A single, just-discovered point mutation is showing the waytoward rescuing many important crop plants from a variety offungal pests. When the tomato leaf-mold fungus, Cladosporiumfulvum, attacks the leaves of Lycopersicon esculentum, itsavirulence gene turns virulent by swapping one amino acidsequence for another.

This week's issue of Nature reports the discovery in a papertitled "Host resistance to a fungal tomato pathogen lost by asingle base-pair change in an avirulence gene." Its principaldiscoverer is Matthieu Joosten, a Dutch post-doctoral plantpathologist in the phytopathology department of WageningenAgricultural University in the Netherlands.

Joosten determined that the point mutation exchanged acysteine TGT codon for a TAT, encoding tyrosine. This DNA baseswap transforms the C. fulvum's harmless avirulence gene intoan aggressive sequence.

Plant pathologist Verna Higgins, who chairs the botanydepartment at the University of Toronto, told BioWorld thatJoosten's finding "is the first situation with a fungal diseasewhere we really understand the genetic change that took placein the fungus, allowing it to be able to attack a resistant plant."Higgins, whose own research explores the still-unknown natureof plant resistance genes, has maintained close contact with thehead of Joosten's laboratory and the principal author of theNature paper, Pierre De Wit.

Now that De Wit and his colleagues have cloned the fungalgene, Higgins said, "if someone were able to clone the plant'sresistance gene, De Wit proposes linking both sequences in theplant PP any plant, not just tomato PP and make it resistant to alot of different fungal diseases."

In a telephone interview Joosten explained to BioWorld thatwhen the non-mutated avirulence gene encounters the plant'sresistance genes on the tomato leaf surface, the leaf cell'sreceptors bind the benign fungal gene in a "gene-for-gene"interaction. "We hope that we can explain the molecular basisof the gene-for-gene interaction," he said. "We think it's auniversal phenomenon in nature because it's also there ininteractions with viruses and bacteria."

Research at Joosten's lab is aimed at elucidating thisrelationship not so much to save greenhouse tomatoes fromfungal attack (which is kept in check by plant breeding orfungicides), but to use the insight gained for moreconsequential crops.

"An enormously important disease is late blight on potatocaused by the fungus Phytophthora infestans," Joosten said."Maybe the results we hope to obtain from this model tomatosystem could be extrapolated to those."

Higgins recalled that in the 1920s and '30s plant growerswould cross-breed tomatoes carrying a resistance gene to C.fulvum, which lives exclusively on P. esculatum. ""The resistantplant wouldn't get the disease for a couple of years," sherecalled. "Then, whammo, the fungus had overcome the geneand the tomato would get infected again," she said.

"The significance of Joosten's work is that he's now shown thetype of change required in the fungus PP one amino acid PP tosuddenly be able to attack that tomato again," Higgins added.

When the fungal avirulence gene is functional, its encodedproduct somehow signals the plant to turn on its defenseresponse gene, she explained. In that response, the tomatostarts to synthesize chemicals toxic to the fungus and enzymesthat break down its cell walls. It also thickens its own leaf cells,presumably to prevent nutrients from leaking out to feed thefungus. Finally, the menaced plant allows some of its leaf cellsto commit suicide to save the rest.

But when mutated, the now-virulent fungal gene fails totelegraph its attack to the plant. Its minimal point mutationteaches it to sneak in unnoticed, and phyto-havoc ensues in thegreenhouse.

Joosten's lab also found a complete deletion of the avirulencegene. "The product that gene expresses as a signal to the plantis no longer expressed," he said, "so the fungus is able to infect."

Higgins predicted that "in the not-too-distant future, probablyby manipulating avirulent and resistance genes in fungus andplant, respectively, we should be able to take what we knowand make a number of plant species resistant to a number ofdifferent diseases of economic importance."

-- David N. Leff Science Editor

(c) 1997 American Health Consultants. All rights reserved.