Molecular Cloning and Expression of Abscisic Acid-Responsive Genes in Embryos of Dormant Wheat Seeds

Abstract

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.

Isolation of a wheat cDNA clone for an abscisic acid-inducible transcript with homology to protein kinases.

“…Supplying ABA to isolated seed embryos has been shown to induce ABA-responsive transcripts (9), and the same result was obtained for PKABA1. Incubation of isolated wheat embryos in ABA as low as 0.1 ,uM resulted in the accumulation of the PKABA1 transcripts compared to a water control (Fig.…”

“…At that time embryos were dissected from the seeds, poly(A)+ RNA was prepared (9), and cDNA was produced by using avian myeloblastosis virus reverse transcriptase. PCR (26) was per-formed on this cDNA in a Coy thermal cycler with a cycle of 1 min at 94°C, 2 min at 50°C, and 3 min at 72°C.…”

“…In vitro studies have shown that a serine-cluster region of the RAB-17 protein can be phosphorylated by maize casein kinase 11 (14). In wheat, potential sites of phosphorylation have been identified in the sequences of cDNA clones including group 3 LEA (pMA2005) and pMA1949 (9,11), based on the substrate specificity of wheat embryo calcium-dependent protein kinase (32). Pre (33).…”

“…A number of ABA-responsive genes encode late embryogenesis abundant (LEA) proteins that accumulate in drying seeds and in water-stressed seedlings (3)(4)(5)(6)(7). In wheat the LEA proteins include Em (8,9), dehydrin (RAB) (9,10), group 3 LEA (11), and group 3 LEA (II) (9). Many of the LEA proteins are hydrophilic and may bind water tightly to function as protectants against desiccation (4).…”

Group 3 Late Embryogenesis Abundant Proteins in Desiccation-Tolerant Seedlings of Wheat (Triticum aestivum L.)

“…The cDNA clones represent transcripts that are found in desiccation-tolerant wheat tissue and comprise six gene families (Curry et al, 1991;Morris et al, 1991;Anderberg and Walker-Simmons, 1992;Curry and Walker-Simmons, 1993). One clone has sequences homologous to protein kinases (Anderberg and Walker-Simmons, 1992), whereas the other sequenced clones are homologous to LEA proteins.…”