Thermoinhibition in Lettuce
When U.S. consumers think of lettuce, they often think of iceberg lettuce. Yet many varieties exist, from heirloom to romaine, to red leaf and green leaf. Most lettuces grown and sold in the U.S. have a mild flavor, some with a hint of sweetness, and some with just a hint of bitter. In recent years, the lettuce industry has grown as consumer demand for lettuce has expanded into all four seasons.
If you're one of the growers producing for our nation's lettuce industry (worth over $2 billion), no doubt you're aware of the challenges in producing a crop to sell in the winter. In California and Arizona, where 90 percent of the country's lettuce grows, winter lettuce must be planted in September, when temperatures are still soaring. Lettuce doesn't sprout at high temperatures unless the seed is treated. Yet seed priming can be costly and difficult, and primed seeds often don't store well.
For over 10 years, researchers at the University of California, Davis, have worked with partners at Arcadia Biosciences, Cal Poly Pomona and Acharya N.G. Ranga Agricultural University in India to determine the factors controlling germination of lettuce seed at high temperatures. Most lettuce seeds are thermoinhibited, which means that when temperatures rise to 80 degrees Fahrenheit or above, seeds go dormant.
Thermoinhibition probably developed in wild lettuce because in the Mediterranean Middle East, where lettuce originated, summers are often hot with little rainfall. Wild lettuce seed that germinated in those conditions would likely die. Under those same conditions, farmers water crops, so the mechanism that causes thermoinhibition is unnecessary for the survival of the crop in today's environment.
At this point in our agricultural history, thermoinhibition is detrimental to a lettuce grower's business if the grower produces in Arizona or the Central Valley of California. To assist growers, researchers explored whether lettuce had the capacity to germinate at high temperatures.
Using the genomic map of lettuce created by UC Davis' Dr. Richard Michelmore, which boasts nearly 14,000 genetic markers, UC Davis Distinguished Professor of Plant Sciences Dr. Kent Bradford and his team examined the progeny of crosses between wild and cultivated lettuce and were able to study the genetic basis of the thermoinhibition trait.
One of the accessions they examined, UC96US23, was discovered in California's Central Valley in a vacant lot outside a gas station. It is a weedy lettuce of the species Lactuca serriola, the progenitor species from which cultivated lettuce (Lactuca sativa) was derived. This parent species is not palatable. Rather, it's a weed; it doesn't form any head, and the leaves are kind of spiny.
This lucky find provided the researchers with a key that could potentially unlock the mystery of thermoinhibition. In the lab, Bradford discovered that UC96US23 seeds can germinate at very high temperatures. With that information in hand, the researcher mapped that trait to a specific region of the lettuce genome. After several years of crossing and backcrossing, Bradford was able to identify the exact gene, now known as NCED4.
The gene encodes an enzyme involved in the production of the plant hormone abscisic acid (ABA), a germination inhibitor. In most lettuce seeds, hydration at high temperatures activates NCED4, which creates an enzyme involved in synthesizing abscisic acid, which inhibits germination. "For some reason, the particular gene we got from this wild parent [UC96] does not respond to that high temperature, so it doesn't make the hormone," reports Bradford.
Without the presence of abscisic acid, there is nothing to inhibit germination, so those seeds will germinate at very high temperatures. Of about 20 accessions of the L. serriola species tested, only three or four have this trait, indicating that the ability to germinate at high temperatures is rare even among the L. serriola species.
How the plant senses temperature remains a mystery to researchers. What is clear is that the heat signal turns on NCED4, which stimulates the DNA to be processed into RNA. The RNA is translated into protein, and the protein makes the ABA. Working with Arcadia Biosciences, Bradford and his team also identified two mutations in NCED4 that restrict the effectiveness of the enzyme that sparks ABA production.
Bradford has relied mostly on natural crossing and backcrossing for this project. Through backcrossing, the researcher can eliminate all the traits many consider unpalatable to create a variety that looks the way the majority of U.S. consumers expect it to look. However, he has also used transgenic methods to completely turn off NCED4. The results were the same.
Although three other related NCED genes in lettuce affect other traits, including the plant's ability to respond to water stress, deactivating the specific NCED4 gene only seems to affect its ability to germinate in high heat.
Other researchers at UC Davis and at Cal Poly Pomona are already crossing the material into germplasm to create desert-ready varieties. Bradford is almost ready to send the seed out to U.S. Department of Agriculture seed breeders and private companies that will breed this trait into commercial varieties.
Breeders can use germplasm from the wild species or from UC Davis' backcrosses from that wild species into cultivated lettuce. "We've removed approximately 98 percent of the wild genome and retained the segment of the DNA that contains that particular version of NCED4, so they have the natural version. We also have the mutation versions that are already in a cultivar," explains Bradford.
With so many different lettuce types growing in the U.S. today, Bradford expects it will take breeding efforts by many companies to distribute this trait among all the varieties that are needed. Growers can begin to look for lettuce that germinates at higher temperatures in a few years.
The author is a freelance writer based in Massachusetts and a monthly contributor to Growing.