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.
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 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 . 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. 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. Goodrich 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 , 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. 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.
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. He found that this rare starch stained red with iodine, in contrast to normal starch which stained blue. Bates, French et al. and Sprague, Brimhall, et al. confirmed that endosperm starch of waxy maize consists nearly exclusively of amylopectin. The presence of amylopectin in rice had been demonstrated previously by Parnell.
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. 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. 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.
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. 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. 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. 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. 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.
“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. 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. There are maize germplasm sources available that range from less than 20 to 100% complement of amylopectin. Waxy maize contains 100% amylopectin.
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.
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 and in a South American variety.
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 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.
In crosses between heterozygous plants for the waxy character, a small but significant deviation from an expected Mendelian ratio in self pollination is produced. Bear 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 for all known other starch forming mutants genes like dull (du), sugary-1 (su1 ) and sugary-2 (su2), it increases sugars and water-soluble polysaccharides (WSP) in a su1 background 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 having found three separate mutations to waxy in three consecutive years in a total population of some 100,000 selfed ears.
Mangelsdorf found also many mutants on his trial fields. Argentine waxy (wx-a), an allele at the waxy locus first reported by Andrés and Bascialli, 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. More than 40 mutant alleles are known for the waxy locus, making up the finest collection of mutations found among higher plants. 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). For a listing of all these mutations, the excellent book of Neuffer, Coe et al. 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 made a fine structure genetic map of most of these mutations.
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 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. 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 short branches do, 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.
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.
When measuring if the activity of the transferase was a function of the Wx dosage in diploid and tetraploid maize, Akatsuka 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. 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 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. 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. 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. 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. This unique structure of aewx maize was confirmed by Yamada et al. [??] and they termed it ”amylo-waxy“ maize.
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.
Unlike the complexities associated with hybrid improvement of high amylose maize, waxy maize breeding programs are generally more conventional and less laborious. This phenomenon is due to the unique expressivity of the waxy gene in maize germplasm and the ease with which it can be transferred between and within breeding populations . A majority of the commercially produced waxy grain is produced under contract to wet milling companies. The contract agreements, in addition to specifying certain grain quality standards, also have waxy purity requirements of waxy grain. Premiums are paid to the waxy grain producer by the wet miller or the waxy grain exporter as compensation for the extra quality control procedures that must be followed .
The amylopectin yield of the kernel ranges from 58,5 to 69% (of dry solid mass) . Wet-milling waxy maize results in starch yields that are only 90% of those of dent maize . 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 tuber starches, 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. Many types 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 .
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