Inhibition of auxin transport from the ovary or from the apical shoot induces parthenocarpic fruit-set in tomato mediated by gibberellins

Abstract

Fruit-set in tomato (Solanum lycopersicum) depends on gibberellins and auxins (GAs). Here, we show, using the cv MicroTom, that application of N-1-naphthylphthalamic acid (NPA; an inhibitor of auxin transport) to unpollinated ovaries induced parthenocarpic fruit-set, associated with an increase of indole-3-acetic acid (IAA) content, and that this effect was negated by paclobutrazol (an inhibitor of GA biosynthesis). NPA-induced ovaries contained higher content of GA(1) (an active GA) and transcripts of GA biosynthetic genes (SlCPS, SlGA20ox1, and -2). Interestingly, application of NPA to pollinated ovaries prevented their growth, potentially due to supraoptimal IAA accumulation. Plant decapitation and inhibition of auxin transport by NPA from the apical shoot also induced parthenocarpic fruit growth of unpollinated ovaries. Application of IAA to the severed stump negated the plant decapitation effect, indicating that the apical shoot prevents unpollinated ovary growth through IAA transport. Parthenocarpic fruit growth induced by plant decapitation was associated with high levels of GA(1) and was counteracted by paclobutrazol treatment. Plant decapitation also produced changes in transcript levels of genes encoding enzymes of GA biosynthesis (SlCPS and SlGA20ox1) in the ovary, quite similar to those found in NPA-induced fruits. All these results suggest that auxin can have opposing effects on fruit-set, either inducing (when accumulated in the ovary) or repressing (when transported from the apical shoot) that process, and that GAs act as mediators in both cases. The effect of NPA application and decapitation on fruit-set induction was also observed in MicroTom lines bearing introgressed DWARF and SELF-PRUNING wild-type alleles.

Figure 1.

Auxin transport in the ovary and apical shoot. A, Comparative effects of IAA, 2,4-D, and NAA application directly to unpollinated ovary versus pedicel on fruit-set and growth. Hormone application was carried out at day 0 to the ovary (in 10 μL of solution at concentrations of 200 ng μL⁻¹ IAA, 20 ng μL⁻¹ 2,4-D, and 200 ng μL⁻¹ NAA) or to the pedicel (in about 20 mg of lanolin at concentrations 10 times higher than to the ovary), and fruits were collected at day 20. Pollinated (Poll.) and unpollinated (Unpoll.) control (C) ovaries were treated with the same volume of solvent solution to the ovary or amount of lanolin to the pedicel. Weight values are means of developed fruits ± se (n = 12). The value in parentheses indicates the number of fruits developed from the 12 ovaries treated; absence of that notation means 100% fruit set. B, Inhibition of basipetal [³H]IAA transport applied to unpollinated and pollinated ovaries (1,670 Bq per ovary) by NPA applied in lanolin (1.5 mg g⁻¹) to the pedicel. The applications were made at day 0, and the material (ovaries and pedicels) was collected 48 h later. Data are means ± se (n = 3, 15 ovaries and pedicels per replicate). Asterisks denote significant differences (P < 0.05, Student's t test) between untreated and treated tissues. C, Inhibition of basipetal [³H]IAA transport applied to the vegetative apex (1,670 Bp per plant) by NPA in lanolin (1.5 mg g⁻¹) applied below the vegetative apex. The applications were made at day 0, and the material was collected 48 h later. Data are means ± se (n = 3, six plants per replicate). Asterisks denote significant differences (P < 0.05) between untreated and treated tissues.

Figure 2.

Effects of auxin transport inhibition on fruit-set and growth. A, Effects of NPA on fruit-set and growth of unpollinated (Unpoll.) and pollinated (Poll.) ovaries from MT, MT-D, and MT-SP plants. Data are means ± se (n = 16). B, Images of representative unpollinated and pollinated MT ovaries nontreated and treated with NPA. Mean weight of unpollinated/−NPA and pollinated/+NPA MT ovaries were 12 and 30 mg, respectively. C, Effects of TIBA on fruit-set and growth and number of seeds in unpollinated and pollinated MT ovaries. Data are means ± se (n = 8). Asterisks denote significant differences (P < 0.05) between untreated and treated organs. D, IAA concentration (ng g⁻¹) in unpollinated and pollinated MT ovaries nontreated and treated with NPA. IAA values are means ± se (n = 3, 15 ovaries/fruits per replicate). Different letters on the bars represent means that are statistically different (P < 0.05). NPA and TIBA were applied in lanolin to the pedicel (at 1.5 and 15 mg g⁻¹, respectively) at day 0, and fruits were collected 20 d (A–C) or 10 d (D) later. For meaning of values in parentheses, see legend of Figure 1A. [See online article for color version of this figure.]

Figure 3.

Effects of NPA on growth of parthenocarpic fruits induced by IAA, 2,4-D, and GA₃. NPA was applied to the pedicel in lanolin (1.5 mg g⁻¹), and IAA (2,000 ng), 2,4-D (2, 20, 200, and 2,000 ng), and GA₃ (2,000 ng) in 10 μL of solution, to unpollinated ovaries at day 0, and material was collected at day 20. Data are means of developed fruits ± se (n = 12). For meaning of values in parentheses, see legend of Figure 1A. Asterisks denote significant differences (* P < 0.05, ** P < 0.01) between untreated and treated tissues.

Figure 4.

PAC inhibition and reversion by GA₃ of parthenocarpic fruit-set and growth of unpollinated MT, MT-D, and MT-SP ovaries induced by NPA applied to the pedicel (A) and of MT unpollinated ovaries induced by IAA applied to the ovary (B). C, Control. NPA was applied in lanolin (1.5 mg g⁻¹), and IAA (2 μg) and GA₃ (2 μg) in 10 μL of solution, to the ovary at day 0. PAC was applied to the roots in 10⁻⁵ m solution. Data are means ± se (n = 16).

Figure 5.

Effects of NPA application to the pedicel on transcript levels of genes of GA biosynthesis and inactivation in unpollinated ovaries. A, SlCPS. B, SlGA20ox1, -2, -3, and -4. C, GA3ox1 and -2. D, SlGA2ox1, -2, -3, -4, and -5. NPA was applied in lanolin (1.5 mg g⁻¹) to the pedicel at day 0, and ovaries were collected 10 and 20 d later. −, Control; +, ovaries treated with NPA. The values are means of three biological replicates ± se.

Figure 6.

Induction of parthenocarpic fruit-set and growth of unpollinated ovaries by plant decapitation. A, Effects of plant decapitation (Decap.) and application of NPA to the apical shoot of intact MT, MT-D, and MT-SP plants, and reversion of the plant decapitation effect by IAA. Data are means ± se (n = 16 for MT and MT-D, n = 5 for MT-SP). C, Control. The inset is a scheme indicating the sites of NPA and IAA application. B, PAC inhibition of fruit-set induced by plant decapitation, and reversion by GA₃ in MT, MT-D, and MT-SP plants. Data are means ± se (n = 16). NPA (1.5 mg g⁻¹) and IAA (1 mg g⁻¹) were applied to the apical shoot in lanolin (see inset in A), and GA₃ (2,000 ng) was applied to the ovary in 10 μL of solution, at day 0, and materials was collected at day 20. PAC was applied as 10⁻⁵ m solution to the roots from day −7 every 2 d. Plant decapitation was done at day 0.

Figure 7.

Effects of plant decapitation on transcript levels of genes of GA biosynthesis and inactivation in unpollinated ovaries. A, SlCPS. B, SlGA20ox1, -2, -3, and -4. C, GA3ox1 and -2. D, SlGA2ox1, -2, -3, -4, and -5. Plant decapitation (Decap.) was carried out at day 0, and ovaries were collected 10 and 20 d later. −, Control; +, decapitated plants. The values are means of three biological replicates ± se.

Figure 8.

Proposed model for auxin (from the ovary and from the apical shoot)-GA interaction on parthenocarpic fruit-set. Auxin accumulated in unpollinated ovary upon NPA application and auxin transported from the apical shoot (without entering in the ovary) regulate GA metabolism genes in the ovary in opposite ways, associated with induction (auxin in the ovary) and repression (auxin from the apical shoot) of fruit-set. For clarity, only GA20ox1, the main GA20ox gene regulated in both cases, is given in the scheme. GA20ox2, GA3ox1, and GA3ox2 transcript levels also increased in NPA-induced ovaries at day 10 after treatment but not in decapitated plants. The question mark (?) means that IAA from the apex does not need to enter the ovary for fruit-set inhibition and that this effect depends on unknown second messenger(s). GGPP, Geranylgeranyl diphosphate.