Rice ecosystems in the tropical coastal areas are subject to flooding, but not all flooding is the same. Sometimes the plants are overwhelmed by water for a short time. Another way they experience flooding is through long-term partial submergence (stagnant flooding). Yoichiro Kato and colleagues have been studying SUB1, a region of the genome that improves submergence tolerance in rice. They wanted to see if Sub1 rice varieties could also be bred to tolerate stagnant flooding.
Professor Kato explained that combining the two traits means getting the plant to respond in two very different ways. “Terrestrial plants generally have two strategies to adapt to flooding stress. One is ‘quiescence’ or partial dormancy of plants, i.e., to quit energy consumption underwater like deep‐sea fish which hardly moves (save energy for recovery after water recession). The other is to actively ‘escape’ flooding stress like a diver with a snorkel. The two strategies sound like the opposite ways, but the natural flooding stress is always a mixture of two flooding stresses. If plants are to adapt to the two stresses, what the adaptive strategies look like? This is particularly an important question for breeders who want to improve rice genotypes.”
You might think of paddy fields for rice as being damp. However Professor Kato said that submergence is a serious problem. “We (or at least Japanese) like hot springs, or even soaking our legs into lukewarm water comforts us. Although it’s not immediately fatal, but nobody likes soaking the body into hot water up to the chin for a long time. Water depth of around 50% is already critical for modern rice (as a consequence of plant breeding to reduce the height – semidwarf stature to increase lodging resistance), causing more than 50% of yield reduction.”
Experiments with Sub1 rice showed that usually, it performs poorly in comparison with stagnant-tolerant rice, under stagnant flood conditions. What Professor Kato and colleagues have managed to do is find varieties of Sub1 rice that can compete with other kinds of rice. Professor Kato said, “Since it was natural or reasonable to think that two adaptation mechanisms are the COMPLETELY opposite (i.e., plants or plant breeders must choose one or the other), nobody has ever seriously said ‘There must be a way, and eventually we will be able to identify unique rice genotypes”, and then worked on the challenge”
The rice is already making its way to the field. Professor Kato said, “Promising genotypes have been already distributed to the target Asian countries where a combination of flooding stresses is prevalent. Plant breeders in each country are testing the general adaptability (such as pest/disease resistance, consumers’ preference of rice paddy), and if it can be directly accepted by farmers, then it will be ready for release soon. At the same time, plant breeders in each country have started to make crosses between these genotypes and popular varieties in their countries.”
While this means there is now a product on its way to farmers, there remains research to be done. Soon there will be a plant that responds in two different ways to flooding, but what triggers a response? Is it the speed of water level rise? Professor Kato is looking for the answers. “This is really a good question to be answered in the future study. But we guess so. For example, if water rises gradually, then we think we try to stretch a periscope to look above the water surface. But if it rises suddenly, then we don’t have any clues how far up the water surface is. Do we think we will still try to use a periscope? We also don’t know what kind of signals are involved in the sense of different types of flooding stress by plants. These questions remain to be answered.”