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V. Genetic Applications of Mutants

V. Genetic Applications of Mutants

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All of the new mutant genes-for semidwarfism, early maturity, waxy endosperm, disease resistance, male sterility, etc.-are useful additions to the array

of marker genes in rice. These genes can be used to define linkage groups more

fully in this important crop. The additional induced mutant genes that may not

have immediate breeding or genetic applications are also useful as marker genes.

These latter genes include such as the three hull color mutants that have been

found in California in recent years.

One is a “yellow panicle” mutant found by the author after 6oCo irradiation of

the semidwarf cultivar M-101, which is characterized by light yellow-green

leaves that are especially noticeable during the seedling stage, by light yellowgreen panicles persisting until about midway between heading and maturity, and

by maturity approximately 7 days earlier than the parent cultivar. The yellow

panicle mutant appears to be controlled as a single recessive gene with a

pleiotropic effect for early maturity (Azzini, 1983). Yield potential has not been

measured, but the mutant appears generally productive.

“Pale green hull” mutants of the cultivars M-101 and M-201 were found

independently by the author and C. W. Johnson, respectively, in 1980. The

mutants are characterized by pale green hulls that are noticeable from heading

until about midway between heading and maturity. The pale green color apparently results from less chlorophyll in the glumes. The mutant found by the author

appears to be recessive, because F, plants from a cross between it and the normal

color germplasm line CI 11051 had normal color glumes. Yield potential has not

been measured but the mutant appears to be as productive as its parent, M-101.

“Gold hull” mutants of the cultivar M-101 also were found independently by

the author and by H:L. Carnahan in 1980. The mutants are characterized by

golden glume colors at maturity. These are assumed to be the same as the

recessive gold hull gene (gh) that has been used widely in breeding programs in

the southem United States. Although it might be anticipated that the gold hull

M-101 lines arose from outcrossing with southern cultivars in the breeding

nurseries, the otherwise normal M-101-like nature of the gold hull lines indicates

that they arose from mutations. Yield of the gold hull mutants has not been

measured, but they also appear to be as productive as the M-101 parent.

Long-culm or tall mutants also may be included in the lists of useful genetic

markers. Okuno and Kawai (1977, 1978a) noted that long-culm mutants are

much rarer than short-culm mutants, but they found eight induced long-culm

mutants in the cultivar Norin 8. In subsequent genetic studies on five of the longculm mutants, Okuno and Kawai (1978b) found that long culm in one mutant,

LM- 1, was controlled by a single recessive gene; however, in another mutant,

LM-3, long culm was controlled by a single dominant gene. Single gene control

was not indicated in the other three mutants.



Hajra et al. (1982) recently reported a recessively inherited, extremely tall

mutant in the tall cultivar Latisail. Earlier, the same group of researchers had

reported a dominant dwarf in the cultivar IR8 (Mallick et al., 1980). In addition

to uses as marker genes, these recessive tall or dominant dwarf mutants may have

applications in producing hybrid rice seed; Rutger and Camahan (1981) reported

the discovery of a putative natural mutation for recessive tall height. They

proposed the use of this recessively inherited tall plant type as a genetic tool for

facilitating hybrid seed production. The tall plant should have a pollen distribution advantage over a semidwarf parent when interplanted with semidwarf maternal plants of a commercial hybrid. Following pollination, the tall males can be

removed from the seed field. Because tall height is recessively inherited, the

resulting hybrids are short as in the female parent, which is desirable in situations

in which short plants are known to be more productive than tall plants. The

recessive tall reported by Rutger and Carnahan (1981) was designated eui for the

elongated uppermost internode that is responsible for the increase in height. The

eui character originated in an F3 population of a cross between a semidwarf and a

tall cultivar, presumably by spontaneous mutation.

Other marker genes include an endosperm mutant induced in California,

which resembles waxy mutants phenotypically. However, thus mutant, termed

“opaque,” apparently has near-normal amylose content (10-15%) compared to

the <0.1% amylose of Calmochi-202. The opaque mutant is inherited as a

simple recessive character (Azzini, 1983) and may be the same as the “crumbly” mutant in rice. Seeds on recessive homozygotes show a xenia effect when

pollinated by normal plants, making the opaque endosperm mutant useful as a

marker gene in outcrossing studies. The possible allelic nature of the opaque

gene and the waxy gene has not been clarified.

Satoh and Omura (1981) induced several different kinds of endosperm mutants

in rice in order to supply materials for improving seed quality. As a rationale for

their work, they noted that several kinds of endosperm mutants have been found

and used in genetic studies in maize but that almost the only known endosperm

mutation in rice was the waxy character. They found endosperm mutants which

they labelled as white core, floury,waxy, dull, wrinkled, or immature grain.

They also found a giant embryo mutant. Each of the induced mutants, except for

one of the flourymutants, was controlled by a single recessive gene. They found

two other endosperm mutants, labeled sugary and shrunken, for which no genetic information was given. Biochemical and other studies are continuing to determine if there are uses for these mutants analogous to their counterparts in maize

(Satoh and Omura, 1981). Regardless of the possible breeding uses of these

mutants, they are valuable additions to the stocks of marker genes.

Other morphological mutants, such as chlorophyll deficiencies, round grain

shape, extreme dwarfs, etc., are observed frequently in irradiated populations.

However, as most mutants seem to be repetitions of known marker genes, they



are not expected to be of much value unless they are in a specific background that

is needed for genetic studies.




A recent IAEA symposium on “Induced Mutations-A Tool in Plant Breeding” highlighted the uses of mutants for developing better understanding of plant

growth and functions. In this symposium, Vose (1981) observed that induced

mutants are useful in studies of physiology because the mutants may differ in

only one major physiological character. He also noted that mutants can provide

previously unknown variation for physiological characters, such as the nitrate

reductase-deficient mutants reported in the same symposium (Feenstra ef al.,

1981; Nilan et al., 1981). In rice, Rutger and Peterson (1981) described the use

of mutants in a three-phase program to develop higher yielding plant models

(Fig. 4).

In Phase 1, induced mutant genes were used with existing genes to develop

near-isogenic comparisons quickly. In Phase 2, the near-isogenics were used to

test agronomic and physiologic hypotheses about the bases for increased yield. In


Develop near-lsogenlcs

for testlng agronomic and

physlol oglc hypotheses

Design higher

ylel ding plants ;

release gemplasm







Use near-lsogenlcs

to &tennine yieldlinltlng factors

FIG. 4. Three-phase program to develop higher yielding models of the rice plant. From Rutger

and Peterson (1981).




Phase 3, the information developed from Phase 2 was used to design more

productive future plants. A critical assumption was that induced mutants for

single gene characters such as height and early maturity were isogenic to the

parent cultivar. Because there is considerable evidence that mutants often have

associated or pleiotropic effects on other characters (Okuno and Kawai, 1977;

Gale et al., 1982), it is usually considered desirable to backcross the “raw”

mutant to its parent so that it may be “cleaned up” and thus more truly isogenic

to its parent. In the California work, there have been minimal side effects

associated with the useful mutants, perhaps because emphasis was placed on

selecting plants normal for attributes other than the mutant characteristics. Thus

the need for clean up of mutants has not seemed critical in most cases


The mutants and various recombinants involving mutants enabled Rutger and

Peterson (1981) to demonstrate that critical elements of high yield included

semidwarfism, seedling vigor, resistance to cold-induced sterility, and early

maturity. In addition, they developed evidence that sink size is a limiting factor

for yield. They proposed that future work involve use of mutants and their

recombinants to manipulate yield components in source-sink and partitioning

studies. Examples of mutants that may be useful in such studies include the

narrow-leaf types found in both California and Japan (Rutger et al., 1982a;

Yamaguchi et al., 1981), which have higher net photosynthesis per unit leaf


Satoh and Omura (198 1) also noted that their various endosperm mutants may

be useful in advancing knowledge of plant development, as each mutant affected

certain grain components. Induced mutants in both the rice host and in the blast

pathogen also have been helpful in understanding host-pathogen interactions and

in demonstrating the reasons that host resistance soon breaks down (Tanaka et

al., 1978).



The best examples of useful mutants in rice have occurred in situations where

only one or two simply inherited changes, such as short stature, early maturity,

waxy endosperm, short grain shape, etc., have been needed in locally adapted

cultivars. Most of these mutants have been recessively inherited. It follows that

emphasis in induced mutation for future work should be placed in similar situations. It is further evident that the benefits of mutants in breeding will be

optimized when the mutants are utilized in standard crossbreeding programs.

Other guidelines include using mutants to supplement natural genetic variability.

Thus, if a desired gene is known in world collections, the breeder will wish to



consider the relative merits of induced mutation versus crossing. Induced mutation may be a means of quickly obtaining a needed gene in an adapted background, but the additional genetic diversity introduced during hybridization with

world collection sources may give greater long-term returns.

Induced mutation research relies heavily on availability or development of

effective screening methods. Most cases to date have been based on visual

screening techniques for semidwarfkm, early maturity, etc. Much current work

on tissue culture is directed at using physiological or biochemical screening

methods for identifying spontaneous or induced mutants. Tissue culture offers

the potential for screening extremely large populations, but this is counterbalanced by the problems of regenerating plants and determining if desired

characteristics expressed in culture also are expressed at the whole plant level. It

is important to remember that some of the physiological screening techniques

currently in use in tissue culture, for herbicide tolerance, salt tolerance, etc., can

also be used on whole-plant populations. Tseng and Seaman (1982), for example, were able to select for increased tolerance in rice breeding lines to the

herbicide thiolcarbamate by a seedling screening test. Similarly, Bright er al.

(1982) selected barley mutants with an altered aspartate kinase enzyme by

screening embryos for growth on a medium containing lysine plus threonine.

Although whole-plant screening may require more space than tissue culture, the

benefits of having a plant in hand at the end of the experiment can outweigh the


A particularly powerful tool for screening for mutants would result if genetic

systems were available for producing easily identified haploid seeds. Recessive

mutants could then be identified in the M, generation. Genetic techniques for

haploid seed production are available in maize (Sarkar, 1974), but so far similar

techniques have not been reported for rice.

Some specific examples of needed diversity in rice that might be obtained

through induced mutation include genes for naked or free-threshing grain, that is,

easy separation of the lemma and palea from the caryopsis as in wheat; longer

floret opening time; cytoplasmic male sterility; and herbicide resistance. According to Vavilov’s proposals (1951) on homologous series and parallel variation,

the naked gene should occur in rice, because it is present in the other cereals.

However, free-threshing, normal kernel shape types have not been found in rice,

probably because the hulls provide a strong selective advantage for survival of

the kernel in aquatic environments. With the advent of peroxide seed coatings,

the needs for a protective hull are diminished, and a naked rice kernel has a better

survival chance. Longer floret opening time would be useful in hybrid rice seed

production, as more time would be available for pollen transfer; rice florets

remain open only for an hour or less, whereas wheat florets remain open for

several hours. A recent report on another member of the grass family, pearl

millet, indicates that the induction of cytoplasmic male sterility is possible (Bur-

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