THE ANALYSIS OF GENETICALLY AND PHYSIOLOGICALLY COMPLEX TRAITS USING CERATOPTERIS - A CASE-STUDY OF NACL-TOLERANT MUTANTS

Genetic and physiological complexities associated with salt tolerance in plants have limited progress in the analysis of specific factors re sponsible for the salt-tolerant phenotype. We have used the homosporou s fern Ceratopteris richardii as a model plant to investigate the phys iological basis of salinity tolerance by selecting single gene mutants that confer tolerance in the gametophyte generation. The unique genet ic system of homosporous ferns permits the generation of mutants in a genetic background nearly isogenic to the wildtype, such that comparat ive studies with the wildtype can identify specific physiological resp onses associated with salt tolerance. One of these mutations, stl2, co nfers a high level of tolerance to Na+ (I-50 approximate to 175 mM NaC l) and generates a complex suite of related phenotypes. For example, i n addition to Na+ tolerance, stl2 exhibits tolerance to Mg(2+)salts, s ensitivity to supplemented K+, higher K+-dependent efflux of K+, alter ed responses to Ca2+ supplementation and moderate tolerance to osmotic stresses. Based upon its physiological attributes, we have proposed t hat the mechanism of action for this mutation involves an enhanced inf lux of K+ and higher selectivity for K+ over Na+ in a KC channel. The direct and indirect consequences of this alteration can account for Na Cl tolerance and the other phenotypes evident in stl2. The complex set of phenotypic responses from such a single gene mutation illustrates the potential for even more extreme pleiotropy in multigenic salt-tole rant strains.