Intermittent pollen-tube growth in pistils of alders ( Alnus )

Abstract

In alders, where fertilization occurs Ϸ8 weeks after pollination, the pollen tube (male gametophyte) grows intermittently in four steps in close association with the development of the ovary and its ovules. Pollen tubes stop growing in the style, at the ovarian locule, and at the chalaza (ovule), before reaching an embryo sac for fertilization. At the stage when the ovary develops an ovule primordium in each of the two locules, many pollen tubes germinate on the stigma, and a few of them reach the style, where they remain for Ϸ7 weeks. Thereafter, a single tube resumes growing; with a short stop in the upper space of the ovarian locule, it reaches the older of the two ovules when it has developed a two-nucleate embryo sac. Except in the last step, where the tube grows from the chalaza to an embryo sac (female gametophyte), an eight-nucleate mature embryo sac is not necessary for pollen-tube guidance in the pistil. Although the intermittent pollen-tube growth appears to play an important role in the selection of a single pollen tube from many and one ovule from two, its detection provides insight into the study of the mechanism of pollen-tube guidance.Betulaceae ͉ chalazogamy ͉ Fagales ͉ fertilization ͉ ovule I n flowering plants, fertilization occurs when the pollen tube (male gametophyte) reaches the embryo sac (female gametophyte) through the tissue of the pistil (sporophyte), unifying a sperm nucleus with an egg nucleus. This process suggests interaction not only between the pollen tube and the embryo sac, but also between the pollen tube (and embryo sac) and an intervening tissue, i.e., the tissue of the pistil. Recent studies using mutants of Arabidopsis thaliana that lacked normally developed ovules or embryo sacs showed that the pollen tubes lost their way to the target within the pistil (1-3). However, these studies mainly paid attention to interaction between pollen tubes and the embryo sac, and analysis of the correlation between the pollen-tube growth and the developmental stage of the pistils is still scarce.In a majority of cases, the length of time between pollination and fertilization ranges from 24 to 48 h, and in some plants it is even shorter (4-8). In fagalean species, however, the pollen tube spends Ͼ1 month in the pistil before fertilization occurs, because the ovules are not yet mature when the pistil receives pollen grains on the stigma (9-11). We recently reported the pollentube growth pattern in Casuarinaceae (Fagales), where a unique mode of fertilization (i.e., chalazogamy) in place of the ordinary mode (i.e., porogamy) was reported (12, 13) (for details regarding chalazogamy and porogamy, see Fig. 5A, which is published as supporting information on the PNAS web site). In Casuarina equisetifolia (12), fertilization occurs in five discontinuous steps of pollen tube growth from the stigma to the embryo sac. We suggested that the pollen tube exhibited repeated steps of growth in accordance with the development of the ovary and ovule(s). However, C. equisetifolia develops four to eight emb...

Glycoprotein composition along the pistil of Malus x domestica and the modulation of pollen tube growth

“…This increase in the rate of growth was previously reported in other species including Petunia [15], peach [9], kiwifruit [50], and apricot [70]. Pollen tube growth in the style also appears to be faster than in the ovary [11,71], and these differences have been related to the requirement for maturation of ovary structures. The acceleration of pollen tube growth rate in the style could be due to developmental events within the pollen tube itself, such as the deposition of the first callose plug, the completion of mitosis II, or by an internally programmed shift to heterotrophic growth.…”

“…The pollen tube traverses different tissues within the pistil on its path to the embryo sac, and the recorded pollen tube growth rate is faster in the style compared to the stigma or the ovary [8,9]. As observed in different species such as peaches [10] or alders [11], slow growth in the ovary has been related to stops and decelerations resulting from the wait for particular structures -the obturator or the ovules- to become receptive to the pollen tube. Indeed, male–female synchrony appears to be a prerequisite for successful fertilization [12].…”

Female gametophyte-controlled pollen tube guidance

“…It has been shown that, before entering the ovary, PT guidance requires intensive communication with the surrounding sporophytic maternal tissues, and the presence of the FG seems not to be necessary for the largest part of the PT's journey [2][3][4]. However, after exit from the TT tissue and emergence on the surface of the placenta, PT guidance is thought to be primarily under gametophytic control, although the maternal sporophytic Key words: Arabidopsis, maize, micropyle, pollen tube guidance, signalling, synergid, Torenia.…”

Pollen Tube Guidance by Attractant Molecules: LUREs

“…Pollen tubes can grow through the stigma and style even in a pistil that lacks a female gametophyte (Hülskamp et al, 1995). This has been demonstrated in a semi-in vitro system using a cut pistil from which the ovary was removed (Higashiyama et al, 1998) and occurs in plants where megasporogenesis occurs after pollination (Sogo and Tobe, 2005). These results suggest that the female gametophyte is not necessary for pollen-tube guidance from the stigma to the base of the style in many species.…”