Pedicel Abscission Research Articles

JOINTLESS Maintains Inflorescence Meristem Identity in Tomato.

JOINTLESS (J) was isolated in tomato (Solanum lycopersicum) from mutants lacking a flower pedicel abscission zone (AZ) and encodes a MADS-box protein of the SHORT VEGETATIVE PHASE/AGAMOUS-LIKE 24 subfamily. The loss of J function also causes the return to leaf initiation in the inflorescences, indicating a pivotal role in inflorescence meristem identity. Here, we compared jointless (j) mutants in different accessions that exhibit either an indeterminate shoot growth, producing regular sympodial segments, or a determinate shoot growth, due to the reduction of sympodial segments and causal mutation of the SELF-PRUNING (SP) gene. We observed that the inflorescence phenotype of j mutants is stronger in indeterminate (SP) accessions such as Ailsa Craig (AC), than in determinate (sp) ones, such as Heinz (Hz). Moreover, RNA-seq analysis revealed that the return to vegetative fate in j mutants is accompanied by expression of SP, which supports conversion of the inflorescence meristem to sympodial shoot meristem in j inflorescences. Other markers of vegetative meristems such as APETALA2c and branching genes such as BRANCHED 1 (BRC1a/b) were differentially expressed in the inflorescences of j(AC) mutant. We also found in the indeterminate AC accession that J represses homeotic genes of B- and C-classes and that its overexpression causes an oversized leafy calyx phenotype and has a dominant negative effect on AZ formation. A model is therefore proposed where J, by repressing shoot fate and influencing reproductive organ formation, acts as a key determinant of inflorescence meristems.

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.

Suppression of a hexokinase gene SlHXK1 in tomato affects fruit setting and seed quality

Hexokinase is considered to be the key molecule in sugar signaling and metabolism. Here, we reported that silencing SlHXK1 resulted in a decrease in flower number, increased rate of flower dropping, abnormal thickening of the anther wall, and reduced pollen and seed viability. An anatomical analysis revealed the loss of small cells and abnormal thickening of anther walls in SlHXK1-RNAi lines. Treatment with auxin and 1-methylcyclopropene inhibited flower dropping from the pedicel abscission zone. qRT-PCR analysis revealed that the effect of SlHXK1 on abscission was associated with the expression levels of genes related to key meristem, auxin, ethylene, cell wall metabolism and programmed cell death. Pollen germination and pollen staining experiments showed that pollen viability was significantly reduced in the SlHXK1-RNAi lines. Physiological and biochemical analyses showed that hexokinase activity and starch content were markedly decreased in the transgenic lines. The expression of genes related to tomato pollen development was also suppressed in the transgenic lines. Although the RNAi lines eventually produced some viable seeds, the yield and quality of the seeds was lower than that of wild-type plants. Yeast two-hybrid and bimolecular fluorescence complementation assays showed that SlHXK1 interacted with SlKINγ. Furthermore, SlPIF4 inhibited the transcriptional expression of SlHXK1. In conclusion, our results demonstrate that SlHXK1 may play important roles in pollen, anther, seed and the pedicel abscission zone by affecting starch accumulation or cell wall synthesis, as well as by regulating the number of the transcripts of genes that are involved in auxin, ethylene and cell wall degradation.

Effects of Low Temperature on Pedicel Abscission and Auxin Synthesis Key Genes of Tomato.

Cold stress usually causes the abscission of floral organs and a decline in fruit setting rate, seriously reducing tomato yield. Auxin is one of the key hormones that affects the abscission of plant floral organs; the YUCCA (YUC) family is a key gene in the auxin biosynthesis pathway, but there are few research reports on the abscission of tomato flower organs. This experiment found that, under low temperature stress, the expression of auxin synthesis genes increased in stamens but decreased in pistils. Low temperature treatment decreased pollen vigor and pollen germination rate. Low night temperature reduced the tomato fruit setting rate and led to parthenocarpy, and the treatment effect was most obvious in the early stage of tomato pollen development. The abscission rate of tomato pTRV-Slfzy3 and pTRV-Slfzy5 silenced plants was higher than that of the control, which is the key auxin synthesis gene affecting the abscission rate. The expression of Solyc07g043580 was down-regulated after low night temperature treatment. Solyc07g043580 encodes the bHLH-type transcription factor SlPIF4. It has been reported that PIF4 regulates the expression of auxin synthesis and synthesis genes, and is a key protein in the interaction between low temperature stress and light in regulating plant development.

The RhLOL1-RhILR3 module mediates cytokinin-induced petal abscission in rose.

In many plant species, petal abscission can be considered the final step of petal senescence. Cytokinins (CKs) are powerful suppressors of petal senescence; however, their role in petal abscission is ambiguous. Here, we observed that, in rose (Rosa hybrida), biologically active CK is accumulated during petal abscission and acts as an accelerator of the abscission process. Using a combination of reverse genetics, and molecular and biochemical techniques, we explored the roles of a LESION SIMULATING DISEASE1 (LSD1) family member RhLOL1 interacting with a bHLH transcription factor RhILR3 in CK-induced petal abscission. Silencing RhLOL1 delays rose petal abscission, while the overexpression of its ortholog SlLOL1 in tomato (Solanum lycopersicum) promotes pedicel abscission, indicating the conserved function of LOL1 in activating plant floral organ abscission. In addition, we identify a bHLH transcription factor, RhILR3, that interacts with RhLOL1. We show that RhILR3 binds to the promoters of the auxin signaling repressor auxin/indole-3-acetic acid (Aux/IAA) genes to inhibit their expression; however, the interaction of RhLOL1 with RhILR3 activates the expression of the Aux/IAA genes including RhIAA4-1. Silencing RhIAA4-1 delays rose petal abscission. Our results thus reveal a RhLOL1-RhILR3 regulatory module involved in CK-induced petal abscission via the regulation of the expression of the Aux/IAA genes.

Transcriptome Analysis of the Development of Pedicel Abscission Zone in Tomato

Plant organ abscission is a common phenomenon that occurs at a specific position called the abscission zone (AZ). The differentiation and development of the pedicel AZ play important roles in flower and fruit abscission, which are of great significance for abscission in tomatoes before harvest. Previous studies have reported some genes involved in AZ differentiation; however, the genes regulating pedicel AZ cell development in tomatoes after AZ differentiation remain poorly understood. In this study, transcriptome analyses of tomato pedicel AZ samples were performed at 0, 5, 15, and 30 days post-anthesis (DPA). Pedicel AZ growth was mainly observed before 15 DPA. A principal component analysis and a correlation analysis were carried out in order to compare the repeatability and reliability for different samples. We observed 38 up-regulated and 31 down-regulated genes that were significantly altered during 0 to 5 DPA, 5 to 15 DPA, and 0 to 15 DPA, which may play key roles in AZ cell enlargement. GO and KEGG enrichment analyses of the selected DEGs under all three different comparisons were conducted. Auxin-signaling-related genes were analyzed, as well as AUX/IAA, GH3, and small auxin up-regulated RNA (SAUR) gene expression patterns. The presented results provide information on pedicel AZ development, which might help in regulating flower or fruit pedicel abscission in tomato production facilities.

A SlCLV3-SlWUS module regulates auxin and ethylene homeostasis in low light-induced tomato flower abscission.

Premature abscission of flowers and fruits triggered by low light stress can severely reduce crop yields. However, the underlying molecular mechanism of this organ abscission is not fully understood. Here, we show that a gene (SlCLV3) encoding CLAVATA3 (CLV3), a peptide hormone that regulates stem cell fate in meristems, is highly expressed in the pedicel abscission zone (AZ) in response to low light in tomato (Solanum lycopersicum). SlCLV3 knockdown and knockout lines exhibit delayed low light-induced flower drop. The receptor kinases SlCLV1 and BARELY ANY MERISTEM1 function in the SlCLV3 peptide-induced low light response in the AZ to decrease expression of the transcription factor gene WUSCHEL (SlWUS). DNA affinity purification sequencing identified the transcription factor genes KNOX-LIKE HOMEDOMAIN PROTEIN1 (SlKD1) and FRUITFULL2 (SlFUL2) as SlWUS target genes. Our data reveal that low light reduces SlWUS expression, resulting in higher SlKD1 and SlFUL2 expression in the AZ, thereby perturbing the auxin response gradient and causing increased ethylene production, eventually leading to the initiation of abscission. These results demonstrate that the SlCLV3-SlWUS signaling pathway plays a central role in low light-induced abscission by affecting auxin and ethylene homeostasis.

The HD-Zip transcription factor SlHB15A regulates abscission by modulating jasmonoyl-isoleucine biosynthesis.

Plant organ abscission, a process that is important for development and reproductive success, is inhibited by the phytohormone auxin and promoted by another phytohormone, jasmonic acid (JA). However, the molecular mechanisms underlying the antagonistic effects of auxin and JA in organ abscission are unknown. We identified a tomato (Solanum lycopersicum) class III homeodomain-leucine zipper transcription factor, HOMEOBOX15A (SlHB15A), which was highly expressed in the flower pedicel abscission zone and induced by auxin. Knocking out SlHB15A using clustered regularly interspaced short palindromic repeats-associated protein 9 technology significantly accelerated abscission. In contrast, overexpression of microRNA166-resistant SlHB15A (mSlHB15A) delayed abscission. RNA sequencing and reverse transcription-quantitative PCR analyses showed that knocking out SlHB15A altered the expression of genes related to JA biosynthesis and signaling. Furthermore, functional analysis indicated that SlHB15A regulates abscission by depressing JA-isoleucine (JA-Ile) levels through inhabiting the expression of JASMONATE-RESISTANT1 (SlJAR1), a gene involved in JA-Ile biosynthesis, which could induce abscission-dependent and abscission-independent ethylene signaling. SlHB15A bound directly to the SlJAR1 promoter to silence SlJAR1, thus delaying abscission. We also found that flower removal enhanced JA-Ile content and that application of JA-Ile severely impaired the inhibitory effects of auxin on abscission. These results indicated that SlHB15A mediates the antagonistic effect of auxin and JA-Ile during tomato pedicel abscission, while auxin inhibits abscission through the SlHB15A-SlJAR1 module.

Hydrogen sulfide inhibits the abscission of tomato pedicel through reconstruction of a basipetal auxin gradient

Abscission is an important developmental process and an essential agricultural trait. Auxin and ethylene are two phytohormones with important roles in the complex, but still elusive signaling network of abscission. Here, we found that hydrogen sulfide (H2S), a newly identified gasotransmitter, inhibits the initiation of tomato pedicel abscission. The underlying mechanism was explored through transcriptome profile analysis in various pedicel tissues with or without H2S treatment in the early abscission stage. The data suggested that H2S strongly influences the global transcription of pedicel tissues, exerts differential expression regulation along the pedicel, and markedly influences both the auxin and ethylene signaling pathways. Computational analysis revealed that H2S reconstructs a basipetal auxin gradient along the pedicel at 4 h after treatment; this finding was further substantiated by the GUS-staining results of DR5::GUS pedicels. The inhibitory effect of H2S to the ethylene signaling pathway might be an indirect action. Moreover, the subtilisin-like proteinase family members involved in the release of peptide signal molecules are critical components of the abscission signaling network downstream of auxin and ethylene.

Overexpression of the MADS-box gene SIMBP21 alters leaf morphology and affects reproductive development in tomato

Fruit yield is the most important horticultural trait of tomato. SlMBP21, a SEPALLATA subclass MADS-box gene has been reported to have functions in regulating pedicel abscission zone identity and development and controlling sepal size in tomato. However, we generated transgenic tomato plants which overexpress SlMBP21 and found the transformants displayed curly leaves, abnormally shaped flowers with twisted and opened stamens, reduced yield parameters, and small and light seeds. Our studies on the gain-of-function phenotype and gene expression level showed that its novel aspects played important roles in determining leaf morphology, flower and inflorescence architecture, and seed size, as well as the fruit yield. Overexpression of SlMBP21 in tomato resulted in curly leaves with fewer leaflets due to the regulation of the critical leaf polarity genes that cause an imbalance between the midvein adaxial–abaxial cell growth. Defects in the architecture of flowers and inflorescences resulted in reduced fruit set. Furthermore, we demonstrated that SlMBP21 plays its role through inhibiting the expression of the genes involved in the determination of seed development in tomato and SlMBP21 protein can interact with other MADS-box protein (SlAGL11, TAGL1 and SlMBP3) to control seed size. Thus, these results suggest that overexpression of SlMBP21 causes multiple types of damage to plant growth and development, especially fruit yield, in tomato.

Role of the KNOTTED1-LIKE HOMEOBOX protein (KD1) in regulating abscission of tomato flower pedicels at early and late stages of the process.

The KNOTTED1-LIKE HOMEOBOX PROTEIN1 (KD1) gene is highly expressed in flower and leaf abscission zones (AZs), and KD1 was reported to regulate tomato flower pedicel abscission via alteration of the auxin gradient and response in the flower AZ (FAZ). The present work was aimed to further examine how KD1 regulates signaling factors and regulatory genes involved in pedicel abscission, by using silenced KD1 lines and performing a large-scale transcriptome profiling of the FAZ before and after flower removal, using a customized AZ-specific microarray. The results highlighted a differential expression of regulatory genes in the FAZ of KD1-silenced plants compared to the wild-type. In the TAPG4::antisense KD1-silenced plants, KD1 gene expression decreased before flower removal, resulting in altered expression of regulatory genes, such as epigenetic modifiers, transcription factors, posttranslational regulators, and antioxidative defense factors occurring at zero time and before affecting auxin levels in the FAZ detected at 4h after flower removal. The expression of additional regulatory genes was altered in the FAZ of KD1-silenced plants at 4-20 h after flower removal, thereby leading to an inhibited abscission phenotype, and downregulation of genes involved in abscission execution and defense processes. Our data suggest that KD1 is a master regulator of the abscission process, which promotes abscission of tomato flower pedicels. This suggestion is based on the inhibitory effect of KD1 silencing on flower pedicel abscission that operates via alteration of various regulatory pathways, which delay the competence acquisition of the FAZ cells to respond to ethylene signaling.

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.

Diverse Functions of IAA-Leucine Resistant PpILR1 Provide a Genic Basis for Auxin-Ethylene Crosstalk During Peach Fruit Ripening.

Auxin and ethylene play critical roles in the ripening of peach (Prunus persica) fruit; however, the interaction between these two phytohormones is complex and not fully understood. Here, we isolated a peach ILR gene, PpILR1, which encodes an indole-3-acetic acid (IAA)-amino hydrolase. Functional analyses revealed that PpILR1 acts as a transcriptional activator of 1-amino cyclopropane-1-carboxylic acid synthase (PpACS1), and hydrolyzes auxin substrates to release free auxin. When Cys137 was changed to Ser137, PpILR1 failed to show hydrolase activity but continued to function as a transcriptional activator of PpACS1 in tobacco and peach transient expression assays. Furthermore, transgenic tomato plants overexpressing PpILR1 exhibited ethylene- and strigolactone-related phenotypes, including premature pedicel abscission, leaf and petiole epinasty, and advanced fruit ripening, which are consistent with increased expression of genes involved in ethylene biosynthesis and fruit ripening, as well as suppression of branching and growth of internodes (related to strigolactone biosynthesis). Collectively, these results provide novel insights into the role of IAA-amino acid hydrolases in plants, and position the PpILR1 protein at the junction of auxin and ethylene pathways during peach fruit ripening. These results could have substantial implications on peach fruit cultivation and storage in the future.

Multifunctional evolution of B and AGL6 MADS box genes in orchids

We previously found that B and AGL6 proteins form L (OAP3-2/OAGL6-2/OPI) and SP (OAP3-1/OAGL6-1/OPI) complexes to determine lip/sepal/petal identities in orchids. Here, we show that the functional L’ (OAP3-1/OAGL6-2/OPI) and SP’ (OAP3-2/OAGL6-1/OPI) complexes likely exist and AP3/PI/AGL6 genes have acquired additional functions during evolution. We demonstrate that the presumed L’ complex changes the structure of the lower lateral sepals and helps the lips fit properly in the center of the flower. In addition, we find that OAP3-1/OAGL6-1/OPI in SP along with presumed SP’ complexes regulate anthocyanin accumulation and pigmentation, whereas presumed L’ along with OAP3-2/OAGL6-2/OPI in L complexes promotes red spot formation in the perianth. Furthermore, the B functional proteins OAP3-1/OPI and OAGL6-1 in the SP complex could function separately to suppress sepal/petal senescence and promote pedicel abscission, respectively. These findings expand the current knowledge behind the multifunctional evolution of the B and AGL6 genes in plants.

SlERF52 regulates SlTIP1;1 expression to accelerate tomato pedicel abscission.

Abscission of plant organs is induced by developmental signals and diverse environmental stimuli and involves multiple regulatory networks, including biotic or abiotic stress-impaired auxin flux in the abscission zone (AZ). Depletion of auxin activates AZ ethylene (ETH) production and triggers acceleration of abscission, a process that requires hydrogen peroxide (H2O2). However, the interaction between these networks and the underlying mechanisms that control abscission are poorly understood. Here, we found that expression of tonoplast intrinsic proteins, which belong to the aquaporin (AQP) family in the AZ was important for tomato (Solanum lycopersicum) pedicel abscission. Liquid chromatography-tandem mass spectrometry and in situ hybridization revealed that SlTIP1;1 was most abundant and specifically present in the tomato pedicel AZ. SlTIP1;1 localized in the plasma membrane and tonoplast. Knockout of SlTIP1;1 resulted in delayed abscission, whereas overexpression of SlTIP1;1 accelerated abscission. Further analysis indicated that SlTIP1;1 mediated abscission via gating of cytoplasmic H2O2 concentrations and osmotic water permeability (Pf). Elevated cytoplasmic levels of H2O2 caused a suppressed auxin signal in the early abscission stage and enhanced ETH production during abscission. Furthermore, we found that increasing Pf was required to enhance the turgor pressure to supply the break force for AZ cell separation. Moreover, we observed that SlERF52 bound directly to the SlTIP1;1 promoter to regulate its expression, demonstrating a positive loop in which cytoplasmic H2O2 activates ETH production, which activates SlERF52. This, in turn, induces SlTIP1;1, which leads to elevated cytoplasmic H2O2 and water influx.

Expression Kinetics of Regulatory Genes Involved in the Vesicle Trafficking Processes Operating in Tomato Flower Abscission Zone Cells during Pedicel Abscission.

The abscission process occurs in a specific abscission zone (AZ) as a consequence of the middle lamella dissolution, cell wall degradation, and formation of a defense layer. The proteins and metabolites related to these processes are secreted by vesicle trafficking through the plasma membrane to the cell wall and middle lamella of the separating cells in the AZ. We investigated this process, since the regulation of vesicle trafficking in abscission systems is poorly understood. The data obtained describe, for the first time, the kinetics of the upregulated expression of genes encoding the components involved in vesicle trafficking, occurring specifically in the tomato (Solanum lycopersicum) flower AZ (FAZ) during pedicel abscission induced by flower removal. The genes encoding vesicle trafficking components included soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), SNARE regulators, and small GTPases. Our results clearly show how the processes of protein secretion by vesicle trafficking are regulated, programmed, and orchestrated at the level of gene expression in the FAZ. The data provide evidence for target proteins, which can be further used for affinity purification of plant vesicles in their natural state. Such analyses and dissection of the complex vesicle trafficking networks are essential for further elucidating the mechanism of organ abscission.

Identification and Characterization of HAESA-Like Genes Involved in the Fruitlet Abscission in Litchi.

Regulation of abscission is an important agricultural concern since precocious abscission can reduce crop yield. INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) peptide and its receptors the HAESA (HAE) and HAESA-like2 (HSL2) kinases have been revealed to be core components controlling floral organ abscission in the model plant Arabidopsis. However, it is still unclear whether the homologs of IDA-HAE/HSL2 in non-model plants are correlated to abscission. Previously, we found LcIDL1, a homolog of IDA from litchi, has a similar role to AtIDA in control of floral organ abscission in Arabidopsis. Here, we further isolated an HAESA-like homolog, LcHSL2, which is likely involved in the fruitlet abscission in litchi. Ectopic expression of LcHSL2 in wild type Arabidopsis has no effect on the floral organ abscission. However, its presence in the hae hsl2 mutant background completely rescued the floral organ abscission deficiency. LcHSL2 is localized in the cell membrane and the LcHSL2 gene is expressed at the pedicel abscission zone (AZ) of litchi and floral AZ of Arabidopsis. Real-time PCR analysis showed that the expression level of LcHSL2 was increased during ethephon-induced fruitlet abscission in litchi. Taken together, our findings suggest that HSL2 homologs have functional conservation in Arabidopsis and litchi, and LcHSL2 might play a critical role in regulation of fruitlet abscission in litchi.

Different expression of ATP-dependent RNA helicases in artificially and naturally induced mango fruitlet abscission

Premature fruit drop is a severe production constraint for mango (Mangifera indica) and recent research demonstrates that various physiological parameters and genes are involved in the abscission process. In addition to hormone and carbohydrate responses in the fruitlet, it is evident that ethylene receptors are involved in abscission zone activation. The mango ethylene receptors MiERS1 and MiETR1 respond differently to abscission-inducing ethephon treatments and their expression pattern indicates plant organ specificity. The study regarding the position of those receptors in the mango genome revealed two putative ATP-dependent RNA helicases, which are located upstream of MiERS1. RNA helicases are important for RNA metabolism, post-transcriptional modulation and gene expression. The mango helicase ADRH 7-1 was downregulated in pedicels of fruitlets prone to natural abscission but upregulated in the pedicel abscission zone after ethephon treatment. In contrast, ADRH 7-2 was not responsive to treatments and its transcription level was not significantly changed in the abscission zone. When ethephon-induced fruitlet abscission is compared to naturally induced abscission there are certain differences in the helicase gene expression pattern, as well as in the ethylene receptor expression pattern, while other physiological responses are known to be similar. The presented expression pattern of the ATP-dependent RNA helicase ADRH 7-1 indicates that it might play a role in the transcriptional regulation of the pedicel abscission zone.

A family of auxin conjugate hydrolases from Solanum lycopersicum and analysis of their roles in flower pedicel abscission

BackgroundAuxin conjugates are hydrolyzed to release free auxin to ensure defined cellular auxin levels or gradients within tissues for proper development or response to environmental signals. The auxin concentration in the abscission zone (AZ) is thought to play an important role in mediating the abscission lag phase.ResultsIn this study, the full cDNA sequences of seven tomato ILR1-like SlILL genes were identified and characterized, All SlILLs were found to have auxin conjugate hydrolysis activity. The effects of different auxin conjugates on abscission identified IAA-Ile as a candidate to determine the auxin conjugate and auxin conjugate hydrolysis functions in abscission. Treatment of pedicel explants with IAA-Ile for different times showed that application before 6 h could effectively delay abscission. IAA-Ile pre-incubation for 2 h was sufficient to inhibit abscission. These results showed that there is not sufficient auxin conjugates in the AZ to inhibit abscission, and the optimal time to inhibit abscission by the application of exogenous auxin conjugates is before 6 h. Treatment with cycloheximide (CHX, a protein biosynthesis inhibitor) indicated that de novo synthesis of auxin conjugate hydrolases is also required to delay abscission. During abscission, SlILL1, 5, and 6 showed abscission-related gene expression patterns, and SlILL1, 3, 5, 6, and 7 showed increasing expression trends, which collectively might contribute to delay abscission. Silencing the expression of SlILL1, 3, 5, 6, and 7 using virus-induced gene silencing showed that SlILL1, 5, and 6 are major mediators of abscission in tomato.ConclusionsIn the process of abscission, auxin inhibition is concentration dependent, and the concentration of auxin in the AZ was regulated by hydrolyzed auxin conjugates. SlILR1, 5, and 6 play a key role in flower pedicel abscission.

Suppression of a Prolyl 4 Hydroxylase Results in Delayed Abscission of Overripe Tomato Fruits.

The tomato pedicel abscission zone (AZ) is considered a model system for flower and fruit abscission development, activation, and progression. O-glycosylated proteins such as the Arabidopsis IDA (INFLORESCENCE DEFICIENT IN ABSCISSION) peptide and Arabinogalactan proteins (AGPs) which undergo proline hydroxylation were demonstrated to participate in abscission regulation. Considering that the frequency of occurrence of proline hydroxylation might determine the structure as well the function of such proteins, the expression of a tomato prolyl 4 hydroxylase, SlP4H3 (Solanum lycopersicum Prolyl 4 Hydroxylase 3) was suppressed in order to investigate the physiological significance of this post-translational modification in tomato abscission. Silencing of SlP4H3 resulted in the delay of abscission progression in overripe tomato fruits 90 days after the breaker stage. The cause of this delay was attributed to the downregulation of the expression of cell wall hydrolases such as SlTAPGs (tomato abscission polygalacturonases) and cellulases as well as expansins. In addition, minor changes were observed in the mRNA levels of two SlAGPs and one extensin. Moreover, structural changes were observed in the silenced SlP4H3AZs. The fracture plane of the AZ was curved and not along a line as in wild type and there was a lack of lignin deposition in the AZs of overripe fruits 30 days after breaker. These results suggest that proline hydroxylation might play a role in the regulation of tomato pedicel abscission.