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Sunny Side Up
by Sarah C.P. Williams
Joanne Chory uses genetics to study how plants respond to changes in their environments—from predictable cycles of day and night to unwanted shade and global warming.
It's a concept that every kindergartner understands after watching a seed sprout roots and shoot a stem out of a paper cup, reaching toward the sun. But plant researchers have struggled to identify the molecular pathway that explains this elementary phenomenon—that plants will do anything to get some extra rays of light. Recently, HHMI investigator Joanne Chory at the Salk Institute for Biological Studies has begun to lead the way out of the shadows.
By studying the interactions between plants and their environment, Chory observes how plants respond to shade as well as to changes in water, day length, and temperature. She studies Arabidopsis—a small flowering mustard plant that is a favorite among biologists because of its fully sequenced genome and easy-to-observe growth changes.
Chory and colleagues recently uncovered a new pathway that is activated when a plant wants to outgrow an encroaching neighbor. Understanding this chain of events, which researchers call “shade avoidance syndrome,” could help scientists engineer food crops that survive in crowded fields, where plants overshadow each other.
For a plant, the consequences of shade are drastic. Desperate for sunlight and striving to outgrow its neighbors, a plant's reaction upon sensing the specific type of shade caused by other plants (plants reflect far-red light and absorb red light, so their shade is low in red light) is to grow straight up, as fast as possible. The plant directs most of its energy into stem growth, sacrificing other important activities: leaf growth, root development, seed production, and immune function.
“There's a lot of stuff that a plant gives up just so it can get up there, above its competitors,” says Chory. The seedlings that have been shaded are tall, but unusually skinny with dwarfed, pale leaves.
To find out what molecules might guide this growth pattern, Yi Tao, a postdoctoral researcher in Chory's group, set up a genetic screen for mutations that would stop plants from shade-induced gangliness. They looked for plants that didn't grow in simulated shade but that grew normally under full light and in complete darkness, reasoning that abnormalities in these two settings would likely mean the plants lacked the ability to elongate. Their screen revealed mutations in a number of genes that seemed to be involved specifically in shade avoidance, and they focused on one—dubbed sav3 (shade avoidance 3). In the shade, sav3 mutants look like they're growing in bright light—they are shorter, darker green, and have fuller leaves than the nonmutant, or wild type, Arabidopsis seedlings grown in the shade.
Photo: Jeffrey Lamont Brown