Thiocarbamate resistance and pollen selection --M.Sari Gorla, E.Ottaviano and M.Villa EPTC is a thiocarbamate herbicide (S-ethyl dipropylthiocarbamate) the main effect of which on plant metabolism is the inhibition of fatty acid biosynthesis. Since the chemical is not completely selective, it is used in association with antidotes serving to prevent injury to corn plants, while remaining active against weed species.

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.

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