Chemical reversion of genic male steriles to produce homozygous male sterile seed has been proposed by several researchers (e.g., Conn. Agric. Exp. Stn. Bull. 550, 1951; MNL 50:116, 1976; MNL 59:87, 1985). Since genic male sterility in maize, as in other species (Can. J. Bot. 51:2473, 1973), might be the result of genetic block in the synthesis or metabolism of endogenous plant growth regulators (MNL 50:116, 1976) or other molecules, treatments with appropriate chemicals might revert the genic male steriles to fertility. The present study was initiated to test the possibility of chemical reversion of genic male steriles of maize using the in vitro tassel culture system described previously (MNL 59:72, 1985; MNL 60:89, 1986). Two genic male steriles of maize, ms14 in Oh43 background and ms24 (UWO stocks, obtained originally from Earl Patterson) were selected for this study because these mutants cause the male gametophyte to degenerate at late microsporogenesis. We assume that these mutants provide a better chance for reversion than do other genic male steriles which fail earlier in development (Can. J. Genet. Cytol. 23:195, 1981; MNL 59:87, 1985).
Immature tassels of these male steriles were cultured in vitro as described previously (MNL 59:72, 1985). While ms14 tassels grew well and produced approximately 100-150 normal spikelets per tassel, the tassels of ms24 developed poorly and produced only 30-80 normal spikelets per tassel. However, the microspore development and breakdown in the cultured tassels of both male steriles was similar to that grown in vivo. Microspores from cultured ms14 tassels degenerated at the late vacuolate microspore stage. Microspores from cultured ms24 tassels underwent normal development through meiosis and the first mitotic division. After the first binucleate pollen stage, the generative nucleus degenerated followed by degeneration of the vegetative nucleus. At maturity, the spikelets on cultured ms24 tassels extruded anthers and occasionally engorged pollen was produced. These results are similar to that of in vivo studies here or reported elsewhere (Can. J. Genet. Cytol. 23:195, 1981; MNL 59:87, 1985). Thus, immature cultured tassels of these genic male steriles (ms14 and ms24) express male sterility without the influence of the rest of the plant. Similarly, the expression of the male sterile-mutant characteristic was observed in the cultured tassels of 2 other genic male steriles (ms2 and ms10).
The effects of 12 different plant growth regulators (PGRs) at various concentrations were studied for the potential reversion of male sterility in vitro (Table 1). None of these treatments reverted either of the 2 genic male steriles to produce normal pollen. In the case of ms24, while a few engorged pollen grains formed in some instances, the frequency of such pollen was not increased with any treatment. One might conclude therefore that at least in these genic male steriles of maize, the metabolic blocks which lead to pollen breakdown are not overcome by the application of specific molecules at the concentrations and in the forms applied. Despite the negative results obtained in this study with PGRs, the possibility of reversion of genic male steriles with other chemicals can not be excluded and therefore, further research is warranted.
Table 1. PGRs used to study their effect on reversion and development of male steriles.
D.R. Pareddy1, R.I. Greyson and D.B. Walden
1Present address: United AgriSeeds, Inc., Champaign, IL
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