Defenseless mutants

--Gurmukh S. Johal and Steven P. Briggs

There is no biochemical explanation for the nature of disease defense mechanisms in maize. Despite considerable research efforts, no phytoalexins have been identified. An ideal way to study this phenomenon would be with a mutant(s) in which the plant's defense is impaired. However, no such mutants have ever been isolated. The main problems are of identification and maintenance, since such mutants will be lethal and be eliminated. To our knowledge, no one has ever searched for these mutants in a systematic way.

We have now devised a genetic strategy to generate and identify such recessive mutants in maize. This approach relies on transposon mutagenesis using Robertson, DS's 'Mutator. Briefly, maize plants containing Mutator are crossed with plants from an inbred line. The resulting F1 progeny are then selfed to generate F2 families. Any recessive mutation in the F1, due to insertional inactivation by Mu, will be expressed in the F2 and approximately 1/4th of the plants will show the mutant phenotype.

The next question is how to uncover mutations in the defense pathway. For this, we have developed a dual screening procedure. First, 48 seeds from each F2 family are planted in the field. The families in which about 1/4th of the plants fail to germinate, die as seedlings, or show root rot symptoms, are replanted in greenhouse under partially sterile conditions. Those families which now germinate and grow normally may have biochemical lesions in the defense pathway. Such families are then subjected to confirming tests.

So far, we have generated about 20,000 F2 families. Last summer, we planted about 10,000 of these in the field to identify those which were deficient in germination, emergence or plant stand. We were unable to make this selection because of constant rain and flooding shortly after planting which affected the germination of the maize crop as a whole. Out of about 46 families that were screened in the greenhouse, one segregated for plants which succumbed to a root rot disease(s) at different times after germination. These results were confirmed when 32 seeds each were planted in both the sterile and nonsterile soil-mix. Five out of 30 plants died in the nonsterile soil-mix, whereas only one out of 32 died in the sterile mix. A soil-borne root rot fungus, Fusarium, has been identified growing on the dead plants. Further characterization of this mutant is in progress.

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