Characterization of the pollen growth transition in self-incompatible Petunia inflata

Abstract

In Petunia inflata, as in other species that shed bicellular pollen, early pollen tube growth in the pistil is slow, then increases 2-to 5-fold depending on the genotype of the female parent. We refer to the time point at which pollen tubes enter the accelerated phase of growth as the pollen growth transition (PGT). Here, we present evidence that pre-PGT and post-PGT growth are quantitatively and qualitatively different, and that the PGT is triggered when pollen tubes reach the transition zone (TZ) below the stigma. The capacity of various pistil zones to precipitate the PGT was tested through 'stump' pollinations: varying lengths of the pistil apex were excised, the cut surface of the remaining pistil (the stump) coated with stigmatic exudates then dusted with compatible pollen. Pollen applied to TZ tissues entered the PGT earlier than pollen growing in intact control pistils; the PGT was delayed in stylar stumps, largely because of delayed germination and reduced pre-PGT growth. In immature pistils, the PGT was delayed by several hours relative to its onset in mature pistils. The PGT fails to occur in pollen cultured in vitro. Collectively, the data suggest that pollen tubes become competent to enter the PGT when they reach a critical size, but the physicochemical environment of the transmitting tissue is necessary for triggering the cellular changes that result in accelerated growth. An analysis of the distribution of pollen tube tips before and after the PGT suggests that pollen competition is most intense during the pre-PGT phase.

Economy, efficiency, and the evolution of pollen tube growth rates

“…We found that N. odorata tubes had significantly thicker walls than did B. schreberi . Wall thickness plays a role in resisting turgor pressure (Styles and Singla, 1987; Lubliner et al, 2003; Parre and Geitmann, 2005b), consistent with our finding that tube walls of N. odorata were thinner when growing within semisolid tissue than within secretions.…”

Winning in style: Longer styles receive more pollen, but style length does not affect pollen attrition in wild Clarkia populations

“…The positive correlation between style length and the number of pollen grains adhering to the stigma may result in a greater opportunity for selection among male gametophytes in flowers with relatively long styles. Large pollen loads may result in higher rates of pollen germination success than small ones (e.g., Brewbaker and Majumder, 1961), and subsequent selection among male gametophytes may be particularly strong as pollen tubes grow from the stigma to the upper style (Erbar, 2003; Lubliner et al, 2003; Stephenson et al, 2003). Consequently, plants with relatively long styles could benefit if receiving more pollen increases seed quality by increasing selection against poor quality male gametophytes during pollen germination and penetration of the stigma.…”

HD-AGPs as Speciation Genes: Positive Selection on a Proline-Rich Domain in Non-Hybridizing Species of Petunia, Solanum, and Nicotiana

“…There is biphasic growth of conspecific pollen tubes in the pistils of Petunia , with a pronounced acceleration of the growth rate by 12 h post-pollination [25]. Biphasic pollen tube growth is seen in P. axillaris pistils after conspecific pollination, but P. exserta pollen also performs well in P. axillaris pistils and their pollen tubes display the same pattern of biphasic growth as conspecific pollen tubes (Figure 4).…”

“…The heterospefic pollen continues to grow slowly for at least 72 h, by which time pollinated flowers start to exhibit floral senescence. Seed set fails in this interspecific pollination presumably because the pollen tube growth transition [25] fails to occur. The dynamics of heterospecific pollen tube growth are consistent with incongruity between P. axillaris pistils and P. integrifolia pollen, and it appears to be the underlying cause of the reproductive barrier between the two species when the former is the pistillate partner.…”

“…In compatible, conspecific pollination, pollen tube growth in Petunia pistils is biphasic, with a relatively slow growth rate in the first 8 h after pollination, followed by a rapid growth phase that coincides with the entry of pollen tubes into the style [25,48]. As seen in Figure 4, P. integrifolia pollen tubes grow more slowly in P. axillaris pistils, and the transition to rapid growth in the style fails to occur in this type of pollination.…”