The Waxy (Wx) protein has been identified as granule-bound starch synthase (GBSS; EC 24.1.21), which is involved in amylose synthesis in plants. Although common wheat (Triticum aestivum L.) has three Wx proteins, "partial waxy mutants" lacking one or two of the three proteins have been found. Using such partial waxy mutants, tetra- and hexaploid waxy mutants with endosperms that are stained red-brown by iodine were produced. Both mutants showed loss of Wx protein and amylose. This is the first demonstration of genetic modification of wheat starch.

Deficiency of the wheat waxy (Wx) proteins (Wx-A1, Wx-B1 and Wx-D1) was studied in 1,960 cultivars derived from several countries. Gel electrophoretic analyses revealed that the null allele for the Wx-A1 protein occurred frequently in Korean, Japanese and Turkish wheats but was relatively rare in cultivars from other countries and regions. About 48% of the wheats deficient for the Wx-B1 protein were from Australia and India. One Chinese cultivar lacked the WxD1 protein. While 9 Japanese cultivars were deficient in both the Wx-A1 and Wx-B1 proteins, no cultivars lacked both the Wx-A1 and Wx-D1 proteins, both the Wx-B1 and Wx-D1 proteins or all three Wx proteins. Two-dimensional gel electrophoresis revealed polymorphisms of the three Wx proteins that varied according to isoelectric points or molecular weight. The Wx-A1 gene coding the Wx-A1 protein and the Wx-B1 gene coding the Wx-B1 protein were localized in the distal regions of chromosome arms 7AS and 4AL, respectively, by deletion mapping using the deletion lines developed in the common wheat cultivar 'Chinese Spring'.

A number of cold responsive ( Cor )/late embryogenesis abundant ( Lea ) genes are induced by both low temperature (LT) and dehydration. To understand the molecular basis of cold acclimation and its relationship with drought stress response in wheat seedlings, we isolated a DREB2 homolog Wdreb2 , which is the candidate gene for a transcription factor of the Cor / Lea genes. The Wdreb2 expression was activated by cold, drought, salt and exogenous ABA treatment. Detailed expression studies of Wdreb2 indicated the involvement of two distinct pathways of its activation, a drought and salt stress-responsive pathway and a cold-responsive pathway. The transient expression analysis showed that the Wrab19 expression was directly activated by the WDREB2 transcription factor in wheat cells. Three transcript forms of Wdreb2 ( Wdreb2 α , Wdreb2 β and Wdreb2 γ ) were produced through alternative splicing. Under drought and salt stress conditions, the amount of the Wdreb2 β form remained fairly constant during 24-hour treatment, while those of the Wdreb2 α and Wdreb2 γ forms showed transient increases. On the other hand, the LT treatment resulted in increased transcript levels of all three forms of Wdreb2 . Thus, under the LT and drought/salt stress conditions the amount of the WDREB2 transcription factors in wheat is differentially controlled by the level of transcription and alternative splicing.

Studies of waxy mutations in wheat and other cereals have shown that null mutations in genes encoding granule-bound starch synthase I (GBSSI) result in amylose-free starch in endosperm and pollen grains, whereas starch in other tissues may contain amylose. We have isolated a cDNA from waxy wheat that encodes GBSSII, which is thought to be responsible for the elongation of amylose chains in non-storage tissues. The deduced amino acid sequences of wheat GBSSI and GBSSII were almost 66% identical, while those of wheat GBSSII and potato GBSSI were 72% identical. GBSSII was expressed in leaf, culm, and pericarp tissue, but transcripts were not detected in endosperm tissue. In contrast, GBSSI expression was high in endosperm tissue. The expression of GBSSII mRNA in pericarp tissue was similar at the midpoints of the day and night periods. The GBSSII genes were mapped to chromosomes 2AL, 2B, and 2D, whereas GBSSI genes are located on group 7 chromosomes. Gelblot analysis indicated that genes related to GBSSII also occur in barley, rice, and maize. The possible role of GBSSII in starch synthesis is discussed.Starch is composed of two distinct polymers; amylopectin, which consists of long chains of (1-4)-linked ␣-Dglucopyranosyl units with extensive branching resulting from (1-6) linkages, and amylose, which is a relatively linear molecule of (1-4)-linked ␣-d-glucopyranosyl units (Whistler and Daniel, 1984). Both types of chains are elongated by starch synthases that transfer ␣-d-Glc from ADPGlc to the growing chain, and specific starch synthases are active in the synthesis of each type of polymer. Whereas a number of starch synthases are thought to catalyze amylopectin synthesis (Dry et al

Cereal Chem. 74(5):576-580The viscoelastic properties and molecular structure of the starch isolated from waxy (amylose-free) hexaploid wheat (WHW) (Triticum aestivum L.) were examined. WHW starch generally had lower gelatinization onset temperature, peak viscosity, and setback than the starch isolated from normal hexaploid wheat (NHW). Differential scanning calorimetry (DSC) showed that WHW starch had higher transition temperatures (T o , T p , and T c ) and enthalpy (∆H) than NHW starch. However, when compared on the basis of amylopectin (AP) content, ∆H of WHW starch was almost statistically identical to that of its parental varieties. Typical Atype X-ray diffraction patterns were observed for the starches of WHW and its parental varieties. Somewhat higher crystallinity was indicated for WHW starch. WHW starch was also characterized by having greater retrogradation resistance. The high-performance size-exclusion chromatography (HPSEC) of amylopectin showed that each amylopectin yielded two fractions after debranching. Although WHW amylopectin had somewhat long B chains, little difference was observed in the ratio of Fr.III/ Fr.II between WHW and its parental varieties.