Abscisic acid-independent and abscisic acid-dependent regulation of proline biosynthesis following cold and osmotic stresses in Arabidopsis thaliana

The role of the phytohormone abscisic acid (ABA) in the regulation of proline synthesis was investigated by following the expression of the At-PSS and At-P5R proline biosynthesis genes in Arabidopsis thaliana wild type, in an ABA-deficient ubal-1 mutant as well as in ABA-insensitive abil-1 and abi2-1 mutants after ABA, cold and osmotic stress treatments. In wild-type and in ABA mutant seedlings, 50 mu M ABA or osmotic stress treatment triggered expression of At-P5S, whereas At-P5R accumulation was scarcely detectable. Expression of either gene was mediated by endogenous ABA since transcript levels were similar in wild-type and in ABA-deficient mutant plants. Proline accumulated to a greater extent after osmotic stress than upon ABA or cold treatment. Thus. ABA-treated abil-1 mutant plants accumulated less proline than the ABA-treated wild type. Upon salt stress, proline accumulated to a lesser extent in abal-1 and abil-1 mutant plants, suggesting an indirect role of ABA on proline accumulation during salt adaptation of the plant. These results indicate that the expression of the genes of the proline biosynthetic pathway is ABA independent upon cold and osmotic treatments, although their expression can be triggered by exogenously applied ABA. However, the endogenous ABA content may affect proline accumulation upon salt stress, suggesting post-transcriptional control of proline biosynthesis in response to NaCl