Levels of endogenous abscisic acid and indole-3-acetic acid influence shoot organogenesis in callus cultures of rice subjected to osmotic stress

Osmotic stress and endogenous hormone levels may have a role in shoot organogenesis, but a systematic study has not yet to investigate the links. We evaluated the changes of the endogenous indole-3-acetic acid (IAA) and abscisic acid (ABA) levels in rice (Oryza sativa L. cv. Tainan 5) callus during shoot organogenesis induced by exogenous plant growth regulator treatments or under osmotic stress. Non-regenerable callus showed low levels of endogenous ABA and IAA, with no fluctuation in level during the period evaluated. The addition of 100 μM ABA or 2 mM anthranilic acid (IAA precursor) into Murashige and Skoog basal induction medium containing 10 μM 2,4-D enhanced the regeneration frequency slightly, to 5 and 35%, respectively, and their total cellular ABA or IAA levels were increased significantly, correspondingly to the treatments. However, the regeneration frequency was greatly increased to 80% after treatment with 0.6 M sorbitol or 100 μM ABA and 2 mM anthranilic acid combined. Both treatments produced high levels of total cellular ABA and IAA at the callus stage, which was quickly decreased on the first day after transfer to regeneration medium. Thus, osmotic stress-induced simultaneous accumulation of endogenous ABA and IAA is involved in shoot regeneration in rice callus.