Genes Enabling Tomatoes to Resist Wilt Disease Discovered

The finding, reported in the current issue of The Plant Cell (volume 9, issue 4), is expected to speed up the breeding of new disease-resistant varieties and reduce the need for chemical spraying.

Wild tomatoes -- but not cultivated varieties -- are naturally resistant to wilt disease spread by the soil-borne Fusarium oxysporum fungus (lycopersici strain). Once a widespread blight plaguing tomato crops, Fusarium wilt disease was largely brought under control through classical breeding techniques.

However, breeding new varieties through this method is a laborious process that can take years because it entails producing many successive generations of hybrid plants in which the undesirable traits have been eliminated but the beneficial ones retained.

The new study facilitates this process by providing molecular markers allowing breeders to simply trace the genetic makeup of new hybrids rather than test for it at each stage. The findings also pave the way for genetically engineered new varieties with an optimal mixture of desired properties.

"In the absence of genes that endow plants with natural resistance to disorders such as wilt disease, farmers must rely on extensive chemical spraying and soil treatment," says Prof. Fluhr, a member of the Weizmann Institute's Plant Genetics Department.

"By imparting natural self-defense capabilities to plants," Fluhr says, "we reduce the amount of dangerous chemicals entering the environment."

Collaborating with Prof. Fluhr were departmental colleague Dr. Dvora Aviv and doctoral student Naomi Ori, along with Prof. Dani Zamir and doctoral student Yuval Eshed of the Hebrew University of Jerusalem's Faculty of Agriculture and Prof. Steve Tanksley of Cornell University.

The newly identified genes are also of primary importance because they form the plant's first line of defense against Fusarium attack. Acting as regulatory switches, they induce the release of various defensive agents such as destructive oxygen molecules that bombard the disease-causing organism, or enzymes that chew away at it.

"A better understanding of how the resistance genes work should allow us to turn on these defense components at will," Fluhr says."Moreover," he adds, "the methodology we have helped develop for isolating resistance genes should make it possible to design new resistance genes and transfer them to various crops where they are lacking."

Fusarium wilt disease is caused by a fungus that penetrates the plant roots, primarily through wounded tissue. The fungus works its way into the plant's vascular system - the vital passageway for water and nutrients - where it thrives.Not only does the Fusarium fungus have a hearty appetite and consume the plant's nourishment, but it also produces toxins. The presence of abundant fungus, therefore, leads to the functional collapse of the vascular system, systemic wilting and often death of the plant.

In the study, the scientists first determined the chromosomal location of the wilt resistance genes, which are known as the Immunity 2 cluster (I2C) gene family. They then cloned the genes and inserted them into nonresistant plants, which thus obtained greatly enhanced resistance to wilt disease.

To further prove that I2C was indeed the long-sought resistance gene family, the researchers introduced a faulty, mutant copy of these genes into resistant plants. As a result, the plants lost their resistance to the fungus and wilted.

In solving the I2C's primary amino acid structure, the researchers found that it bears some striking similarities to several recently found resistance genes from other plants, such as one endowing tobacco leaves with resistance to viruses and another that makes the leaves of a weed resistant to bacteria.

This resemblance is intriguing since the plant's mechanism for defending the vascular system is very different from the one used for defending leaves. When a leaf is invaded, the plant can choke off the precise area under siege, killing infected tissue along with the infiltrator but leaving the rest of the plant intact.

But when countering an attack on the vascular system -- its essential lifeline -- the plant must employ more precise strategies, such as producing special cellular structures that confine the fungus to the lower part of the root and thus localize the disease.

"Not only do the products of diverse plant resistance genes share structural features," says Fluhr, "but one such feature, resembling a component of biological receptors, can be found in animals as well. This underscores the unity of the plant and animal kingdoms."