To live or Die in the Shade

Shade avoidance syndrome or SAS. When it is hot most people enjoy the shade but not plants which live for the sun. They have SAS. Now, the molecular details of SAS have been brought to light by researchers at the Salk Institute for Biological Studies. To step out of their neighbors' shade, plants switch on a natural chemical factory for the synthesis of the plant growth hormone auxin that lets a plant grow and ultimately stretch toward the sun. The Salk researchers report in an article published in the April 4, 2008 issue of the journal Cell. Understanding this response at a molecular level will allow scientists to naturally manipulate this response to increase yield in crops ranging from rice to wheat. Shade avoidance is a set of responses that plants display when they are subjected to the shade of another plant. It often includes elongation, altered flowering time, and altered partitioning of resources. This set of responses is collectively called the shade-avoidance syndrome (SAS). Plants can tell the difference between the shade of an inanimate object (e.g. a rock) and the shade of another plant. In the shade of a plant, far red light is present in a higher irradiance than red light, as a result of the absorption of the red light by the pigments involved in photosynthesis. When a plant detects competition from neighboring plants, it initiates a set of responses, called collectively the shade avoidance syndrome, that alter its growth and physiology. A rapid and transient increase of newly synthesized auxin via a newly discovered auxin synthesis pathway allows plants to elongate and grow toward the sun. Plants can sense and respond to the presence of other plants in their neighborhood by the relative increase in incoming far-red light resulting from absorption of red light by canopy leaves and reflection of far-red light from neighboring plants. To secure their place in the sun, plants direct their growth resources toward stem elongation and away from bulking up harvestable portions such as leaves and seeds. "If all else fails, the plants put out what I like to call a premature 'desperation flower' to produce at least a couple of seeds that might find better growing conditions during the next season," explains Chory one of the authors. Despite the importance of auxin for plant growth and development, the details of how auxin is synthesized continue to puzzle plant biologists. Multiple biochemical pathways for the production of auxin have been identified or proposed but the specific function of each pathway and how they intersect is not known. Now, the role of at least one pathway has become clearer. "When the major photoreceptor for shade avoidance detects neighbors, it triggers the TAA1 pathway resulting in a rapid increase in free auxin, which is transported to sites in the stem where it can participate in the growth response," explains Chory. "Although we showed earlier that at least two additional biosynthetic routes to auxin exist in Arabidopsis, these other pathways are unable to compensate for the loss of the TAA1-dependent pathway."