We have previously reported the recovery of many plants resistant to methomyl following in vitro selection using 6-8 month old callus derived from normally sensitve cms-T inbreds and hybrids (MNL 61: 59-60). Field tests were conducted this past summer using first (R1) and second (R2) generation progeny from regenerated plants (R0). Lannate (whose active ingredient is methomyl) was applied at 1.12 lbs/A, twice the rate recommended for sweet corn, before evaluation. Results indicate a maternal mode of inheritance for the methomyl resistance trait. All methomyl-resistant plants were also male-fertile and HmT toxin resistant. Segregation for resistance and sensitivity was seen among ear-to-row progeny from some self- and cross-pollinated R0: 4/21 rows from sensitive R0 and 3/69 rows from resistant R0. None of the R2 examined showed segregation in the field. The N- and T-cytoplasm checks were uniformly resistant and sensitive, respectively. Test crosses indicate that restoration of male-fertility was not due to a novel or newly activated dominant cms-T restorer gene.
Regenerated plants were also recovered from callus cultured for 14-16 months. Many (19/31 or 61%) methomyl-resistant R0 were obtained from non-selected control callus. Only 2% of the plants regenerated from callus 6-8 months in culture showed such spontaneous resistance. This result suggests that prolonged time in culture increases spontaneous resistance and concurrent reversion to male-fertility. Not all genotypes responded similarly, suggesting an effect of nuclear background on frequency of these traits.
Analysis of phosphorylation by mitochondria isolated from resistant R1 and R2 and N-cytoplasm checks, shows that resistant mitochondria are unaffected by the addition of 1mM methomyl. Mitochondria isolated from sensitive R1 and R2, and T-cytoplasm checks, show over a 70% decrease in their phosphorylation activity in the presence of ImM methomyl. These results suggest that the tissue culture-derived resistance may arise from an alteration in the mitochondrion. Molecular studies of mitochondrial DNA from resistant and sensitive lines are currently in progress.
A.R. Kuehnle and E.D. Earle
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