Female receptivity and the cessation of gynoecial growth

Often in horticulture, two or more varieties are planted together in a specific planting or orchard design to maximize cross-pollination for hybrid production or yield. The female receptive period is an important final component of the floral maturation process and has a direct bearing on fruit set and initiation, since viable female and male components must both exist in space and time, while the plant sufficiently conserves essential resources. The receptive period has also been referred to as the effective pollination period (EPP) and is the mutual or partial sum of the longevity for the stigma, style and ovule, while taking into account the time taken for the pollen tube to grow and fertilize the ovule (Williams, 1966; reviewed Sanzol and Herrero, 2001; Page et al., 2006). During the maturation and receptive periods, specific molecular pathways restrict the growth of the pistil and accessory tissues and, thus, stop them from developing into fruit.

Despite the wealth of data in crop species and extensive analysis of female gametophyte development, female receptivity has not been the subject of any significant genetic analysis. Female receptivity can be quantified by emasculating flowers before anthesis and allowing a sample of flowers to be pollinated on successive days post-anthesis (Williams, 1966). Final seed set reflects the period when pistils were most receptive to pollination. As a rule, pollen-tube growth on floral tissues is temperature dependent and different results would be expected if pistils were incubated at sub-optimal temperatures for stigmatic receptivity, pollen-tube development or ovule longevity (Sanzol and Herrero, 2001). Often the optimum for pollen-tube elongation and the optimum for support on the female component do not coincide, and the optima are usually higher for pollen-tube growth (Hedhly et al., 2005a,b).

In Arabidopsis, female receptivity, as assessed by seed set, lasts up to 3 days post-anthesis and effectively correlates with the integrity of the female game-tophyte which deteriorates shortly thereafter (Christensen et al., 1998; Vivian-Smith and Koltunow, 1999; Vivian-Smith and Offringa, unpublished). There are marked differences in female receptivity duration between the ecotypes Landsberg and Columbia (Vivian-Smith and Koltunow, 1999). Nevertheless, the receptivity periods in Arabidopsis ovules are significantly shorter than the period the pistil remains receptive to exposures of 10 nmol GA3 that stimulate fruit development (Vivian-Smith and Koltunow, 1999). A longer period of gibberellin perception suggests that the viability of the gametophyte and ovule is completely independent to the perception and signalling of a GA3-mediated growth in the pistil and that the gibberellin-mediated restriction maybe directly occurring in the carpel.

Mutations in the Auxin Response Factor 8 gene (ARF8), which lead to parthenocarpic fruit initiation, dramatically shorten the duration of female receptivity and lead to reduced seed set (Vivian-Smith et al., 2001). ARF8 mutants also initiate fruit development precociously and the pistil protrudes far enough to prevent proper contact between the stigma and anthers to effect proper self-pollination (Vivian-Smith et al., 2001). Taken alone, however, the arf8 mutant data may suggest an indirect link with female receptivity. On the contrary, mutations in ARF8 together with the related gene ARF6 lead to complete sterility and dramatically prevent flower maturation in numerous aspects (Nagpal et al., 2005; Wu et al., 2006). This data suggest a global role for both genes in flower maturation, female receptivity and pollen-tube growth.

Distinct genetic pathways halt further development of the egg cell and the central cell at maturity and this has been demonstrated with the use of Arabidopsis gametophytic and sporophytic mutants (see sections below). Evidence that the female gametophyte reciprocally exerts control over the developing sporophyte comes from transcriptional profiling studies where mutants lack a viable gametophyte (Johnston et al., 2007). Significant modulation of the sporophytic genes has been observed for SUPERMAN (SUP), Small Auxin Upregulated RNA (SAUR), C3HC4-type RING finger proteins, the homeobox gene SHOOT MERISTEMLESS (STM) and the STYLISH2 (STY2)

transcription factor. However, these genes are just a few examples amongst 527 genes identified (Johnston et al., 2007). Many of these genes could be candidates linking female receptivity, the female gametophyte and the regulation of fruit initiation.

Other studies have also implicated the phytohormone cytokinin in game-tophyte development and maintenance of receptivity. Pischke et al. (2002) and Hejatko et al. (2003) demonstrated that CKI is expressed in the female gametophyte until fertilization and it is essential for gametophyte viability. Previous research showed that overexpression of CKI results in cytokinin independency in somatic tissues (Kakimoto, 1996; Glover et al., 2008). If CKI functions in a similar manner in the female gametophyte, it may play a significant role in maintenance of gametophyte viability via a cytokinin-related pathway. Female receptivity can also be positively influenced by the application of nitrogen fertilizer (Williams, 1965; Tromp et al., 1994), and by stigmatic secretions induced by pollination, that can help release carbohydrates from the transmitting tissue and prolong embryo sac viability (Herrero, 1992).

While the beginning of female receptivity is demarcated by the period when pollen tubes can grow on the stigma, style and transmitting tissue (Kandasamy et al., 1994), the end of female receptivity is onset by an irreversible initiation of floral senescence (O'Neill, 1997; O'Neill and Nadeau, 1997; Lewis et al, 2006).

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