Discovery of pollen tube-dependent ovule enlargement morphology phenomenon, a new step in plant reproduction

In animals, when semen is discharged into the uterus, the seminal plasma carries the sperm to the egg. In plants, the function of pollen tube contents (PTC) is analogous to that of the seminal plasma in animals, i.e., carrying sperm cells to the ovules for fertilization. Because the function of the seminal plasma is essential for fertilization in animals, we propose that the function of PTC must be important for plant fertilization. To understand the function of PTC, we examined the transcriptional variation after the release of PTC into the embryo sac. The phenotypic analysis revealed that ovules were enlarged without fertilization when the PTC was released into the ovule, entirely consistent with the transcriptome analysis. We identified a new plant phenomenon, pollen tube-dependent ovule enlargement morphology (POEM) phenomenon that occurs only when the ovule accepts PTC, irrespective of fertilization. POEM is a new phase between the pollen tube guidance and fertilization phases as a reproductive step. Here we established the in vitro POEM assay, which effectively measures POEM activity. Using this assay, we identified that a simple dose of plant hormone(s) cannot induce POEM. We also showed that this assay could be a powerful tool for identifying POEM factor(s).