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Root enzymes are in control.

Root Enzymes Are in Control

Plants need iron to live and get it from the soil, through their roots. But in many soils, iron is found in forms that plants can't absorb.

To change it into a form they can take up, roots of some plants acidify the soil, which makes iron more soluble. Then the roots use an enzyme that converts the iron to a form the plant can take up.

While iron is absolutely required for plant survival, too much is toxic and deadly. And too little iron stresses some plants, yellowing the leaves (a condition called chlorosis) and reducing crop yields.

Douglas G. Luster and Marcia J. Holden, plant physiologists, are trying to understand how the root "turns on" an enzyme - called an iron chelate reductase - in response to the stress of iron deficiency. (A chelate binds with metals, such as iron, to make them more available in the soil for plant use.)

Iron is found in two major forms: ferric (F[e.sup.3+]) and ferrous (F[e.sup.2+]). Plant roots can only take up the ferrous form. The roots can chemically transfer an electron to ferric iron - an electrochemical process called reduction - transforming it to ferrous iron.

Some plants, such as tomatoes, turn on and off the mechanism for converting iron much better than other plants do.

These iron-efficient plants respond to low levels of soil iron by producing more roots and root hairs that have more of the enzyme needed to make iron available. When there is enough iron of the right form in the soil, the plants don't turn on this mechanism.

Luster and Holden are with the Foreign Disease-Weed Science Research Laboratory in Frederick, Maryland. In studying the characteristics of the proteins that make up this iron chelate reductase enzyme, they have collaborated with Rufus L. Chaney of the Environmental Chemistry Laboratory in Beltsville.

The iron chelate reductase enzyme is located in the plasma membrane, a thin membrane that surrounds root cells. Membranes are composed mostly of fatty components and enzymes, which are made up of proteins.

Holden wants to see how the stress of iron deficiency affects the root's enzyme. "We want to know whether there's actually more of this enzyme being produced in the membrane, or if iron stress merely activates the system that's already there," she says. "A third option is that it's a unique enzyme made only under iron stress."

To study the enzyme, Holden isolates it from the plasma membranes of root cells of tomato plants grown with insufficient iron. She separates the enzyme from the plasma membrane by adding a detergent to separate the proteins from the fat. "The root cell plasma membrane has |icebergs' of protein floating in a fat layer," she says. "The proteins, including the reductase enzyme, will move into the detergents and become water soluble." This step makes the enzyme easier to purify.

It was Chaney who originally realized that the reduction of ferric to ferrous iron was a necessary step for a plant to take up iron. He theorized that the enzyme must be positioned across the cell-membrane, taking electrons from inside the cell - to change the iron outside the cell to ferrous iron, the form the plant can use.

He found a way to show where the enzyme is active - bathing roots with a stain that shows on reduced (ferrous) iron. In stressed plants - that have been grown with insufficient iron - the roots will turn blue at enzyme sites; in unstressed plants, little dye appears.

Luster and Holden's work with the enzyme in the plasma membrane shows that the enzyme on the root cell surface is, in fact, an important plant response to iron deficiency. Activity of this enzyme (iron chelate reductase) is a measure of the intensity of a plant's reaction to iron stress.

The scientists measure the enzyme's activity on electrophoretic polyacrylamide gels - a standard laboratory technique that uses a small electric current to separate various proteins into layers within a gel material. Where the enzyme is active, stained "bands" (of a stain to detect ferrous iron) appear on the gel.

"We see more than one enzyme band on the gels," says Luster, "which means that there are several forms of the enzyme, but only some of them are activated when the plant is under iron stress."

So far, they've found that the properties of the enzyme are similar in unstressed and stressed plants. This means that probably the same enzymes are produced by both, but more enzyme is made when the plants are stressed.

Once they identify all parts of the mechanism that responds to iron stress, the important genes - the ones that turn the mechanism "on" and "off" - can be identified. It will then be possible to clone and transfer the gene to plants that are less efficient at responding to iron stress.

PHOTO : Plant physiologists Marcia Holden and Douglas Luster inspect tomato plants for iron deficiency.
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Title Annotation:iron deficiency in plants
Author:Konstant, Dvora Aksler
Publication:Agricultural Research
Date:Sep 1, 1991
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