Abstract
Author SummarySperm cells of animals and lower plants are mobile and can swim to the oocyte or egg cell. In contrast, flowering plants generate immobile sperm encased in a pollen coat to protect them from drying out and are transported via the pollen tube cell towards the egg apparatus to achieve double fertilization. Upon arrival the pollen tube tip bursts to deliver two sperm cells, one fusing with the egg cell to generate the embryo and the other fusing with the central cell to generate the endosperm. Here, we report the mechanisms leading to pollen tube burst and sperm discharge in maize. We found that before fertilization the defensin-like protein ZmES1-4 is stored in the secretory zone of the egg apparatus cells and that pollen tubes cannot discharge sperm in ZmES1-4 knock-down plants. Application of chemically synthesized ZmES4 leads to pollen tube burst within seconds in maize, but not in other plant species, suggesting this mechanism may be species specific. Finally, we identified the pollen tube-expressed potassium channel KZM1 as a target of ZmES4, which opens after ZmES4 treatment and probably leads to K+ influx and sperm release after osmotic burst.
Highlights
Flowering plants emerged some 180–140 million years ago (MYA) [1] and have since inhabitated most ecological environments, which are often far away from humid conditions
We found that before fertilization the defensin-like protein ZmES1-4 is stored in the secretory zone of the egg apparatus cells and that pollen tubes cannot discharge sperm in ZmES1-4 knock-down plants
We identified the pollen tube-expressed potassium channel K+ channel Zea mays 1 (KZM1) as a target of Zea mays embryo sac 4 (ZmES4), which opens after ZmES4 treatment and probably leads to K+ influx and sperm release after osmotic burst
Summary
Flowering plants (angiosperms) emerged some 180–140 MYA [1] and have since inhabitated most ecological environments, which are often far away from humid conditions. Adaptive selection led to the reduction of the haploid male gametophyte to a three-cellular pollen grain and pollen tube, respectively, which is able to be transported over long distances and to grow deeply inside female reproductive tissues. The central events preceding fertilization involve signaling towards the pollen tube to arrest growth and to induce discharge of the two sperm cells, a process first described by the famous German-Polish botanist Eduard Strasburger in 1884 [4]. It took more than 120 years to identify the first molecular players involved in these processes
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