Abscisic Acid Content'

Developing kernels of the maize (Zea mays) hybrid W64A x W182E germinated precociously following fluridone treatment. Likewise, following premature drying, the kernels germinated upon subsequent rehydration. Tolerance of the aleurone layer to premature desiccation considerably preceded that of the embryo. The increase in a-amylase activity following premature drying was substantial and was equal to, or exceeded, the increase which occurred following normal maturation drying. In contrast, there was only a small increase in enzyme activity, regardless of the concentration of the supplied gibberellic acid, following fluridone treatment. Both fluridone and drying cause a decrease in abscisic acid content within the developing kernels. While this decline in growth regulator may permit kernels to germinate, alone this is not sufficient to permit an increase in a-amylase activity. Thus drying is necessary to sensitize the aleurone layer to gibberellin, and thereby elicit enzyme synthesis. For this tissue to achieve its full potential to produce a-amylase, it must not only be free of the inhibitory effects of abscisic acid, but it must also be competent to respond to gibberellin. During development, the aleurone layer serves as a storage tissue, whereas following germination, its function is largely hydrolytic, as a source of enzymes for the mobilization of stored reserves (6). While it must be ensured that the catabolic events of postgermination do not occur during development, a switch in metabolism must occur to ultimately trigger reserve mobilization and its associated enzyme synthesis and release. The nature of this switch remains elusive, but it is possible that drying and the growth regulator ABA, which are known to affect both development and germination of the embryo (reviewed in ref. 13), also play a role in the aleurone layer. Drying, when prematurely imposed on developing grains, enables the aleurone layer of wheat and barley to respond to GA and produce a-amylase (1, 2, 5, 17), a starch-hydrolyzing This work is supported by National Science and Engineering Research Council of Canada grant A2210 to J.D. B. M.Y. O. has been the recipient of a National Science and Engineering Research Council of Canada Postgraduate Scholarship and an Ontario Graduate Scholarship. enzyme which is the predominant protein synthesized by this tissue (6). Drying, it is argued, renders the tissue sensitive to gibberellin (1, 5). Conversely, others have suggested that high levels of ABA prevent the developing aleurone layer from initiating a-amylase biosynthesis, a function that ABA is known to effect in germinated grains (14, 18). These authors propose that drying serves to reduce endogenous ABA, allowing the aleurone layer to produce enzyme. This contention is supported by studies of viviparous (vp) mutants of corn; the ABA-deficient and -insensitive aleurone layer of these mutants produces elevated levels of a-amylase late in development while still attached to the cob (24). There has been no previous attempt to simultaneously assess the relative significance of drying and ABA depletion in the triggering of a-amylase biosynthesis in the aleurone layer or how, if at all, the two are interrelated. Here, we compare the responses of maize kernels to premature drying and application of fluridone, an inhibitor of ABA-precursor synthesis (8); both treatments are known to elicit the precocious germination of maize kernels. If drying serves merely to reduce inhibitory levels of ABA in the developing tissue, then reducing the endogenous levels of ABA with fluridone in developing kernels should allow the aleurone layer to respond and thus result in the precocious production of enzyme in the absence of a drying treatment. It is evident, however, that germination and a-amylase synthesis cannot be attributed only to a modulation in ABA levels. MATERIALS AND METHODS