Bacteria Talk, Plants Listen: The Discovery of Plant Immune Receptors, an Interview with Pamela Ronald

A series of graduate student conversations with leading women biologists, at the Women in Science Symposium at Cornell April 2-3.

Prof. Pamela Ronald, a Professor in Plant Pathology at University of California, Davis and director of Grass Genetics at the Joint Bioenergy Institute, studies genes that control the plant response to stress.

In her presentation for the Frontiers in Life Sciences symposium at Cornell University, Ronald described the isolation of a novel bacterial signal that is key to bacterial communication. In a process called quorum sensing, bacteria use this molecule, called Ax21, to communicate with each other, essentially transforming these single celled organisms into a coordinated team of fierce invaders. Remarkably, the rice XA21 receptor can detect these small molecules. That detection triggers a robust defense response. As she said, ?Bacteria talk, plants listen.?

In 1995, the Ronald Laboratory isolated and characterized the XA21 immune receptor (Song et al. Science 1995; Lee et al., Science 2009). Subsequent discoveries in flies (Lemaitre et al., 1996), humans (Medzhitov, et al. 1997), mice (Poltorak et al, 1998), and Arabidopsis (Gomez-Gomez, 2000) revealed that animals and other plant species also carry membrane-anchored receptors with striking structural similarities to XA21 and that these receptors also play key roles recognition of microbial signatures and host defense (Ronald and Beutler, Science 2010). The importance of this work is reflected in the 2011 Nobel Prize in Physiology or Medicine to Beutler and Hoffman, who discovered the animal receptors.

Ronald?s team also led the isolation of a gene, Sub1A, that makes rice tolerant to flooding for over 2 weeks. This is a vast improvement over the two or three days that most conventional strains can tolerate. In flood-prone regions, submergence tolerance is critical for survival of this staple food crop and to the people who depend on it for sustenance. Her collaborator, David Mackill, has now introduced Sub1 A into six different varieties using a marker assisted breeding approach. In 2011, 1 million farmers grew Sub1 rice.

Not one to shy away from a chance for discussion, Ronald has been around the world engaging the public on the importance of science-based approaches to issues of food security, organic farming, and plant genetics. Together with her husband, organic farmer Raoul Adamchak, Ronald co-wrote the book Tomorrow?s Table: Organic Farming, Genetics, and the Future of Food. This book, and the blog that followed, are accessible accounts of genetics, food and farming and the importance of sustainable agriculture.

I had the opportunity to interview Prof. Ronald following her talk. I asked what prompted her to write Tomorrow?s Table, both the book and the blog. ?I was frustrated with the quality of information available to the public,? she responded. ?If scientists could directly communicate with the public, there would be less distortion of science.? Dr. Ronald fits blogging around her research, busy grant-writing schedule, and a revision of the book. This kind of communication is particularly important as the debate about the role of GE foods continues, more than 15 years after the introduction of the first GE crops.

I also asked Ronald how she started on the Sub1 project. ?David Mackill approached me about the project when he was still at UC Davis,? Dr. Ronald explains. Mackill knew of a long-abandoned strain of rice from India that could tolerate extended periods of submersion, and he had found the general region of the genome that conferred this trait. ?I had recently isolated XA21 using a positional cloning approach and Dave asked me to use the same approach to isolate Sub1. We thought if we could isolate the gene for submergence we could introduce it into varieties favored by farmers using marker assisted breeding or genetic engineering. We applied for a grant from the USDA who funded our work to isolate the gene. My laboratory led the cloning and demonstrated that Sub1A encoded a master regulator, a transcription factor.?

Mackill?s group led the breeding. He crossed the ancient strain with locally adapted rice varieties, and used markers from the Sub1 genomic region to track the introduction of Sub1 into the resulting progeny. The advantage of this approach is that he could select for progeny that carried the Sub1A gene. This ?precision breeding? process also reduces the transfer of undesirable traits, like reduced yield and poor taste, that are difficult to screen out through conventional breeding approaches. The gene isolation and precision breeding approach successfully produced Sub1 varieties that have been embraced by farmers in India and Bangladesh.

How does one get to be a scientist that saves lives? Issues of social justice and food security came up often throughout Dr. Ronald?s career path. Her key decision, she says, came at the transition from graduate student to post-doc. Initially, she focused on plant?microbe communication because she loved the science and also because she saw potential to reduce the use of pesticides. As she chose her post-doctoral position, she switched from working on tomatoes and peppers to working on rice. It was there, in a crop feeding half of the world?s people, that she saw even greater potential for her science to contribute to the public good and enhance food security.

As a Plant Biology graduate student myself, I had to ask what drew Ronald to plant sciences in the first place. The turning point, Ronald says, was an early moment when she discovered one could even build a career out of studying plants. Backpacking with her brother and a friend through California?s Sierra Nevada range, she came across a teacher and his student, botanists, with their noses buried in a field guide. Early exposure to science, and a family that fostered an appreciation for learning and the outdoors, are recurring themes among the life scientists that I know, myself included. Prof. Ronald?s efforts to reach out to the public herself carry on this tradition by increasing public exposure to science and scientists.

When I asked her advice for junior scientists interested in communicating with the public, Dr. Ronald replied: ?Of course, the love of science always comes first. Just practice communicating, keep reading and listening to other good science communicators. Think about your audience and keep dropping the technical lingo. It takes a lot of time, but there?s a great science writing community. After a while you just get sucked in.?

Previously in this series:

Serendipity and Science: 30 Minutes with Dr. Sharon Long
The Co-Evolution of Insects, Plants and a Career
Empirically dancing your way to the top ? How Nicole Dubilier does it!
From babies to baboons: one woman?s path to success
At home underwater and on land: a conversation with Dr. Mary Power

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