October 24, 2016
Scientists trace plant hormone pathway back 450 million years
WEST LAFAYETTE, Ind. – Purdue scientists got a glimpse into more than 450 million years of evolution by tracing the function of a hormone pathway that has been passed along and co-opted by new species since the first plants came onto land.
Flowering plants today, known as angiosperms, use the phytohormone abscisic acid (ABA) to keep seeds dormant until ready for germination and to open and close stomates, tiny openings on leaves used to control gas exchange.
“This hormone is important for drought tolerance,” said Jody Banks, Purdue professor of botany and plant pathology. “When plants are water-stressed, ABA levels shoot up and close the stomates so the plants won’t wilt as quickly.”
It was unclear, however, what kind of role ABA played in ferns and other lycophytes, which Banks studies. Like many of her peers, Banks assumed that ABA would also play a role in stomate function.
But when she developed a line of mutant ferns that could not process ABA, she found that there was no difference between her mutants and wild type ferns that were water-stressed.
Banks shelved the research for nearly two decades before teaming with scientists at Australia’s University of Tasmania and Germany’s University of Würzburg. Together, they determined that ABA plays a key role in determining the sex of ferns, using a mechanism that was co-opted by flowering plants to tolerate desiccation.
Matching the genes of Arabidopsis, a model flowering plant, and the fern Ceratopteris richardii, scientists at the University of Tasmania found the homologous fern gene responsible for ABA signaling. Scientists at the University of Würzburg then found that the proteins produced when the ABA signaling pathway is turned on do not interact with proteins that would open and close stomates. They realized that regulating stomate closing by ABA was novel to angiosperms, which evolved from ferns about 150 million years ago.
ABA, they found, promotes femaleness in ferns. When a wild type plant is exposed to ABA, the plant becomes female. When ABA pathways are disrupted, as with the mutants Banks studied, the plants become male, even in the presence of ABA. They also discovered that ABA is linked to spore dormancy in ferns, just as ABA is linked to seed dormancy in angiosperms.
“Promoting a dormant state was likely the original function of ABA as plants came up out of the water onto the land,” Banks said. “You wouldn’t need that dormancy if you were living in water. But on land, you need to have dormancy to survive desiccation.”
Banks and her colleagues will continue studying other ABA pathways in lycophytes and ferns, as well as hormones that control of sex of plants.
The findings were published in the Proceedings of the National Academy of Sciences the week of Monday (Oct. 24) and are available at http://dx.doi.org/10.1073/pnas.1606614113
The National Science Foundation, Australian Research Council, King Abdullah Institute for Nanotechnology of King Saud University and a Deutsche Forshungsgemeinshaft grant funded the research.
Writer: Brian Wallheimer, 765-532-0233, email@example.com
Contact: Natalie van Hoose, 765-496-2050, firstname.lastname@example.org
Source: Jody Banks, 765-494-5895, email@example.com
Abscisic acid controlled sex before transpiration in vascular plants
S. A. M. McAdam, T. J. Brodribb, J. A. Banks, R. Hedrich, N. M. Atallah, C. Cai, M. A. Geringer, C. Lind, D. S. Nichols, K. Stachowski, D. Geiger, F. C. Sussmilch
1 School of Biological Sciences, University of Tasmania, Hobart, TAS, Australia
2 Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN, USA
3 Institute for Molecular Plant Physiology and Biophysics, University of Würzburg, Würzburg, Germany
4 Central Science Laboratory, University of Tasmania, Hobart, TAS, Australia
Sexual reproduction in animals and plants shares common elements including sperm and egg production, but unlike animals, little is known about the regulatory pathways that determine the sex of plants. Here we use mutants and gene silencing in a fern species to identify a core regulatory mechanism in plant sexual differentiation. A key player in fern sex differentiation is the phytohormone abscisic acid (ABA) which regulates the sex ratio of male to hermaphrodite tissues during the reproductive cycle. Our analysis shows that in the fern Ceratopteris richardii, a gene homologous to the core ABA transduction proteins in flowering plants (SNF1-Related Kinase2s (SnRK2s)) is primarily responsible for the hormonal control of sex determination. Furthermore we provide evidence that this ABA-SnRK2 signalling pathway has transitioned from determining the sex of ferns, to controlling seed dormancy in the earliest seed plants, before being co-opted to control transpiration and CO2 exchange in derived seed plants. By tracing the evolutionary history of this ABA signalling pathway from plant reproduction through to its role in the global regulation of plant-atmosphere gas exchange over the last 450 million years we highlight the extraordinary impact of the ABA-SnRK2 signalling pathway in plant evolution and vegetation function.