The inheritance of two glyphosate tolerant mutants in the fern Ceratopteris

The herbicide glyphosate ([ N-phosphonomethyl] glycine) is a highly effective and widely used broad spectrum herbicide which also possesses properties that reduce its potential harmful effect on the environment. Because of this, there is considerable interest in developing glyphosate tolerant crop plants. However, genes that confer tolerance to glyphosate must first be identified. The fern Ceratopteris richardii can be used to select for and characterize mutations conferring herbicide tolerance. In this study two different confirmed glyphosate tolerant mutant strains, HαG492 and HαG343, of the fern Ceratopteris richardii were characterized in both the haploid gametophyte and diploid sporophyte generation.

The effects of glyphosate were assayed by the comparison of spore germination in gametophytes and growth both in the gametophyte and sporophyte generation. Germination of the wild type and one mutant strain, HαG343, was affected by increasing concentrations of glyphosate while the HαG492 mutant strain was unaffected. Both mutants showed increased growth in comparison to the wild type in the presence of glyphosate. One strain, HαG492, showed high tolerance to glyphosate while the HαG343 strain showed moderate tolerance.

The mutant strains were analyzed to determine the inheritance of the tolerance traits. Segregation in the gametophyte generation showed that for both strains, tolerance to glyphosate is due to a single nuclear gene. Segregation analysis of the double mutant hybrid indicated that the mutations involve separate loci but are linked. Genetic analysis in the sporophyte generation showed that the mutation carried by the HαG343 strain is recessive while that in the HαG492 strain is expressed as a semidominant trait. The expression of this semidominant mutation is also evident in sporophytes containing both mutations in the trans configuration. The mutations that are responsible for conferring tolerance to the HαG343 and HαG492 strains have been designated g343 and G492, respectively.