How plants allocate photosynthetic products.

New discoveries about how plants distribute nutrients internally could lead to crops that are more nutritious, and which can produce higher yields or overcome environmental challenges. As animals have a signaling system between the brain and the stomach to tell them when to stop or start eating, plants also have a signaling system that regulates nutrient distribution.

The signaler in the system is sucrose, the major form of sugar transported in the plant's vascular system. A plant responds to sucrose's signal by increasing or decreasing nutrient flow to roots, seeds and storage organs, known as sink tissues. These tissues are called sinks because they import sugars and amino acids to support plant growth and development.

When sucrose exits plant leaves, it flows through elongated structures called phloem cells, which lie end to end and form a continuous conduit in the plant's vascular system. A specialized sucrose transport protein loads the sugar into the phloem. Inside the phloem cells, the concentration of sucrose is 100 times greater than that outside.

This attracts water into the cells. The release of sucrose into sink tissues causes the water to leave the phloem cells, creating a hydraulic pressure difference between the leaf and the sink phloem that drives long-distance nutrient transport. This is similar to the pressure-driven flow of blood pumped through the human body.

By learning more about how plants allocate photosynthetic products, scientists may be able to modify plant growth to increase a crop's yields, alter nutritional value, or overcome environmental challenges, such as elevated carbon dioxide levels. Further information. Daniel Bush, USDA/ARS Photosynthesis Research Unit, 190 PABL, University of Illinois, 1201 W. Gregory, Urbana, IL 61801; phone: [phone]; fax: [phone].