Waxy corn


Waxy corn

Waxy corn (maize) was found in China in 1909. As this plant showed many peculiar traits, the American breeders long used it as a genetic marker to tag the existence of hidden genes in other maize breeding programs. In 1922 a researcher found that the endosperm of waxy maize contained only amylopectin and no amylose in opposition to normal dent maize varieties that contain both. Until World War II, the main source of amylopectin was cassava but when Japan severed the supply lines of the States, they forced processors to turn to waxy maize. Amylopectin is used in food products, in the textile, adhesive, corrugating and paper industry.

Feeding trials later showed that waxy maize produces more efficient gains than normal dent. Interest in waxy maize suddenly mushroomed. Geneticists could show that waxy maize has a defect in metabolism precluding the synthesis of amylose in the endosperm. It is coded by a single recessive gene (wx). Waxy maize yield about 3.5% less than their normal dent counterparts and has to be isolated from any nearby normal maize fields by at least 200 meters.

History

The exact history of waxy maize is unknown. The first mentions of it were found in the archives of the U.S. Department of Agriculture (USDA). In 1908, the Rev. J. M. W. Farnham, a Presbyterian missionary in Shanghai, sent a sample of seeds to the U.S. Office of Foreign Seed and Plant Introduction. A note with the seeds called it: “A peculiar kind of corn. There are several colours, but they are said to be all the same variety. The corn is much more glutinous than the other varieties, so far as I know, and may be found to be of some use, perhaps as porridge.” These seeds were planted on May 9, 1908, near Washington, D.C., by a botanist named G.N. Collins. He was able to grow 53 plants to maturity and made a thorough characterisation of these plants, including photographs, which were published in a USDA bulletin issued in December 1909. [Collins, G. N. (1909). ”A new type of Indian Corn from China.” Bureau of Plant Industry (Bulletin) 161: 1-30.] In 1915, the plant was rediscovered in Upper Burma and in 1920 in the Philippines. Kuleshov, [Kuleshov, N. N. (1954). ”Some peculiarities in the maize of Asia.” (original version in Russian, St-Petersbourg, 1928) Annals of the Missouri Botanical Garden 41(3):271-299.] when screening the distribution of maize in Asia, found it in many other places.

The discovery in China of a distinct type of maize bears the historical question whether maize was known in the Orient before the discovery of America. The question was closed at the end of the 19th century by De Candolle [De Candolle, A. (1883). Origine des plantes cultivées. Marseille, Editions Jeanne Laffitte (republished in 1984).] who stated: ”Maize is of American origin, and has only been introduced into the old world since the discovery of the new. I consider these two assertions as positive, in spite of the contrary opinion of some authors.“ But the finding of this unique variety of maize suggested a re-examination of the question. He also states that Portuguese arrived in China in 1516, simultaneously introducing maize.

Collins supposed that waxy maize has arisen by a way of mutation in Upper Burma [Collins, G. N. (1920). ”Waxy maize from upper Burma.” Science 52(1333): 48-51.] . In this case, it was difficult to conceive that from 1516 on the American plant had had time to penetrate into a wild country inaccessible to foreigners, to produce a mutation, and as such a mutant to spread from the Philippines to Northern Manchuria and the Primorsky region within three to four hundred years. [Kuleshov, N. N. (1954). ”Some peculiarities in the maize of Asia.” (original version in Russian, St-Petersbourg, 1928) Annals of the Missouri Botanical Garden 41(3):271-299.] We are able to counterpart both of these arguments nowadays. At first we know that the waxy mutation is quite common (see #Genetics). Secondly, the fact that maize, if introduced into Asia in Post-Columbian times, must have been rapidly accepted merely indicates that, like the potato in Ireland, it met an acute and pressing need. [Mangelsdorf, P. C. (1974). Corn, Its Origin, Evolution and Improvement. Cambridge, Massachusetts, The Belknap Press of Harvard University Press.] Goodrich [Goodrich, L. C. (1938). ”China’s first knowledge of the Americas.” Geog. Rev. 27: 400-411.] states that there are now in China some 6000 local histories called gazetteers written from A.D. 347 on. Maize was first accurately described in one of them, published in the sixteenth century.

Ho [Ho, P. T. (1956). ”The introduction of American food plants into China.” Am. Anthrop. 57: 191-201.] , an eminent Chinese historian, stated: “Summing up the introduction of maize into China, we may say that maize was introduced into China two or three decades before 1550 . . . ” It might be, as various students concluded, that maize reached Asia before 1492, but currently we are not aware of a single plant fragment, artifact, illustration, or written record to prove it. Therefore, any undocumented statement about its occurrence there in earlier times is to be regarded with scepticism until substantiated. [Mangelsdorf, P. C. (1974). Corn, Its Origin, Evolution and Improvement. Cambridge, Massachusetts, The Belknap Press of Harvard University Press.] [Weatherwax, P. (1950). ”The history of corn.” The scientific monthly 71(1): 50-60.] Thus, the two assertions of De Candolle are still valid.

In his publication, Collins characterised the new plants as possessing a number of unique characters. No indications of these characters in any recorded form of Zea mays had thus far been found. Several of the unique features combine to enable the plant to resist the drying out of the silks by dry, hot winds at the time of flowering. Although the plants produced such small ears that they could find no place in direct competition with the improved varieties, the possession of this adaptation gave the new type an economic interest, particularly in some parts of the semiarid Southwest. Consequently, the effort has been made to combine by hybridising the desirable characters of this small variety with those of larger and more productive types. [Collins, G. N. (1914). ”Inheritance of Endosperm Texture in Sweet x Waxy Hybrids of Maize.” The American Naturalist 48(574): 584-594.]

When Collins found such a distinct difference in the appearance of normal and waxy maize endosperm, he suspected a difference in chemical composition, but the analysis did not yield any unusual results. The percentages of starch, oil, and protein were all within the normal range. Yet, he was intrigued by the physical nature of the starch, and wrote: “In view of the recent development of specialised maize products as human food, the unique type of starch may be of some economic importance."

Actually, for many years the main use of waxy maize was a genetic marker for other maize breeding programs. Breeders were able to use some of the traits to “tag” the existence of hidden genes and follow them through breeding programs. It is possible that waxy maize would have become extinct again in the USA without this special application in breeding.

In 1922, another researcher, P. Weatherwax of Indiana University in Bloomington, reported that the starch in waxy maize was entirely of a “rare” form called “erythrodextrin”, known today as amylopectin [Weatherwax, P. (1922). ”A rare carbohydrate in waxy maize.” Genet. 7: 568-572.] . He found that this rare starch stained red with iodine, in contrast to normal starch which stained blue. Bates, French "et al." [Bates, L. L., D. French, "et al." (1943). ”Amylose and amylopectin content of starches determined by their iodine complex formation.” J. Am Chem. Soc. 65.] and Sprague, Brimhall, "et al." [Sprague, G. F., B. Brimhall, "et al." (1943). ”Some affects of the waxy gene in corn on properties of the endosperm starch.” J. Am. Soc. Agron. 35: 817-822.] confirmed that endosperm starch of waxy maize consists nearly exclusively of amylopectin. The presence of amylopectin in rice had been demonstrated previously by Parnell. [Parnell, F. R. (1921). ”Note on the detection of segregation by examination of the pollen of rice.” J. Genet. 11: 209-212.]

In 1937, just before World War II, G.F. Sprague and other plant breeders at what was then called Iowa State College had begun a crossbreeding program to attempt to introduce the waxy trait into a regular high-yielding hybrid maize. By this time, the waxy plant no longer had the peculiar structural traits noted by Collins, probably due to years of crossing into various genetic stocks. Only the unique endosperm had been retained.

At this time, waxy maize was not so important because the main source of pure amylopectin still was the cassava plant, a tropical shrub with a large underground tuber. [FAO (2004). ”Global cassava market study.” http://www.fao.org/docrep/007/y5287e/y5287e00.htm (14. Jan. 2006).] This situation maintained until World War II, when the Japanese severed the supply lines of the States and forced processors to turn to waxy maize. [ Schopmeyer, H. H. (1943). ”Waxy cornstarch as a replacement for tapioca.” Ind. Eng. Chem. 35: 1168-1172.] Waxy maize appeared to be especially suitable for this purpose because it could be milled with the same equipment already extensively used for ordinary maize. [ Kiesselbach, T. A. and N. F. Petersen (1926). ”The segregation of carbohydrates in crosses between waxy and starchy types of maize.” Genet. 11(5): 407-422.]

H. H. Schopmeyer has advised that the production of waxy maize in Iowa for industrial use amounted to approximately 356 metric tons in 1942 and 2540 tons in 1943. [Schopmeyer, H. H. (1943). ”Waxy cornstarch as a replacement for tapioca.” Ind. Eng. Chem. 35: 1168-1172.] In 1944, there were only 5 varieties of waxy maize available for waxy starch production. In 1943, to cover all the special requirements for amylopectin, approximately 81650 tons of grains were produced. From World War II until 1971, all the waxy maize produced in the U.S. was grown under contract for food or industrial processors. In fact, most of the maize was grown in only a few counties in Iowa, Illinois, and Indiana. [Crookston, R. K. (1979). ”The story of waxy corn maize varieties, uses and history.” Crops and soils 32(9): 11-13.] But in 1970, as most maize growers remember well, the Southern corn leaf blight epidemic (Helminthosporium maydis Nisik. and Miyake) swept the U.S. corn belt.

At the same time, at least 80% of the maize being grown in the U.S. was susceptible to the blight because this maize contained the “Texas type” male-sterile cytoplasm, which allowed production of hybrid seed without mechanical or hand detasseling. So quite naturally, there was a mad scramble in 1971 to find any kind of maize that had normal cytoplasm – cytoplasm that would resist the blight. Consequently, some seed of waxy maize worked its way into the market.

Backcrossing also has been used extensively to transfer individual genes such as wx (waxy), o2 and the Ht gene for resistance to the leaf blight. [Walden, D. B. (1978). Maize breedings and genetics. US, Wiley-Interscience publication.] Some farmers who fed this waxy grain to their beef cattle observed that animals thrived on it. Feeding trials were set up which suggested that the waxy maize produced more efficient weight gains than normal dent. Interest in waxy maize suddenly mushroomed, and this maize type abandoned the status of botanical curiosity and speciality product to become the subject of major research importance. [Crookston, R. K. (1979). ”The story of waxy corn maize varieties, uses and history.” Crops and soils 32(9): 11-13.] In 2002, an estimated 1,200,000 to 1,300,000 tonnes of waxy maize was produced in the United States on about 2,000 km², representing only 0,5% of the total maize production. [US Grain Council (2002). ”Value enhanced-corn (VEC) quality report 2001-2002.” http://www.vegrains.org/documents/2002veg_report/toc/tablecont.html (30.01.06).]

Biology

Chinese Maize

Collins noted, among others, these unusual traits of the Chinese maize:
*Several unique structural features that enabled the plants to resist the drying out of the silks by wind at the time of flowering
*Unusual growth behaviour in that the top four or five leaves all appeared on the same side of the main stem of the plant. Extremely erect leaves of the upper nodes, while the lower leaves were more spread and drooping
*One of the main things he noted was the composition of the endosperm of the maize kernels. He wrote:

“The texture of the endosperm is one of the unique features of this maize. Cut in any direction it separates with a sort of cleavage, exposing a dull, smooth surface. The texture suggests that of the hardest waxes, though it is still harder and more crystalline. From this optical resemblance to wax the term cereous or waxy endosperm is suggested.”

The moisture content of the kernel must be 16% or lower before the waxy trait can be recognised visually. [Fergason, V. (2001). High Amylose and Waxy Corns (pp. 63-84). Specialty Corns. A. R. Hallauer, Boca Raton, CRC Press: 479 pp.] The starch of normal dent maize is characterised by a content of about 25% amylose with the remainder being amylopectin and the intermediate fraction (see 3.5 Biochemistry). But these percentages vary among cultivars and with kernel development. For example, amylose percentage ranged from 20 to 36% for 399 cultivars of normal maize. [Deatherage, W. L., M. M. Macmasters, "et al." (1955). ”A partial survey of amylose content in starch from domestic and foreign varieties of corn, wheat and sorghum and from some other starch-bearing plants.” Trans. Am. Assoc. Cereal Chem. 13(31).] [ Whistler, R. L. (1984). History and future expectation of starch use. Starch: Chemistry and Technology, R. L. Whistler, J. N. BeMiller, E. F. Paschell, academic Press, Orlando, FL.] There are maize germplasm sources available that range from less than 20 to 100% complement of amylopectin. [ Fergason, V. (2001). High Amylose and Waxy Corns (pp. 63-84). Specialty Corns. A. R. Hallauer, Boca Raton, CRC Press: 479 pp.] Waxy maize contains 100% amylopectin. [Sprague, G. F., B. Brimhall, "et al." (1943). ”Some affects of the waxy gene in corn on properties of the endosperm starch.” J. Am. Soc. Agron. 35: 817-822.]

It is of main interest because fractionation of normal starch to obtain pure amylose or amylopectin is very costly. Waxy endosperm is inherently a defect in metabolism, and its low frequency in most maize populations in the face of recurring mutations indicates that it is acted against by natural selection. [Mangelsdorf, P. C. (1974). Corn, Its Origin, Evolution and Improvement. Cambridge, Massachusetts, The Belknap Press of Harvard University Press.]

Genetic drift

Experiments by Sprague [Sprague, G. F. (1939). ”An estimation of the number of the top crossed plants required for adequate representation of a corn variety.” J. Am. Soc. Agron. 31: 11-16.] have shown that ten to twenty plants are required for adequate representation of genetic diversity in an open-pollinated maize variety. Since the number of ears saved for seed by ancient Asian maize cultivators with only small plots of land at their disposal was often smaller than this and, indeed, since new maize populations are sometimes established by growing the progeny of a single ear, it follows that there must often have been genetic drift – changes in gene frequencies resulting from the creation of small breeding populations.

A striking example of genetic drift in maize is the occurrence in parts of Asia of varieties with waxy endosperm. In maize races of America such a variety is unknown, but the waxy character itself has been discovered in non-waxy varieties: in a New England flint maize [ Mangelsdorf, P. C. (1924). ”Waxy endosperm in New England Maize.” Science 60 (1549): 222-3.] and in a South American variety. [Andrés, J. M. and P. C. Bascialli (1941). ”Characteres hereditarios aislaidos en maices cultivados en la Argentina.” Univ. Buenos Aires Inst. Genet. 2: 1.] [Breggar, T. (1928). ”Waxy endosperm in Argentine maize.” J. Hered. 19: 111.]

The fact that waxy maize occurs so commonly in a part of the world that also possesses waxy varieties of rice, sorghum, and millet can be attributed to artificial selection. The people of Asia being familiar with waxy varieties of these cereals and accustomed to using them for special purposes recognised the waxy character in maize after it was introduced into Asia following the discovery of America and purposely isolated varieties purely for waxy endosperm. But the fact that waxy endosperm came to their attention in the first place is probably due to genetic drift. The gene for waxy endosperm, which has a low frequency in American maize, apparently attained a high frequency in certain samples of Asian maize.

Indeed, the practice reported by Stonor and Anderson [ Stonor, C. R. and E. Anderson (1949). ”Maize among the hill people of Assam.” Ann. Missouri Bot. Gard. 36: 355-404.] of growing maize as single plants among other cereals would result in some degree of self-pollination and, in any stock in which the waxy gene was present, would inevitably lead in a very short time to the establishment of pure waxy varieties with special properties that people accustomed to the waxy character in other cereals could hardly fail to recognise. [Mangelsdorf, P. C. (1974). Corn, Its Origin, Evolution and Improvement. Cambridge, Massachusetts, The Belknap Press of Harvard University Press.]

Genetics

A single recessive gene (wx), located on the short arm of chromosome 9, codes for the waxy endosperm of the kernel (Wx codes for endosperm with normal starch). [Neuffer, M. G., E. H. Coe, "et al." (1997). Mutants of maize. New York, Cold Spring Harbor Laboratory Press.] This was first shown by Collins [Collins, G. N. (1909). ”A new type of Indian Corn from China.” Bureau of Plant Industry (Bulletin) 161: 1-30.] and Kempton. [Kempton, J. H. (1919). ”Inheritance of waxy endosperm in maize.” USDA Bull. 754.] The structure of the wildtype waxy (wx+) locus has been determined through DNA sequence analysis. [Klösgen (1986). Molekulare Analyse des waxy Gens aus Zea mays. Köln, Universit¨at zu Köln: 55 pp.] The gene has 3718 bp (14 exons and 13 introns). Waxy endosperm is the counterpart in maize of the “glutinous” character in rice. [Mangelsdorf, P. C. (1974). Corn, Its Origin, Evolution and Improvement. Cambridge, Massachusetts, The Belknap Press of Harvard University Press] There is a wide range of species also presenting the waxy mutation, including rice, sorghum, millet, barley and wheat, which were characterised by starch granules staining red with iodine.

In crosses between heterozygous plants for the waxy character, a small but significant deviation from an expected Mendelian ratio in self pollination is produced. [Kiesselbach, T. A. (1944). ”Character, field performance, and commercial production of waxy corn.” J. of Am. Soc. of Agr. 36(8): 668-682.] [Walden, D. B. (1978). Maize breedings and genetics. US, Wiley-Interscience publication.] Bear [Bear, R. P. (1944). ”Mutations for waxy and sugary endosperm in inbred lines lines of dent corn.” J. Am. Soc. Agron. 36: 89-91.] obtained from 71 segregated ears on the F1 generation 23,77% of waxy kernels and 76,23% of non-waxy kernels. This is evidenced by the two heterozygous types, Wx Wx wx and wx wx Wx. The waxy gene is epistatic [Boyer, C. D., D. L. Garwood, "et al." (1976). ”The interaction of the Amylose-Extender and Waxy Mutants of Maize (Zea Mays L.).” Starch 28: 405-410.] [Creech, R. G. (1968). ”Carbohydrate synthesis in maize.” Adv. Agron. 20: 275.] for all known other starch forming mutants genes like dull (du), sugary-1 (su1 ) and sugary-2 (su2), [ White, P. J. (1994). Properties of corn starch (pp. 29-54). Specialty corns, Boca Raton, CRC Press: 410 pp.] it increases sugars and water-soluble polysaccharides (WSP) in a su1 background [Andrew, R. H., R. A. Brink, "et al." (1944). ”Some effects of the waxy and sugary genes on endosperm development in maize.” J. Agr. Res. 69: 355-371.] and it causes dramatic increases in sugars and reduction in starch with ae or ae du. The mutation from Wx to wx is not uncommon in Corn Belt varieties, Bear [Bear, R. P. (1944). ”Mutations for waxy and sugary endosperm in inbred lines lines of dent corn.” J. Am. Soc. Agron. 36: 89-91.] having found three separate mutations to waxy in three consecutive years in a total population of some 100,000 selfed ears.

Mangelsdorf [Mangelsdorf, P. C. (1947). ”The inheritance of amylaceous sugary endosperm and its derivatives in maize.” Genet. 32: 448-458.] found also many mutants on his trial fields. Argentine waxy (wx-a), an allele at the waxy locus first reported by Andrés and Bascialli, [Andrés, J. M. and P. C. Bascialli (1941). ”Characteres hereditarios aislaidos en maices cultivados en la Argentina.” Univ. Buenos Aires Inst. Genet. 2: 1.] is known to produce small amount of amylose (< 5%) and gives an intermediate staining reaction with iodine. Other mutant alleles at the waxy locus have been reported which possess similar starch properties to those observed with wx. [Bear, R. P. (1944). ”Mutations for waxy and sugary endosperm in inbred lines lines of dent corn.” J. Am. Soc. Agron. 36: 89-91.] [Nelson, O. E. (1968). ”The waxy locus in maize. II. The location of the controlling element alleles.” Genet. 60: 507-524.] [Neuffer, M. G., E. H. Coe, "et al." (1997). Mutants of maize. New York, Cold Spring Harbor Laboratory Press.] More than 40 mutant alleles are known for the waxy locus, making up the finest collection of mutations found among higher plants. [Wessler, S. R. and M. J. Varagona (1985). ”Molecular basis of mutations at the waxy locus of maize: correlation with the fine structure genetic map.” Genet. 82: 4177-4181.] Some of them are very stable whereas others are very unstable.

The phenotype of the stable mutants remains unchanged whereas the one of unstable mutants changes because of the insertion of transposable elements (5-8). [Wessler, S. R. and M. J. Varagona (1985). ”Molecular basis of mutations at the waxy locus of maize: correlation with the fine structure genetic map.” Genet. 82: 4177-4181.] For a listing of all these mutations, the excellent book of Neuffer, Coe "et al." [Neuffer, M. G., E. H. Coe, "et al." (1997). Mutants of maize. New York, Cold Spring Harbor Laboratory Press.] is greatly recommended. Because the waxy mutation is expressed in an easy identifiable nonlethal phenotype, it has been the subject of major research during the 20th century. Nelson [Nelson, O. E. (1968). ”The waxy locus in maize. II. The location of the controlling element alleles.” Genet. 60: 507-524.] made a fine structure genetic map of most of these mutations.

Genotype and characterisation with iodine

The wx locus is expressed in the endosperm, in the male gametophyte (pollen) [Demerec, M. (1924). ”A case of Pollen Dimorphism in Maize.” Am. J. of Botany 11(7): 461-464.] [Weatherwax, P. (1922). ”A rare carbohydrate in waxy maize.” Genet. 7: 568-572.] as well as in the female gametophyte (embryo sac). [Brink, R. A. (1925). ”Mendelian ratios and the gametophyte generetion in angiosperms.” Genetics 10: 359-388.] Amylose and amylopectin have different iodine binding-properties, with maize amylose and amylopectin giving iodine affinity (IA) values of about 19 to 20 and 1%, respectively, depending upon the source. Weatherwax discovered this process in 1922. [Weatherwax, P. (1922). ”A rare carbohydrate in waxy maize.” Genet. 7: 568-572.]

The amount of apparent amylose can be determined either by measuring the absorbency of the starch-iodine complex (blue-value) and relating this value to that of pure amylose and amylopectin standards [ Shannon, J. C. and D. L. Garwood (1984). Genetics and physiology of starch development. Starch: Chemistry and Technology, R. L. Whistler, J. N. Be- Miller, E. F. Paschell, Academic Press, Orlando, FL: 25.] or by measuring the amount of iodine (mg) bound per 100 mg of starch in a potentiometric titration and relating the value to the amount bound by an amylose standard. [ Banks, W., C. T. Greenwood, "et al." (1974). ”The characterization of starch and its components. Part VI. A critical comparison of the estimation of amylosecontent by colorimetric determination and potentiometric titration of the iodine-complex.” Starch 26: 73-78.] Values used on the iodine binding, however, are only estimates of amylose content because of differences in the binding abilities (and structure) of amylose and amylopectin among starch types. For example, amylopectin molecules with long external branches bind more iodine than those with shortbranches do, [Banks, W. and C. T. Greenwood (1975). The reaction of starch and its components with iodine. Starch and its components. Edinburgh, University Press: 67.] [Banks, W., C. T. Greenwood, "et al." (1970). ”The properties of synthetic amylopectin with long external chains.” Starch 22: 292-296.] resulting in a small measure of apparent amylose. Chromatographic profiles of wx-containing starches, however, reveal no amylose peak. The wave-length at which a starch-iodine complex has maximum absorbency is referred to as the lambda max. [ White, P. J. (1994). Properties of corn starch (pp. 29-54). Specialty corns, Boca Raton, CRC Press: 410 pp.]

Plants which are heterozygous on the waxy gene (Wx:wx) can be characterised by staining the pollen with iodine. Half of the pollen will be blue and half brown whereas the kernels will stay blue (very helpful in backcrossing program). If the plant is homozygous recessive (wx:wx) the whole pollen will be brown and the kernel too. Being homozygously dominant (Wx:Wx) the iodine will appear only blue.

Biochemistry

Normal dent maize has two different pathways for starch formation: one leading to branched chain (amylopectin) and the other to straight-chain polysaccharides (amylose). [Nelson (1962). ”The waxy locus in maize. I. Intralocus recombination frequency estimates by pollen and by conventional analysis.” Genet. 47: 737-742.] The amylopectin consists of chain of α-D-(1-4) and α-D-(1-6)-glucosidic linkages that form a branched molecule. Amylose is primarily linear with α-D-(1-4)-linked glucoseresidues. However the full starch content is the same in both genotypes. [Nelson (1962). ”The waxy locus in maize. I. Intralocus recombination frequency estimates by pollen and by conventional analysis.” Genet. 47: 737-742.] The locus wx code for a starch granule-bound nucleotide diphosphate-starch glycosyl transferase (UDPG) responsible for amylose biosynthesis. [Echt, C. S. and D. Schwartz (1981). ”Evidence for the inclusion of controlling elements within the structural gene at the waxy locus in maize.” Genet. 99: 275-284.] [Nelson (1962). ”The waxy locus in maize. I. Intralocus recombination frequency estimates by pollen and by conventional analysis.” Genet. 47: 737-742.] It catalyses the 1–4 linkage from glucose residues to amylose synthesis in the developing endosperm. This enzyme is located in the amyloplasts and is the major component of the starchbound protein in maize. [Tsai, C. Y. (1974). ”The Function of the Waxy Locus in Starch Synthesis in Maize Endosperm.” Biochem. Genet. 11(2): 83-96.] Nelson showed that starch granules from wx wx wx endosperm had very low starch granule-bound glucosyl transferase activity.

When measuring if the activity of the transferase was a function of the Wx dosage in diploid and tetraploid maize, Akatsuka [Akatsuka, T. and O. E. Nelson (1969). ”Studies on starch synthesis in maize mutants.” J. Jap. Starch Sci. 17: 99-115.] noticed a linear proportionality between a preparation of Wx Wx Wx and Wx Wx Wx Wx Wx Wx . Nevertheless the amylose content was the same in both types suggesting that activity of the transferase is not directly linked to the amylose content.

In maize and some other plants, there is evidence of a starch molecule that is intermediate in size to amylose and amylopectin. [ White, P. J. (1994). Properties of corn starch (pp. 29-54). Specialty corns, Boca Raton, CRC Press: 410 pp.] The intermediate fraction contains chains of (1–4)-linked alpha-D-anhydroglucose residues, but the average length of these chains and the number of chains per molecule are different from those in either amylopectin or amylose. Several researchers [Boyer, C. D., D. L. Garwood, "et al." (1976). ”The interaction of the Amylose-Extender and Waxy Mutants of Maize (Zea Mays L.).” Starch 28: 405-410.] [Lansky, S., S. Kooli, "et al." (1949). ”Properties of the fractions and linear subfractions from various starches.” J. Am. Chem. Soc. 71: 4066.] demonstrated the presence in normal maize starch of about 5 to 7% intermediate polysaccharides, basing their conclusions on indirect evidence from IA.

As early as in 1956, it was stated that amylopectin contained three different types of chains. [Peat, S., W. J. Whelan, "et al." (1956). ”The enzymic synthesis and degradation of starch. XXII Evidence of multiple branching in waxy-maize starch.” J. of the Chem. Soc.: 3025-3030] In each macromolecule there is one C-chain, which carries the only reducing group. The B-chains are linked to the macromolecules linked by their potential reducing group, and may contain one or more A-chains that are similarly linked. The ratio of A-B chains (1:1 to 1,5:1) is a measure of the degree of multiple branching and is an important property describing amylopectin. Nevertheless the exact arrangement of chains within the amylopectin molecule is still not clear. [ White, P. J. (1994). Properties of corn starch (pp. 29-54). Specialty corns, Boca Raton, CRC Press: 410 pp.] The aewx starch contain 21% apparent amylose and has a lambda max. of 580 for the iodine-starch complex. The aewx outer chains are longer than those of wx and fewer in number per weight of starch. In general, the aewx starch had a unique structure that is similar to the anomalous amylopectin (intermediate fraction) reported in ae starch.

Increased dosage at the ae locus, regardless of the genotype at the wx locus, resulted in amylopectin with increased linearity. Short-chained amylose (approximately 100 glucose units) was observed in all ae genotypes in a homozygous Wx background. [Boyer, C. D., D. L. Garwood, "et al." (1976). ”The interaction of the Amylose-Extender and Waxy Mutants of Maize (Zea Mays L.).” Starch 28: 405-410.] Amylopectin of the aewx mutants had an increased proportion of long B-chains and a decreased proportion of short B-chains compared with wx amylopectin, whereas amylopectin of the duwx mutant had a decreased proportion of long B-chains and an increased proportion of short B-chains, thus confirming the novel nature of aewx and duwx amylopectin.

The A:B chain ratios, however, for amylopectin from aewx, aewxfl2, aewxsu1, aewxsu2, btwx, duwx and su2wx were in the range of 1.1 to 1.4 and all similar to the wx amylopectin. [Fuwa, H., D. V. Glover, "et al." (1987). ”Chain length distribution of amylopectin of double and triple mutants containing the waxy gene in the inbred Oh 43 maize background.” Starch 39: 295-298.] This unique structure of aewx maize was confirmed by Yamada "et al." [??] and they termed it ”amylo-waxy“ maize.

Agronomic features

Growing maize with pure amylopectin starch is not easy. The waxy gene being recessive, the waxy maize has to be isolated from any nearby normal maize field by at least 200 meters. In maize mono-cropping, volunteers are not uncommon in regions without severe frost. A few volunteers in a waxy field will be enough to contaminate the whole field even if it is isolated [19] . One alternative to this is to use the outside 15 to 20 rows of the field as a buffer strip. They can be fed to cattle. The necessity of detecting some contamination from normal maize type could support the creation of a waxy variety with white kernels. This would allow to select the kernels with a photoelectric cell and to reject the contaminated seeds [30] .

No information could be found about the eventual application of this great idea. Responses to fertilisers, diseases, insects, environmental stresses, etc. . . theoretically should not differ between waxy hybrids and their dent counterparts because no physical or chemical differences should exist between these two grain types, except in the kernel. Sucrose is the sugar of transport, but it is not normally converted into starch until it is translocated to the maize endosperm.

Yield

The production quality and disease resistance of waxy maize appears to be identical tonormal types. Kiesselbach indicated that the weight of waxy kernel is slightly higher. Collins obtained an increase of 16% in kernel weight of waxy maize outcrossed by dent maize. Schopmeyer [57] reported that waxy maize starch has approximately 1% highermoisture equilibrium than ordinary maize starch at various moisture and temperatures.Gallais reported a 2 to 3% higher moisture content than normal maize at harvest [30] .The observation that the waxy character may result in approximately 3% less translocationfrom the stalk to the grain suggests that two hybrids, identical in all genes exceptwaxy, would differ about 3% in yield of grain per ha [37] .

Breeding

The waxy locus is easy to introduce through back-crossing, but this causes a loss of productivityof about 3 to 10% [30] . From the onset of many waxy inbreds development programs,the back-cross method has probably been the most popular. There is one compelling reasonto choose this breeding method for waxy hybrid improvement. Competition betweenthe seed companies to provide better hybrids as fast as possible encouraged many breedersto choose the backcross breeding method. Conversion of elite dent lines provide the fastestand most positive result [24] . However, the new hybrids are only expected to have equalbut not to exceed the performance of their counterparts. And meanwhile, there are newerand better dent hybrid developments. That is the reason why another breeding method iswished. Initially, most private waxy breeding programs did not have adequate reserves ofwaxy converted germplasm to permit their utilisation in more complex breeding schemes.Nowadays, enough germplasm material exists and a recurrent selection program would bepossible. But due to the limited resources and markets for waxy maize, no long-term programhas yet been conducted. Actually, it does not seem that long-term recurrent selectionprograms are really necessary for waxy maize breeding [60] . In a pedigree-selection schemeperformed within a breeding population derived from crossing two elite inbred sources, it isnecessary that only one of the two inbred lines contributes the waxy gene to the population.

Companies

Most maize seed companies do not have a waxy maize breeding program, or if they do, it isapparently a small and insignificant part of their total breeding effort. In the U.S., in 1994,there were fewer than six private seed companies devoting any significant breeding efforts to the development of waxy maize hybrids. They were about 12 to offer waxy hybrids forretail sales. Most of these companies offer fewer than five waxy hybrids covering a verynarrow range in relative maturity. There was only one exception offering 20 waxy hybridsranging in relative maturity from 83 to 122 days.

Unlike the complexities associated with hybrid improvement of high amylose maize,waxy maize breeding programs are generally more conventional and less laborious. Thisphenomenon is due to the unique expressivity of the waxy gene in maize germplasm andthe ease with which it can be transferred between and within breeding populations [24] .A majority of the commercially produced waxy grain is produced under contract to wetmilling companies. The contract agreements, in addition to specifying certain grain qualitystandards, also have waxy purity requirements of waxy grain. Premiums are paid to thewaxy grain producer by the wet miller or the waxy grain exporter as compensation for theextra quality control procedures that must be followed [24] .

Utilization

The amylopectin: industrial uses

Starch is the reserve carbohydrate in the plant kingdom. Although starch occurs throughout the plant world, there are only a few plants used to produce it commercially, and maizeis the major source of starch produced world-wide [73] . At the second range comes potato, then wheat and to a lesser extent rice. Maize starch was first produced in the U.S. in 1844at the plant owned by William Colgate in Jersey City, New Jersey [72] .

The amylopectin yield of the kernel ranges from 58,5 to 69% (of dry solid mass) [59] . Wet-milling waxy maize results in starch yields that are only 90% of those of dent maize [73] . The wx starch is relatively easy to gelatinise and produces a clear viscous paste with a sticky or tacky surface, rather than one with sharp edges. This paste resembles pastes of root or tuberstarches, such as potato or tapioca. Most starches in their native or unmodified form have limited use in various industries. Therefore, most starches including waxy maize starch are modified either to improve or repress their inherent properties as may be required for special use applications. Manytypes of modified waxy starches have a multitude of applications in the paper, textile, corrugating, and adhesive industries in addition to an enormous array of application in the food industry [47] .

Food products

Modified waxy maize starches serve essential functions in foods, including the improvement of uniformity, stability, and texture in various food products. The clarity and stability of amylopectin starch make it especially suitable for thickening fruit pies. It improves smoothness and creaminess of canned food and dairy products as well as freeze-thaw stability of frozen foods. It gives a more desirable texture and appearance to dry foods and mixes [24] . Waxy maize starch is also the preferred starting material for the production of maltodextrins because of improved water solubility after drying and greater solution stability and clarity [59] .As of recent, the dietary supplement industry has seen an increase in the usage of Swedish Waxy Maize Starch. It does have a moderate Glycemic index, however, it is its unique osmolity that allows it to pass through the digestive system relatively quickly and help pull other nutrients with it into the bloodstream. It's purported effect is assumed to be a greater insulin release and having a higher amount of desired materials reach the blood stream in a smaller time frame.

Textile industry

Waxy starch is of interest for the textile industry because of its ability to make transparentfilms [38] .6.1.3 Adhesive industryStarch from waxy maize differs from regular maize starch in both molecular structureand pasting characteristics. According to Watson [67] , pastes made from waxy starch arelong and cohesive; whereas, pastes made from regular maize starch are short and heavybodied. Waxy maize starch is a major starch component in adhesives used for makingbottle labels. This waxy starch based adhesive imparts resolubilizing resistance to thelabels which prevents their soaking off the bottle if immersed in water or being subjectedto very high humidity conditions. Moreover, waxy maize starches are commonly used inthe manufacture of gummed tapes and envelope adhesives.

Papermaking

Major advancements in papermaking technology are in part related to the availability ofnew types of modified waxy starches. These starches essentially provide binding or bondingqualities to the papermaking process, as well as adding other essential features, includingimproved sizing for greater paper strength and printing properties [24;42] . Papermakingis a highly technical science and the writer cannot cover this more thoroughly because itwould be a major theme by itself. For more details about papermaking consult the chap. 13of the book of Wurzburg [42] .

tarches

Although there is already a large array of available modified starch from normal or waxymaize, there is a growing interest in producing all-natural starches. In fact, consumersprefer it and the rules for regulatory approval of chemical processes are more and morerestrictive, thus encouraging the finding of native natural materials to partly replace thechemically modified ones [73] . Several patents have been deposed on double mutant ofmaize. Two patents have been deposed on the wxsu2 maize genotype, one to use starchas thickener with improved low-temperate stability [74] , and the other as an antistalentin bread [77] . In the second case, the bread was said to have a softer, moisture crumbafter baking and a fresh texture and appearance after storage than bread made without it.Several patents from the American Maize-products Company are based on new starchesthat can replace chemically modified ones, all of them containing the wx gene [25–28] .These starches all present a modified viscosity, clarity, paste appearance or freeze-thawstability.

Livestock, dairy and poultry feeding

The feeding of waxy maize began in the 1940s. Beginning with a research report in 1944, waxy maize seemed to have the potential to increase feed conversion efficiencies [48] . Manyother feeding trials involving swine, beef and dairy cattle, lambs and poultry were designed to compare the feeding value of waxy to normal dent grain [11;32;46;54] . Generally, the trials indicated an advantage for feeding waxy grain. Seldom have the investigations shown any negative or adverse effects from feeding waxy grains. Increases of both milk production and butterfat content are not uncommon when waxy maize is fed to lactating dairy cattle [24] . Increases of more than 20% in average daily weight gains in fattening lambs were observed when waxy grain was compared with normal dent [46] . In addition, a 14% increase in feed efficiency was noted in favor of waxy grain. Likewise an increase in feed efficiency approaching 10% was obtained in trials where waxy grain was compared with the dent counterparts when fed to finishing beef cattle. The pancreatic digestibility of starches was analysed for several genotypes [56] . They noticed that ae has very low value. We could come to the conclusion that high amylose content is correlated with bad digestibility, but we see that du and su2, also characterised by a high amylose content, present an excellent digestibility. Thus, it cannot be linked. Sandsted [56] suggested that digestibility could lie in the structure of starch granule, in differences in bonding of the starch molecules and in possible anomalous linkages betweenmolecules.

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