Hydrated dormant cereal seeds do not germinate even when environmental conditions are favorable for germination. By using cDNA cloning and differential screening, we have identified mRNAs from five gene families that are abundant in the embryos of imbibed, but developmentally arrested wheat (Triticum aestivum L.) seeds. Gene transcript levels of these mRNAs are maintained and even increase in embryos of imbibed dormant seeds for as long as the seeds remain dormant. In contrast, transcript levels decline in nondormant seeds after imbibition and disappear as germination occurs. All the identified genes are ABA responsive. Based on these data we conclude that wheat seeds in the hydrated dormant state exhibit prolonged expression of ABAresponsive genes.Upon hydration, most mature seeds germinate readily over a range of environmental conditions. By contrast, dormant seeds do not germinate under conditions favorable for germination and can remain viable in a hydrated state for prolonged periods without cell elongation until a signal is recognized by the seed embryo which initiates germination. The mechanism of seed dormancy poses an intriguing problem in plant molecular biology and has major agricultural importance. For the first few hours after imbibition dormant and nondormant seeds exhibit similar physiological responses, including the same rates of water uptake and new protein synthesis (6). In dormant seeds, however, there appear to be molecular and biochemical restrictions which prevent cell expansion and germination. The intent of this research is to contribute toward the identification of the restrictions which regulate seed dormancy in mature seeds.In some plant species seed dormancy is released by prolonged storage of dry seeds (afterripening), while other species require hydration at low temperatures (6,20). Proposed dormancy mechanisms in mature seeds have focused on regulation by inhibitors acting alone or in combination. Such mechanisms have included those attributed to the seed coat such 'Contribution from the U.S.

Dormant seeds do not germinate when imbibed in water even when conditions are favorable for germination. These hydrated seeds remain viable, but growth-arrested for weeks due to unknown restrictions within the embryo. As a model system for the study of the molecular processes occurring in dormant seeds, we have chosen to examine gene expression in Bromus secalinas, a grass species that produces seeds with high levels of embryonic dormancy. Using differential screening for mRNAs present in hydrated dormant embryos, we have identified a cDNA clone, pBS128, that encodes a mRNA transcript found in the embryos of hydrated seeds of B. secalinus as well as in embryos from mature dry seeds. Striking differences in pBS128 transcript levels appear upon hydration of dormant and nondormant seeds. Upon imbibition pBS128 transcript levels increase over four-fold in dormant seeds, but rapidly decline and disappear in nondormant seeds, which subsequently germinate. The pBS128 transcript appears to be embryo-specific since the transcript is not detectable in either non-stressed or dehydrated seedling tissue. Application of 50 microM ABA to nondormant seeds arrests germination and enhances pBS128 transcript levels. The nucleotide sequence of the nearly full-length pBS128 cDNA shows no homology to other reported genes, and the putative protein sequence does not exhibit the hydrophilic characteristics of the ABA-responsive LEA (late embryogenesis abundant) proteins.

Abstract. A new endonuclease activity from Escherichia coli which cleaves circular, single-stranded DNA in the presence of added nucleoside triphosphate has been purified. The activity has not been detected in extracts from certain rec-strains and is therefore implicated in genetic recombination.