PNAS January 24, 2023; 120 (5) e2220550120

Figure: Portrait of Pamela Ronald. Image credit: Reproduced with permission. © The Regents of the University of California, Davis campus.

Elected to the National Academy of Sciences in 2019, Ronald has received numerous honors, including the 2019 American Society of Plant Biologists Leadership in Science Policy Public Service Award, the Global Confederation of Higher Education Associations for Agricultural and Life Sciences World Agriculture Prize in 2020, the 2022 Wolf Prize in Agriculture, and the 2022 Vinfuture Prize for Innovative Female Scientist, which recognized her pioneering work on disease resistance and environmental stress tolerance in rice. Ronald's Inaugural Article (2) reviews her group’s research on the interaction between rice and the bacterial pathogen Xanthomonas oryzae—a system that her team has developed as a key model for studies of infectious disease biology.

Parental Influences, Early Mentors

Ronald was raised in Northern California by her parents. Her father was a Jewish refugee whose family in Berlin escaped the Nazis during World War II—a story detailed in his 1997 memoir Last Train to Freedom. Ronald says, “He reminded my brothers and I to value our education, not take it for granted, and to use our privilege to help others.”

Of her mother, an environmentalist, Ronald says, “She introduced me to native ecosystems and the importance of conserving pristine wilderness.” From a young age, Ronald spent her summers backpacking with her brothers in the Sierra Nevada wilderness. It was during one such trip that Ronald ran into botanists identifying wildflowers. She says, “It was the first time that I understood that one could study plants as a profession.” The songwriter Rita Hosking wrote the song “Sierra Bound” about that watershed day and dedicated the song to Ronald when it was released in 2013

After high school, Ronald entered Reed College. In 1981, during her junior year, she attended the Université Louis-Pasteur in Strasbourg, France, and earned a diploma in the French language. She returned to Reed, where she was mentored by biologist Helen Stafford and studied the microbial connections between fungi and trees. For her senior thesis, Ronald documented the recolonization of mycorrhizal fungi following the 1980 Mount St. Helens eruption. She earned a bachelor’s degree in biology in 1982 from Reed and a Master’s degree, also in biology, from Stanford University 2 years later.

Through a Fulbright Scholarship to attend Uppsala University in Sweden, Ronald studied under biologist and mycorrhizal fungi expert Nils Fries. There, she received a second Master’s degree, in plant physiology, in 1985. More than three decades later, in acknowledgment of her achievements, Ronald received an honorary doctorate from the Swedish University of Agricultural Sciences and in 2022 was elected as an International Fellow of the Royal Swedish Academy of Agriculture and Forestry.

Inspired by her mentors at Stanford—plant geneticists Patricia Bedinger, Judy Callis, Kathy Newton, Davis Stern, and Virginia Walbot—Ronald decided to pursue plant molecular biology as a career. In 1990, she received a PhD in molecular and physiological plant biology from the University of California, Berkeley, where she was mentored by plant and microbial biologist Brian Staskawicz, scientific director of agricultural genomics at the Innovative Genomics Institute at UC Berkeley.

From 1990 to 1992, Ronald did a postdoctoral stint with geneticist Steve Tanksley. Ronald says, “I chose a postdoc to work on rice because it is a staple food for half the human population and envisioned that rice genetics research would sustain my interest through a long career. I wanted to learn from rice experts from diverse countries so that I could help improve this staple grain for the poorest people in the world. I was fortunate to learn from Susan McCouch and Steve Tanksley.” On the heels of her postdoctoral apprenticeship, Ronald accepted an assistant professorship in the department of plant pathology at UC Davis, advancing through the ranks to her present position.

Plant Resistance Gene Encoding a Receptor Kinase

When Ronald began her research, genetic loci conferring disease resistance had been used for breeding crops for more than a century, allowing farmers to reduce or eliminate pesticide use. The identity of genes underlying such disease resistance, however, was unknown. Because researchers anticipated that isolation of the genes would help unravel the molecular basis of plant defense against pathogen invasion, effort was directed toward cloning genes conferring resistance to a variety of infections.

In the 1990s, several labs made discoveries using genetic approaches to isolate the first plant disease resistance genes. In 1995, Ronald and her team, led by postdoctoral associates Guoliang Wang and Wenyuan Song, isolated and characterized the first-known gene encoding a transmembrane innate immune receptor kinase, XA21 (4). Subsequent discoveries in flies, humans, mice, and Arabidopsis thaliana 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 in the immune response (5). For example, in animals, recognition of microbial molecules at the cell surface is mainly accomplished by the Toll-like receptor family that contains leucine-rich repeat receptors in the extracellular domain

Climate-Resilient Rice

Ronald has made significant contributions to the scientific understanding of environmental stress tolerance in plants, knowledge that is especially important in the light of climate change. The Intergovernmental Panel on Climate Change predicts that floods will increase in duration and intensity as the climate changes, posing challenges for 70 million rice farmers who live in flood-prone regions (10). Ronald says, “Many of these farmers survive on less than $3/day. In South and Southeast Asia, 4 million tons of rice, enough to feed 30 million people, are lost every year to flooding.”

Unlike most commercially grown rice varieties, the ancient Indian rice landrace Dhalputtia is unusual in its ability to endure complete submergence for more than 14 days. For over 40 years, breeders at the Philippines-based International Rice Research Institute worked to introduce Dhalputtia’s submergence tolerance (Sub1) trait into farmer-grown rice varieties. Because breeding was performed with genetic tools based mainly on visual selection, the resulting varieties lacked many traits farmers desired. Without knowledge of the exact genes needed to confer submergence tolerance, breeders unknowingly included undesirable traits along with Sub1, reducing yield and grain quality.

In 1996, UC Davis plant geneticist David Mackill and colleagues mapped the Sub1 trait to rice chromosome 9. In 1995, the Ronald and Mackill labs launched a map-based cloning effort to isolate Sub1 using the expertise Ronald had established to isolate the gene Xa21. In 2000, the teams generated a high-resolution map that laid the foundation for physical isolation of Sub1 (11). Postdoctoral associate Kenong Xu in Ronald’s lab then sequenced the 182-kb Sub1 region delimited by the mapping and identified three genes that encode for proteins with ethylene response factor (ERF) domains (12). Her team showed that japonica rice Nipponbare, which is intolerant of submergence, lacked one of these genes that she named Sub1A and that Sub1A is up-regulated rapidly in response to submergence. Through detailed analysis of hundreds of rice lines engineered with Sub1A, Ronald’s team demonstrated that Sub1A is sufficient to confer 2 weeks of submergence tolerance on otherwise intolerant rice varieties.

The isolation of Sub1A launched in-depth studies of its molecular mechanism of action. With UC Riverside geneticist Julia Bailey-Serres, Ronald and colleagues showed that the rice ERF genes ERF66 and ERF67 are transcriptionally up-regulated by Sub1A (13). ERF66 and ERF67 were later shown by the team of Ming-Che Shih of the National Taiwan University to be direct transcriptional targets of Sub1A that are stabilized under hypoxia and promote survival during submergence. The Bailey-Serres lab also showed that Sub1A down-regulates cell elongation and carbohydrate breakdown and induces ethanolic fermentation. Together, these activities conserve the plant shoot meristem and energy reserves during the transient flooding period, resulting in submergence tolerance.

The availability of the Sub1A sequence and adjacent sequences facilitated the precise introduction—by Mackill, Abdelbagi Ismail, and colleagues—of the Sub1A gene into rice varieties popular in South and Southeast Asia. Ronald says, “In the 2017 season, Sub1 varieties were grown by nearly 6 million subsistence farmers in India and Bangladesh, providing a greater than 60% yield advantage. Because most of the world’s poorest people get their food and income by farming small plots of land, the availability of Sub1 rice varieties has made a major difference in food security for these farmers.” Recent studies have shown that the introduction of Sub1 rice overwhelmingly benefits the world's poorest farmers who have been historically allocated the most flood-prone land for rice cultivation (14).

Driving Force

As of 2022, Ronald has been at UC Davis for a quarter of a century. She says, “I am lucky to have spent my career at UC Davis, which is an ideal place to pursue plant science; and I am grateful for my husband, Raoul, and our two children, Audrey and Cliff, for hiking with me in the mountains.”

In addition to her active research program, teaching, and leadership at UC Davis, Ronald is affiliated with the Innovative Genomics Institute, the Joint BioEnergy Institute, Lawrence Berkeley National Laboratory, and the Center on Food Security and the Environment at Stanford. At present, Ronald and her collaborators are working to use CRISPR genome editing and other tools to engineer plants to enhance soil organic carbon storage to help fight climate change.

Ronald maintains an active public speaking schedule. A 2015 TED talk by Ronald (15) that described plant biology research and made a case for engineering food has been viewed more than 2 million times. “I am driven to contribute something to society and to this planet”, she says. “We have only one life. My parents often reminded myself and my brothers that we should try to leave Earth a better place than we found it.”

See https://www.pnas.org/doi/10.1073/pnas.2220550120