WASHINGTON - In a landmark discovery, scientists at the University of California, Davis, have found that a hormone called auxin is responsible for egg production in plants.
It is the first definitive study of a plant hormone acting as a morphogen-a substance that directs the pattern of development of cells based on its concentration.
In addition, the results provide tantalizing new insights into the evolutionary pathway that flowering plants took 135 million years ago when they split off from gymnosperms-the “naked-seeded” plant group that includes conifers, cycads and ginkgo trees.
The researchers also used their discovery to make additional egg cells within plant reproductive structures, raising the prospects that these techniques might someday be used for enhancing the reproduction and fertility of crop plants.
“So the sequence becomes clear now. The plant triggers auxin synthesis at one end of the female reproductive unit called the embryo sac, creating an auxin gradient. The eight nuclei in the sac are then exposed to different levels of auxin, but only the nucleus in the correct position in the gradient becomes an egg cell. And that cell is subsequently fertilized to make the next generation,” said Venkatesan Sundaresan, the UC Davis professor of plant biology and plant sciences who led the study.
Earlier, the research team used genetic tools to shift the position of a single nucleus at one end of an embryo sac in the plant Arabidopsis.
Upon examining the mature sac, they found that it had produced two egg cells instead of one.
They recognised that a pattern shift like this was similar to the response that had been reported two decades earlier in Drosophila fruit flies in experiments that provided the first direct evidence for the existence of morphogens.
Thus, Sundaresan began searching for a substance in Arabadopsis that might be acting as a morphogen.
They then discovered that auxin was accumulating at the open end of the ovule, and turned their attention to this ubiquitous hormone.
After multiple tests, the researchers found that during embryo sac formation, auxin concentrations did indeed follow a gradient, with the highest levels occurring in the ovule at the end of the embryo sac where the pollen enters and lowest levels occurring at the opposite end of the sac.
They confirmed their hypothesis after varying auxin concentrations in the embryo sac of the plant.
They found that auxin concentrations determined the fate of the nuclei and, depending on whether auxin levels were high or low, they could predict the appearance or disappearance of egg cells at different positions within the embryo sac.
Finally, the group employed a long series of bio-manipulative techniques to determine that the auxin gradient they had discovered within the embryo sac was due to on-site synthesis rather than transport from a source outside the sac.
“What we have found about the way auxin works here is amazing. The idea that you can have a small molecule like this being maintained in a gradient within this eight-nucleate structure through synthesis alone is mind-boggling,” said Sundaresan.
The study has been published in the journal Science’s online site, Science Express. (ANI)
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