Helminthosporium carbonum is a fussy fungus that attacksonly one small, oddball proportion of Zea mays -- corn -- thathappens to have no built-in resistance to the specific fungaltoxin it releases.
Plant pathologist Steven P. Briggs at Pioneer-Hi-BredInternational Inc. of Johnston, Iowa, has uncovered themolecular genetics that drive that toxin, and the mechanismthat most breeds of maize deploy to resist it. His paper in lastFriday's Science tells the story: "Reductase Activity Encoded bythe HM1 Disease Resistance Gene in Maize."
An eminent expert on fungal toxins, Professor Olen Yoder atCornell University, hails Briggs' research as embodying at leasttwo "firsts."
For one thing, Yoder told BioWorld, "it reports the first plant-disease resistance gene ever cloned. For another, it's the firstexample where we know the product of the gene, and how itacts to confer resistance."
Most breeds of maize, Briggs pointed out, do not develop fungalleaf spot because of their inherent resistance. This, he hasdiscovered, takes the form of an enzyme, a toxin reductaseencoded by a specific resistance gene, which inactivates thefungal toxin.
Pioneer Hi-Bred has filed for a process patent on this finding,but does not expect to commercialize the resistance factor assuch. Briggs explained that because plant resistance to H.carbonum through the commercial germ plasm is sowidespread, "the main thrust of our discovery is to use thegene as a selectable marker."
He noted that "other people have worked on this from time totime, but I don't think I have any competitors right now."
Since submitting his paper to Science last spring, he and hispost-doc, Gurmukh S. Johal, have turned to a second gene, onanother maize chromosome, which also encodes a resistancefactor to H. carbonum, (also known as Cochliobolus carbonum).Unlike the HM1 gene, which acts locally, HM2 switches onsimultaneously all over the entire plant, converting it all atonce from susceptible to resistant.
Briggs is now in the process of cloning HM2 and elucidating itsmode of action. "We have expression of the HM1 gene intransgenic cultures," he said. "We don't have plants yet."
Yoder suggested that once the mechanism is understood, "moregene shuffling could be done" to introduce the resistance factorinto other plants.
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.