The landing of pollen on a compatible stigma marks the beginning of pollination and fertilization. The pollen will then adhere, hydrate, and germinate to produce the pollen tube, a structure specialized in the delivery of the sperm cells to the ovule. In a genome-wide transcriptomic approach using stigmatic tissue from rice, it was found that several auxin-signaling genes were overrepresented in the stigma gene set (Li et al. 2007
). In agreement with this, a very detailed study of auxin distribution in the stigma and style of Nicotiana tabacum
suggests that auxin distribution plays an important role in the pollination process (Chen and Zhao 2008
). In this study, high levels of free auxin in stigmas and styles during pollen germination and pollen-tube growth were detected by immunolocalization. The auxin levels (immuno detection) and response (DR5::GUS
) increased in the part of the transmitting tract where pollen tubes would enter and then rapidly declined in the part where pollen tubes had penetrated, suggesting that auxin is important for growth of the pollen tube tip.
Although these results indicate that fruit initiation is coordinated by phytohormone biosynthesis during seed development, it is only now that molecular data to support this idea are beginning to emerge. In a facultative parthenocarpic Arabidopsis
mutant, dramatically reduced IAA levels were found in unpollinated versus pollinated fruits, even though the unpollinated grew to almost the same length (Fuentes and Østergaard, unpubl.). A detailed study showed that the only difference between these fruits is the presence or absence of seeds, suggesting that the seeds are the main source of auxin (Fuentes et al., submitted). In another example, it was recently reported that gibberellin (GA) biosynthesis genes are up-regulated on pollination of pea fruits and that this regulation depends on the presence of seeds (Ozga et al. 2009
). However, the auxin 4-chloro-IAA (4-Cl-IAA) was found to mimic this effect in the absence of fertilization, thereby directly confirming previous results that auxin and gibberellin pathways interact to mediate fruit initiation (Sastry and Muir 1963
; Koshioka et al. 1994
). Moreover, Ozga et al. (2009)
also found that GA biosynthesis was induced by applying 4-Cl-IAA to deseeded pericarp, supporting the hypothesis that auxin is synthesized in the seed on fertilization and transported to the pericarp to induce GA-mediated fruit growth.
Central to GA signaling are DELLAs, nuclear proteins characterized by a conserved DELLA-motif in their amino-terminal domain (Peng et al. 1997
; Silverstone et al. 1998
). According to the “relief of restraint” model (Harberd 2003
), DELLA proteins act as growth repressors, and GA-mediated DELLA degradation is required to overcome this restraint. The crucial role of DELLA proteins in GA signaling is well understood and their role in fruit development has recently been identified by Marti et al. (2007)
, Dorcey et al. (2009)
and Fuentes et al. (submitted), who showed that lack of DELLA proteins results in parthenocarpic fruit development.
Based on the molecular data described previously, a model is emerging for fruit initiation in which auxin is synthesized in the ovules on fertilization and then transported to the pericarp, where it induces GA biosynthesis. In turn, the newly synthesized GA will lead to DELLA degradation and thereby relief of growth repression to allow fruit elongation (). This description is highly simplified, and there are most certainly additional layers of regulation. For example, it has been shown that a threshold level of GA in the gynoecium is required to initiate the auxin biosynthesis, providing a feedback circuit (Vivian-Smith and Koltunow 1999
). Further elucidation of the hormonal interactions during fruit initiation will provide exciting insight into the setup of fruit development.
Figure 3. Model for hormonal activities on fertilization. The figure shows schematic cross sections of an Arabidopsis gynoecium. Before fertilization (top), DELLA proteins repress growth and elongation of the ovary. On fertilization (bottom), auxin (IAA) is produced (more ...)
Parthenocarpy in Crops
There are major advantages of parthenocarpy in crops. For example, fruit seedlessness is advantageous for fruit processing (e.g., tomato paste), it can improve fruit quality (e.g., eggplant), or seedlessness can simply be a feature appreciated by consumers (e.g., grape and melon). Moreover, in parthenocarpic plants, fruit set and production is less affected by environmental factors adverse for pollination and fertilization.
Parthenocarpy can either be elicited by artificial means or via genetic modifications. In agriculture, auxin and gibberellin are the most common phytohormones used to trigger artificial parthenocarpy. Genetic parthenocarpy can be either facultative or obligate. In facultative parthenocarpy, the fruits are seedless only when grown under conditions adverse for pollination and/or fertilization or when the flowers are emasculated. Parthenocarpy is obligate when the fruit is always seedless. In the past, obligate and facultative genetic parthenocarpy was either obtained by alterations of ploidy level or caused by mutations. In recent years, parthenocarpy has also been achieved via transgenesis, as described in the following.
Auxin synthesis within unpollinated ovary has been achieved by expressing the iaaM
gene from Pseudomonas syringae
under control of the DefH9
(Deficiens Homolog 9) promoter from Antirrhinum majus
(Rotino et al. 1997
), conferring placenta–ovule-specific expression of tryptophan-2-monooxygenase, known to convert l
-tryptophan to indole-3-acetamide (Kosuge et al. 1966
). Within plant cells, indole-acetamide is converted by either spontaneous or enzymatic hydrolysis to indole-3-acetic acid (IAA) and ammonia. The DefH9-iaaM
construct has been used to induce parthenocarpy in several crop plant species (eggplant, tobacco [Rotino et al. 1997
]; tomato [Ficcadenti et al. 1999
]; raspberry, strawberry [Mezzetti et al. 2004
]; cucumber [Yin et al. 2006
]). When cultivated in vitro, wild-type preanthesis tomato flowers need exogenous auxin to develop fruits (Nitsch 1952
), whereas DefH9-iaaM
preanthesis flower buds develop fruits in medium not supplemented with auxin. Altogether, these data show that enhanced auxin synthesis within the placenta/ovules triggers parthenocarpic fruit development in species belonging to distinct botanical families (e.g., Solanaceae
, and Cucurbitaceae
) and with morphologically different fruit types (e.g., tomato, raspberry, and tobacco). Auxin-synthesis parthenocarpy is facultative, because fruits are seedless only under conditions prohibitive for fertilization/pollination.
Parthenocarpy can also be achieved by modifying auxin signaling. Auxin-dependent transcriptional regulation is controlled by the Aux/IAA
and AUXIN RESPONSE FACTOR
) gene families (Leyser 2006
; Guilfoyle and Hagen 2007
). In the absence of auxin, Aux/IAA proteins bind ARF transcription factors to prevent expression of auxin-response genes. Presence of auxin induces Aux/IAA degradation, thereby allowing the ARFs to activate gene expression. Manipulation of the activity of specific Aux/IAA and ARF family members has now been reported to affect fruit development. In Arabidopsis
, parthenocarpy has been obtained by genetic alterations of ARF8 function (Vivian-Smith et al. 2001
; Goetz et al. 2006
; Goetz et al. 2007
). ARF8 is an ovule-specific transcription factor that negatively regulates fruit set, and ARF8
gene expression is therefore normally turned off after fertilization (Goetz et al. 2006
; Goetz et al. 2007
). In tomato, the SlARF7
gene is expressed in placental and ovule tissues and down-regulated soon after pollination. Silencing of the SlARF7
gene leads to parthenocarpic fruit development, showing that SlARF7
functions as a negative regulator of fruit set (de Jong et al. 2009
). Similarly, silencing of the IAA9
gene expression also conferred parthenocarpy (Wang et al. 2005
). The down-regulation of IAA9
mRNAs most likely mimics the degradation of IAA9 protein caused by the increased auxin content of the ovules/ovary that follows fertilization.
In addition to auxin, gibberellin-based approaches have also been used to exploit genetic parthenocarpy in crops. One example is the tomato pat-3
line, which was reported to induce parthenocarpic capacity in fruits by increasing the levels of active GAs in the ovary before fertilization (Fos et al. 2001
). In a more directed approach, Marti et al. (2007)
obtained parthenocarpic tomatoes by RNA silencing of a DELLA
gene, providing an additional tool in the box to control this important trait.
The observations in this section show that genetic parthenocarpy can be obtained by manipulating hormone dynamics in various ways in most plant species. Genetic parthenocarpy is advantageous over artificial parthenocarpy because it is environmentally friendlier than spraying hormones in the field and it is uniform and easier to control. Moreover, this area of research provides an illustrative example of how fundamental research that has been invested to understand the biology of hormone activity can be hugely beneficial for crop improvement programs.