Tropical plants grow fast and flower quickly to take advantage of the blistering sun year-round. In higher latitudes, the vegetation patiently waits for the time of year with more intense sunlight. Now two independent groups have for the first time found genetic differences that account for these adaptations in one species of mustard plant.
The amount of sunlight is an important cue. Plants rely on day length to decide when to bloom, for example, with temperate plants requiring longer days than their tropical counterparts. Plants also use day length and light intensity to judge the best time to metamorphose from a pale yellow sprout into a green plant, a process known as etiolation. In northern regions, seedlings turn green only after basking in a great deal of sunlight, whereas plants in the tropics have lower standards.
To analyze how etiolation varies with geography, Joanne Chory and her colleagues at the Salk Institute for Biological Studies in La Jolla, California, collected 141 Arabidopsis thalania strains from around the world. One tropical variant known as Lm-2 required 100 times less far-red light to etiolate than the rest of the plants, they report in the 26 November online issue of Nature Genetics. The researchers also discovered the cause: Compared to other plants, Lm-2 had less of a receptor, called phytochrome A, of red and far-red light. When team members sequenced its phytochrome A gene to understand the basis of this drastic decrease, they found that it differs from normal plants by only a single nucleotide.
In the same issue of Nature Genetics, a separate research team simultaneously reports that single nucleotide mutations also affect flowering time. Maarten Koornneef and his colleagues at Wageningen University in the Netherlands explored a gene region of Arabidopsis that previous studies had suggested to be somehow linked to flowering. The team discovered that the region contained the gene encoding a blue-light photoreceptor called cryptochrome-2. A tropical Arabidopsis variant called cvi has a point mutation in the gene, subtly shifting the daily cycle of cryptochrome-2 levels. That mutation, the researchers found, was responsible for the variant's decision to flower sooner after it encounters long days than its northern counterparts.
Both variations occur in only one species, and other plants probably have found different ways to adapt, the authors say. Still, notes plant physiologist Michael Purugganan of North Carolina State University in Raleigh, the studies are the first to examine natural variation in light response. "Now we are beginning to understand how the plant out in nature is handling light perception," Purugganan says.