Abscisic acid causes changes in gene expression involved in the induction of the landform of the liverwort Riccia fluitans L.

The conversion of the submerged form of Riccia fluitans to the landform either by transfer to a moist solid surface or by treatment with abscisic acid (ABA), is accompanied by the formation of a set of new polypeptides and concomitant down-regulation of other polypeptides. Changes in gene expression were analyzed by two-dimensional separations of proteins and differential screening of a cDNA library. One of the landform-specific proteins might depend on the expression of the newly discovered Ric 1 gene. The deduced amino acid sequence of the isolated Ric 1 cDNA clone codes for a protein with a molecular mass of 30.1 kDa. This polypeptide possesses two amino acid sequences which are repeated five times each and it is largely hydrophilic with the exception of a hydrophobic carboxyl-terminal region. Under ABA treatment the expression of the Ric 1 mRNA had already reached its maximum after 1 h of incubation. Transferring submerged thalli onto an agar surface resulted in a slower induction. The Ric 1 gene product shows homology to an embryo-specific polypeptide of carrot seeds and to the group 3 of late-embryogenesis-abundant (LEA) proteins. Interestingly, ABA treatment improved the desiccation tolerance of the submerged thalli. Additionally, ABA stimulated the synthesis of a protein which is immunologically related to a tonoplast protein. This finding, together with the fact that the ABA-induced landform exhibits an increased activity of several vacuolar enzymes, may indicate a special role of the tonoplast and the vacuole during ABA-induced conversion of the thallus from the submerged to the terrestrial form.