Role of polyamines in gibberellin-induced internode growth in peas

To determine the requirement for polyamines in gibberellin (GA) induced internode growth polyamine content was measured in internodes of peas of various internode phenotypes (slender, tall, dwarf, nana) with and without applied gibberellin (GA₃) and polyamine synthesis inhibitors. Polyamines were assayed as dansyl derivatives which were separated by reverse phase high performance liquid chromatography and detected by fluorescence spectrophotometry. The amounts of polyamines in the different genetic lines of peas, which differed in internode lengths and extractable GA content, correlated with the extent of internode elongation. High polyamine concentrations were associated with young internodes and decreased with internode expansion. Extremely short internodes of nana plants without GA exhibited equal or higher amine concentrations relative to internodes of other lines of peas and GA-stimulated nana seedlings. The polyamine synthesis inhibitors, α-difluoromethylornithine and α-difluoromethylarginine, independently or in combination, inhibited polyamine accumulation and internode elongation of tall peas and GA-stimulated nana plants. Agmatine and putrescine restored growth and endogenous polyamine content to variable degrees. However, exogenous polyamines were not effective in promoting growth unless intracellular amines were partially depleted.

These results suggest that polyamines do not have a role in cell elongation, but may be required to support cell proliferation. Polyamines do not mediate the entire action of GA in internode growth of peas since GA induction of growth involves both cell division and cell elongation, whereas polyamines appear to affect cell division only.

     

Internode length in Lathyrus odoratus. The involvement of gibberellins

Evidence was obtained by gas chromatography-mass spectrometry and gas chromatography-selected ion monitoring for the presence of gibberellin A₂₀), GA₁, GA₂₉, GA₈ and 2-epiGA₂₉ in vegetative shoots of tall sweet pea, Lathyrus odoratus L. Both tall (genotype L –) and dwarf (genotype II ) sweet peas elongated markedly in response to exogenous GA₁ attaining similar internode lengths at the highest dose levels. Likewise internode length in both genotypes was reduced by application of the GA biosynthesis inhibitor, PP333. The ratio of leaflet length to width was reduced by application of PP333 to tall plants and this effect was reversed by GA₁. When applied to plants previously treated with PP333, GA₂₀ promoted internode elongation of L – plants as effectively as GA₁, but GA₂₉ was not as effective as GA₁ when applied to II plants. In contrast, GA₂₀ and GA₁ were equally effective when applied to the semidwarf lb mutant but GA-treated lblb plants did not attain the same internode length as comparable GA-treated Lb – plants. The difference in stature between the tall and dwarf types persisted in dark-grown plants. It is concluded that GA₁ may be important for internode elongation and leaf growth in sweet pea. Mutant l may influence GA₁ synthesis by reducing 3β-hydroxylation of GA₂₀ whereas mutant lb appears to affect GA sensitivity.     

Role of Assimilates and Phosphates in the Control of Internode Elongation in Tall and Dwarf Indica Rice Varieties

Six indica rice varieties belonging to three duration groups,each group containing one tall and one dwarf variety, were grownunder irrigated field conditions during the wet season of 1986.At the time of culm elongation, the distal 5–6 internodesof a variety elongated. The pattern of extension was hierarchical,each successive internode dominating the one immediately belowit and hence the final internode lengths increased in acropetalfashion from the bottom to the top. Between the tall and dwarfvarieties, the lengths of the individual intemodes did not differmuch in the apical and basal positions, but significant differenceswere found between the lengths of the middle internodes. Withincrease in length, the internodes became visibly thinner anddry matter content per unit length decreased. Analyses of solublecarbohydrates, amino acids, and phosphate contents of the internodesindicated that the concentration of the metabolites increasedin acropetal succession from the base to the apex. In general,the dwarf varieties contained at least the same, if not a higherconcentration of the metabolites as the tall varieties. Thepossible role of the metabolites in internode extension is discussed. Key words: Rice, internode extension, assimilates, phosphates, sink     

Peroxidase Ontogeny in a Dwarf Pea Stem as Affected by Gibberellin and Decapitation

The ontogeny of peroxidase activity and isoenzyme pattern wasinvestigated in the stem of dwarf pea plants. Peroxidase activityper unit soluble protein was a given internode is highest inthe youngest growth stage, drops during elongation, remainsconstant upon cessation of growth, and increase at senescence.The lower the internode on the stem the higher is its peroxidaseactivity. These developmental differences are already apparentat the youngest growth stage of the internodes adn increaseduring elongation. Several anodic and five cathodic isoperoxidasesare apparent after starch gel electrophoresis. This patternis constant for all internodes at all growth stages, but therelative importance of particular isoenzymes changes with time. Gibberellic acid (GA₃) treatment causes greatly elongated internodes,decreased soluble protein, and inhibition of the rise in peroxidaseactivity within 4–8 h. Application of GA₃ to young internodesleads to a persistent depression in peroxidase activity, whiletreated older internodes suffer only a temporary depression.GA₃ causes no qualitative changes in the isoenzyme pattern butproduces some quantitative alterations in internodes in whichits influence on peroxidase activity is persistent. Decapitation of untreated and GA₃-treated dwarfs has littleinfluence on internode elongation, causes an increase in peroxidaseactivity, especially in the upper internodes, and alters therelative activity of particular isoenzymes. By contrast, decapitationinhibits elongation of young internodes in genetically tallpea plants.     

Effects of Gibberellin on the Arrangement and the Cold Stability of Cortical Microtubules in Epidermal Cells of Pea Internodes

Longitudinal microtubules are predominant in epidermal cellsof the 3rd internodes of dwarf pea (Pisum sativum L. cv. LittleMarvel) seedlings. In more than 50% of the cells, cortical microtubulesare running parallel to the cell axis. GA₃ promotes elongation of the internodes and gives rise toa predominance of transverse microtubules. In more than 60%of the GA₃-treatd cells, cortical microtubules are running transverseto the cell axis. Longitudinal microtubules in the GA₃-untreated cells are resistantto low-temperature treatment, but transverse microtubules inthe GA₃-treated cells are sensitive to it. Longitudinal microtubulesare present in GA₃-treated epidermal cells with low frequency.They are resistant to low-temperature treatment. Longitudinal, oblique and transverse microtubules are presentwith almost the same frequency in epidermal cells of the 3rdinternodes of tall pea (cv. Early Alaska) seedlings. GA₃ promoteselongation of the internodes also in tall pea seedlings, butit does not alter the direction of cortical microtubules sodistinctly as it does in dwarf pea seedlings. As in dwarf pea seedlings, longitudinal microtubules are resistantto low-temperature treatment, and transverse microtubules aresensitive to it in tall pea seedlings. (Received September 19, 1986; Accepted December 26, 1986)     

蒺藜苜蓿株高控制基因COSTIN的克隆及功能研究

株高是影响植物株型建成的重要农艺性状之一,直接决定作物的倒伏性和生物产量,但目前关于苜蓿等豆科牧草株高性状形成的分子调控机制尚不清楚。通过定向筛选豆科模式植物蒺藜苜蓿Tnt1逆转座子插入突变体库,分离鉴定了一个蒺藜苜蓿矮化突变体compact stalk internodes(costin),该突变体的矮化表型是由于茎节伸长受到抑制所致。通过基因表型连锁分析成功克隆了COSTIN基因,该基因编码一个钙离子交换蛋白,与拟南芥的CALCIUM EXCHANGER 7(CAX7) 基因高度同源。qRT-PCR检测发现COSTIN基因在茎、叶和果荚等组织中有较高的表达。进一步研究发现在costin突变体中赤霉素合成途径关键基因MtCPS、MtKAO1、MtGA20ox4、MtGA20ox7和MtGA3ox1表达下调;外施赤霉素GA3可以恢复costin突变体的矮化表型。上述研究表明COSTIN基因通过影响植物激素赤霉素的生物合成来调控蒺藜苜蓿的茎节伸长。     

Phytohormonal regulation of S-adenosylmethionine synthetase and S-adenosylmethionine levels in dwarf pea epicotyls.

A significant stimulation (2- to 2.5-fold) of AdoMet synthetase was witnessed in glibberellicd acid (GA3, 1 microM)-treated epicotyls of the dwarf pea (Pisum sativum). This was accompanied by a 2.4-fold increase in the endogenous pool of S-adenosylmethionine. Both abscisic acid (10 microM) and cycloheximide (20 micrograms/ml) inhibited the GA3-mediated enhancement of AdoMet synthetase activity. Three isozymes of AdoMet synthetase were detected in GA3-treated epicotyls, whereas a single activity peak was observed in controls. Thus, GA3 seems to control the induction of two new isozymes of AdoMet synthetase in the dwarf pea. By contrast, the tall pea exhibited three isozymes of AdoMet synthetase even in the absence of GA3 treatment. High concentration of L-methionine (2 mM) mimicked the GA3-elicited induction of two new isozymes of AdoMet synthetase in dwarf pea epicotyls.     

ent-Kaurene Biosynthesis in Cell-Free Extracts of Excised Parts of Tall and Dwarf Pea Seedlings

Investigations on the sites of ent-kaur-16-ene (ent-kaurene) biosynthesis were conducted with cell-free extracts from several excised parts of 10-, 13-, and 16-d-old tall and dwarf pea (Pisum sativum L.) seedlings. [¹⁴C]Mevalonic acid was incorporated into ent-kaurene in cell-free extracts from young developing leaves and elongating internodes of tall (`Alaska') and dwarf (`Progress No.9') pea seedlings at all three stages of development. ent-Kaurene biosynthesis also occurred readily in cell-free extracts from shoot tips, petioles, and stipules near the young elongating internodes. The ent-kaurene-synthesizing activity found in young developing tissues declined as tissues matured. Little or no activity was detectable in enzyme extracts from cotyledons and root tips at different stages. In light grown tall pea internodes ent-kaurene-synthesizing activity was low as they began to elongate, reached a maximum when the internodes reached about 2 cm in length and declined as they matured. Activity in extracts of dwarf shoot tips and internodes was generally lower than in equivalent tall plants, but the activity in dwarf leaves and stipules was somewhat higher than in tall plants. With the exception of root tips, there is a strong correlation between growth potential of a tissue and the rate of ent-kaurene biosynthesis in extracts from that tissue.     

The role of a class III gibberellin 2‐oxidase in tomato internode elongation

A network of environmental inputs and internal signaling controls plant growth, development and organ elongation. In particular, the growth‐promoting hormone gibberellin (GA) has been shown to play a significant role in organ elongation. The use of tomato as a model organism to study elongation presents an opportunity to study the genetic control of internode‐specific elongation in a eudicot species with a sympodial growth habit and substantial internodes that can and do respond to external stimuli. To investigate internode elongation, a mutant with an elongated hypocotyl and internodes but wild‐type petioles was identified through a forward genetic screen. In addition to stem‐specific elongation, this mutant, named tomato internode elongated ‐1 (tie‐1) is more sensitive to the GA biosynthetic inhibitor paclobutrazol and has altered levels of intermediate and bioactive GAs compared with wild‐type plants. The mutation responsible for the internode elongation phenotype was mapped to GA2oxidase 7, a class III GA 2‐oxidase in the GA biosynthetic pathway, through a bulked segregant analysis and bioinformatic pipeline, and confirmed by transgenic complementation. Furthermore, bacterially expressed recombinant TIE protein was shown to have bona fide GA 2‐oxidase activity. These results define a critical role for this gene in internode elongation and are significant because they further the understanding of the role of GA biosynthetic genes in organ‐specific elongation.     

Genetic Dissection of the Relative Roles of Auxin and Gibberellin in the Regulation of Stem Elongation in Intact Light-Grown Peas

Exogenous gibberellin (GA) and auxin (indoleacetic acid [IAA]) strongly stimulated stem elongation in dwarf GA1-deficient le mutants of light-grown pea (Pisum sativum L.): IAA elicited a sharp increase in growth rate after 20 min followed by a slow decline; the GA response had a longer lag (3 h) and growth increased gradually with time. These responses were additive. The effect of GA was mainly in internodes less than 25% expanded, whereas that of IAA was in the older, elongating internodes. IAA stimulated growth by cell extension; GA stimulated growth by an increase in cell length and cell number. Dwarf lkb GA-response-mutant plants elongated poorly in response to GA (accounted for by an increase in cell number) but were very responsive to IAA. GA produced a substantial elongation in lkb plants only in the presence of IAA. Because lkb plants contain low levels of IAA, growth suppression in dwarf lkb mutants seems to be due to a deficiency in endogenous auxin. GA may enhance the auxin induction of cell elongation but cannot promote elongation in the absence of auxin. The effect of GA may, in part, be mediated by auxin. Auxin and GA control separate processes that together contribute to stem elongation. A deficiency in either leads to a dwarfed phenotype.     

Magnitude and Kinetics of Stem Elongation Induced by Exogenous Indole-3-Acetic Acid in Intact Light-Grown Pea Seedlings

Exogenously applied indole-3-acetic acid (IAA) strongly promoted stem elongation over the long term in intact light-grown seedlings of both dwarf (cv Progress No. 9) and tall (cv Alaska) peas (Pisum sativum L.), with the relative promotion being far greater in dwarf plants. In dwarf seedlings, solutions of IAA (between 10-4 and 10-3 M), when continuously applied to the uppermost two internodes via a cotton wick, increased whole-stem growth by at least 6-fold over the first 24 h. The magnitude of growth promotion correlated with the applied IAA concentration from 10-6 to 10-3 M, particularly over the first 6 h of application. IAA applied only to the apical bud or the uppermost internode of the seedling stimulated a biphasic growth response in the uppermost internode and the immediately lower internode, with the response in the latter being greatly delayed. This demonstrates that exogenous IAA effectively promotes growth as it is transported through intact stems. IAA withdrawal and reapplication at various times enabled the separation of the initial growth response (IGR) and prolonged growth response (PGR) induced by auxin. The IGR was inducible by at least 1 order of magnitude lower IAA concentrations than the PGR, suggesting that the process underlying the IGR is more sensitive to auxin induction. In contrast to the magnitude of the IAA effect in dwarf seedlings, applied IAA only doubled the growth in tall seedlings. These results suggest that endogenous IAA is more growth limiting in dwarf plants than in tall plants, and that auxin promotes stem elongation in the intact plant probably by the same mechanism of action as in isolated stem segments. However, since dwarf plants to which IAA was applied failed to reach the growth rate of tall plants, auxin cannot be the only limiting factor for stem growth in peas.     

The pea gene NA encodes ent-kaurenoic acid oxidase

The gibberellin (GA)-deficient dwarf na mutant in pea (Pisum sativum) has severely reduced internode elongation, reduced root growth, and decreased leaflet size. However, the seeds develop normally. Two genes, PsKAO1 and PsKAO2, encoding cytochrome P450 monooxygenases of the subfamily CYP88A were isolated. Both PsKAO1 and PsKAO2 had ent-kaurenoic acid oxidase (KAO) activity, catalyzing the three steps of the GA biosynthetic pathway from ent-kaurenoic acid to GA(12) when expressed in yeast (Saccharomyces cerevisiae). In addition to the intermediates ent-7alpha-hydroxykaurenoic acid and GA(12)-aldehyde, some additional products of the pea KAO activity were detected, including ent-6alpha,7alpha-dihydroxykaurenoic acid and 7beta-hydroxykaurenolide. The NA gene encodes PsKAO1, because in two independent mutant alleles, na-1 and na-2, PsKAO1 had altered sequences and the five-base deletion in PsKAO1 associated with the na-1 allele cosegregated with the dwarf na phenotype. PsKAO1 was expressed in the stem, apical bud, leaf, pod, and root, organs in which GA levels have previously been shown to be reduced in na plants. PsKAO2 was expressed only in seeds and this may explain the normal seed development and normal GA biosynthesis in seeds of na plants.     

Spiral growth and cell wall properties of the gibberellin-treated first internodes in the seedlings of a wheat cultivar tolerant to deep-sowing conditions

The Hong Mang Mai wheat cultivar is tolerant to deep-sowing conditions because it has an elongated first internode that is sensitive to gibberellin (GA₃). The cells in the GA-treated first internodes were approximately 4.2 mm long, twice as long as the untreated Hong Mang Mai first internode cells. The elongation of the first internode of Hong Mang Mai, particularly when treated with GA₃, was accompanied by remarkable spiral growth. In contrast, the first internodes of the GA-insensitive cultivar Norin 10 did not exhibit GA₃-induced elongation or spiral growth. The walls of the first internode cells of GA₃-treated Hong Mang Mai seedlings showed increased extensibility and higher (1→3), (1→4)- β - d -glucanase activity, autolysis and glucan contents than the cell walls of untreated Hong Mang Mai first internodes. The changes in the cell wall extensibility due to GA₃ treatment correlated strongly with the GA₃-induced changes in cell wall glucan content, autolysis, and glucanase activity. GA₃-treated Hong Mang Mai seedlings showed elevated expression of Glucanase EI gene in the first internode compared to GA₃-treated Norin 10. Thus, GA aids first internode elongation in Hong Mang Mai by enhancing glucan turnover and thus increasing cell wall loosening. The spiral growth of the first internode also helps the plant elongate against soil resistance, thereby promoting the deep-sowing tolerance of this cultivar.     

The Relation between Cell-wall and Protein Synthesis in Dwarf Pea Plants treated with Gibberellic Acid

The amount of protein and soluble nitrogen present in expandinginternodes of intact dwarf pea seedlings, was investigated atdifferent times after treatment of plants with gibberellic acid(GA). There was a marked increase in rate of protein synthesisfollowing GA treatment, but the rate of synthesis did not keeppace with internode expansion, so that the amount of proteinper unit length fell. The rate of cell-wall synthesis was alsoincreased, and, in contrast to protein, the amount of cell wallper unit length remained approximately constant during internodeexpansion. It is suggested that the increased rate of cell-wallsynthesis which follows GA treatment is mediated by a changein protein metabolism. The amount of soluble nitrogen presentin expanding internodes was also increased. There was littleeffect of GA upon the protein content of internodes which werealmost fully expanded at the time of treatment.     

Developmental changes in the gibberellin-induced growth response in stem segments of light-grown pea genotypes

The effects of GA on stem elongation were studied using segments from one tall and three dwarf light-grown pea genotypes varying in endogenous hormone content. Stem segments were cut at two distinct ages: when the fourth internode was at about 6–13% of full expansion (early-expansion) or at 18–25% of full expansion (mid-expansion). Light microscopy and flow cytometry were used to demonstrate that GA does not induce cell division in excised pea stem segments. The growth studied here was strictly elongation. Measurement of final segment length after 48 hours and high resolution measurement of growth kinetics over 20 hours using an angular position transducer were done on segments treated with hormone solutions. Our data indicate that the action of GA on stem elongation can be classified into two distinct modes. The first, apparent in early-expansion stem segments, shows distinct growth kinetics and is independent of the endogenous IAA concentration of the segments. Quantitation of IAA by GC/MS in early-expansion segments of wild type pea incubated with gibberellin shows that an increase in IAA concentration is part of the GA response in such segments. The second mode of GA action is evinced in mid-expansion segments. Whereas there is no short term (<20 h) response to GA alone (as determined by growth kinetics), there is a long term (48 h) response whose magnitude decreases across the genotypes with decreasing endogenous hormone content. Growth responses indicate that in mid-expansion segments exogenous GA acts by enhancing IAA action but appears to be unable to augment endogenous IAA content. Contradictory reports of the response of excised stem segments to GA can be reconciled when tissue genotype and developmental stage are considered.     

Co-regulation Of ear growth and internode elongation in corn

Ear is the harvest part of corn (Zea mays L.) and we are interested in studying its growth and development in our effort in corn yield improvement. In our current study, we examined the relationship between ear growth and internode characteristics using different approaches. Correlations between stem growth rate and number of ears per plant (prolificacy) were assessed among several genotypes. Internode elongation of 2 genotypes was modified by plant hormones and by population density manipulations. Among the 7 genotypes examined that have different prolificacy levels, there was a general correlation of slower stem elongation at middle growth stages and larger ear number. When the internode elongation was enhanced by application of gibberellic acid (GA), ear growth was suppressed; and when a GA synthesis inhibitor uniconazole was applied at early stages, internode length was reduced and ear growth was promoted in terms of both ear size and visible ear number at silking stage. Higher population density caused longer internodes and fewer ears per plant and the effect of lower density was the opposite. Our results suggested that internode elongation in the middle section of corn plants was linked to suppression of ear development in corn.     

Auxin Physiology of Dwarfism in Pisum sativum

Similar levels of diffusible auxin are measured for the apices of both Little Marvel (dwarf) and Alaska (normal) cultivars of the pea when grown in sunlight and darkness. In sunlight, however, diffusible auxin disappears in the subtending internode of the Little Marvel plant but remains at 50 per cent of the level of the apex in the subtending internode of the Alaska plant. The enzyme preparation from the apex of the dwarf plant converts tryptophan and tryptamine to IAA more readily than that from the normal plant. Indoleacetyl aspartate synthetase activity is also higher in the dwarf plant than in the normal plant and the dwarf plant contains four times as much conjugate as the normal plant with or without treatment with gibberellic acid. Gibberellic acid (GA) does not affect the induction of the synthetase enzyme nor the enzymatic formation of indoleacetyl aspartate. The growth induced by GA is the result of an increased synthesis of auxin.     

Control of Internode Length in Pisum sativum (Further Evidence for the Involvement of Indole-3-Acetic Acid)

The effects of altered endogenous indole-3-acetic (IAA) levels on elongation in garden pea (Pisum sativum L.) plants were investigated. The auxin transport inhibitors 2,3,5-triiodobenzoic acid (TIBA) and 9-hydroxyfluorene-9-carboxylic acid (HFCA) were applied to elongating internodes of wild-type and mutant lkb plants. The lkb mutant was included because elongating lkb internodes contained 2- to 3-fold less free IAA than those of the wild type. In the wild type, TIBA reduced both the IAA level and internode elongation below the site of application. Both TIBA and HFCA strongly promoted the elongation of lkb internodes and also raised IAA levels above the application site. The synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) also markedly increased internode elongation in lkb plants and virtually restored petioles and tendrils to their wild-type length. In contrast, treatment of wild-type plants with TIBA, HFCA, or 2,4-D caused little or no increase in elongation above the application site. The ethylene synthesis inhibitor aminoethoxyvinylglycine also increased stem elongation in lkb plants, and combined application of HFCA and aminoethoxy-vinylglycine restored lkb internodes to the wild-type length. It is concluded that the level of IAA in wild-type internodes is necessary for normal elongation, and that the reduced stature of lkb plants is at least partially attributable to a reduction in free IAA level in this mutant.     

鹅掌楸属种间杂种优势与内源植物激素的关系

研究了鹅掌楸属( Liriodendron )种间F1杂种(杂种马褂木 L.chinense × L.tulipifera )及其亲本种(中国马褂木 L.chinense (Hemsl.) Sarg.和北美鹅掌楸 L.tulipifera L.)在生长性状和内源GA1/3、IAA和iPA含量上的差异性.结果表明:(1)杂种马褂木高生长性状杂种优势主要是由节间的相对伸长造成的,鹅掌楸属节间伸长区位于顶芽下第1～3节间,其中第1节间伸长量最大、是产生杂种优势的主要节间; (2)中国马褂木、北美鹅掌楸和杂种马褂木内源GA1/3、IAA和iPA含量差异很大,顶芽下第1节间GA1/3和iPA含量杂种家系分别为两个亲本种中含量相对较高的中国马褂木的133.58%～284.17%和396.64%～1 264.28%.杂种马褂木顶芽下第1节间中GA1/3和iPA含量的大量增加可能与高生长性状杂种优势有关; (3)杂种家系3年生时其苗高生长量排序与顶芽下第1节间中GA1/3和iPA含量排序并不一致,不能以第1节间GA1/3和iPA含量的高低作为预测杂种家系间杂种优势大小的依据.     

Genetic and physiological characterization of tomato cv. Micro-Tom

Based on its compact habit, Micro-Tom, a dwarf cultivar of tomato (Solanum lycopersicum L.), has been proposed as a preferred variety to carry out molecular research in tomato. This cultivar, however, is poorly characterized. It is shown here that Micro-Tom has mutations in the SELF-PRUNING (SP) and DWARF (D) genes. In addition to this, it is also shown that Micro-Tom harbours at least two independently segregating resistance loci to the plant pathogen Cladosporium fulvum. The presence of the self-pruning mutation in Micro-Tom, that generates a determinate phenotype, was confirmed by crossing and sequence analysis. It was also found that Micro-Tom has a mutation in the DWARF gene (d) that leads to mis-splicing and production of at least two shorter mRNAs. The d mutation is predicted to generate truncated DWARF protein. The d sequence defect co-segregates with dark-green and rugose leaves, characteristics of brassinosteroid biosynthesis mutants. Micro-Tom also carries at least another mutation producing internode length reduction that affects plant height but not active gibberellin (GA) levels, which were similar in dwarf and tall Micro-TomxSeverianin segregants. GAs and brassinosteroids act synergistically in Micro-Tom, and the response to GA depends on brassinosteroids because the elongation of internodes was at least six times higher when GA(3) was applied simultaneously with brassinolide. A novel variety, Micro-0 that is fully susceptible to C. fulvum and almost as dwarf as Micro-Tom, has been generated from the cross of Cf0xMicro-Tom. This line represents a valuable resource for future analysis of Cf resistance genes through breeding or transformation.