The group’s groundbreaking discovery that a plant hormone called auxin is responsible for egg production has several major implications.
First, this is the first definitive report of a plant hormone acting as a morphogen, that is, a substance that directs the pattern of development of cells based on its concentration.
Also, the study’s 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.
Finally, the group used their discovery to make additional egg cells within plant reproductive structures, raising the prospects that these techniques may someday be used for enhancing the reproduction and fertility of crop plants.
“So the sequence becomes clear now,” said Venkatesan Sundaresan, the UC Davis professor of plant biology and plant sciences who led the study. “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.”
A paper describing the study was published June 4 in the journal Science’s online site, Science Express, in advance of its publication in the journal later this month.
At the start of the process of egg-cell development, a “mother cell” in the ovule divides several times, in a sequence involving both meiosis and mitotic divisions. These divisions result in the creation of an oblong, cell-like structure called the embryo sac, which contains eight nuclei, three of which are clustered near the open end of the ovule.
Within hours cell membranes start forming, eventually, creating seven cells: the all-important egg cell near the ovule opening where pollen will enter, and six other supporting cells, with essential functions for seed formation.
“The big question in our field for the past 50 years or more has been: How does this process happen in such a beautifully orchestrated pattern?” Sundaresan said. “It’s been clear that there’s a program here telling the plants exactly what to do, and that it is working not on cells, but on nuclei.”Auxin concentrations determine fate of nuclei
Sundaresan recognized 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.
This prompted him to begin searching for a substance in Arabadopsis that might be acting as a morphogen. When the group discovered that auxin was accumulating at the open end of the ovule, they turned their attention to this ubiquitous hormone, which is known to play myriad signaling roles in plant growth and behavioral processes. (The hormone’s existence was first guessed by Charles Darwin when he was studying how plants grow towards light.)
After many tests, Sundaresan and his group 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.
To test the theory that this gradient was determining the fate of nuclei in the sac, Sundaresan and his group created a series of genetically manipulated Arabadopsis plants. In some plants they ratcheted up production of auxin in the embryo sac, and in others they decreased the sac’s sensitivity to auxin, creating the same effect that a decline in auxin would make.
When they examined these experimental plants, their hypothesis was confirmed: Auxin concentrations determined the fate of the nuclei. Knowing whether auxin levels were high or low, it became possible to 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,” Sundaresan said. “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.”Implications for flowering plant evolution
Yet the fossil record reveals very little about the stages that led from gymnosperm seed production to angiosperm seed production when the transition occurred around 135 million years ago. The rapid expansion of flowering plants and their eventual domination of the Earth’s vegetation was called “an abominable mystery” by Darwin.
By elucidating the mechanism of embryo sac development, Sundaresan and his team have opened the door to new work into the evolutionary pathway between these two major plant groups. The discovery supports what is known as the modular theory, which posits that the first angiosperms underwent a drastic reduction of their female reproductive unit compared to the gymnosperms, allowing flowering plants to reproduce more efficiently and eventually supplant their naked-seeded forebears.
Most remarkably, perhaps, the new work suggests that the eight nuclei of the angiosperm embryo sac have retained developmental plasticity in their evolution from gymnosperms. “It’s amazing that even though the split supposedly happened over a hundred million years ago,” Sundaresan said, “all these nuclei still have the capacity to become egg cells.”
Collaborators in the study are lead author Gabriela Pagnussat and Monica Alandete-Saez, who were postdoctoral researchers with Sundaresan when they did the work, and John L. Bowman, a professor of plant biology at UC Davis at the time of the study, now at Monash University in Melbourne, Australia.
The work was supported by grants from the National Science Foundation.About UC Davis
Liese Greensfelder | EurekAlert!
Rainbow colors reveal cell history: Uncovering β-cell heterogeneity
22.09.2017 | DFG-Forschungszentrum für Regenerative Therapien TU Dresden
The pyrenoid is a carbon-fixing liquid droplet
22.09.2017 | Max-Planck-Institut für Biochemie
Plants and algae use the enzyme Rubisco to fix carbon dioxide, removing it from the atmosphere and converting it into biomass. Algae have figured out a way to increase the efficiency of carbon fixation. They gather most of their Rubisco into a ball-shaped microcompartment called the pyrenoid, which they flood with a high local concentration of carbon dioxide. A team of scientists at Princeton University, the Carnegie Institution for Science, Stanford University and the Max Plank Institute of Biochemistry have unravelled the mysteries of how the pyrenoid is assembled. These insights can help to engineer crops that remove more carbon dioxide from the atmosphere while producing more food.
A warming planet
Our brains house extremely complex neuronal circuits, whose detailed structures are still largely unknown. This is especially true for the so-called cerebral cortex of mammals, where among other things vision, thoughts or spatial orientation are being computed. Here the rules by which nerve cells are connected to each other are only partly understood. A team of scientists around Moritz Helmstaedter at the Frankfiurt Max Planck Institute for Brain Research and Helene Schmidt (Humboldt University in Berlin) have now discovered a surprisingly precise nerve cell connectivity pattern in the part of the cerebral cortex that is responsible for orienting the individual animal or human in space.
The researchers report online in Nature (Schmidt et al., 2017. Axonal synapse sorting in medial entorhinal cortex, DOI: 10.1038/nature24005) that synapses in...
Whispering gallery mode (WGM) resonators are used to make tiny micro-lasers, sensors, switches, routers and other devices. These tiny structures rely on a...
Using ultrafast flashes of laser and x-ray radiation, scientists at the Max Planck Institute of Quantum Optics (Garching, Germany) took snapshots of the briefest electron motion inside a solid material to date. The electron motion lasted only 750 billionths of the billionth of a second before it fainted, setting a new record of human capability to capture ultrafast processes inside solids!
When x-rays shine onto solid materials or large molecules, an electron is pushed away from its original place near the nucleus of the atom, leaving a hole...
For the first time, physicists have successfully imaged spiral magnetic ordering in a multiferroic material. These materials are considered highly promising candidates for future data storage media. The researchers were able to prove their findings using unique quantum sensors that were developed at Basel University and that can analyze electromagnetic fields on the nanometer scale. The results – obtained by scientists from the University of Basel’s Department of Physics, the Swiss Nanoscience Institute, the University of Montpellier and several laboratories from University Paris-Saclay – were recently published in the journal Nature.
Multiferroics are materials that simultaneously react to electric and magnetic fields. These two properties are rarely found together, and their combined...
19.09.2017 | Event News
12.09.2017 | Event News
06.09.2017 | Event News
22.09.2017 | Life Sciences
22.09.2017 | Medical Engineering
22.09.2017 | Physics and Astronomy