Characterization of a hydroxyproline tolerant mutant in the fern ceratopteris

Drought and salinity stresses can greatly reduce crop yields. Due to the polygenic nature of drought and salt tolerance, progress in the development of tolerant crops has been limited. An understanding of the cellular and molecular mechanisms of tolerance would aid attempts to obtain tolerant crops. In this regard, single gene mutants would be useful in studying the mechanisms of tolerance. Tolerant mutants can be obtained either by direct selection on salt and osmotic stress medium or by indirect selection of potential proline overproducers on hydroxyproline or proline analogues. This study documents the genetic analysis of a hydroxyproline tolerant mutant in the fern, Ceratopteris richardii, and compares its growth with the wild type under a variety of conditions. The role of proline and other amino acids in osmoregulation was also studied. The mutant had reduced growth at low temperatures in both the sporophytic and gametophytic generations. Reduced growth at low temperatures was also associated with higher levels of tolerance to hydroxyproline, NaCl, and mannitol. Genetic analysis of the mutant indicated a single, recessive, nuclear gene basis for hydroxyproline tolerance. Reduced growth at low temperatures appeared to cosegregate with hydroxyproline tolerance, suggesting single gene control for both phenotypes. Amino acid analyses revealed that the mutant had higher levels of proline and alanine at low temperature on basal medium. This indicated that accumulation of proline may be its basis of tolerance to hydroxyproline. However, only minor differences in levels of proline and other amino acids between the mutant and the wild type on medium supplemented with 75 mM NaCl indicated that some other factor(s) was involved in tolerance to NaCl. It is suggested that tolerance at low temperatures may be associated with a reduced metabolic rate and reduced uptake of NaCl, hydroxyproline, and mannitol.