However, maize genotypes are characterized by different degrees of susceptibility; we analyzed about 40 inbred lines of different genetic origin with regard to their response to EPTC treatment. Seeds were soaked for 20 hours in EPTC solution (0.55 ml/l) and then planted in sand in pots covered with a plastic bag to prevent volatilization of the product. During seedling growth water was given without removing the bag, and after 12 days the effect of EPTC was evaluated: on the basis of plant injury, seedlings were scored from 1 (no damage) to 5 (completely deformed). A wide variability was observed among genotypes, ranging from complete susceptibility to resistance.
Two well-differentiated lines were chosen for the genetic analysis of the trait. By the procedure described above, P1, P2, F1, BC and F2 generation performances were evaluated; the data indicated that the character is controlled by few genes (2 or 3) and that resistance is due to recessive alleles. The observed segregations can be interpreted on the basis of the mechanism of EPTC metabolization in plants. The herbicidal compound is formed on metabolic sulphoxidation of EPTC by monoxygenase enzymes; the detoxification of EPTC-sulphoxide is mainly due to the formation of an EPTC-sulphoxide-glutathione conjugate, catalyzed by the glutathione S-transferase enzyme. The effect of corn-protective antidotes is in fact based on inducing high levels of glutathione and enhancing glutathione S-transferase activity; this process does not occur in weeds. Thus it is reasonable to think that EPTC resistance in maize can be the result of lack of sulphoxide production (inactivity of monoxygenase) or of a very efficient detoxification process (high levels of glutathione and of glutathione S-transferase). Work is in progress to characterize the lines with regard to the glutathione levels, and glutathione S-transferase and monoxygenase activity in the absence or presence of EPTC, antidote or both.
The effect of EPTC was also studied
on pollen of the same lines: susceptibility or tolerance was evaluated
on the basis of the germination percentage and pollen tube length on artificial
medium supplemented with EPTC, in comparison with controls (germination
and tube growth on standard medium). Since a high gametophytic-sporophytic
correlation with regard to EPTC tolerance was observed, an experiment of
gametophytic selection was carried out. The selection pressure was applied
during the last phases of pollen maturation or during pollen function.
Male inflorescences of F1 plants between susceptible and resistant genotypes
were enclosed in a plexiglass chamber containing EPTC vapors for different
times (from 3 to 24 h) during the pollen shedding period; the pollen produced
was used to pollinate female plants of recessive genotype. Another group
of female plants was treated in the same way after pollination, during
pollen germination and the beginning of tube growth in the silks. The backcross
progeny produced was evaluated for EPTC tolerance; seedlings of the BC
progeny from treated pollen proved to be more tolerant than non-treated
BC progeny. This response to selection, applied within plant, indicates
that the observed gametophytic-sporophytic correlation for the trait is
due to the expression of the same genes conferring herbicide resistance
in both phases of the plant life cycle. It also indicates that the inclusion
of pollen selection in breeding programs will serve to increase the effectiveness
of selection methods.
to the MNL 64 On-Line Index
Return to the Maize Newsletter Index
Return to the Maize Genome Database Page