Genetic variability of maize GST and relation to herbicide tolerance
--M. Sari Gorla, S. Ferrario, L. Rossini and C. Frova

Metabolic detoxification is probably the most general major mechanism involved in plant tolerance to herbicides. In particular, the glutathione (GSH)/glutathione-S-transferase (GST) system appears to be widespread among plants and to confer protection against toxic chemicals by catalysing the conjugation of glutathione to an electrophile center of hydrophobic molecules by means of the SH group. Species tolerant or susceptible to a wide spectrum of herbicides (S-triazines, acetoanilides, thiocarbamates) are characterized by high and low levels of GST respectively, and an increase in specific GST activity in response to some herbicide treatment has been reported in a few cases. Thus the system appears to be involved in the determination of plant tolerance to these classes of molecules.

The system is complex: in maize, at least 3 different GSTs (GST I, II and III) have been detected. They are all functionally dimers and two of them (I and III) have been molecularly characterized. The genetic characterization of the system, however, is far behind. In particular, no information is available about the genetic variability existing within species with regard to the efficiency of the system and of its components.

We have analyzed GST expression by gel electrophoresis in different tissues of several maize inbred lines. Fig. 1a illustrates the enzyme electrophoretic profile in 4 tissues of inbred H99, while the results from 9 genotypes have been pooled together in Fig 1b. On the basis of these zymograms, four GST activity zones (A, B, C, D) have been identified on the gels: some are common to all tissues, while others are found only in the scutellum. Taking into account also the relative band intensities in the tissues analyzed, the presence of two different sets of genes is hypothesized, one expressed mainly in roots and pollen and one in the scutellum. Moreover, although the multiplicity of the protein species observed could be due to the expression of many genes as well as to posttranslational modifications and random association of different monomeric molecules, the analysis of the enzymatic variants and of their combination in different genotypes suggests that at least five genes control GST isozymes in maize.

In order to explore the existence of genetic variability for the efficiency of the system to detoxify EPTC (a thiocarbamate) and Alachlor (a chloroacetanilide), we measured GST activity, basal and after herbicide application, in roots, leaves, scutella and pollen of different genotypes. A wide variability in the basal level was observed in all tissues. Moreover, even though the levels of activity were not equal in all tissues (very high in pollen, scutella and roots, lower in leaves), they were correlated in the different tissues.

In roots (Fig. 2), an increase of GST activity was detected after herbicide treatment, in almost all the lines in response to Alachlor, only in some in response to EPTC. A similar pattern was observed in leaves, while scutella and pollen do not seem to respond to herbicide application.

Figure 1. a: Schematic representation of GST isozyme profile in scutellum (S), roots (R), leaves (L) and pollen (P) of inbred H99. b: GST variants expressed in different tissues (pooled data) of nine genotypes.

Figure 2. GST activity in roots of fifteen genotypes in control conditions and after treatment with EPTC or Alachlor. SE=standard error.

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