SlBEL11 regulates flavonoid biosynthesis, thus fine-tuning auxin efflux to prevent premature fruit drop in tomato.

Auxin regulates flower and fruit abscission, but how developmental signals mediate auxin transport in abscission remains unclear. Here, we reveal the role of the transcription factor BEL1-LIKE HOMEODOMAIN11 (SlBEL11) in regulating auxin transport during abscission in tomato (Solanum lycopersicum). SlBEL11 is highly expressed in the fruit abscission zone, and its expression increases during fruit development. Knockdown of SlBEL11 expression by RNA interference (RNAi) caused premature fruit drop at the breaker (Br) and 3 d post-breaker (Br+3) stages of fruit development. Transcriptome and metabolome analysis of SlBEL11-RNAi lines revealed impaired flavonoid biosynthesis and decreased levels of most flavonoids, especially quercetin, which functions as an auxin transport inhibitor. This suggested that SlBEL11 prevents premature fruit abscission by modulating auxin efflux from fruits, which is crucial for the formation of an auxin response gradient. Indeed, quercetin treatment suppressed premature fruit drop in SlBEL11-RNAi plants. DNA affinity purification sequencing (DAP-seq) analysis indicated that SlBEL11 induced expression of the transcription factor gene SlMYB111 by directly binding to its promoter. Chromatin immunoprecipitation-quantitative polymerase chain reaction and electrophoretic mobility shift assay showed that S. lycopersicum MYELOBLASTOSIS VIRAL ONCOGENE HOMOLOG111 (SlMYB111) induces the expression of the core flavonoid biosynthesis genes SlCHS1, SlCHI, SlF3H, and SlFLS by directly binding to their promoters. Our findings suggest that the SlBEL11-SlMYB111 module modulates flavonoid biosynthesis to fine-tune auxin efflux from fruits and thus maintain an auxin response gradient in the pedicel, thereby preventing premature fruit drop.

The F-box protein RhSAF destabilizes the gibberellic acid receptor RhGID1 to mediate ethylene-induced petal senescence in rose.

Roses are among the most popular ornamental plants cultivated worldwide for their great economic, symbolic, and cultural importance. Nevertheless, rapid petal senescence markedly reduces rose (Rosa hybrida) flower quality and value. Petal senescence is a developmental process tightly regulated by various phytohormones. Ethylene accelerates petal senescence, while gibberellic acid (GA) delays this process. However, the molecular mechanisms underlying the crosstalk between these phytohormones in the regulation of petal senescence remain largely unclear. Here, we identified SENESCENCE-ASSOCIATED F-BOX (RhSAF), an ethylene-induced F-box protein gene encoding a recognition subunit of the SCF-type E3 ligase. We demonstrated that RhSAF promotes degradation of the GA receptor GIBBERELLIN INSENSITIVE DWARF1 (RhGID1) to accelerate petal senescence. Silencing RhSAF expression delays petal senescence, while suppressing RhGID1 expression accelerates petal senescence. RhSAF physically interacts with RhGID1s and targets them for ubiquitin/26S proteasome-mediated degradation. Accordingly, ethylene-induced RhGID1C degradation and RhDELLA3 accumulation are compromised in RhSAF-RNAi lines. Our results demonstrate that ethylene antagonizes GA activity through RhGID1 degradation mediated by the E3 ligase RhSAF. These findings enhance our understanding of the phytohormone crosstalk regulating petal senescence and provide insights for improving flower longevity.

Abscisic acid-induced transcription factor PsMYB306 negatively regulates tree peony bud dormancy release.

Bud dormancy is a crucial strategy for perennial plants to withstand adverse winter conditions. However, the regulatory mechanism of bud dormancy in tree peony (Paeonia suffruticosa) remains largely unknown. Here, we observed dramatically reduced and increased accumulation of abscisic acid (ABA) and bioactive gibberellins (GAs) GA1 and GA3, respectively, during bud endodormancy release of tree peony under prolonged chilling treatment. An Illumina RNA sequencing study was performed to identify potential genes involved in the bud endodormancy regulation in tree peony. Correlation matrix, principal component, and interaction network analyses identified a downregulated MYB transcription factor gene, PsMYB306, the expression of which positively correlated with 9-CIS-EPOXYCAROTENOID DIOXYGENASE 3 (PsNCED3) expression. Protein modeling analysis revealed 4 residues within the R2R3 domain of PsMYB306 to possess DNA binding capability. Transcription of PsMYB306 was increased by ABA treatment. Overexpression of PsMYB306 in petunia (Petunia hybrida) inhibited seed germination and plant growth, concomitant with elevated ABA and decreased GA contents. Silencing of PsMYB306 accelerated cold-triggered tree peony bud burst and influenced the production of ABA and GAs and the expression of their biosynthetic genes. ABA application reduced bud dormancy release and transcription of ENT-KAURENOIC ACID OXIDASE 1 (PsKAO1), GA20-OXIDASE 1 (PsGA20ox1), and GA3-OXIDASE 1 (PsGA3ox1) associated with GA biosynthesis in PsMYB306-silenced buds. In vivo and in vitro binding assays confirmed that PsMYB306 specifically transactivated the promoter of PsNCED3. Silencing of PsNCED3 also promoted bud break and growth. Altogether, our findings suggest that PsMYB306 negatively modulates cold-induced bud endodormancy release by regulating ABA production.

A petunia transcription factor, PhOBF1, regulates flower senescence by modulating gibberellin biosynthesis.

Flower senescence is commonly enhanced by the endogenous hormone ethylene and suppressed by the gibberellins (GAs) in plants. However, the detailed mechanisms for the antagonism of these hormones during flower senescence remain elusive. In this study, we characterized one up-regulated gene PhOBF1, belonging to the basic leucine zipper transcription factor family, in senescing petals of petunia (Petunia hybrida). Exogenous treatments with ethylene and GA3 provoked a dramatic increase in PhOBF1 transcripts. Compared with wild-type plants, PhOBF1-RNAi transgenic petunia plants exhibited shortened flower longevity, while overexpression of PhOBF1 resulted in delayed flower senescence. Transcript abundances of two senescence-related genes PhSAG12 and PhSAG29 were higher in PhOBF1-silenced plants but lower in PhOBF1-overexpressing plants. Silencing and overexpression of PhOBF1 affected expression levels of a few genes involved in the GA biosynthesis and signaling pathways, as well as accumulation levels of bioactive GAs GA1 and GA3. Application of GA3 restored the accelerated petal senescence to normal levels in PhOBF1-RNAi transgenic petunia lines, and reduced ethylene release and transcription of three ethylene biosynthetic genes PhACO1, PhACS1, and PhACS2. Moreover, PhOBF1 was observed to specifically bind to the PhGA20ox3 promoter containing a G-box motif. Transient silencing of PhGA20ox3 in petunia plants through tobacco rattle virus-based virus-induced gene silencing method led to accelerated corolla senescence. Our results suggest that PhOBF1 functions as a negative regulator of ethylene-mediated flower senescence by modulating the GA production.

A pear S1-bZIP transcription factor PpbZIP44 modulates carbohydrate metabolism, amino acid, and flavonoid accumulation in fruits.

Fruit quality is defined by attributes that give value to a commodity. Flavor, texture, nutrition, and shelf life are key quality traits that ensure market value and consumer acceptance. In pear fruit, soluble sugars, organic acids, amino acids, and total flavonoids contribute to flavor and overall quality. Transcription factors (TFs) regulate the accumulation of these metabolites during development or in response to the environment. Here, we report a novel TF, PpbZIP44, as a positive regulator of primary and secondary metabolism in pear fruit. Analysis of the transient overexpression or RNAi-transformed pear fruits and stable transgenic tomato fruits under the control of the fruit-specific E8 promoter demonstrated that PpZIP44 substantially affected the contents of soluble sugar, organic acids, amino acids, and flavonoids. In E8::PpbZIP44 tomato fruit, genes involved in carbohydrate metabolism, amino acid, and flavonoids biosynthesis were significantly induced. Furthermore, in PpbZIP44 overexpression or antisense pear fruits, the expression of genes in the related pathways was significantly impacted. PpbZIP44 directly interacted with the promoter of PpSDH9 and PpProDH1 to induce their expression, thereby depleting sorbitol and proline, decreasing citrate and malate, and enhancing fructose contents. PpbZIP44 also directly bound to the PpADT and PpF3H promoters, which led to the carbon flux toward phenylalanine metabolites and enhanced phenylalanine and flavonoid contents. These findings demonstrate that PpbZIP44 mediates multimetabolism reprogramming by regulating the gene expression related to fruit quality compounds.

Ethylene controls cambium stem cell activity via promoting local auxin biosynthesis.

The vascular cambium is the main secondary meristem in plants that produces secondary phloem (outside) and xylem (inside) on opposing sides of the cambium. The phytohormone ethylene has been implicated in vascular cambium activity, but the regulatory network underlying ethylene-mediated cambial activity remains to be elucidated. Here, we found that PETAL MOVEMENT-RELATED PROTEIN1 (RhPMP1), an ethylene-inducible HOMEODOMAIN-LEUCINE ZIPPER I transcription factor in woody plant rose (Rosa hybrida), regulates local auxin biosynthesis and auxin transport to maintain cambial activity. Knockdown of RhPMP1 resulted in smaller midveins and reduced auxin content, while RhPMP1 overexpression resulted in larger midveins and increased auxin levels compared with the wild-type plants. Furthermore, we revealed that Indole-3-pyruvate monooxygenase YUCCA 10 (RhYUC10) and Auxin transporter-like protein 2 (RhAUX2), encoding an auxin biosynthetic enzyme and an auxin influx carrier, respectively, are direct downstream targets of RhPMP1. In summary, our results suggest that ethylene promotes an auxin maximum in the cambium adjacent to the xylem to maintain cambial activity.

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.

Overexpression of Myrothamnus flabellifolia MfWRKY41 confers drought and salinity tolerance by enhancing root system and antioxidation ability in Arabidopsis.

Myrothamnus flabellifolia is the only woody resurrection plant discovered so far and could recover from extreme desiccation condition. However, few genes related to its strong drought tolerance have been characterized, and the underlying molecular mechanisms remains mysterious. Members of WRKY transcription factor family are effective in regulating abiotic stress responses or tolerance in various plants. An early dehydration-induced gene encoding a WRKY transcription factor namely MfWRKY41 was isolated from M. flabellifolia, which is homologous to AtWRKY41 of Arabidopsis. It contains a typical WRKY domain and zinc finger motif, and is located in the nucleus. Comparing to wild type, the four transgenic lines overexpressing MfWRKY41 showed better growth performance under drought and salt treatments, and exhibited higher chlorophyll content, lower water loss rate and stomatal aperture and better osmotic adjustment capacity. These results indicated that MfWRKY41 of M. flabellifolia positively regulates drought as well as salinity responses. Interestingly, the root system architecture, including lateral root number and primary root length, of the transgenic lines was enhanced by MfWRKY41 under both normal and stressful conditions, and the antioxidation ability was also significantly improved. Therefore, MfWRKY41 may have potential application values in genetic improvement of plant tolerance to drought and salinity stresses. The molecular mechanism involving in the regulatory roles of MfWRKY41 is worthy being explored in the future.

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.

Heterologous Expression of MfWRKY7 of Resurrection Plant Myrothamnus flabellifolia Enhances Salt and Drought Tolerance in Arabidopsis.

Drought and salinity have become major environmental problems that affect the production of agriculture, forestry and horticulture. The identification of stress-tolerant genes from plants adaptive to harsh environments might be a feasible strategy for plant genetic improvement to address the challenges brought by global climate changes. In this study, a dehydration-upregulated gene MfWRKY7 of resurrection Plant Myrothamnusflabellifolia, encoding a group IId WRKY transcription factor, was cloned and characterized. The overexpression of MfWRKY7 in Arabidopsis increased root length and tolerance to drought and NaCl at both seedling and adult stages. Further investigation indicated that MfWRKY7 transgenic plants had higher contents of chlorophyll, proline, soluble protein, and soluble sugar but a lower water loss rate and malondialdehyde content compared with wild-type plants under both drought and salinity stresses. Moreover, the higher activities of antioxidant enzymes and lower accumulation of O2- and H2O2 in MfWRKY7 transgenic plants were also found, indicating enhanced antioxidation capacity by MfWRKY7. These findings showed that MfWRKY7 may function in positive regulation of responses to drought and salinity stresses, and therefore, it has potential application value in genetic improvement of plant tolerance to abiotic stress.

A WRKY Protein, MfWRKY40, of Resurrection Plant Myrothamnus flabellifolia Plays a Positive Role in Regulating Tolerance to Drought and Salinity Stresses of Arabidopsis.

WRKY transcription factors (TFs), one of the largest transcription factor families in plants, play an important role in abiotic stress responses. The resurrection plant, Myrothamnus flabellifolia, has a strong tolerance to dehydration, but only a few WRKY proteins related to abiotic stress response have been identified and functionally characterized in M. flabellifolia. In this study, we identified an early dehydration-induced gene, MfWRKY40, of M. flabellifolia. The deduced MfWRKY40 protein has a conserved WRKY motif but lacks a typical zinc finger motif in the WRKY domain and is localized in the nucleus. To investigate its potential roles in abiotic stresses, we overexpressed MfWRKY40 in Arabidopsis and found that transgenic lines exhibited better tolerance to both drought and salt stresses. Further detailed analysis indicated that MfWRKY40 promoted primary root length elongation and reduced water loss rate and stomata aperture (width/length) under stress, which may provide Arabidopsis the better water uptake and retention abilities. MfWRKY40 also facilitated osmotic adjustment under drought and salt stresses by accumulating more osmolytes, such as proline, soluble sugar, and soluble protein. Additionally, the antioxidation ability of transgenic lines was also significantly enhanced, represented by higher chlorophyll content, less malondialdehyde and reactive oxygen species accumulations, as well as higher antioxidation enzyme activities. All these results indicated that MfWRKY40 might positively regulate tolerance to drought and salinity stresses. Further investigation on the relationship of the missing zinc finger motif of MfWRKY40 and its regulatory role is necessary to obtain a better understanding of the mechanism underlying the excellent drought tolerance of M. flabellifolia.

Overexpressing PhytochromeInteractingFactor 8 of Myrothamnus flabellifolia Enhanced Drought and Salt Tolerance in Arabidopsis.

Myrothamnus flabellifolia is the only woody resurrection plant found in the world and can survive from long-term desiccation. Therefore, M. flabellifolia could be considered as a valuable resource for study of plant adaptation to abiotic stress. However, few genes related to its drought tolerance have been functionally characterized and the molecular mechanisms underlying the stress tolerance of M. flabellifolia are largely unknown. The phytochrome interacting factor (PIF) family is a group of basic helix-loop-helix (bHLH) transcription factors and functions as the core regulator in plant growth and development. However, less is known of its participation in abiotic stress response. In this study, we isolated and characterized a dehydration-inducible PIF gene MfPIF8 from M. flabellifolia. Heterologous expression of MfPIF8 in Arabidopsis enhanced tolerance to drought and salinity stresses at seedling and adult stages. It significantly increased primary root length and stomatal aperture (ration of length/width) under stress treatments and decreased water loss rate. Compared with WT, the transgenic lines overexpressing MfPIF8 exhibited higher chlorophyll content and lower malondialdehyde accumulation. The abilities of osmotic adjustment and reactive oxygen species scavenging were also enhanced in MfPIF8 transgenic lines. These results suggest that MfPIF8 may participate in the positive regulation of abiotic stress responses. Additional investigation of its mechanism is needed in the future.

Heterologous Expression of Dehydration-Inducible MfbHLH145 of Myrothamnus flabellifoli Enhanced Drought and Salt Tolerance in Arabidopsis.

Myrothamnus flabellifolia is the only woody resurrection plant found in the world. It has a strong tolerance to drought and can survive long-term exposure to desiccated environments. However, few genes related to its drought tolerance have been functionally characterized and the molecular mechanisms underlying the stress tolerance of M. flabellifolia are largely unknown. In this study, we isolated a dehydration-inducible bHLH transcription factor gene MfbHLH145 from M. flabellifolia. Heterologous expression of MfbHLH145 enhanced the drought and salt tolerance of Arabidopsis. It can not only promote root system development under short-term stresses, but also improve growth performance under long-term treatments. Further investigation showed that MfbHLH145 contributes to enhanced leaf water retention capacity through the promotion of stomatal closure, increased osmolyte accumulation, and decreased stress-induced oxidative damage through an increase in antioxidant enzyme activities. These results suggest that MfbHLH145 may be involved in the positive regulation of stress responses in M. flabellifolia. This study provides insight into the molecular mechanism underlying the survival of M. flabellifolia in extreme dehydration conditions.

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.

The chrysanthemum DEAD-box RNA helicase CmRH56 regulates rhizome outgrowth in response to drought stress.

Plants have evolved complex mechanisms to reprogram growth in response to drought stress. In herbaceous perennial plant species, the rhizome, which is normally an organ for propagation and food storage, can also support plant growth in stressful environments, and allows the plant to perennate and survive stress damage. However, the mechanisms that regulate rhizome growth in perennial herbs during abiotic stresses are unknown. Here, we identified a chrysanthemum (Chrysanthemum morifolium) DEAD-box RNA helicase gene, CmRH56, that is specifically expressed in the rhizome shoot apex. Knock down of CmRH56 transcript levels decreased the number of rhizomes and enhanced drought stress tolerance. We determined that CmRH56 represses the expression of a putative gibberellin (GA) catabolic gene, GA2 oxidase6 (CmGA2ox6). Exogenous GA treatment and silencing of CmGA2ox6 resulted in more rhizomes. These results demonstrate that CmRH56 suppresses rhizome outgrowth under drought stress conditions by blocking GA biosynthesis.

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 4 h 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.

A Cytokinin Analog Thidiazuron Suppresses Shoot Growth in Potted Rose Plants via the Gibberellic Acid Pathway.

Application of thidiazuron (N-phenyl-N'-1,2,3-thiadiazol-5-ylurea, TDZ), a cytokinin analog, to inhibit the leaf yellowing that occurs after pinching potted rose plants, resulted in compact plants with shorter shoots and thicker internodes. Two weeks after treatment with 100 µM of TDZ, new shoots were half as long as those in control plants, and stem diameters were about 40% greater. This effect of TDZ is associated with changes in cell architecture. Although TDZ treatment stimulated ethylene production by the plants, inhibitors of ethylene biosynthesis (2-aminoethoxyvinyl glycine) or action (silver thiosulfate) did not affect the response of plants to TDZ. We found that TDZ treatment significantly suppressed the expression of bioactive gibberellic acid (GA) biosynthesis genes encoding GA3 and GA20 oxidases and slightly increased the expression of GA catabolism genes encoding GA2 oxidase. Application of GA3 and TDZ together resulted in normal elongation growth, although stem diameters were still somewhat thicker. Our results suggest that TDZ regulates shoot elongation and stem enlargement in potted rose plants through the modulation of bioactive GA biosynthesis.