A sprouting-resistant and a sprouting-susceptible wheat cultivar were utilized to examine the role of ABA levels and sensitivity responses in wheat embryonic germination. Endogenous embryonic ABA levels were measured in both cultivars throughout grain maturation utilizing a new and sensitive ABA immunoassay. Embryonic ABA levels of each cultivar were similar with the sprouting-susceptible cultivar having about a 25% lower ABA level than that of the sprouting-resistant cultivar. Larger differences between the cultivars were noted in sensitivity to ABA, as measured by capability of ABA to block embryonic germination. ABA inhibited embryonic germination much more effectively in the sproutingresistant cultivar.ABA has been demonstrated to have an important role in regulating seed embryonic maturation and germination (13,22 inhibition of GA3-induction of a-amylase synthesis (3, 10, 32). Not only is the grain embryo considered to be the site of ABA action, but the earliest significant sprouting damage caused by a-amylase has recently been shown to occur in the grain embryo (16-18). It has been previously difficult to measure actual embryonic ABA levels due to the small amount of tissue in developing embryos.Wheat cultivars vary in susceptibility to preharvest sprouting and in the duration of seed dormancy (25). Grains removed from the maturing plant of a sprouting-susceptible cultivar and placed in water will easily germinate. In contrast, grains removed from a maturing sprouting-resistant cultivar will not sprout easily. Similar cultivar differences in preharvest sprouting susceptibility occur in field-grown grains still within the wheat spike under rainy weather conditions. We have been utilizing a sprouting-susceptible and a sprouting-resistant cultivar to examine the role of embryonic ABA in regulating embryonic germination in wheat. Under field conditions, 50 Mm ABA more effectively blocked embryonic germination in a sprouting-resistant than in a sprouting-susceptible cultivar (9). However, the amount of ABA sensitivity and endogenous levels of ABA varied widely in response to different environmental conditions (9).The objectives of this research were to determine, under controlled growth conditions, if embryonic ABA levels in sprouting susceptible and resistant cultivars account for sprouting differences, and to determine if embryonic sensitivities to ABA correspond to whole seed germination capabilities of the cultivars. A sensitive immunoassay utilizing a monoclonal antibody to (+)ABA was developed to measure ABA in small amounts of embryonic tissue. Embryonic ABA sensitivity in the two cultivars was compared by measuring the effectiveness of a broad range of ABA concentrations to inhibit embryonic germination. Finally, it was demonstrated that the embryonic ABA sensitivity responses reflect the capacity of whole seeds to germinate during maturation. MATERIALS AND METHODS Plant Material. Triticum aesitivum L. cultivars Brevor andGreer were grown in a greenhouse with 22°C day/15°C night temperatures and...

Genes detected by wheat expressed sequence tags (ESTs) were mapped into chromosome bins delineated by breakpoints of 159 overlapping deletions. These data were used to assess the organizational and evolutionary aspects of wheat genomes. Relative gene density and recombination rate increased with the relative distance of a bin from the centromere. Single-gene loci present once in the wheat genomes were found predominantly in the proximal, low-recombination regions, while multigene loci tended to be more frequent in distal, high-recombination regions. One-quarter of all gene motifs within wheat genomes were represented by two or more duplicated loci (paralogous sets). For 40 such sets, ancestral loci and loci derived from them by duplication were identified. Loci derived by duplication were most frequently located in distal, high-recombination chromosome regions whereas ancestral loci were most frequently located proximal to them. It is suggested that recombination has played a central role in the evolution of wheat genome structure and that gradients of recombination rates along chromosome arms promote more rapid rates of genome evolution in distal, high-recombination regions than in proximal, low-recombination regions.

The abscisic acid (ABA)-induced protein kinase PKABA1 is present in dormant seeds and is a component of the signal transduction pathway leading to ABA-suppressed gene expression in cereal grains. We have identified a member of the ABA response element-binding factor (ABF) family of basic leucine zipper transcription factors from wheat (Triticum aestivum) that is specifically bound by PKABA1. This protein (TaABF) has highest sequence similarity to the Arabidopsis ABA response protein ABI5. In two-hybrid assays TaABF bound only to PKABA1, but not to a mutant version of PKABA1 lacking the nucleotide binding domain, suggesting that binding of TaABF requires prior binding of ATP as would be expected for binding of a protein substrate by a protein kinase. TaABF mRNA accumulated together with PKABA1 mRNA during wheat grain maturation and dormancy acquisition and TaABF transcripts increased transiently during imbibition of dormant grains. In contrast to PKABA1 mRNA, TaABF mRNA is seed specific and did not accumulate in vegetative tissues in response to stress or ABA application. PKABA1 produced in transformed cell lines was able to phosphorylate synthetic peptides representing three specific regions of TaABF. These data suggest that TaABF may serve as a physiological substrate for PKABA1 in the ABA signal transduction pathway during grain maturation, dormancy expression, and ABA-suppressed gene expression.Abscisic acid (ABA) is required during seed development for the acquisition of desiccation tolerance and dormancy and plays an important role in mediating many plant responses to the environment (Busk and Pages, 1998). As ABA levels increase during the seed development program or in response to environmental stress, a number of ABA-induced genes are expressed. These include the LEA (late embryogenesis abundant) genes, which may serve to protect the developing seed from desiccation (Ried and Walker-Simmons, 1993), as well as genes such as KIN1, which encode proteins similar to antifreeze proteins (Wang and Cutler, 1995). In addition to stimulating the transcription of a suite of ABAinducible genes, ABA also suppresses the expression of GA-induced genes encoding ␣-amylase and Cys proteinase in aleurone layers of cereal grains (Bethke et al., 1997).Because many physiological responses to ABA are dependent upon ABA-mediated gene expression, the signal transduction pathway involved in this process has received considerable attention (Lovegrove and Hooley, 2000; Rock, 2000). Detailed studies of the promoters of ABA-induced genes have identified an ABA response complex that is necessary and sufficient for ABA-induced transcription (Shen et al., 1996). This complex consists of the ABA response element (ABRE) (T/C)ACGTGGC together with a coupling element (CE) containing the consensus sequence CGCGTG. A number of transcription factors involved in ABA-induced gene expression have also been identified. The VP1 protein is able to activate ABA-induced genes, and requires the presence of ABREs to do so. However, VP1 does not bind...