Waxy mutants

Amano, E. Genetic and biochemical characterization of waxy mutants in cereals. Environ. Health. 

Waxy (wx) or glutinous (gl) loci have been identified in maize, sorghum, rice (different species), barley, millets and Job s tears (Coix lachryma-jobi).254 257 Waxy mutants... 

The capacity of starch to stain blue-black with iodine suggests that some of the amylose is present in the starch in the V-form. The lipids present in cereal starch would bind to amylose if it were in the V-form, and yet X-ray analysis does not show the presence of the V-polymorph in cereal starches (i.e., most of the amylose would be in the amorphous form). The conclusion is that although a significant part of the amylose is probably in the helical form, the three-dimensional order necessary to give a crystalline diffraction pattern is absent. Indeed, the crystalline nature of starch is now attributed to the presence of amylopectin and not to amylose. Starch from waxy mutants contains only amylopectin (and no amylose), but this starch has the same degree of crystallinity and the same X-ray pattern as the regular starches that contain both components. 

Isozymes of plant starch synthases (2, 16-20) and branching enzymes (2, 16, 21-26) have been characterized. Presumably, they play distinct roles in amylopectin and amylose synthesis. Also, plant and Chlamydomonas reinhardtii granule-bound starch synthases (27-33) are responsible for synthesis of amylose. Mutants defective in this enzyme are known as waxy mutants and contain starch granules that have little or no amylose. 

Experiments with starch granule-bound ADP-D-glucose starch glucosyltransferase isolated from the embryos of non-waxy com (maize) seeds indicated that it is different from that of similar preparations isolated from the endosperms. This is compatible with the observation that, in the waxy mutants of com, the activity is lowered to a very low level, whereas the activity of the embryo preparation remains the same. 

One of these spontaneous mutants gave rise to the precious sticky rice of Thailand. The sticky rice lacks the starch amylose, which constitutes up to 30% of the total starch in nonsticky rice endosperm. The lack of amylose is due to a mutation in a gene called Waxy, which encodes an enzyme required for amylose synthesis (Sano 1984). Sticky rice is an important culinary and cultural component throughout East Asia and is used in festival foods and desserts. In upland regions of Southeast Asia, it is a staple food in many homes. Ten percent of the rice traded each year is sticky rice. 

Uses of partial-waxy and waxy wheat in Asian noodles, bread and tortillas have been reviewed.269,270,298 French bread made from double-null partial waxy wheat flour retained 1-2% more moisture in its crumb in 24 hours compared to bread from a wild type flour. French bread made from the mutant wheat had a softer crumb immediately and up to 48 hours after baking.299 Wheat noodles are made from flour and low levels of salts, so starch plays a major role in noodle quality. Flours from partial waxy wheats with —10% protein and 21-24% apparent amylose (starch basis) are favored for white salted (sodium chloride) noodles because partial waxy starch swells during cooking somewhat more than non-waxy wheat starch. For good appearing noodles, the flour should be from a white wheat low in polyphenol oxidase, and... 

S9, with higher starch synthase activity, had 36% of the amylose content observed in the nonmutant endosperm, whereas mutant S5, with an even lower starch synthase activity of 32%, had only 21% of the nonmutant maize amylose content. These data further support the view that the waxy protein is involved in amylose synthesis. 

Delrue, B., Fontaine, T., Routier, F., Decq. A., Wieruszeski, J.-M., Van Den Koomhuyse, N., Maddelein, M.-L., Fournet, B., and Ball, S. 1992. Waxy Chlamydomonas reinhardtii Monocellular algal mutants defective in amylose synthesis and granule-bound starch synthase activity accumulate a structurally modified amylopectin. J. BacterioL 174,3612-3620. 

Native starches contain approximately 25% amylose, yet there are some mutant varieties of com that contain 85% amylose (high-amylose com) or no amylose (waxy maize) (Whistler and Daniel, 1985). Amylose leaches from starch granules during... 

The amounts of amylose and amylopectin differ for starches from different botanical sources. Most so-called normal starches have 20-30% amylose and 80-70% amylopectins, respectively [29,30]. There are mutant varieties, such as waxy maize, waxy rice, and waxy potato, that are composed of 100% amylopectin. There also are the high amylose varieties, such as amylomaize-V that consists of 53% amylose and 47% amylopectin and amylomaize-Vn that is 70% amylose and 30% amylopectin, just the reverse of the normal starches. Many of the normal starches have been found to have an intermediate component that is slightly branched amylose with 0.5-3% a-(1 6) branch linkages [26,27,29,30]. 

The ratio of amylose to amylopectin also varies, depending on the source of the starch it ranges from 17 to 70% amylose and a corresponding 83 to 30% amylopectin. SeeO Table 2 for a list of different starches and their composition of amylose and amylopectin. Usually amylopectin is present as the major component, with a common ratio of 1 3 or 25% amylose and 75% amylopectin. There are mutants, however, such as, waxy barley, waxy maize, waxy rice, waxy potato, and waxy sorghum that are 100% amylopectin and there are the high-amylose starches. 

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