TIR1-like auxin-receptors are involved in the regulation of plum fruit development

Auxin responsive factor MdARF17 promotes ethylene synthesis in apple fruits by activating MdERF003 expression

KEY MESSAGE

Auxin (AUX) promotion of apple fruit ripening is ethylene-dependent, and AUX-MdARF17-MdERF003 plays a role in AUX-promoted ethylene synthesis in apple. Phytohormones play important roles in plant growth and fleshy fruit ripening, and the phytohormone auxin (AUX) can either promote or inhibit the ripening of fleshy fruits. Although AUX can influence ethylene (ETH) synthesis in apple (Malus domestica) fruits by affecting ETH system II, this mechanism remains to be explored. Here, we identified an ETH response factor (ERF) family transcription factor, MdERF003, whose expression could be activated by naphthalene acetic acid. The transient silencing of MdERF003 inhibited ETH synthesis in fruits, and MdERF003 could bind to the MdACS1 promoter. To explore the upstream target genes of MdERF003, we screened the MdARF family members by yeast one-hybrid assays of the MdERF003 promoter, and found that the transcription factor MdARF17, which showed AUX-promoted expression, could bind to the MdERF003 promoter and promote its expression. Finally, we silenced MdERF003 in apple fruits overexpressing MdARF17 and found that MdERF003 plays a role in MdARF17-promoted ETH synthesis in apple. Thus, AUX-MdARF17-MdERF003 promotes ETH synthesis in apple fruits.

Molecular mapping of genomic regions and identification of possible candidate genes associated with gynoecious sex expression in bitter gourd

Bitter gourd is an important vegetable crop grown throughout the tropics mainly because of its high nutritional value. Sex expression and identification of gynoecious trait in cucurbitaceous vegetable crops has facilitated the hybrid breeding programme in a great way to improve productivity. In bitter gourd, gynoecious sex expression is poorly reported and detailed molecular pathways involve yet to be studied. The present experiment was conducted to study the inheritance, identify the genomic regions associated with gynoecious sex expression and to reveal possible candidate genes through QTL-seq. Segregation for the gynoecious and monoecious sex forms in the F₂ progenies indicated single recessive gene controlling gynoecious sex expression in the genotype, PVGy-201. Gynoecious parent, PVGy-201, Monoecious parent, Pusa Do Mausami (PDM), and two contrasting bulks were constituted for deep-sequencing. A total of 10.56, 23.11, 15.07, and 19.38 Gb of clean reads from PVGy-201, PDM, gynoecious bulk and monoecious bulks were generated. Based on the ΔSNP index, 1.31 Mb regions on the chromosome 1 was identified to be associated with gynoecious sex expression in bitter gourd. In the QTL region 293,467 PVGy-201 unique variants, including SNPs and indels, were identified. In the identified QTL region, a total of 1019 homozygous variants were identified between PVGy1 and PDM genomes and 71 among them were non-synonymous variants (SNPS and INDELs), out of which 11 variants (7 INDELs, 4 SNPs) were classified as high impact variants with frame shift/stop gain effect. In total twelve genes associated with male and female gametophyte development were identified in the QTL-region. Ethylene-responsive transcription factor 12, Auxin response factor 6, Copper-transporting ATPase RAN1, CBL-interacting serine/threonine-protein kinase 23, ABC transporter C family member 2, DEAD-box ATP-dependent RNA helicase 1 isoform X2, Polygalacturonase QRT3-like isoform X2, Protein CHROMATIN REMODELING 4 were identified with possible role in gynoecious sex expression. Promoter region variation in 8 among the 12 genes indicated their role in determining gynoecious sex expression in bitter gourd genotype, DBGy-1. The findings in the study provides insight about sex expression in bitter gourd and will facilitate fine mapping and more precise identification of candidate genes through their functional validation.

The Ubiquitin-26S Proteasome Pathway and Its Role in the Ripening of Fleshy Fruits

The 26S proteasome is an ATP-dependent proteolytic complex in eukaryotes, which is mainly responsible for the degradation of damaged and misfolded proteins and some regulatory proteins in cells, and it is essential to maintain the balance of protein levels in the cell. The ubiquitin-26S proteasome pathway, which targets a wide range of protein substrates in plants, is an important post-translational regulatory mechanism involved in various stages of plant growth and development and in the maturation process of fleshy fruits. Fleshy fruit ripening is a complex biological process, which is the sum of a series of physiological and biochemical reactions, including the biosynthesis and signal transduction of ripening related hormones, pigment metabolism, fruit texture changes and the formation of nutritional quality. This paper reviews the structure of the 26S proteasome and the mechanism of the ubiquitin-26S proteasome pathway, and it summarizes the function of this pathway in the ripening process of fleshy fruits.

Salicylic acid delays pear fruit senescence by playing an antagonistic role toward ethylene, auxin, and glucose in regulating the expression of PpEIN3a

Salicylic acid (SA) and ethylene (ET) are crucial fruit senescence hormones. SA inhibited ET biosynthesis. However, the mechanism of SA delaying fruit senescence is less known. ETHYLENE INSENSITIVE 3 (EIN3), a key positive switch in ET perception, functions as a transcriptional activator and binds to the primary ET response element that is present in the promoter of the ETHYLENE RESPONSE FACTOR1 gene. In this study, a gene encoding putative EIN3 protein was cloned from sand pear and designated as PpEIN3a. The deduced PpEIN3a contains a conserved EIN3 domain. The evolutionary analysis results indicated that PpEIN3a belonged to the EIN3 superfamily. Real-time quantitative PCR analysis revealed that the accumulation of PpEIN3a transcripts were detected in all tissues of this pear. Moreover, PpEIN3a expression was regulated during fruit development. Interestingly, the expression of PpEIN3a was downregulated by SA but upregulated by ET, auxin, and glucose. Additionally, the contents of free and conjugated SA were higher than those of the control after SA treatment. While the content of ET and auxin (indole-3-acetic acid, IAA) dramatically decreased after SA treatment compared with control during fruit senescence. The content of glucose increased when fruit were treated by SA for 12 h and then there were no differences between SA treatment and control fruit during the shelf life. SA also delayed the decrease in sand pear (Pyrus pyrifolia Nakai. 'Whangkeumbae') fruit firmness. The soluble solid content remained relatively stable between the SA treated and control fruits. This study showed that SA plays an antagonistic role toward ET, auxin, and glucose in regulating the expression of PpEIN3a to delay fruit senescence.

The molecular mechanism on suppression of climacteric fruit ripening with postharvest wax coating treatment via transcriptome

Wax coating is an important means to maintain fruit quality and extend fruit shelf life, especially for climacteric fruits, such as apples (Malus domestica). Here, we found that wax coating could inhibit ethylene production, chlorophyll degradation, and carotenoid synthesis, but the molecular mechanism remains unclear. The regulatory mechanism of wax coating on apple fruit ripening was determined by subjecting wax-treated apple fruits to transcriptome analysis. RNA-seq revealed that 1,137 and 1,398 genes were upregulated and downregulated, respectively. These differentially expressed genes (DEGs) were shown to be related to plant hormones, such as ethylene, auxin, abscisic acid, and gibberellin, as well as genes involved in chlorophyll degradation and carotenoid biosynthesis. Moreover, we found that some genes related to the wax synthesis process also showed differential expression after the wax coating treatment. Among the DEGs obtained from RNA-seq analysis, 15 were validated by quantitative RT-PCR, confirming the results from RNA-seq analysis. RNA-seq and qRT-PCR of pear (Pyrus ussuriensis) showed similar changes after wax treatment. Our data suggest that wax coating treatment inhibits fruit ripening through ethylene synthesis and signal transduction, chlorophyll metabolism, and carotenoid synthesis pathways and that waxing inhibits endogenous wax production. These results provide new insights into the inhibition of fruit ripening by wax coating.

Bioinformatics Study of Aux/IAA Family Genes and Their Expression in Response to Different Hormones Treatments during Japanese Apricot Fruit Development and Ripening

Auxin/indole-3-acetic acid (Aux/IAA) is a transcriptional repressor in the auxin signaling pathway that plays a role in several plant growth and development as well as fruit and embryo development. However, it is unclear what role they play in Japanese apricot (Prunus mume) fruit development and maturity. To investigate the role of Aux/IAA genes in fruit texture, development, and maturity, we comprehensively identified and expressed 19 PmIAA genes, and demonstrated their conserved domains and homology across species. The majority of PmIAA genes are highly responsive and expressed in different hormone treatments. PmIAA2, PmIAA5, PmIAA7, PmIAA10, PmIAA13, PmIAA18, and PmIAA19 showed a substantial increase in expression, suggesting that these genes are involved in fruit growth and maturity. During fruit maturation, alteration in the expression of PmIAA genes in response to 1-Methylcyclopropene (1-MCP) treatment revealed an interaction between auxin and ethylene. The current study investigated the response of Aux/IAA development regulators to auxin during fruit ripening, with the goal of better understanding their potential application in functional genomics.

AcERF1B and AcERF073 Positively Regulate Indole-3-acetic Acid Degradation by Activating AcGH3.1 Transcription during Postharvest Kiwifruit Ripening

Ethylene can accelerate the postharvest ripening process of kiwifruit, while indole-3-acetic acid (IAA) delays it. However, the molecular mechanism by which ethylene regulates IAA degradation is unclear. Here, we found that ethephon promotes the degradation of free IAA in kiwifruit. Furthermore, ethylene can promote the expression of AcGH3.1 and enhance its promoter activity. Two ethylene response factors (ERFs), AcERF1B and AcERF073, were obtained using an AcGH3.1 promoter as bait for a yeast one-hybrid screening library. Both AcERF1B and AcERF073 bind to the AcGH3.1 promoter to activate it. Also, AcERF1B/073 enhanced AcGH3.1 expression, decreased the free IAA content, and increased the IAA-Asp content in kiwifruit. In addition, we found that the AcERF1B and AcERF073 proteins directly interact, and this interaction enhanced their binding to the AcGH3.1 promoter. In summary, our results suggest that AcERF1B and AcERF073 positively regulate IAA degradation by activating AcGH3.1 transcription, which accelerated postharvest kiwifruit ripening.

Recent Advances in Phytohormone Regulation of Apple-Fruit Ripening

Apple (Malus domestica) is, globally, one of the largest fruits in terms of cultivated area and yield. Apple fruit is generally marketed after storage, which is of great significance for regulating the market supply in the off-season of fruit production. Apple-fruit ripening, which culminates in desirable changes in structural and textural properties, is governed by a complex regulatory network. Much is known about ethylene as one of the most important factors promoting apple-fruit ripening. However, the dynamic interplay between phytohormones also plays an important part in apple-fruit ripening. Here, we review and evaluate the complex regulatory network concerning the action of phytohormones during apple-fruit ripening. Interesting future research areas are discussed.

Functional characterization of a gibberellin F-box protein, PslSLY1, during plum fruit development

Fruit development is orchestrated by a complex network of interactions between hormone signaling pathways. The phytohormone gibberellin (GA) is known to regulate a diverse range of developmental processes; however, the mechanisms of GA action in perennial fruit species are yet to be elucidated. In the current study, a GA signaling gene PslSLY1, encoding a putative F-box protein that belongs to the SLY1 (SLEEPY1)/GID2 (gibberellin-insensitive dwarf2) gene family, was isolated from Japanese plum (Prunus salicina). PslSLY1 transcript abundance declined as fruit development progressed, along with potential negative feedback regulation of PslSLY1 by GA. Subcellular localization and protein-protein interaction assays suggested that PslSLY1 functions as an active GA signaling component that interacts with the ASK1 (Arabidopsis SKP1) subunit of an SCF-ubiquitin ligase complex and with PslDELLA repressors, in a GA-independent manner. By using a domain omission strategy, we illustrated that the F-box and C-terminal domains of PslSLY1 are essential for its interactions with the downstream GA signaling components. PslSLY1 overexpression in wild-type and Arabidopsissly1.10 mutant backgrounds resulted in a dramatic enhancement in overall plant growth, presumably due to triggered GA signaling. This includes germination characteristics, stem elongation, flower structure, and fertility. Overall, our findings shed new light on the GA strategy and signaling network in commercially important perennial crops.

Auxin-activated MdARF5 induces the expression of ethylene biosynthetic genes to initiate apple fruit ripening

The gaseous plant hormone ethylene induces the ripening of climacteric fruit, including apple (Malus domestica). Another phytohormone, auxin, is known to promote ethylene production in many horticultural crops, but the regulatory mechanism remains unclear. Here, we found that auxin application induces ethylene production in apple fruit before the stage of commercial harvest, when they are not otherwise capable of ripening naturally. The expression of MdARF5, a member of the auxin response factor transcription factor (TF) family involved in the auxin signaling pathway, was enhanced by treatment with the synthetic auxin naphthaleneacetic acid (NAA). Further studies revealed that MdARF5 binds to the promoter of MdERF2, encoding a TF in the ethylene signaling pathway, as well as the promoters of two 1-aminocyclopropane-1-carboxylic acid synthase (ACS) genes (MdACS3a and MdACS1) and an ACC oxidase (ACO) gene, MdACO1, all of which encode key steps in ethylene biosynthesis, thereby inducing their expression. We also observed that auxin-induced ethylene production was dependent on the methylation of the MdACS3a promoter. Our findings reveal that auxin induces ethylene biosynthesis in apple fruit through activation of MdARF5 expression.

Genome-wide characterization and expression analysis of the auxin response factor (ARF) gene family during melon (Cucumis melo L.) fruit development

ARFs in plants mediate auxin signaling transduction and regulate growth process. To determine genome-wide characterization of ARFs family in melon (Cucumis melo L.), ARFs were identified via analysis of information within the melon genomic database, and bioinformatic analyses were performed using various types of software. Based on different treatment methods involving dipping with the growth regulator Fengchanji No. 2 and artificial pollination, Jingmi No. 11 melon was used as the test material, and melon plants with unpollinated ovaries served as controls. The expression of ARFs during the early development of melon was analyzed via qRT-PCR. Seventeen genes that encode ARF proteins were identified in the melon genome for the first time. The expression of these ARFs differed in different tissues. The expression levels of CmARF2, CmARF16-like, CmARF18-like2, and CmARF19-like were especially high in melon fruits. The expression of ARFs during the early development of melon fruits differed in response to the different treatments, which suggested that CmARF9, CmARF16-like, CmARF19-like, CmARF19, CmARF1, CmARF2, CmARF3, and CmARF5 may be associated with melon fruit growth during early development. Interestingly, the increase in the transverse diameter of fruits treated with growth regulators was significantly greater than that of fruits resulting from artificial pollination, while the increase in the longitudinal diameter of the fruits resulting from artificial pollination was significantly greater.

Evidence for pre-climacteric activation of AOX transcription during cold-induced conditioning to ripen in European pear (Pyrus communis L.)

European pears (Pyrus communis L.) require a range of cold-temperature exposure to induce ethylene biosynthesis and fruit ripening. Physiological and hormonal responses to cold temperature storage in pear have been well characterized, but the molecular underpinnings of these phenomena remain unclear. An established low-temperature conditioning model was used to induce ripening of 'D'Anjou' and 'Bartlett' pear cultivars and quantify the expression of key genes representing ripening-related metabolic pathways in comparison to non-conditioned fruit. Physiological indicators of pear ripening were recorded, and fruit peel tissue sampled in parallel, during the cold-conditioning and ripening time-course experiment to correlate gene expression to ontogeny. Two complementary approaches, Nonparametric Multi-Dimensional Scaling and efficiency-corrected 2-(ΔΔCt), were used to identify genes exhibiting the most variability in expression. Interestingly, the enhanced alternative oxidase (AOX) transcript abundance at the pre-climacteric stage in 'Bartlett' and 'D'Anjou' at the peak of the conditioning treatments suggests that AOX may play a key and a novel role in the achievement of ripening competency. There were indications that cold-sensing and signaling elements from ABA and auxin pathways modulate the S1-S2 ethylene transition in European pears, and that the S1-S2 ethylene biosynthesis transition is more pronounced in 'Bartlett' as compared to 'D'Anjou' pear. This information has implications in preventing post-harvest losses of this important crop.

Integrated Transcriptomic, Proteomic, and Metabolomics Analysis Reveals Peel Ripening of Harvested Banana under Natural Condition

Harvested banana ripening is a complex physiological and biochemical process, and there are existing differences in the regulation of ripening between the pulp and peel. However, the underlying molecular mechanisms governing peel ripening are still not well understood. In this study, we performed a combination of transcriptomic, proteomic, and metabolomics analysis on peel during banana fruit ripening. It was found that 5784 genes, 94 proteins, and 133 metabolites were differentially expressed or accumulated in peel during banana ripening. Those genes and proteins were linked to ripening-related processes, including transcriptional regulation, hormone signaling, cell wall modification, aroma synthesis, protein modification, and energy metabolism. The differentially expressed transcriptional factors were mainly ethylene response factor (ERF) and basic helix-loop-helix (bHLH) family members. Moreover, a great number of auxin signaling-related genes were up-regulated, and exogenous 3-indoleacetic acid (IAA) treatment accelerated banana fruit ripening and up-regulated the expression of many ripening-related genes, suggesting that auxin participates in the regulation of banana peel ripening. In addition, xyloglucan endotransglucosylase/hydrolase (XTH) family members play an important role in peel softening. Both heat shock proteins (Hsps) mediated-protein modification, and ubiqutin-protesome system-mediated protein degradation was involved in peel ripening. Furthermore, anaerobic respiration might predominate in energy metabolism in peel during banana ripening. Taken together, our study highlights a better understanding of the mechanism underlying banana peel ripening and provides a new clue for further dissection of specific gene functions.

Hormone balance in a climacteric plum fruit and its non-climacteric bud mutant during ripening

Hormone balance plays a crucial role in the control of fruit ripening. We characterized and compared hormone balance in two Japanese plum cultivars (Prunus salicina Lindl.), namely Santa Rosa, a climacteric type, and Sweet Miriam, its non-climacteric bud-sport mutant. We assessed hormonal changes in gene expression associated with hormone biosynthesis, perception and signaling during ripening on-the tree and throughout postharvest storage and in response to ethylene treatments. Non-climacteric fruit displayed lower ethylene levels than climacteric fruit at all stages and lower auxin levels during the initiation of ripening on-the-tree and during most of post-harvest storage. Moreover, 1-MCP-induced ethylene decrease also resulted in low auxin contents in Santa Rosa, supporting the role of auxin in climacteric fruit ripening. The differences in auxin contents between Santa Rosa and Sweet Miriam fruit could be the consequence of different routed auxin biosynthesis pathways as indicated by the significant negative correlations between clusters of auxin metabolism-associated genes. Ethylene induced increased ABA levels throughout postharvest storage in both ripening types. Overall, ripening of Santa Rosa and Sweet Miriam fruit are characterized by distinct hormone accumulation pathways and interactions.

TIR1 auxin receptors are implicated in the differential response to 4-Cl-IAA and IAA in developing pea fruit

The auxins indole-3-acetic acid (IAA) and 4-chloroindole-3-acetic acid (4-Cl-IAA) occur naturally in pea (Pisum sativum); however, only 4-Cl-IAA mimics the presence of seeds in stimulating pericarp growth. To examine if this differential auxin effect is mediated through TIR1/AFB auxin receptors, pea TIR1 and AFB2 homologs were functionally characterized in Arabidopsis, and receptor expression, and auxin distribution and action were profiled in developing pea fruits. PsTIR1a, PsTIR1b, and PsAFB2 restored the auxin-sensitive root growth response to the mutant Arabidopsis seedlings Attir1-10 and/or Attir1-10 afb2-3. Expression of PsTIR1 or AtTIR1 in Attir1-10 afb2-3 mutants also restored the greater root inhibitory response of 4-Cl-IAA compared to that of IAA, implicating TIR1 receptors in this response. The ability of 4-Cl-IAA to stimulate a stronger DR5::GUS auxin response than IAA at the same concentration in pea pericarps was associated with its ability to enrich the auxin-receptor transcript pool with PsTIR1a and PsAFB2 by decreasing the transcript abundance of PsTIR1b (mimicking results in pericarps with developing seeds). Therefore, the markedly different effect of IAA and 4-Cl-IAA on pea fruit growth may at least partially involve TIR1/AFB receptors and the differential modulation of their population, resulting in specific Aux/IAA protein degradation that leads to an auxin-specific tissue response.

Biosynthesis, Metabolism and Function of Auxin, Salicylic Acid and Melatonin in Climacteric and Non-climacteric Fruits

Climacteric and non-climacteric fruits are differentiated by the ripening process, in particular by the involvement of ethylene, high respiration rates and the nature of the process, being autocatalytic or not, respectively. Here, we focus on the biosynthesis, metabolism and function of three compounds (auxin, salicylic acid and melatonin) sharing not only a common precursor (chorismate), but also regulatory functions in plants, and therefore in fruits. Aside from describing their biosynthesis in plants, with a particular emphasis on common precursors and points of metabolic diversion, we will discuss recent advances on their role in fruit ripening and the regulation of bioactive compounds accumulation, both in climacteric and non-climacteric fruits.

Cloning and Characterization of a Flavonol Synthase Gene From Litchi chinensis and Its Variation Among Litchi Cultivars With Different Fruit Maturation Periods

Litchi (Litchi chinensis) is an important subtropical fruit tree with high commercial value. However, the short and centralized fruit maturation period of litchi cultivars represents a bottleneck for litchi production. Therefore, the development of novel cultivars with extremely early fruit maturation period is critical. Previously, we showed that the genotypes of extremely early-maturing (EEM), early-maturing (EM), and middle-to-late-maturing (MLM) cultivars at a specific locus SNP51 (substitution type C/T) were consistent with their respective genetic background at the whole-genome level; a homozygous C/C genotype at SNP51 systematically differentiated EEM cultivars from others. The litchi gene on which SNP51 was located was annotated as flavonol synthase (FLS), which catalyzes the formation of flavonols. Here, we further elucidate the variation of the FLS gene from L. chinensis (LcFLS) among EEM, EM, and MLM cultivars. EEM cultivars with a homozygous C/C genotype at SNP51 all contained the same 2,199-bp sequence of the LcFLS gene. For MLM cultivars with a homozygous T/T genotype at SNP51, the sequence lengths of the LcFLS gene were 2,202-2,222 bp. EM cultivars with heterozygous C/T genotypes at SNP51 contained two different alleles of the LcFLS gene: a 2,199-bp sequence identical to that in EEM cultivars and a 2,205-bp sequence identical to that in MLM cultivar 'Heiye.' Moreover, the coding regions of LcFLS genes of other MLM cultivars were almost identical to that of 'Heiye.' Therefore, the LcFLS gene coding region may be used as a source of diagnostic SNP markers to discriminate or identify genotypes with the EEM trait. The expression pattern of the LcFLS gene and accumulation pattern of flavonol from EEM, EM, and MLM cultivars were analyzed and compared using quantitative real-time PCR (qRT-PCR) and high-performance liquid chromatography (HPLC) for mature leaves, flower buds, and fruits, 15, 30, 45, and 60 days after anthesis. Flavonol content and LcFLS gene expression levels were positively correlated in all three cultivars: both decreased from the EEM to MLM cultivars, with moderate levels in the EM cultivars. LcFLS gene function could be further analyzed to elucidate its correlation with phenotype variation among litchi cultivars with different fruit maturation periods.

Phospholipase D inhibition by hexanal is associated with calcium signal transduction events in raspberry

Raspberry (Rubus spp.) is an economically important crop with a restricted growing season and very limited fruit shelf-life due to its extreme tenderness. In order to prolong its shelf life, an aqueous composition containing hexanal as the key active ingredient (HC) was applied as a preharvest spray during fruit development. The effects of HC were assessed using physiological, biochemical and anatomical parameters on the treated fruits and compared with the effects of mock inoculation which lacked hexanal. Sugars and acidity did not show a significant change in response to HC treatment, while the pulling force (the tension required to detach the berry from the receptacle) significantly improved in the HC-treated fruits, compared to control. Scanning electron microscope (SEM) analysis revealed a high correlation between the presence of rigid epidermal hairs and a stronger degree of attachment between berries and their receptacle in the HC treated fruits. Further, electron micrographs also showed abnormal crystalline depositions on the epidermal drupelets of the treated berries. Energy Dispersive X-ray Spectroscopy (EDS) analysis showed those crystals to be largely composed of calcium. HC treatment also resulted in the reduction of transcript level of three phospholipase D genes, as well as altered expression pattern of five members of the annexin gene family, and four calmodulin-binding transcription activators. Quantification of PLD activity showed that hexanal inhibited PLD activity in treated berries. The potential crosstalk between hexanal, phospholipase D activity and calcium and this crosstalk's role in delaying fruit softening and in prolonging storage life of fruits shelf life is discussed.

New insights into the roles of cucumber TIR1 homologs and miR393 in regulating fruit/seed set development and leaf morphogenesis

BACKGROUND

TIR1-like proteins act as auxin receptors and play essential roles in auxin-mediated plant development processes. The number of auxin receptor family members varies among species. While the functions of auxin receptor genes have been widely studied in Arabidopsis, the distinct functions of cucumber (Cucumis sativus L.) auxin receptors remains poorly understood. To further our understanding of their potential role in cucumber development, two TIR1-like genes were identified and designated CsTIR1 and CsAFB2. In the present study, tomato (Sonanum lycopersicum) was used as a model to investigate the phenotypic and molecular changes associated with the overexpression of CsTIR1 and CsAFB2.

RESULTS

Differences in the subcellular localizations of CsTIR1 and CsAFB2 were identified and both genes were actively expressed in leaf, female flower and young fruit tissues of cucumber. Moreover, CsTIR1- and CsAFB2-overexpressing lines exhibited pleotropic phenotypes ranging from leaf abnormalities to seed germination and parthenocarpic fruit compared with the wild-type plants. To further elucidate the regulation of CsTIR1 and CsAFB2, the role of the miR393/TIR1 module in regulating cucumber fruit set were investigated. Activation of miR393-mediated mRNA cleavage of CsTIR1 and CsAFB2 was revealed by qPCR and semi-qPCR, which highlighted the critical role of the miR393/TIR1 module in mediating fruit set development in cucumber.

CONCLUSION

Our results provide new insights into the involvement of CsTIR1 and CsAFB2 in regulating various phenotype alterations, and suggest that post-transcriptional regulation of CsTIR1 and CsAFB2 mediated by miR393 is essential for cucumber fruit set initiation. Collectively, these results further clarify the roles of cucumber TIR1 homologs and miR393 in regulating fruit/seed set development and leaf morphogenesis.

SlTIR1 is involved in crosstalk of phytohormones, regulates auxin-induced root growth and stimulates stenospermocarpic fruit formation in tomato

TIR1 and its homologs act as auxin receptors and play important roles in plant growth and development in Arabidopsis thaliana. An auxin receptor homolog Solanum lycopersicum TIR1 (SlTIR1) has been isolated from tomato cultivar Micro-Tom, and SlTIR1 over-expression results in parthenocarpic fruit formation. In this study, the promoter driving the β-glucuronidase (GUS) expression vector was constructed and stably transformed into Micro-Tom seedlings. The SlTIR1 promoter driving GUS expression accumulated predominantly in the leaf and vasculature in transgenic seedlings. Promoter analysis identified an auxin-response element (AuxRE) and two gibberellic acid (GA)-response elements in the SlTIR1 promoter. Quantitative PCR showed that SlTIR1 transcript level was down-regulated by naphthaleneacetic acid, ethephon and abscisic acid and up-regulated by GA. Furthermore, because of the lack of ability to form reproductive seeds in SlTIR1 over-expressing Micro-Tom, this limits further exploration of potential roles of SlTIR1 in auxin signaling. Here, an antisense vector and an over-expression vector of the SlTIR1 gene were stably transformed into Micro-Tom and Ailsa Craig tomato, respectively. Phenotypes and physiological analyses indicated that SlTIR1 regulated primary root growth and auxin-associated lateral root formation in Micro-Tom. Meanwhile, SlTIR1 also stimulated abnormal seed development, so-called stenospermocarpy, in Ailsa Craig. Transcript accumulations of auxin-signaling genes determined by quantitative PCR were consistent with the idea that SlTIR1 regulated plant growth and development, partially mediated by controlling the mRNA levels of auxin-signaling genes. Our work demonstrates that SlTIR1 regulated auxin-induced root growth and stimulated stenospermocarpic fruit formation. SlTIR1 may be a key mediator of the crosstalk among auxin and other hormones to co-regulate plant growth and development.

Overexpression of plum auxin receptor PslTIR1 in tomato alters plant growth, fruit development and fruit shelf-life characteristics

BACKGROUND

TIR1-like proteins are F-box auxin receptors. Auxin binding to the F-box receptor proteins promotes the formation of SCF(TIR1) ubiquitin ligase complex that targets the auxin repressors, Aux/IAAs, for degradation via the ubiquitin/26S proteasome pathway. The release of auxin response factors (ARFs) from their Aux/IAA partners allows ARFs to mediate auxin-responsive changes in downstream gene transcription. In an attempt to understand the potential role of auxin during fruit development, a plum auxin receptor, PslTIR1, has previously been characterized at the cellular, biochemical and molecular levels, but the biological significance of this protein is still lacking. In the present study, tomato (Solanum lycopersicum) was used as a model to investigate the phenotypic and molecular changes associated with the overexpression of PslTIR1.

RESULTS

The findings of the present study highlighted the critical role of PslTIR1 as positive regulator of auxin-signalling in coordinating the development of leaves and fruits. This was manifested by the entire leaf morphology of transgenic tomato plants compared to the wild-type compound leaf patterning. Moreover, transgenic plants produced parthenocarpic fruits, a characteristic property of auxin hypersensitivity. The autocatalytic ethylene production associated with the ripening of climacteric fruits was not significantly altered in transgenic tomato fruits. Nevertheless, the fruit shelf-life characteristics were affected by transgene presence, mainly through enhancing fruit softening rate. The short shelf-life of transgenic tomatoes was associated with dramatic upregulation of several genes encoding proteins involved in cell-wall degradation, which determine fruit softening and subsequent fruit shelf-life.

CONCLUSIONS

The present study sheds light into the involvement of PslTIR1 in regulating leaf morphology, fruit development and fruit softening-associated ripening, but not autocatalytic ethylene production. The results demonstrate that auxin accelerates fruit softening independently of ethylene action and this is probably mediated through the upregulation of many cell-wall metabolism genes.

Genome-wide identification and characterization of auxin response factor (ARF) family genes related to flower and fruit development in papaya (Carica papaya L.)

Background

Auxin and auxin signaling are involved in a series of developmental processes in plants. Auxin Response Factors (ARFs) is reported to modulate the expression of target genes by binding to auxin response elements (AuxREs) and influence the transcriptional activation of down-stream target genes. However, how ARF genes function in flower development and fruit ripening of papaya (Carica papaya L.) is largely unknown. In this study, a comprehensive characterization and expression profiling analysis of 11 C. papaya ARF (CpARF) genes was performed using the newly updated papaya reference genome data.

Results

We analyzed CpARF expression patterns at different developmental stages. CpARF1, CpARF2, CpARF4, CpARF5, and CpARF10 showed the highest expression at the initial stage of flower development, but decreased during the following developmental stages. CpARF6 expression increased during the developmental process and reached its peak level at the final stage of flower development. The expression of CpARF1 increased significantly during the fruit ripening stages. Many AuxREs were included in the promoters of two ethylene signaling genes (CpETR1 and CpETR2) and three ethylene-synthesis-related genes (CpACS1, CpACS2, and CpACO1), suggesting that CpARFs might be involved in fruit ripening via the regulation of ethylene signaling.

Conclusions

Our study provided comprehensive information on ARF family in papaya, including gene structures, chromosome locations, phylogenetic relationships, and expression patterns. The involvement of CpARF gene expression changes in flower and fruit development allowed us to understand the role of ARF-mediated auxin signaling in the maturation of reproductive organs in papaya.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2182-0) contains supplementary material, which is available to authorized users.