Optical isomers of abscisic acid in carrot somatic embryos have the same effect on induction of dormancy and desiccation tolerance

Abstract

Carrot (Daucus carota) somatic embryos have been extensively used as an experimental system for studying plant embryogenesis. In maturing zygotic embryos, the phytohormone abscisic acid (ABA) is involved in induction of dormancy and desiccation tolerance. Somatic embryos contain only low levels of endogenous ABA and lack desiccation tolerance and dormancy, but both tolerance and dormancy can be induced by application of exogenous ABA. We analyzed the effects of the optical isomers of ABA, (S)-(� )-2-cis-ABA ((� )-ABA) and (R)-(� )-2-cis-ABA ((� )-ABA), and a racemic mixture of these two isomers (racemic ABA) on the carrot system. Somatic embryos treated with (� )-ABA, (� )- ABA, or racemic ABA showed the same levels of growth inhibition and desiccation tolerance. Similarly, each ABA isomer and racemic ABA had the same effect on accumulation of transcripts for ABA-inducible genes. The results indicate that both (� )-ABA and (� )-ABA cause the same amount of activity in these physiological phenomena. Our findings suggest that strict steric requirements of the ABA signaling pathways are absent in carrot somatic embryos, and we propose that commercially available racemic ABA is as effective as the natural isomer (� )-ABA in induction of dormancy and desiccation tolerance in carrot somatic embryos.