Higher Yields, Lower Pollution

 

Like miniature samurai, some species of fungi carry around an arsenal of personal weapons for use in overcoming assaults against the plants they are sworn to protect. Plants, like prosperous lords, grow faster and more luxuriantly when a member of the fungus genus Trichoderma is nearby. The fungal weapons, which include the biological, chemical and conventional, have made this group of fungi a favorite agent of biological plant disease control over the last decade.

 

Many fungi are known to attack plants. However, Trichoderma curiously sides with the plants, raising the possibility of "fighting fungus with fungus" instead of using pesticides (which are dangerous to humans and harmful to the environment).

 

Found in soil all over the world, Trichoderma is known to latch on to harmful fungi that attack plants and destroy them. By using specialized tools, Trichoderma coils around the body of the invading fungus and penetrates its outer cell walls using several enzymes, including potent chitin-eating enzymes called chitinases. (Chitin is the hard material found in the shells of beetles and crabs as well as in the cell walls of most fungi.)

 

Work in the lab of Prof. Ilan Chet, President of the Weizmann Institute, focuses on this important fungus and the chitinase genes that make it such a powerhouse of plant defense. Chet began his work on Trichoderma at the Hebrew University of Jerusalem, where he discovered two out of five versions of chitinase known to exist in Trichoderma. Each version is encoded by a different gene; and each gene is programmed to switch on in response to a different set of stimuli.

 

Today, at the Weizmann Institute's Biological Chemistry Department, he has isolated and sequenced a previously unknown gene responsible for producing a key chitinase enzyme, in research performed with team member Dr. Ada Viterbo, Prof. Aviv Zilberstein and Dr. Smadar Penini of Tel Aviv University, as well as scientists from the Hebrew University's Faculty of Agriculture. Chet's team then set out to determine what it takes to "turn on" the gene -  that is, produce the enzyme.

 

 

The strongest stimulus they found for firing up chitinase production came from the presence of chitin in the cell walls of nearby, harmful fungi. This particular chitinase gene begins producing its cell-wall-eating enzyme even before there is physical contact between Trichoderma and the attacking fungus. The gene appears to use a remote sensing mechanism that detects foreign chitin by picking up on tiny molecules that the attacking fungus releases into the surrounding medium.

 

Other stresses, such as low levels of available nutrients and temperature extremes, can also trigger chitinase production.

 

The team also showed that when chitanase works in tandem with other, similar enzymes, a synergistic effect ensues, providing augmented firepower against invaders.

 

 

Trichoderma not only acts as a biocontrol agent -  it also functions as an extra helping of fertilizer. In the latest research to come out of the Weizmann Institute Trichoderma lab, Chet and postdoctoral fellows Michal Shoresh and Iris Yedidya have shown why this happens.

 

Trichoderma works its way through the outer layers of the plant's root tissue and into the spaces between cells, where it remains without injuring the plant. The researchers found that the plant responds by activating a part of its immune system. It thus seems that Trichoderma acts as a kind of "early warning" system, putting the plant on high alert and improving its readiness to deal with an actual attack. Plants hosting Trichoderma had quicker response times to onslaughts of harmful bacteria in any part of the plant and were able to defeat the invading microbes more easily. Because Trichoderma-inoculated plants are better equipped to ward off infection, they are free to devote more of their energy to growth.

 

The researchers noted a number of chemical changes in the leaves, indicating that signals are relayed systematically up the plant from the roots, triggering a coordinated chain of responses on the way. By analyzing the pattern of chemical changes, they determined that a specific defense mechanism, known as "induced systemic resistance," had been activated in the plant.

 

Once the weapons used by Trichoderma are understood, the knowledge can be applied in several ways. Current disease control methods that employ the fungus can be improved. In addition, the genes that carry the instructions for the weapons' manufacture are now being engineered for other organisms -  such as bacteria grown specifically for assorted pesticide applications or plants that will carry the disease resistance traits themselves.

 

 

Friendly bacteria or fungi used in biocontrol agents are especially susceptible to the sun's ultraviolet rays and can also be destroyed by micro-organisms in the soil. Prof. Ilan Chet, together with Prof. Amos Nussinovitch of the Hebrew University's Faculty of Agriculture and doctoral student Cheinat Zohar-Perez, recently developed a "time-release" system that keeps the helpful microorganisms safe while allowing steady amounts of their disease-fighting enzymes to reach the plants.

 

In a trial conducted in cucumber plants, the spread of disease dropped by 80 percent. The system is based on the creation of tiny beads, some no more than a few microns in size,which are made from a water soluble polymer. Inside each bead, the scientists trap roughly a billion friendly bacteria or fungal spores along with enough nutrients to sustain them over time. Thus, the bacteria and fungal spores are able to continue producing enzymes that attack various disease-causing microorganisms.

 

The ecologically safe, biodegradable polymer breaks down over time, slowly releasing the microorganisms into the soil.

Options for producing the beads on a commercial basis are currently being explored.

 

 

Greenhouses are ideal breeding grounds for many kinds of plant diseases, including fungal infections. High humidity and other greenhouse conditions contribute to the problem. Chemical applications are problematic, as workers are constantly exposed to the air and soil, and the chemicals often break down more slowly in the closed greenhouses. In addition, some fungal infections do not respond to known chemical pesticides, while others become resistant to particular chemicals over time.

 

For these reasons, biocontrol products, such as Trichoderma-based preparations, have made important inroads into the greenhouse market, especially for growers raising vulnerable seedlings. Thanks to Chet and his years of work on Trichoderma, Israel is a world leader in research and development of these products.

 

Outside the greenhouse, the demand is mounting for biological control methods as awareness of the risks of pesticides to consumers and the environment grows. For instance, production of one widely used chemical for control of fungal disease in soil, methyl bromide, will be banned world-wide in 2005 under the Rio Convention because it destroys the ozone layer. Therefore, developing resistant plants and creating biocontrol applications that can be used efficiently on all kinds of field crops are of the utmost importance.