November 16, 2001
La Jolla, CA – In the first global survey of its kind, scientists at The Salk Institute have isolated a genetic switch that controls how plants growing at different latitudes respond to light. The variation ensures that plants in northern climes near the poles, for example, are more sensitive to light than their counterparts closer to the equator.
Scientists say the finding may help fine-tune the growth and increase the yields of agricultural crop plants.
“If plants in light-intensive regions were as sensitive to light as those in dimmer areas, seedlings would start unfolding their leaves before they penetrated the soil,” said Salk Professor Joanne Chory, lead author of the study, appearing in the current issue of Nature Genetics. “These plants would never reach the light and would die an early death under the ground.”
“We knew a mechanism should exist for fine-tuning light sensitivity, but we didn’t know what form it would take,” said coauthor Salk Associate Professor Detlef Weigel.
Chory and Weigel led a comparative survey that examined 141 variations of Arabidopsis thaliana, gathered from regions ranging from the American Southwest to Sweden. Arabidopsis is a small mustard plant, adopted as the ‘lab rat’ of the plant research community.
All 141 strains were grown from seed under dark and standardized light conditions. As expected, behavior in the dark was similar among the varietals. All the plants germinated but were ‘etiolated’ leaves remained tucked close to stems and chloroplast development was kept on hold.
“Light turns on ‘de-etiolation,’ the unfurling of leaves and chloroplast production, so that photosynthesis can begin,” said Chory. “When that process happens, elongation of the hypocotyl, the embryonic stem, stops.”
Therefore, by holding light constant and examining stem length, Chory and her colleagues could compare the various strains for their sensitivity to light.
“More sensitive strains will stop growing sooner and display shorter hypocotyl length,” Weigel said. “It’s a good quantitative measure of light sensitivity.”
The team found, as might be expected, a correlation between stem length and latitude of origin. Strains from northern climes were more sensitive to light, displaying shorter stem lengths than strains collected from southern latitudes.
Next, the investigators sought clues to the nature of the sensitivity by growing the plants under segmented light – specifically blue, red and infrared light.
“Plants use specific photoreceptors – molecules that detect light – for different wavelengths of light,” said Chory. “So finding differences in sensitivities to particular wavelengths would point us to specific photoreceptor pathways.”
The strains displayed a variety of patterns, but one strain called Lm-2 from Le Mans, France was about a hundredfold less sensitive to far-red light than a standard laboratory strain used for comparison. Lm-2 was found to have a variation in its photoreceptor gene PHYA (Phytochrome A) that results in a single amino acid change in the receptor protein.
“It’s remarkable that plants within the same species can grow in so many diverse light environments. To understand the molecular basis for these strain differences will greatly advance our knowledge of how plants optimize their growth and development,” said Chory.
“Such knowledge could be very useful applied to crop plants,” she added, “particularly in growing plants outside their native regions.”
The study, called ‘Natural variation in light sensitivity of Arabidopsis’ was conducted in collaboration with Salk co-authors Julin Maloof, Justin Borevitz, Tsegaye Dabi, Jason Lutes, Ramlah Nehring, Joanna Redfern, Gabriel Trainer, and Jeanne Wilson. Authors from other institutions included Tadao Asami from the Institute of Physical and Chemical Research in Saitama, Japan and Charles Berry from the University of California, San Diego. The study was funded by the National Institutes of Health, the Howard Hughes Medical Institute, and the Torrey Mesa Research Institute/Syngenta. Maloof is supported by a Helen Hay Whitney Fellowship. Chory is a Howard Hughes Medical Institute investigator.
The Salk Institute for Biological Studies, located in La Jolla, Calif., is an independent nonprofit institution dedicated to fundamental discoveries in the life sciences, the improvement of human health and conditions, and the training of future generations of researchers. The Institute was founded in 1960 by Jonas Salk, M.D., with a gift of land from the City of San Diego and the financial support of the March of Dimes Birth Defects Foundation.