Fertilization recovery after defective sperm cell release in Arabidopsis

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. Plant fertilization has thus been thought to depend on the fertility of a single pollen tube. 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, one of which degenerates upon pollen tube discharge. By semi-in vitro live-cell imaging 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.