fruitlet growth and harvest in ‘Gala’ and ‘Red Delicious’ apples. NIR spectra was collected for freeze-dried fruit samples using a benchtop near infrared spectrometer. Sorbitol was the major carbohydrate early in the growing season (~40% of total carbohydrates). However, the relative contribution of sorbitol to total carbohydrates rapidly decreased by 59 days after full bloom (<10%). The proportion of fructose to total carbohydrates increased throughout fruit development (40–50%). Three distinct periods of fruit development, early, mid-season, and late, were found over all sampling dates using principal component analysis. The first (PC1) and second (PC2) principal components accounted for 90% of the variation in the data, samples separated among sampling date along PC1. Partial least squares regression was used to build the models by calibrating carbohydrates quantified with HPLC and measured reflectance spectra. The NIR models reliably predicted the content of fructose, glucose, sorbitol, sucrose, starch, and total soluble sugars for both ‘Gala’ and ‘Red Delicious’; r2 ranged from 0.60 to 0.96. These results show that NIR can accurately estimate carbohydrates throughout the growing season and offers an efficient alternative to liquid or gas chromatography.

A LcDOF5.6-LcRbohD regulatory module controls the reactive oxygen species-mediated fruitlet abscission in litchi.

Reactive oxygen species (ROS) have been emerging as a key regulator in plant organ abscission. However, the mechanism underlying the regulation of ROS homeostasis in the abscission zone (AZ) is not completely established. Here, we report that a DOF (DNA binding with one finger) transcription factor LcDOF5.6 can suppress the litchi fruitlet abscission through repressing the ROS accumulation in fruitlet AZ (FAZ). The expression of LcRbohD, a homolog of the Arabidopsis RBOHs that are critical for ROS production, was significantly increased during the litchi fruitlet abscission, in parallel with an increased accumulation of ROS in FAZ. In contrast, silencing of LcRbohD reduced the ROS accumulation in FAZ and decreased the fruitlet abscission in litchi. Using invitro and invivo assays, we revealed that LcDOF5.6 was shown to inhibit the expression of LcRbohD via direct binding to its promoter. Consistently, silencing of LcDOF5.6 increased the expression of LcRbohD, concurrently with higher ROS accumulation in FAZ and increased fruitlet abscission. Furthermore, the expression of key genes (LcIDL1, LcHSL2, LcACO2, LcACS1, and LcEIL3) in INFLORESCENCE DEFICIENT IN ABSCISSION signaling and ethylene pathways were altered in LcRbohD-silenced and LcDOF5.6-silenced FAZ cells. Taken together, our results demonstrate an important role of the LcDOF5.6-LcRbohD module during litchi fruitlet abscission. Our findings provide new insights into the molecular regulatory network of organ abscission.

Physiological Study of Ethephon- and ACC-Induced Fruitlet and Leaf Abscission in Peach Trees Under Different Conditions of Temperature

The effect of temperature on the ability of 2-chloroethylphosphonic acid (ethephon, ETH) and the 1-aminocyclopropane carboxylic acid (ACC) to induce ethylene production in fruitlets and abscission of fruitlets and leaves when applied at postbloom (~15 mm fruit diameter) was studied using 5-year-old ‘Sweet Lady’/Rootpac-20 peach [Prunus persica (L.) Batsch] trees kept in pots in environment-controlled growth rooms. ETH at 150 mg L−1 and ACC at 500 mg L−1 effectively thinned peaches. Increasing the temperature from 10 to 20 °C pronounced the ETH-induced fruit and leaf abscission and the ACC-induced leaf abscission and decreased stomatal conductance in leaves. However, the ACC thinning effect was not affected by increasing temperature, as well as ETH- and ACC-induced ethylene production in fruitlets and in the quantum yield of PSII photochemistry. Changes in ethylene peak in fruitlets and stomatal conductance in leaves were related to the abscission response.

Ringing branches reduces fruitlet abscission by promoting PIN1 expression in ‘Orri’ mandarin

• ‘Orri’ mandarin has a low parthenocarpic ability, in accordance with its low CcGA3ox1 gene expression, but the application of GA 3 does not increase fruit set. • Ringing branches performed 35-40 days after anthesis increases fruitlet growth rate. This effect correlates inversely with the fruitlet abscission rate, and positively with cell division, i.e., with the CcCYCA1.1 gene expression in the fruitlet, but not with the GA biosynthesis, i. e., with the CcGA3ox1 gene expression. • Ringing branches significantly increased CcPIN1 gene expression in the fruitlet. • Results suggest that ringing protects the fruitlet abscission zone mediated by polar auxin transport from the fruit, which allows the fruitlet to maintain carbohydrate assimilation and continue growing. Ringing branches is a technique which is widely used to increase the yield of Citrus cultivars with low parthenocarpic ability. When performed during the physiological fruitlet abscission stage it prevents fruitlet drop and increases the number of fruits harvested. This effect has been related with an increased carbohydrate supply, which requires an enhanced photosynthesis efficiency of leafy flowering shoots. Since ringing also reduces vegetative growth, both the number of shoots and the leaves per shoot, the mechanism by which the carbohydrate supply is increased should be revised. Our results show that ringing carried out at this stage maintains the ability of the ovary for cell division mediated by the availability of carbohydrates, as indicated by an increased CcCYCA1.1 expression. But this effect is not linked with an increase in GA 1 biosynthesis ( CcGA3ox1 expression), as this occurs during fruit set; hence, hormones other than gibberellin must be controlling the physiological fruitlet abscission in response to ringing. We found that an increased expression of the auxin efflux carrier CcPIN1 gene suggests that ringing induces the auxin export out of the fruitlet and transport to the abscission zone (AZ-C), thus inhibiting its activation and allowing carbohydrates supply to the fruitlet which, thus, prevents abscission and continues growth.

Limiting carbohydrates to trunk and roots improves bud fruitfulness, fruit set and yield in cv. Malbec

Many commercial vineyards of Malbec, the most cultivated grapevine in Argentina, show unstable yield because of variations in bud fruitfulness and the occurrence of “shatter”, characterised by poor fruit set and fruitlet abscission. Shatter can be due to plant material, growing conditions and meteorological events. Among the parameters that determine fruit set efficiency and vine yield, the availability of carbohydrates (CH) plays an essential role. We previously showed that controlling CH partitioning by removing part of the phloem tissue through an annular incision at the base of the fruit shoots (shoot girdling) reduced shatter in Malbec. The objective of this research was to evaluate the partitioning of CH for the different sink organs of the aerial part of the plant when an interruption of phloem flux from leaves to storage organs is imposed by a girdle. Shoot and trunk girdling trials were conducted during the 2018 and 2019 growing seasons, respectively. At flowering, girdling was performed on different plant lots either at the base of the shoot (Base G), above the distal cluster of the shoot (Top G), to the trunk (Trunk G) or no girdling (Control). Most of the yield components were increased by Base G and Trunk G with no significant impact on vegetative growth or fruit quality. Total shoot biomass was not affected in Base G, while a 39 % reduction was observed in Top G, as compared to Control. The partitioning pattern within the shoot was modified by shoot girdling, and Base G favoured the accumulation of CH towards clusters at the expense of lateral shoots. Shoot girdling increased node diameter, bud fruitfulness and inflorescence dry weight of the shoot. Trunk-girdled vines showed an increase in fruit set and total yield at harvest, with no significant impact on vegetative growth. The restriction of CH flow to the trunk and roots by Base G and Trunk G at flowering increased yield components at harvest without affecting vegetative growth or grape quality. These results reinforce our hypothesis that the root system of the vines under study constitutes a strong sink during the flowering period that competes for CH.

Genome-Wide Identification of the DOF Gene Family Involved in Fruitlet Abscission in Areca catechu L.

Fruitlet abscission frequently occurs in Areca catechu L. and causes considerable production loss. However, the inducement mechanism of fruitlet abscission remains mysterious. In this study, we observed that the cell architecture in the abscission zone (AZ) was distinct with surrounding tissues, and varied obviously before and after abscission. Transcriptome analysis of the “about-to-abscise” and “non-abscised” AZs were performed in A. catechu, and the genes encoding the plant-specific DOF (DNA-binding with one finger) transcription factors showed a uniform up-regulation in AZ, suggesting a role of the DOF transcription in A. catechu fruitlet abscission. In total, 36 members of the DOF gene family distributed in 13 chromosomes were identified from the A. catechu genome. The 36 AcDOF genes were classified into nine subgroups based on phylogenic analysis. Six of them showed an AZ-specific expression pattern, and their expression levels varied according to the abscission process. In total, nine types of phytohormone response cis-elements and five types of abiotic stress related cis-elements were identified in the promoter regions of the AcDOF genes. In addition, histochemical staining showed that lignin accumulation of vascular bundles in AZ was significantly lower than that in pedicel and mesocarp, indicating the specific characteristics of the cell architecture in AZ. Our data suggests that the DOF transcription factors might play a role in fruitlet abscission regulation in A. catechu.

Genome-wide identification and expression analysis of carotenoid cleavage oxygenase genes in Litchi (Litchi chinensis Sonn.)

BackgroundCarotenoid cleavage oxygenases (CCOs) include the carotenoid cleavage dioxygenase (CCD) and 9-cis-epoxycarotenoid (NCED), which can catalize carotenoid to form various apocarotenoids and their derivatives, has been found that play important role in the plant world. But little information of CCO gene family has been reported in litchi (Litchi chinensis Sonn.) till date.ResultsIn this study, a total of 15 LcCCO genes in litchi were identified based on genome wide lever. Phylogeny analysis showed that LcCCO genes could be classified into six subfamilies (CCD1, CCD4, CCD7, CCD8, CCD-like, and NCED), which gene structure, domain and motifs exhibited similar distribution patterns in the same subfamilies. MiRNA target site prediction found that there were 32 miRNA target sites in 13 (86.7%) LcCCO genes. Cis-elements analysis showed that the largest groups of elements were light response related, following was plant hormones, stress and plant development related. Expression pattern analysis revealed that LcCCD4, LcNCED1, and LcNCED2 might be involving with peel coloration, LcCCDlike-b might be an important factor deciding fruit flavor, LcNCED2 and LcNCED3 might be related to flower control, LcNCED1 and LcNCED2 might function in fruitlet abscission, LcCCD4a1, LcCCD4a2, LcCCD1, LcCCD4, LcNCED1, and LcNCED2 might participate in postharvest storage of litchi.ConclusionHerein, Genome-wide analysis of the LcCCO genes was conducted in litchi to investigate their structure features and potential functions. These valuable and expectable information of LcCCO genes supplying in this study will offer further more possibility to promote quality improvement and breeding of litchi and further function investigation of this gene family in plant.