Assessment of gene copy number in the homosporous ferns Ceratopteris thalictroides and C. richardii ( Parkeriaceae ) by restriction fragment length polymorphisms

Homosporous ferns are generally considered polyploid due to high chromosome numbers, but genetically diploid since the expression of isozymes is generally controlled by a single locus. Gene silencing over evolutionary time is one means by which this apparent contradiction can be explained. A prediction of this hypothesis is that silenced gene sequences still reside in the genomes of homosporous ferns. We examined the genomes of Ceratopteris richardii and C. thalictroides for sequences which are similar to expressed gene sequences. Genomic DNA blots hybridized with C. richardii cDNA clones showed that the majority of these clones detected multiple fragments, suggesting that most gene-like sequences are duplicated in Ceratopteris. Hybridization signal intensity often varied between fragments of the same size between accessions, sometimes dramatically, which indicates that not all sequences are equivalent, and may represent the products of silenced genes. Observed reciprocal differences in intensity could be due to reciprocally silenced genes. In addition, an unusual segregation pattern for one locus followed by one probe may indicate homeologous chromosome pairing and segregation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/41638/1/606_2004_Article_BF00939726.pd

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 responsible for the salt-tolerant phenotype. We have used the homosporous fern Ceratopteris richardii as a model plant to investigate the physiological basis of salinity tolerance by selecting single gene mutants that confer tolerance in the gametophyte generation. The unique genetic system of homosporous ferns permits the generation of mutants in a genetic background nearly isogenic to the wildtype, such that comparative studies with the wildtype can identify specific physiological responses associated with salt tolerance. One of these mutations, stl2, confers a high level of tolerance to Na+ (I-50 approximate to 175 mM NaCl) and generates a complex suite of related phenotypes. For example, in addition to Na+ tolerance, stl2 exhibits tolerance to Mg2+salts, sensitivity to supplemented K+, higher K+-dependent efflux of K+, altered responses to Ca2+ supplementation and moderate tolerance to osmotic stresses. Based upon its physiological attributes, we have proposed that the mechanism of action for this mutation involves an enhanced influx of K+ and higher selectivity for K+ over Na+ in a KC channel. The direct and indirect consequences of this alteration can account for NaCl 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-tolerant strains

Sodium/Potassium Selectivity and Pleiotropy in stl2, a Highly Salt-tolerant Mutation of Ceratopteris richardii

The roles of Na+ and K+ (Rb+) uptake were further studied in a NaCl-tolerant strain of Ceratopteris richardii containing the stl2 mutation by direct comparison with the wild-type strain. In addition to Na+ tolerance, stl2 also confers tolerance to Mg2+ and sensitivity to K+. In addition to higher K+ (Rb+) uptake at concentrations commonly associated with low-affinity K+ transport, stl2 maintained higher uptake down to 0·1 mol m–3 Rb+. Up to a 25-fold excess of Na+ had little effect in either genotype on K+ (Rb+) uptake at low concentrations, i.e. 0·2 and 0·5 mol m–3 RbCl. Pretreatment with K+ (20 mol m–3) inhibited uptake of K+ (Rb+) in the wild type, whereas concurrent inclusion of K+ inhibited uptake of Rb+ more in stl2. In the absence of K+, Na+ uptake (0·01–60 mol m–3) was nearly identical in the wild type and stl2. K+ inhibited Na+ uptake more effectively in stl2 than the wild type, especially at 60 mol m–3 Na+. Greater inhibition of K+ uptake in stl2 occurred with MgCl2 or TEA (tetraethylammonium chloride) preincubation or with simultaneous inclusion of Al3+ (Al2SO4). The higher effective velocity of K+ uptake at a wide range of concentrations and the enhanced selectivity for K+ and against Na+ contribute to the preservation of higher cytosolic K+ and lower Na+ under salinity stress