Papers of the month (July 2012)
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Two exciting research articles in Current Biology have shed light on an intriguing question: what is the temporal relationship between pollen tube repulsion and double fertilization? These two elegant articles have revealed that as long as double fertilization between female gametes and sperm cells have not occurred, pollen tube repulsion by ovules does not initiate, allowing multiple pollen tubes to enter an ovule.
1. Gamete Fusion Is Required to Block Multiple Pollen Tubes from Entering an Arabidopsis Ovule
Kristin M. Beale, Alexander R. Leydon, Mark A. Johnson
Department of Molecular Biology, Cell Biology, and Biochemistry, 185 Meeting Street, Brown University, Providence, RI 02912, USA
http://dx.doi.org/10.1016/j.cub.2012.04.041,
Reference: Beale et al (2012) Current Biology , Vol. 22 no.12, pp. 1090–1094
Abstract
In double fertilization, a reproductive system unique to flowering plants, two immotile sperm are delivered to an ovule by a pollen tube. One sperm fuses with the egg to generate a zygote, the other with the central cell to produce endosperm [1]. A mechanism preventing multiple pollen tubes from entering an ovule would ensure that only two sperm are delivered to female gametes. We use live-cell imaging [ [1] and [2]] and a novel mixed-pollination assay that can detect multiple pollen tubes and multiple sets of sperm within a single ovule to show that Arabidopsis efficiently prevents multiple pollen tubes from entering an ovule. However, when gamete-fusion defective hap2(gcs1) or duo1 sperm are delivered to ovules, as many as three additional pollen tubes are attracted. When gamete fusion fails, one of two pollen tube-attracting synergid cells persists, enabling the ovule to attract more pollen tubes for successful fertilization. This mechanism prevents the delivery of more than one pair of sperm to an ovule, provides a means of salvaging fertilization in ovules that have received defective sperm, and ensures maximum reproductive success by distributing pollen tubes to all ovules.
2. Fertilization recovery after defective sperm cell release in Arabidopsis.
Kasahara RD, Maruyama D, Hamamura Y, Sakakibara T, Twell D, Higashiyama T.
Division of Biological Science, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan; JST ERATO Higashiyama Live-Holonics Project, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan.
Reference: Kasahara et al (2012) Current Biology , Vol. 22 no.12, pp. 1084-1090.
Abstract
In animal fertilization, multiple sperms typically arrive at an egg cell to "win the race" for fertilization. However, in flowering plants, only one of many pollen tubes, conveying plant sperm cells, usually arrives at each ovule that harbors an egg cell [1, 2]. Plant fertilization has thus been thought to depend on the fertility of a single pollen tube [1]. Here we report a fertilization recovery phenomenon in flowering plants that actively rescues the failure of fertilization of the first mutant pollen tube by attracting a second, functional pollen tube. Wild-type (WT) ovules of Arabidopsis thaliana frequently (∼80%) accepted two pollen tubes when entered by mutant pollen defective in gamete fertility. In typical flowering plants, two synergid cells on the side of the egg cell attract pollen tubes [3-5], one of which degenerates upon pollen tube discharge [3, 6]. By semi-in vitro live-cell imaging [7, 8] we observed that fertilization was rescued when the second synergid cell accepted a WT pollen tube. Our results suggest that flowering plants precisely control the number of pollen tubes that arrive at each ovule and employ a fertilization recovery mechanism to maximize the likelihood of successful seed set.
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