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Essay 19.5

Auxin and Fruit Initiation

Stephen M. Swain and Anna M. Koltunow, Commonwealth Scientific and Industrial Research Organization, Division of Plant Industry, Merbein (Vic) and Adelaide, Australia

October, 2006

Fruit and Seed Development

In flowering plants, seed set and fruit development normally occur in a coordinated manner following pollination of the stigma and subsequent double fertilization in the ovule inside the flower (Gillaspy et al. 1993; Figure 1). Different parts of the flower can contribute to the final structure of dry and fleshy fruits (Coombe 1975). The final form of the fruit is dependent upon the contributing number and type of floral organ components, the position of the contributing organs, and how the different tissues within them grow and differentiate. During the formation of the dry Arabidopsis thaliana fruit, the ovary containing two fused carpels enlarges to form a silique with localized dehiscence zones at each carpel boundary that split open to release the mature seeds (see Figure 1). By contrast, the ovary of tomato expands after fertilization and the locule spaces around the developing seeds fill with pulp to form this familiar fleshy fruit.

Figure 1 Fertilization and seed set are usually essential for fruit initiation. A) Photo of an open Arabidopsis flower at anthesis (pollen shedding). The sepals (green) partly enclose the petals (white), the stamens (yellow anthers, top), and the central gynoecium consisting of the stigma, style, and ovary. The anthers have just begun to deposit pollen onto the stigma. B) Schematic diagram of a WT Arabidopsis flower with sepals and petals removed. In this essentially self-pollinating species, pollen grains produced by the anther germinate on the stigma to produce pollen tubes (shown in red) containing two sperm cells that grow through the transmitting tract toward the ovules. Once a pollen tube reaches an ovule it enters inside and into the female gametophyte where it releases the sperm cells. Double fertilization occurs as one sperm cell fuses with the egg cell to initiate embryo formation and the other fuses with the central cell nuclei to initiate endosperm development (see Figure 3). At this point, seed set occurs and as embryo and endosperm development proceed, the surrounding ovule tissues differentiate into a seed coat. C) ARF8 prevents fruit initiation in the absence of seed set. The gynoecium of unpollinated WT flowers (left) in which no seed set occurs, exhibit little growth beyond the stage shown in (A). Pollination and seed set promote fruit initiation and subsequent growth (center). By contrast, loss-of-function arf8 alleles allow fruit initiation and growth in the absence of seed set (right). (Click image to enlarge.)

The dependence of fruit initiation on seed set in flowering plants suggests that fertilization-dependent pollen and seed-derived signals are required for fruit initiation and subsequent development (Raghavan 2003). Fruit growth, shape, and form are known to be modified by differences in seed genotype (Denny 1992) and seed number (Sedgley and Griffin 1989). For example, localized but unknown signals from seeds influence silique growth in Arabidopsis (Cox and Swain 2006). Various phytohormones, including gibberellins (GAs), cytokinin, and auxin, are involved in the growth and development of both seeds and fruit (Gustafson 1936; Nitsch 1952; Coombe 1960; Garcia-Martinez and Hedden 1997; Swain et al. 1997; Fos et al. 2000, 2001). Both pollen and developing seeds contain plant hormones, and they may serve as sources of some of these hormones (Nitsch 1970; Eeuwens and Schwabe 1975; Archbold and Dennis 1985; Talon et al. 1990a; Garcia-Martinez et al. 1991; Ben-Cheikh et al. 1997; Ozga et al. 2002). However, movement of hormones from pollen and seeds directly into fruit progenitor tissues has not, to our knowledge, been demonstrated.

Plant Hormones and Fertilization-Independent Fruit Formation

Despite the usual requirement for seed set, fruit development can be uncoupled from fertilization and seed development to generate seedless (parthenocarpic) fruit (Talon et al. 1992; Fos and Nuez 1996; Robinson and Reiners 1999; Varoquaux et al. 2002). Parthenocarpy has a genetic basis (Pike and Peterson 1969; Lin et al. 1984; Potts et al. 1985; de Menezes et al. 2005) and has been exploited by farmers and plant breeders for the production of seedless fruits (Sykes and Lewis 1996). Elevated endogenous phytohormone levels have been observed during parthenocarpic fruit set (George et al. 1984; Talon et al. 1990b, 1992), suggesting that increased supply of phytohormones to fruits from sources other than pollen and seeds may be sufficient to induce fruit growth. Accordingly, parthenocarpy can be induced in Arabidopsis and in diverse agricultural species by the exogenous application of auxins, cytokinins, or GAs (Gillaspy et al. 1993; Vivian-Smith and Koltunow 1999), by increasing auxin levels or response in ovaries and ovules (Rotino et al. 1997; Carmi et al. 2003; Mezzetti et al. 2004), or by increasing GA response (Potts et al. 1985).

What are the Signaling Events Regulating the Transition between A Mature Flower and the Initiation of Fruit and Seed Development?

While the physiological basis for seed and fruit initiation and growth has long been attributed to changes in plant hormone levels, the primary hormone cue and the sequence of signal transduction events leading to the coupling of fruit and seed development have remained unresolved. Recent molecular analyses of fertilization-independent fruit formation in both tomato and Arabidopsis have identified auxin signaling as one of the early events in the fruit initiation cascade. Furthermore, components of the auxin signaling pathway are also involved in repressing fruit initiation until the fertilization cue (Vivian-Smith et al. 2001; Wang et al. 2005; Goetz et al. 2006). Some background information concerning auxin signaling in plants needs to be covered before we can discuss this data further and propose some models concerning the control of fruit initiation.

Auxin Signaling in Plants

Auxin action is mediated in part by targeted protein degradation initiated by the binding of auxin to its receptor, Transport Inhibitor Response1 (TIR1; Dharmasiri et al. 2005; Kepinski and Leyser 2005). In addition to its role as an auxin receptor, TIR1 also functions as the F-box component of a SCF ubiquitin ligase complex that targets a specific class of auxin signaling proteins to the proteasome degradation pathway. These targets are members of the AUXIN/INDOLE-3-ACETIC ACID (Aux/IAA) transcription factor family, which are short-lived proteins sharing four highly conserved domains. Domain I contains a functionally characterised transcriptional repressor motif (Tiwari et al. 2004), while domain II interacts with the SCF complex mentioned above. Domains III and IV act as C-terminal dimerization domains that mediate homo- and heterodimerization between Aux/IAA family members and dimerization with related domains found in AUXIN RESPONSE FACTOR (ARF) proteins (Kim et al. 1997; Ulmasov et al. 1997; Ouellet et al. 2001). ARFs are transcriptional activators/repressors that bind to auxin-response cis- elements found in the promoters of early auxin-responsive genes. ARFs can also influence the transcription of the Aux/IAA family suggesting that auxin signaling is self-regulating (Ulmasov et al. 1997).

Current models for auxin response therefore involve auxin-induced degradation of Aux/IAA proteins (Gray et al. 1999, 2001; Rogg and Bartel 2001; Hellmann and Estelle 2002; Dharmasiri and Estelle 2004; Jenik and Barton 2005), which then allows ARFs, as either homo- or heterodimers and acting as repressors or activators, to regulate expression of downstream genes (Gray et al. 2001; Kepinski and Leyser 2004). These changes ultimately lead to alterations in cell division, extension, and differentiation at the cellular level, and consequently to a wide range of changes at the whole plant level, including fruit set and growth (Figure 2).

Figure 2 Auxin signaling is mediated by Aux/IAA and ARF proteins. Specific Aux/IAA proteins are able to form heterodimers with ARFs as part of complexes that can either directly block transcription of target genes, or act indirectly by preventing the ARF from functioning as a transcriptional activator or repressor. In the example shown here, the protein complex including an Aux/IAA and an ARF blocks transcriptional activation and, in the absence of the Aux/IAA, the ARF acts as an activator of early auxin responsive genes. Auxin acts by binding to its receptor, TIR1, promoting degradation of Aux/IAA proteins via the SCF^TIR1 ubiquitin ligase complex. (Click image to enlarge.)

Auxin Signalling Components and Parthenocarpy

Recent research has begun to integrate our knowledge of auxin signal transduction with the effects of auxin on fruit development. Wang et al. (2005) have used tomato as a model system to investigate the role of a specific nuclear-localized Aux/IAA protein, IAA9, using an antisense (AS) gene silencing approach to specifically reduce IAA9 expression. IAA9 is expressed in most plant organs and like most Aux/IAA genes its expression can be induced by auxin application. Compared with wild-type (WT) plants, several independent AS-IAA9 lines exhibited phenotypic changes consistent with defects in auxin-regulated developmental processes. For example, antisense plants exhibited enhanced hypocotyl/stem elongation, increased leaf vascularization, and reduced apical dominance. AS-IAA9 plants were also more sensitive to applied auxin in a variety of physiological assays and for auxin-induced expression of another Aux/IAA gene of unknown function, IAA3. These phenotypes are consistent with IAA9 acting as a negative regulator of auxin response, consistent with the model in Figure 2.

AS-IAA9 plants also initiated fruit development before fertilization, giving rise to seedless, parthenocarpic fruit in a high proportion of flowers. This phenotype suggests that IAA9 normally functions in part to prevent premature fruit growth and also growth in the absence of fertilization and seed set. Given that Aux/IAA proteins can form protein complexes with ARF proteins, the simplest biochemical models are that either (i) IAA9 interacts with, and inhibits the action of, unknown ARF(s) that act to promote fruit set, and/or (ii) a heterodimer consisting of IAA9 and an unknown ARF(s) acts directly to prevent fruit set prior to fertilization. In both models, the assumption is that a fertilization-dependent burst of auxin promotes degradation of IAA9, via SCF^TIR1 (thereby changing the activity of specific ARF proteins) and allows fruit initiation to occur. Obvious potential sources of auxin in this model are the products of fertilization within the ovules.

Using Arabidopsis as a model system, Goetz et al. (2006) have shown that the FRUIT WITHOUT FERTILIZATION (FWF) locus encodes ARF8, a member of the ARF family described above. Loss-of-function arf8 alleles, including the original fwf (now called arf8-4) mutation, result in parthenocarpic fruit development (Vivian-Smith et al. 2001; see Figure 1). Similar to AS-IAA9 tomato plants, arf8 siliques exhibit precocious growth before fertilization, consistent with early and fertilization-independent fruit initiation. In contrast with AS-IAA9 lines, arf8 mutants closely resemble WT plants with only relatively subtle vegetative and flower phenotypes (Tian et al. 2004; Nagpal et al. 2005). Protoplast transformation experiments support the concept that the transcriptional activity of the ARF8 protein is inhibited by heterodimerization with Aux/IAA proteins (Guilfoyle et al. 1998; Ulmasov et al. 1999a, 1999b; Guilfoyle and Hagen 2001; Rogg and Bartel 2001; Liscum and Reed 2002; Tiwari et al. 2003). Several studies have also demonstrated that ARF8 can mediate auxin- induced gene activation (Ulmasov et al. 1999b; Tiwari et al. 2003; Nagpal et al. 2005), and physical interactions between ARF8 and members of both the Aux/IAA repressor and ARF protein families have been demonstrated (Hardtke et al. 2004; Tatematsu et al. 2004). Thus, while studies have shown that ARF8 transcription is not regulated by auxin per se (Ulmasov et al. 1999a; Pufky et al. 2003; Okushima et al. 2005), it is probable that ARF8 activity is regulated by auxin via degradation of, as yet unidentified, Aux/IAA proteins. This remains to be demonstrated experimentally.

In addition to the biochemical analysis of ARF8 function, genetic evidence also suggests that ARF8 contributes to the regulation of plant development by auxin. Plants with either increased or decreased ARF8 expression possess hypocotyl elongation and root growth phenotypes consistent with altered auxin response, and ARF8 regulates expression of some members of the GH3 gene family, auxin-induced genes which encode putative auxin-conjugating enzymes (Tian et al. 2004). Furthermore, the Arabidopsis genome contains another gene, ARF6, which is very similar to ARF8. Partial functional redundancy exists between these two genes such that the single mutants have subtle defects in stamen development and the arf6 arf8 double mutant is unable to complete flower development (Nagpal et al. 2005). Consistent with a failure in auxin-regulated development, several auxin responsive genes have been identified as candidates for direct regulation by these two ARFs in microarray experiments (Nagpal et al. 2005). An increase in total auxin levels within WT flower buds during various developmental stages was not observed (Nagpal et al. 2005), suggesting that localized changes in auxin levels in specific tissues may be important in regulating ARF8 function.

Although more complex models are possible, the simplest is that both Arabidopsis and tomato possess ARF8- and IAA9-related proteins that physically interact to regulate fruit set in the two species, as indicated in Figure 2. Based on sequence similarity and expression profiles, the closely related Arabidopsis genes AtIAA8 and AtIAA9 are the most likely candidates for orthologs of tomato IAA9. Similarly, tomato contains genes that are similar to—and likely to be—orthologs of Arabidopsis ARF8. Assuming that this model is correct, the genetic analysis of ARF8 function allows the two models of IAA9 action described above to be reevaluated. The observation that ARF8 has an active role in preventing fruit set suggests that the second model, in which an ARF8/IAA9 complex directly represses fruit initiation, is correct. Thus, in the absence of functionally redundant genes, genetic ablation of either protein allows parthenocarpic fruit growth by allowing fruit initiation in the absence of fertilization.

Since fertilization occurs within ovules, and this event is the key trigger for fruit initiation, the ovules represent a likely site of action for IAA9 and ARF8, and for localized changes in auxin levels. Expression has been analyzed in most depth for ARF8, using both in situ hybridization and GUS reporter constructs (Figure 3). ARF8 is regulated at a number of levels, including post-transcriptional control by microRNAs that is essential for ovule and anther function (Wu et al. 2006). The ARF8 translational GUS fusion (ARF8-GUS) displays distinct developmental regulation in floral tissues involved in pollination and fertilization and in the carpel wall (Goetz et al. 2006; Wu et al. 2006). The expression in the ovule before fertilization occurs in the stalk or funiculus and in the embryo sac, where the pollen tube deposits the sperm cells and double fertilization takes place (see Figure 3). After fertilization, the level of expression declines in the developing seed, while in ovules destined to senesce, ARF8-GUS expression spreads throughout the ovule. These results are consistent with the proposed role for ARF8 in restricting signal transduction processes in ovules and preventing pistil growth prior to the fruit initiation cue (Goetz et al. 2006).

Figure 3 ARF8 expression and model for IAA9/ARF8’s role in ovules at fertilization. A,B Schematic model of an Arabidopsis ovule and ARF8 expression, based on an ARF8-GUS translational fusion construct, in an ovary at stage 13 (anthesis) (Goetz et al. 2006). The pattern of expression is illustrated by the shading in (A). The embryo sac contains the egg and central cells, each of which is fertilized by a sperm cell released from the pollen tube. C) Model for IAA9/ARF8 action in ovules following fertilization. The red line represents a pollen tube. The green lines represent the inhibitory effects of the putative IAA9/ARF8 complex and the unknown signal from surrounding floral whorls. The blue lines represent fertilization-dependent auxin that promotes IAA9 degradation. The blue arrows represent the proposed seed-derived signal that promotes fruit initiation and growth. D) In plants either lacking ARF8 with outer floral organs removed (Arabidopsis) or with reduced IAA9 expression (tomato), fertilization is not required to remove the inhibitory signals, and fruit initiation occurs without seed set. (Click image to enlarge.)

Interaction with Other Floral Organs

Given the central role of ovules in fertilization, it is perhaps not surprising that mutations that alter ovule structure can influence parthenocarpy. For example, aberrant testa shape (ATS) is required for the two integument layers of the ovule to form correctly (Leon-Kloosterziel et al. 1994). ATS is the same gene as KANADI4 (KAN4; McAbee et al. 2006), a member of the KANADI family which encodes putative transcription factors that regulate abaxial/adaxial organ identity and vasculature development (e.g., Eshed et al. 2004). Loss-of-function kan4 alleles have a detectable effect only on ovule integument development, and kan4 enhances the arf8 parthenocarpy phenotype (Vivian-Smith et al. 2001). While the mechanism by which KAN4 influences parthenocarpy is not clear, it is presumably by influencing ovule-derived signals that promote fruit initiation.

In addition to ovules, other floral organs also appear to be involved in regulating fruit initiation. For example, one interesting aspect of ARF8′s role in fruit initiation is that, when KAN4 is functional, parthenocarpy is only observed in plants in which pollination is prevented and when the floral organs surrounding the carpel are physically removed (Vivian-Smith et al. 2001). This inhibitory effect does not depend on pollen grain germination or pollen tube growth since parthenocarpic fruit growth is diminished when pollen grain germination is prevented by loss of pollen-pistil interaction1 (POP1) function (Preuss et al. 1993).

Although the effect of the outer floral organs on parthenocarpy in AS-IAA9 lines has not been examined in detail, AS-IAA9 plants appear to be able to produce full-sized parthenocarpic fruit from intact flowers because precocious ovary growth often disrupts stamen positioning and/or function preventing self-pollination and seed set. Although it is possible that the role of the outer floral organs is different between the two species, it is likely that arf8 represents a weaker parthenocarpy phenotype than AS-IAA9. This is consistent with the inability of parthenocarpic arf8 fruit to reach the size of fully-seeded WT fruit (see Figure 1) unless ovule development is also altered by loss of KAN4 function. Based on these observations, we speculate that an interfloral whorl communication mechanism may function in the coordination of pollination with fertilization and fruit growth to maximize the numbers of seeds formed and their dispersal.

Auxin and Fruit Initiation

Based on the results described above, the following model for the role of ARF8/IAA9 during the transition from carpel to fruit growth can be proposed, assuming that IAA9 and ARF8 interact as shown in Figure 2. In addition to its role with ARF6 in flower development prior to pollination and fertilization, ARF8 is bound to the promoters of a range of primary auxin responsive genes that play an essential role in fruit initiation and development. Transcription of these genes is repressed at this stage by a protein complex, including ARF8 bound to the Arabidopsis ortholog(s) of IAA9, which functions as a repressor. As described above, this repressor function of ARF8/IAA9 is likely to occur in ovules (see Figure 3). This model further proposes that in WT flowers a fertilization-induced auxin burst promotes degradation of IAA9, which, in turn, removes the ARF8/IAA9 transcriptional block and allows expression of genes that promote fruit initiation. Whether ARF8 is involved in actively promoting transcription of these genes is not known, but this could explain why parthenocarpic arf8 fruit are smaller than fully-seeded WT fruit. ARF8 is clearly not essential, since fruit initiation occurs in arf8 mutants, but it is possible that ARF8 is functionally redundant in fruit growth with other ARFs, such as the closely related ARF6.

Following removal of the IAA9/ARF8 transcriptional block, an unknown signal is generated that communicates with the rest of the ovary and promotes fruit initiation. While the identity of this signal is not known, possibilities include additional auxin, GA, or cytokinin—all hormones that can promote fruit growth when supplied exogenously to unpollinated flowers. A link between auxin and GA is further supported by the partial dependence of arf8 parthenocarpy on GA activity (Vivian-Smith et al. 2001) and by work with garden peas suggesting that seeds produce a modified auxin which stimulates GA production in the surrounding carpel tissues (Ozga et al. 2002, 2003).

Conclusion

Recent genetic and physiological analyses have allowed the increasingly detailed information on auxin signaling to be linked with the developmental changes associated with seed set and fruit initiation. It appears that flowers use repressive mechanisms involving the auxin signaling pathway to prevent the development of a flower into a fruit. Disruption of the expression and function of components involved in this repression can uncouple fruit formation from pollination and fertilization. Several aspects of the presented model need to be experimentally confirmed. For example, the Aux/IAA partner of Arabidopsis ARF8 and the ARF partner of tomato Aux/IAA9, need to be determined. The sources of auxin in fertilization-dependent fruit and seed growth need identification, with likely candidates including pollen tubes or the initial products of fertilization. Neither the nature of the seed-derived signals communicated to the carpel to initiate fruit growth are known, nor is which of the floral whorls contribute to the inhibition of fruit growth in Arabidopsis. An analysis of how changes in ovule structure influences parthenocarpic fruit growth in Arabidopsis is currently underway as is the influence of ARF8 on fruit growth in tomato and the role of other plant hormones in fruit initiation and growth.

Acknowledgments

Thanks to Adam Vivian-Smith and Marc Goetz for help with Figures 1 and 3, respectively. This work was supported in part by a Horticulture Australia Ltd. grant as part of the Key Genes for Horticultural Markets project.

References

Archbold, D. D., and Dennis, F. G. (1985) Strawberry receptacle growth and endogenous IAA content as affected by growth regulator application and achene removal. J. Am. Soc. Hortic. Sci. 110: 816–820.

Ben-Cheikh, W., Perez-Botella, J., Tadeo, F. R., Talon, M., and Primo-Millo, E. (1997) Pollination increases gibberellin levels in developing ovaries of seeded varieties of Citrus. Plant Physiol. 114: 557–564.

Carmi, N., Salts, Y., Dedicova, B., Shabtai, S., and Barg, R. (2003) Induction of parthenocarpy in tomato via specific expression of the rolB gene in the ovary. Planta 217: 726–735.

Coombe, B. G. (1960) Relationship of growth and development to changes in sugars, auxins, and gibberellings in fruit of seeded and seedless varieties of vitis vinifera. Plant Physiol. 35: 241–250.

Coombe, B. G. (1975) The development of fleshy fruits. Annu. Rev. Plant Physiol. 27: 507–528.

Cox, C. M., and Swain, S. M. (2006) Localised and non-localised promotion of fruit development by seeds in Arabidopsis. Funct. Plant Biol. 33: 1–8.

de Menezes, C. B., Maluf, W. R., de Azevedo, S. M., Faria, M. V., Nascimento, I. R., Nogueira, D. W., Gomes, L. A. A., and Bearzoti, E. (2005) Inheritance of parthenocarpy in summer squash (Cucurbita pepo L.). Genet. Mol. Res. 4: 39–46.

Denny, O. J. (1992) Xenia includes metaxenia. HortScience 27: 722–728.

Dharmasiri, N., and Estelle, M. (2004) Auxin signaling and regulated protein degradation. Trends Plant Sci. 9: 302–308.

Dharmasiri, N., Dharmasiri, S., and Estelle, M. (2005) The F-box protein TIR1 is an auxin receptor. Nature 435: 441–445.

Eeuwens, C. J., and Schwabe, W. W. (1975) Seed and pod wall development in Pisum sativum L. in relation to extracted and applied hormones. J. Exp. Bot. 26: 1–14.

Eshed, Y., Izhaki, A., Baum, S. F., Floyd, S. K., and Bowman, J. L. (2004) Asymmetric leaf development and blade expansion in Arabidopsis are mediated by KANADI and YABBY activities. Development 131: 2997–3006.

Fos, M., and Nuez, F. (1996) Molecular expression of genes involved in parthenocarpic fruit set in tomato. Physiol. Plant. 98: 165–171.

Fos, M., Nuez, F., and Garcia-Martinez, J. L. (2000) The gene, pat-2, which induces natural parthenocarpy, alters the gibberellin content in unpollinated tomato ovaries. Plant Physiol. 122: 471–480.

Fos, M., Proaño, K., Nuez, F., and García-Martínez, J. L. (2001) Role of gibberellins in parthenocarpic fruit development induced by the genetic system pat-3/pat-4 in tomato. Physiol. Plant. 111: 545–550.

Garcia-Martinez, J. L., and Hedden, P. (1997) Gibberellins and fruit development. In Phytochemistry of Fruit and Vegetables, F. A. Tomás-Barberán and R.J. Robins, (eds.). Clarendon Press: Oxford, pp. 263–286.

Garcia-Martinez, J. L., Marti, M., Sabater, T., Maldonado, A., and Vercher, Y. (1991) Development of fertilized ovules and their role in the growth of the pea pod. Physiol. Plant. 83: 411–416.

George, W. L., Scott, J. W., and Splittstoesser, W. E. (1984) Parthenocarpy in tomato. Hortic. Rev. 6: 65–84.

Gillaspy, G., Ben-David, H., and Gruissem, W. (1993). Fruits: A developmental perspective. Plant Cell 5: 1439–1451.

Goetz, M., Vivian-Smith, A., Johnson, S. D., Koltunow, A. M. (2006) AUXIN RESPONSE FACTOR8 is a negative regulator of fruit initiation in Arabidopsis. Plant Cell 18: 1873–1886.

Gray, W. M., Kepinski, S., Rouse, D., Leyser, O., and Estelle, M. (2001) Auxin regulates SCFTIR1-dependent degradation of AUX/IAA proteins. Nature 414: 271–276.

Gray, W. M., del Pozo, J. C., Walker, L., Hobbie, L., Risseeuw, E., Banks, T., Crosby, W. L., Yang, M., Ma, H., and Estelle, M. (1999) Identification of an SCF ubiquitin-ligase complex required for auxin response in Arabidopsis thaliana. Genes Dev. 13: 1678–1691.

Guilfoyle, T. J., and Hagen, G. (2001) Auxin Response Factors. J. Plant Growth Regul. 20: 281–291.

Guilfoyle, T., Hagen, G., Ulmasov, T., and Murfett, J. (1998) How does auxin turn on genes? Plant Physiol. 118: 341–347.

Gustafson, F. G. (1936) Inducement of fruit development by growth promoting chemicals. Proc. Natl. Acad. Sci. USA 22: 629–636.

Hardtke, C. S., Ckurshumova, W., Vidaurre, D. P., Singh, S. A., Stamatiou, G., Tiwari, S. B., Hagen, G., Guilfoyle, T. J., and Berleth, T. (2004) Overlapping and non-redundant functions of the Arabidopsis auxin response factors MONOPTEROS and NONPHOTOTROPIC HYPOCOTYL 4. Development 131: 1089–1100.

Hellmann, H., and Estelle, M. (2002) Plant development: Regulation by protein degradation. Science 297: 793–797.

Jenik, P. D., and Barton, M. K. (2005) Surge and destroy: The role of auxin in plant embryogenesis. Development 132: 3577–3585.

Kepinski, S., and Leyser, O. (2004) Auxin-induced SCFTIR1-Aux/IAA interaction involves stable modification of the SCFTIR1 complex. Proc. Natl. Acad. Sci. USA 101: 12381–12386.

Kepinski, S., and Leyser, O. (2005) The Arabidopsis F-box protein TIR1 is an auxin receptor. Nature 435: 446–451.

Kim, J., Harter, K., and Theologis, A. (1997) Protein-protein interactions among the Aux/IAA proteins. Proc. Natl. Acad. Sci. USA 94: 11786–11791.

Leon-Kloosterziel, K. M., C. J. Keijzer, and Koornneef, M. (1994) A seed shape mutant of Arabidopsis that is affected in integument development. Plant Cell 6: 385–392.

Lin, S., George, W. L., and Splittstoesser, W. E. (1984) Expression and inheritance of parthenocarpy in "Severianin" tomato. J. Hered. 75: 62–66.

Liscum, E., and Reed, J. W. (2002) Genetics of Aux/IAA and ARF action in plant growth and development. Plant Mol. Biol. 49: 387–400.

McAbee, J. M., Hill, T. A., Skinner, D. J., Izhaki, A., Hauser, B. A., Meister, R. J., Venugopala Reddy, G., Meyerowitz, E. M., Bowman, J. L., and Gasser, C. S. (2006) Aberrant testa shape encodes a KANADI family member, linking polarity determination to separation and growth of Arabidopsis ovule integuments. Plant J. 46: 522–531.

Mezzetti, B., Landi, L., Pandolfini, T., and Spena, A. (2004) The defH9-iaaM auxin-synthesizing gene increases plant fecundity and fruit production in strawberry and raspberry. BMC Biotechnol. 4: 4.

Nagpal, P., Ellis, C. M., Weber, H., Ploense, S. E., Barkawi, L. S., Guilfoyle, T. J., Hagen, G., Alonso, J. M., Cohen, J. D., Farmer, E. E., Ecker, J. R., and Reed, J. W. (2005) Auxin response factors ARF6 and ARF8 promote jasmonic acid production and flower maturation. Development 132: 4107–4118.

Nitsch, J. P. (1952) Plant hormones in the development of fruits. Q. Rev. Biol. 27: 33–57.

Nitsch, J. P. (1970) Hormonal factors in growth and development. In The Biochemistry of Fruits and their Products, A. C. Hulme, (ed.). Academic Press: New York, pp. 427–472.

Okushima, Y., Overvoorde, P. J., Arima, K., Alonso, J. M., Chan, A., Chang, C., Ecker, J. R., Hughes, B., Lui, A., Nguyen, D., Onodera, C., Quach, H., Smith, A., Yu, G., and Theologis, A. (2005) Functional genomic analysis of the AUXIN RESPONSE FACTOR gene family members in Arabidopsis thaliana: Unique and overlapping functions of ARF7 and ARF19. Plant Cell 17: 444–463.

Ouellet, F., Overvoorde, P. J., and Theologis, A. (2001) IAA17/AXR3: Biochemical insight into an auxin mutant phenotype. Plant Cell 13: 829–841.

Ozga, J. A., van Huizen, R., and Reinecke, D. M. (2002) Hormone and seed-specific regulation of Pea fruit growth. Plant Physiol. 128: 1379–1389.

Ozga, J. A., Yu, J., Reinecke, D. M. (2003) Pollination-, development-, and auxin-specific regulation of gibberellin 3β-hydroxylase gene expression in pea fruit and seeds. Plant Physiol. 131: 1137–1146.

Pike, L. M., and Peterson, C. R. (1969) Inheritance of parthenocarpy in the cucumber (Cucumis sativus L.). Euphytica 18: 101–105.

Potts, W. C., Reid, J. B., and Murfet, I. C. (1985) Internode length in Pisum: Gibberellins and the slender phenotype. Physiol. Plant. 63: 357–364.

Preuss, D., Lemieux, B., Yen, G., and Davis, R. W. (1993) A conditional sterile mutation eliminates surface components from Arabidopsis pollen and disrupts cell signaling during fertilization. Genes Dev. 7: 974–985.

Pufky, J., Qiu, Y., Rao, M., Hurban, P., and Jones, A. (2003) The auxin-induced transcriptome for etiolated Arabidopsis seedlings using a structure/function approach. Funct. Integr. Genomics 3: 135–143.

Raghavan, V. (2003) Some reflections on double fertilization, from its discovery to the present. New Phytol. 159: 565–583.

Robinson, R. W., and Reiners, S. (1999) Parthenocarpy in summer squash. HortScience 34: 715–717.

Rogg, L. E., and Bartel, B. (2001) Auxin signaling: Derepression through regulated proteolysis. Dev. Cell 1: 595–604.

Rotino, G. L., Perri, E., Zottini, M., Sommer, H., and Spena, A. (1997) Genetic engineering of parthenocarpic plants. Natl. Biotech. 15: 1398–1401.

Sedgley, M., and Griffin, A. R. (1989) Sexual reproduction of tree crops. Academic Press: London.

Swain, S. M., Reid, J. B., and Kamiya, Y. (1997) Gibberellins are required for embryo growth and seed development in pea. Plant J. 12: 1329–1338.

Sykes, S. R., and Lewis, S. (1996) Comparing Imperial mandarin and Silverhill satsuma mandarins as seed parents in a breeding program aimed at developing new seedless citrus cultivars for Australia. Aust. J. Exp. Agric. 36: 731–738.

Talon, M., Hedden, P. and Primo-Millo, E. (1990b) Gibberellins in Citrus sinensis: A comparison between seeded and seedless varieties. J. Plant Growth Regul. 9: 201–206.

Talon, M., Zacarias, L. and Primo-Millo, E. (1990a) Hormonal changes associated with fruit set and development in mandarins differing in their parthenocarpic ability. Physiol. Plant 79: 400–406.

Talon, M., Zacarias, L., and Primo-Millo, E. (1992) Gibberellins and parthenocarpic ability in developing ovaries of seedless mandarins. Plant Physiol. 99: 1575–1581.

Tatematsu, K., Kumagai, S., Muto, H., Sato, A., Watahiki, M. K., Harper, R. M., Liscum, E., and Yamamoto, K. T. (2004) MASSUGU2 encodes Aux/IAA19, an auxin-regulated protein that functions together with the transcriptional activator NPH4/ARF7 to regulate differential growth responses of hypocotyl and formation of lateral roots in Arabidopsis thaliana. Plant Cell 16: 379–393.

Tian, C., Muto, H., Higuchi, K., Matamura, T., Tatematsu, K., Koshiba, T., and Yamamoto, K. T. (2004) Disruption and overexpression of Auxin Response Factor 8 gene of Arabidopsis affect hypocotyl elongation and root growth habit, indicating its possible involvement in auxin homeostasis in light condition. Plant J. 40: 333–343.

Tiwari, S. B., Hagen, G., and Guilfoyle, T. (2003) The roles of Auxin Response Factor domains in auxin-responsive transcription. Plant Cell 15: 533–543.

Ulmasov, T., Hagen, G., and Guilfoyle, T. J. (1997) ARF1, a transcription factor that binds to auxin response elements. Science 276: 1865–1868.

Ulmasov, T., Hagen, G., and Guilfoyle, T. J. (1999a) Dimerization and DNA binding of Auxin Response Factors. Plant J. 19: 309–319.

Ulmasov, T., Hagen, G., and Guilfoyle, T. J. (1999b) Activation and repression of transcription by Auxin-Response Factors. Proc. Natl. Acad. Sci. USA 96: 5844–5849.

Varoquaux, F., Blanvillain, R., Delseny, M., and Gallois, P. (2002) Less is better: New approaches for seedless fruit production. Trends Biotechnol. 18: 233–242.

Vivian-Smith, A., and Koltunow, A. M. (1999) Genetic analysis of growth-regulator-induced parthenocarpy in Arabidopsis. Plant Physiol. 121: 437–452.

Vivian-Smith, A., Luo, M., Chaudhury, A., and Koltunow, A. (2001) Fruit development is actively restricted in the absence of fertilization in Arabidopsis. Development 128: 2321–2331.

Wang, H., Jones, B., Li, Z., Frasse, P., Delalande, C., Regad, F., Chaabouni, S., Latché, A., Pech, J.-C., and Bouzayen, M. (2005) The tomato Aux/IAA transcription factor IAA9 is involved in fruit development and leaf morphogenesis. Plant Cell 17: 2676–2692.

Wu, M.-F., Tian, Q., and Reed, J. W. (2006) Arabidopsis microRNA167 controls patterns of ARF6 and ARF8 expression, and regulates both female and male reproduction. Development 133: 4211–4218.