ABA AND LOW-TEMPERATURE INDUCE FREEZING TOLERANCE VIA DISTINCT REGULATORY PATHWAYS IN WHEAT

The role of ABA in the induction of freezing tolerance was investigate d in two wheat (T. aestivum L.) cultivars, Glenlea (spring var) and Fr edrick (winter var). Exogenous application of ABA (5 x 10(-5) M for 5 days at 24-degrees-C) increased the freezing tolerance of intact plant s by only 3-degrees-C (LT50) in both cultivars. Maximal freezing toler ance (LT50 of -9-degrees-C for Glenlea and -17-degrees-C for Fredrick) could only be obtained with a low temperature treatment (6/2-degrees- C; day/night) for 40 days. These results show that exogenously applied ABA cannot substitute for low temperature requirement to induce freez ing tolerance in intact wheat plants. Furthermore, there was no increa se in the endogenous ABA level of wheat plants during low temperature acclimation, suggesting the absence of an essential role for ABA in th e development of freezing tolerance in intact plants. On the other han d, ABA application (5 x 10(-5) M for 5 days at 24-degrees-C) to embryo genic wheat calli resulted in an increase of freezing tolerance simila r to that achieved by low temperature. However, as in intact plants, t here was no increase in the endogenous A.BA level during low temperatu re acclimation of calli. These results indicate that the induction of freezing tolerance by low temperature is not associated with an increa se in ABA content. Using an antibody specific to a protein family asso ciated with the development of freezing tolerance, we demonstrated tha t the induction of freezing tolerance by ABA in embryogenic wheat call i was correlated with the accumulation of a new 32 kDa protein. This p rotein is specifically induced by ABA but shares a common antigenicity with those induced by low temperature. These results suggest that ABA induces freezing tolerance in wheat calli via a regulatory mechanism different from that of low temperature.