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VI. Improvement of Crop Yield

VI. Improvement of Crop Yield

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regard to both genomic and plasmonic factors has not been fully utilized for the

purposeful and efficient enhancement of yield-limiting processes that at present

limit crop productivity. Identified mitochondria1 and chloroplast genes specifically code subunit polypeptide components of enzyme systems that play key

roles in respiratory and photosynthetic electron transport and phosphorylation. In

the most thoroughly studied species, like Chlamydomonas reinhardii, Saccharomyces cerevisiae, the geranium Pelargonium, and Oenothera, organelle

genes are known to mutate, segregate, and recombine just like nuclear genes. In

these cases there is good reason to believe that most or all zygotes receive intact

organelles from both parents. Should we not expect, therefore, that if technical

difficulties were surmounted and a higher plant system were to afford the opportunity, then selection for recombinant cytoplasmic genomes and the development of cytoplasmic hybrids (cybrids) would lead to corresponding advantages in

yield and other desirable agronomic attributes? Treat (1978) has recently shown

that, although most yeast diploids are homoplasmic after about 10-20 generations of growth from zygote, some remain heteroplasmic for much longer

periods. Thus crop yield improvement, taking advantage of organelle genes,

seems an achievable goal in the near future.

It has been customary to think of cytoplasmic inheritance as uniparental inheritance, that is, as involving transmission of organelle genes through the

female but not the pollen parent. If, in fact, there were no exceptions to

the rule that plasmon genes are transmitted through the female parent,

it would mean that there is no sexual mechanism in higher plants to bring

together in a common cytoplasm the heteroplasmon of different individuals.

Many instances of biparental transmission, as illustrated earlier, raise the hope

that ways may be found to induce it in economically important crop species. On

the face of it, evolutionary advantages should result from biparental transmission

in terms of both interorganelle or intraorganelle recombination. If mt-DNA or

ct-DNA from both gametes is included in the zygote formed from them, their

genetic heterogeneity could result in heterotic vigor by complementation in zygote organelle functions, especially if there is recombination between parental

organelle genomes. By contrast, the limitation of transmission to the egg results

in restriction of plasmon heterogeneity to the factors that happen to be in the egg.

The only genetic changes that can take place are from random mutations occurring one at a time. The evolutionary coadaptation between the cytoplasmic genes

of a population of plants and their nuclear genes is a specific product of the

natural selection pressures to which both are subjected. For species that show

significant proportions of biparental zygotes, it seems that the most appropriate

and fruitful course is to consider organelle genetics as a problem in intracellular

population genetics. Intraorganism competition between cellular organelles and

between organelles and their host cell nucleus must therefore be considered

relatively stable phenomena upon which natural selection would operate for



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improved reproduction. Persistent heterozygotes have been found among

antibiotic-sensitive revertants from antibiotic-dependent plastome mutants in

Chlamydomonas (Sager, 1977). Mitochondria1 gene recombination has very recently been demonstrated in two fungi, Aspergillus nidulans (Mason and Turner,

1975; Lazarus and Turner, 1977) and Podospora anserina (Belcour and Begel,

1977). Recombination of chondriome and plastome genes cannot be studied in

many systems because suitable markers are lacking or because paternal organelle

DNA is too diluted in the zygote to be experimentally detected.

Unipaternal inheritance of organelle genes is phylogenetically ubiquitous and

ancient (Birky, 1978). It was earlier thought that there would be a single

mechanism of organelle genome transmission, equally ubiquitous and primitive,

but this is not the case. The mechanism(s) used by different eukaryotic organisms

to bring about preponderance of only one type of organelles (or their genomes)

during maternal or uniparental inheritance may well involve any one of the

following possibilities: (1) one type of organelle could cause the destruction of

genetically different organelles that are in the minority due to interorganelle

competition; (2) one type of organelle could have strong input bias and favor its

own inclusion in the gametes; (3) one type could reproduce more rapidly after

inhibiting the reproduction of other types within a variety of cells very early in

development so that its chances of inclusion within gametes were enhanced; and

(4) one type possesses more favorable replication sites in its DNA than the other,

and thus could multiply faster for its predominance in the progeny. However, if

there are very low levels of paternal gene transmission and recombination, these

must be measured, for they may become very important over evolutionary time

scales. When crosses occur between two distinct species that have different

plasmon genes, there may be abnormalities in the hybrids that result from a lack

of genotypic incompatibilities between the nuclear genes of one parent and the

plasmon genes of the other, i.e., lack of a proper intergenomic interaction. The

evolutionary significance of biparental inheritance would seem to be primarily in

the production of heterogeneity through recombination of organelle genes. Recent evidence has shown that some subunits of key organelle enzymes, including

RBPCase and the F,-F,, of chloroplasts, cytochrome oxidase, cytochromes a and

b, and the F,-ATPase of mitochondria are coded by organelle DNAs, and other

subunits of the same enzyme are coded by nuclear DNAs. Like viruses, eukaryotic organelles can have only a small number of genes and must rely on the

“host,” i.e., the nuclear genome, to code for most organelle components. Intergenomic interaction between heterogeneous nuclear and cytoplasmic genes as

a result of biparental transmission for maintaining heterotic advantages of organelle enzymes would be expected to have greater evolutionary significance

over the maintenance of genetic uniformity by blocking recombination and allelic

polymorphism in the case of uniparental or maternal inheritance of organelle

DNAs. Hybrid vigor thus results from an initial superiority of the heterozygous



INTERGENOMIC INTERACTION, HETEROSIS, AND CROP YIELD



177



zygote produced under the influence of genome-plasmone interaction. Some

evidence in support of this hypothesis comes from the experiment of Demarley

(1976), where it was demonstrated that a higher rate of cell division during the

first three mitosis cycles is sufficient to induce the superiority of a heterozygous

individual throughout its life.

9. MITOCHONDRIAL

EFFICIENCY

A N D PREDICTION

OF YIELD



The advance prediction of the degree of yield heterosis of F, combinations

from parental observations at young seedling stages could be of immense advantage in crop breeding experiments. Heterosis is generally manifested in quantitative characters and is expressed in vigor of vegetative organs, in increased grain

yield, and in resistance to diseases, climatic factor fluctuations, etc. Substantial

evidence is available to suggest that hybrids are endowed with greater homeostasis by offering alternative genetic pathways (Lewis, 1954; Mackey, 1970).

This reaction will give them an advantage under conditions of stress such as

dense plant population, adverse weather, and pathogen attacks. In contrast, other

expressions of heterosis might not be fully developed unless the plant is given

optimal conditions to exhibit its genetic capacity. The principal components of

yield in wheat are heads per unit area, spikelets per head, and seeds per spikelet,

and in maize are cobs per plant, kernel rows per cob, kernels per row, and weight

per grain. In some cases the main components may be further divided, retaining

the multiplicative principle, into tillers per unit area times the percentage of

fertile tillers (Thomas and Grafius, 1976). The prediction methods based on

parental yield components can be more accurate and meaningful than those based

on yield alone. Most of these methods, however, rely on a geometric approach

involving the biometrical interrelationship of the components and crop yield. The

fairly recent information on the importance of different physiological (photosynthesis, respiration, ferment activity, mitochondria1 activity, chloroplast activity,

isozyme spectrum, composition of a number of endogenic regulators) and

molecular-genetic (synthetic intensity of DNA, RNA, and genome active sites)

processes in the manifestation of yield heterosis leads us to use such contributory

parameters in the choice of potential parents, evaluation of heterosis effects, and

its prognosis. Some of the physiological parameters commonly used today for the

rapid screening of potential inbred lines include effective photosynthetic area,

flag leaf size and duration, nitrate reductase activity, photorespiration rate, estimates of photosynthesis and carbohydrate translocation under field conditions,

chlorophyll a content, cytochrome c oxidase activity, and adenosine triphosphatase activity (Srivastava, 1975; Lupton, 1976; Planchon, 1976). A new

method for the prognosis of heterosis under field conditions using the immunochemical technique of antigen and serum analyses of inbred lines, their



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hybrids, and back crosses in maize (Zea mays) has also been worked out (Dimitrov et al., 1974). In the inbred lines the authors found three protein fractions

common to all of them, and a fourth antigen, contained only in those inbred lines

that produced heterotic hybrids upon crossing. The gene coding for the synthesis

of this individual antigen resides in the nucleus. The heterotic maize hybrids,

however, were found to possess an additional fifth protein fraction with increased

number of subfractions.

Many wheat hybrids exhibit heterosis under field conditions, but the exploitation of this vigor in the form of economic yield has proved very difficult and

requires the identification of parental combinations exhibiting greater combining

ability (Lupton, 1976). Heterosis may be expressed at any stage in crop development, and the purpose of the breeder is to identify those stages at which

heterosis may most conveniently be assessed, and when correlation with yielding

capacity is greatest. On the other hand, it is known in wheat that the photosynthetic activity of the flag leaf blade during the blossoming-maturity period determines the yielding capacity to a large extent (Lupton, 1968; Planchon, 1968). In

order to obtain better understanding of the combining ability of various wheat

cultivars, Planchon (1976) analyzed the photosynthetic activity of the flag leaf

blade in the parents and their F, hybrids. The activity in this study was measured

using two parameters: chlorophyll a content in the flag leaf and net CO, exchange rate per unit surface area-a trait strictly associated with photorespiration. The results showed a positive correlation between chlorophyll a content and

grain yield. It was suggested that parents possessing both a low photorespiration

rate and high chlorophyll a content must be crossed to produce highly heterotic

hybrids.

Several studies which have considered the effects of heterosis prior to maturity

have all reported that hybrids exhibiting a high level of grain yield heterosis also

show vigor for growth and development at early seedling stages (Sagi et a / . ,

1976; Gibson and Schertz, 1977). Organelle heterosis and complementation have

been shown to be significantly correlated with grain yield heterosis in many

crops. These studies have led us to believe that mitochondrial and chloroplast

activity may be rate-limiting in plants; thus organelle activity and efficiency are

basic to the control of growth and yield. Organelle complementation is a challenge to conventional biometrical methods and estimations, since apparently both

plasmon and nuclear genes are involved. The use of mitochondrial complementation as a tool to preselect well-combining parents under laboratory conditions in

crop plants is more amenable, and by this method plant breeders can save

considerable time, work, and expense. A relatively good positive correlation

r = 0.61) between grain yield per plot of the natural hybrids at low plant density

and cytochrome c oxidase activity of the “mitochondrial” hybrids (1 : 1 parental

“complementing” mixture) made in v i m has been observed (Sagi et al., 1976).

This correlation is in concordance with that found by McDaniel (1972) between



INTERGENOMIC INTERACTION, HETEROSIS, AND CROP YIELD



179



mitochondrial heterosis and grain yield of several heterotic barley hybrids (r =

0.69). Lupton (1976) also reports a significant positive correlation (r = 0.845)

between yield heterosis and mitochondrial complementation based on ADP : 0

ratio in wheat. Both mitochondrial ADP : 0 and respiratory control ratios have

further been found to be highly correlated ( r = 0.94) with average grain yield

(Flavell and Barratt, 1977). Because of their great significance in plant breeding,

our original findings on mitochondrial heterosis and complementation, together

with the currently verified results in many other laboratories, demonstrate the

possibility of using estimates of ADP : 0 ratios or cytochrome c oxidase activity

as an initial screen, independent of such complications as genetic heterogeneity

(segregating seed material) in the seeds used, regional adaptation, or disease susceptibility, in assessing the potential of varieties as parents for hybrid breeding.



c.



PHOTOSYNTHETIC



EFFICIENCY

AND



YIELD



Low photosynthetic efficiency of our modem crop varieties has become a

“bottleneck” for yield improvement. Efficiency of solar energy utilization for

many crops does not exceed 1%. Improved varieties of such crops as wheat,

cotton, and others have been found to possess lower rates of photosynthesis than

their ancestral types (Evans, 1975; Evans and Wardlaw, 1976). However, some

improvement i n yield has been made by leaf area increase, by changes in the

biomass of reproductive organs to that of vegetative ones, and by other

morphological factors. Selection for desirable morphological properties and traits

has been practiced, with greater emphasis given to increasing the size of sink and

storage organs-the number and size of ears, bolls, tubers, etc. Enhancement of

the plant sink and storage capacity could result in an increase of the agronomic

yield as long as the photosynthetic potential permitted. However, the imbalance

between the accumulation and partition of photosynthetic products in the newly

selected varieties and hybrids leads to puniness and immaturity of seeds, falling

of ovaries and bolls, reduction in sugar content, and many other disorders

(Nasyrov, 1978). Besides, an excessive increase in the sink power can provoke,

by the negative feedback mechanism, a decrease in the activity of the photosynthetic apparatus due to the withdrawal of nitrogen from leaves into other

organs, thereby causing their premature senescence (McArthur et al., 1975).

The improvement of the chloroplast efficiency on physiological and genetic

bases is a brand new field in plant breeding research. Breeding of improved high

yielding cultivars in cereals and other crops based on chloroplast and mitochondrial efficiency will probably continue to arouse greater interest in modem breeding technology in future, just as dwarf crop varieties presently dominates the

scene in plant breeding. Two genetic approaches could be applied to maximize

yield potential through chloroplast breeding: (1) germ plasm evaluation for genet-



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ic diversity of chloroplast function and photosynthetic efficiency, and (2) induction of mutations to produce considerable shifts in the structural and biochemical

organization of chloroplasts. Ecological and geographical populations of crop

species have been found to exhibit a large genetic variability of chloroplast

photochemical and enzymatic activities, which are significantly correlated with

photosynthetic efficiency and grain yield potential (Wilson and Cooper, 1969;

Treharne, 1972). Photosynthetically “plus” (efficient) mutants have been obtained in Chlorella (Mukhamadiev and Zalensky, 1972), pea (Highkin et al.,

1969), and cotton (Usmanov et al., 1975). An economically promising mutant

variety “duplex” in cotton has been released by gamma irradiation (Nasyrov,

1977). It has a compact type of branching, high frequency of double sympodial

bolls, early maturing, and high agronomic yield. Its harvest index amounts to

50%, which points to efficient partitioning and utilization of photosynthetic

products. The economic yield of this mutant variety is at the rate of 1 ton/ha of

seed cotton higher than that of the parent variety 108-F. Physiological and

biochemical analyses have demonstrated that the mutant “duplex” is characterized by a higher photochemical and enzymic activity of the chloroplast during

the reproductive period. In this mutant the endogenous delay of leaf senescence

is observed, whereas in the parent variety, as well as in many other crops, a

decrease in the specific activity of key Calvin cycle enzymes occurs on the

20th-30th day after the emergence of leaves, when there are no visible alterations

of chloroplast ultrastructure (Nasyrov, 1975). Physiologic-genetic prolongation

of the functionally active state of chloroplasts and prevention of premature leaf

senescence are therefore important factors in high photosynthetic productivity.

D. CONVERSION

OF C3 PLANTS

TO C4 PATHWAY



The C4 pathway is observed not only in tropical grasses, but it is also widespread among dicotyledons (Dowton, 1975). Even within the same genus both C3

and C4 species together with their intermediate forms have been reported (Kestler

et al., 1975). Zelitch (1973, 1975) and Bassham (1977) have produced data

showing that in C, plants (wheat, rice, soybean, potato, peanut, barley, sugarbeet, cassava, banana, alfalfa, chlorella, eucalyptus, and algae) photosynthetic

product losses upon photorespiration can reach 50% of the net assimilation. This

is further evidenced by the data on the photosynthetic efficiency of C4 plants

(maize, sorghum, millet, sugarcane, and napier grass) in which photorespiration

is practically absent (Lorimer and Andrews, 1973; Zelitch, 1975). The main

source of CO, release in the process of photorespiration is decarboxylation of

intermediates of glycolate metabolism (Bishop and Reed, 1976). The glycolate

pathway is closely linked with the Calvin cycle, particularly with oxygenation of RBPCase resulting in the formation of phosphoglycolic acid (Tolbert,



INTERGENOMIC INTERACTION, HETEROSIS, AND CROP YIELD



181



1971). It is generally accepted that the key enzyme RBPCase plays a bifunctional

role, catalyzing carboxylation and oxygenation reactions. Oxygen and CO, compete for the reaction substrate ribulose 1,5-diphosphate (Laisk, 1977).

In the enzyme oxygenase reaction, the sulfhydryl (SH) groups take part; their

total amount is 96, but the number of easily modified, “accessible” ones is 12

(Akhmedov et al., 1976). It has been found that the photorespiration inhibitor

glycidate irreversibly inactivates oxygenation, the carboxylase activity of the

enzyme not being affected (Wildner and Henkel, 1976). Evidently oxygenationspecific inhibition is connected with blocking of enzyme SH groups located in its

active site. In its native state the conformation of this oligomeric protein complex

and its functional activity depend on conformation and dimensions of large and

small subunits (von Wettstein et al., 1978). Analyzing the peptide composition

of RBPCase in 60 tobacco species and 10 plants belonging to variable phylogenetic groups-from blue-green algae to ginkgo-Kung (1976) has identified that in

all species studied the large subunit contains only three polypeptides, while the

number of polypeptides in the small subunit varies from one to four.

How could the C3plants be converted to the C4 pathway? The more efficient

C4 plants characteristically have two kinds of photosynthetic tissue (dimorphic

chloroplasts). The mesophyll cells fix CO, by way of phosphoenolpymvate

(PEP) carboxylase to form oxaloacetate and thence the four-carbon compounds

malate or aspartate. A current hypothesis is that one of these acids, depending on

the plant species, is transferred to the cells sheathing the vascular bundles of the

leaves where decarboxylation occurs (Day, 1977). The peculiar anatomy of C4

species involving complementation between dimorphic chloroplasts is perhaps

responsible for some part of their greater yield. In the genus Atriplex, conventional sexual hybridizations between C, and C4 species have been made. Among

F, and F3 progeny of hybrids between A . hastata and A . rosea, individuals with

the anatomy of the C4parent A . rmea were present, but none had photosynthetic

rates that approached it (Bjorkman, 1976). These approaches for selecting plants

with C4 anatomy and chloroplast dimorphisms in segregating lines after hybridization are being camed out in many other laboratories (Nasyrov, 1978). Other

genetical screening experiments have distinguished a tobacco population from

the parent variety “Havana seed” in which 25% of the progeny had low

photorespiration (Zelitch, 1975). Some promising varieties with low photorespiration have been obtained through exhaustive genetic screening within populations of C3 species (Wilson, 1972).

The C4 pathway is connected with the leaf kranz anatomy, and therefore it

seems rather impossible to impart properties of cooperative photosynthesis to C3

plants. Several other experimental approaches to improve photosynthetic efficiency of our crop species include (1) mutant isolation and selection of C3

cultivars with low rates of photorespiration and high rates of net photosynthesis;

(2) enhancement of the auxiliary COP fixation through PEP carboxylase; (3)



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induction of desirable C3 plant mutants by mutagen treatments, and (4)incorporation of C4 dimorphic chloroplasts into protoplasts by somatic cell hybridization. Although plants with low rates of photorespiration and high rates of net

photosynthesis were observed in a tobacco cultivar, attempts to establish this

phenotype in several generations of pedigree selection were unsuccessful (Zelitch

and Day, 1973). Several mutagen-treated tobacco haploid cell lines possessing

slower than normal rates of photorespiration and vigorous autotrophic growth in

low concentration of C 0 2 have been identified (Zelitch et al., 1977). The possibility of the enhancement of the auxiliary C02 fixation through PEP carboxylase

among C , plant mutants produced by genetic manipulations also deserves mentioning. The formation of PEP carboxylase is controlled by nuclear genes, but the

enzyme operates in the cytoplasm. It has a higher affinity for HCO, than for C O z

(Filmer and Cooper, 1970); therefore it can bind the hydrated Cot more efficiently and transfer it to the chloroplasts, thus decreasing the limitation of photosynthesis by carboxylation resistance. The activity of PEP carboxylase in C ,

plants is 60 times lower than that in C4 plants (Slack and Hatch, 1967). Somatic

cell genetics has already found widespread use in mutant isolation and selection.

It may be feasible in future to transfer C4 dimorphic chloroplasts to C3 species

through protoplast fusion and thus improve the photosynthetic efficiency and

crop yield. The assumption that the C4 pathway originated in the course of

evolution as a consequence of plant adaptation to arid and hot climates favors the

experimental possibility of any such conversion of C , plants to the C4 pathway.



VII. SUMMARY AND CONCLUSIONS

Both mitochondria and chloroplasts are self-replicating organelles, arising

only by growth and division of preexisting mitochondria and chloroplasts. The

organelle genetic systems are thus genetically autonomous, obeying their own

unique laws of uniparental or biparental (non-Mendelian) inheritance. It is apparent that organelle genomes are present in multiple (polyploidy) copies per cell,

and that they are packed in one to many organelles and, within each organelle, in

one to many nucleoids (DNA gene centers). The population of organelle

genomes within a cell may be divided into subpopulations in individual organelles

and in nucleoids. These “populations within populations” complicate the

analysis of organelle transmission genetics. The application and interpretation of

some generalized laws of organelle genetics are made more difficult by the lack

of good quantitative data on the number of nucleoids and of DNA molecules per

nucleoid, or on the behavior of nucleoids during sexual reproduction in higher

plants. It is likely that the segregating units in organelles are groups of

mt-DNA or ct-DNA molecules, perhaps corresponding to the nucleoids or



INTERGENOMIC INTERACTION, HETEROSIS, AND CROP YIELD



183



even to whole mitochondria or segments thereof in some organisms. The transmission of organelles in higher plants appears to be predominantly maternal or

uniparental, but the possibility of biparental zygotes resulting due to equal or

unequal contributions of organelles from maternal and paternal gametes cannot

be ruled out. Paternal organelles are visibly destroyed in the zygotes of some

mammals, algae, and higher plants. However, there is evidence suggesting that

paternal chloroplast or mitochondrial genes, though in minority, do survive in the

biparental zygote and may have a significant role in maintaining plasmon

heterogeneity.

Heterosis and complementation with regard to mitochondrial and chloroplast

activities are observed in several economically important crops. The present

evidence indicates that mitochondrial heterosis is the result of complementation

between polymorphic mitochondria that may be present in the hybrid as a result

of inter- or intraorganelle recombinations. The significance of mitochondrial

polymorphism or heterogeneity in the manifestation of heterosis and homeostasis

is also illustrated. The genomes of organelles are insufficient to code for all the

functional and structural proteins of mitochondria and chloroplasts. Close cooperation between nuclear and organelle genes seems to dominate the scene and

determine cell life strategies during plant growth, development, and maturation,

for most organelle enzyme systems also contain polypeptides coded by nuclear

genes. The results on biosynthesis of key enzymes (RBPCase in chloroplasts,

and cytochrome oxidase and ATPase complex in mitochondria) of organelle

systems influenced by both nuclear and organelle genomes provide insights into

the physicobiochemical and genetical mechanisms responsible for heterotic expressions. A hypothesis based on intergenomic interaction to interpret the operational mechanism of heterosis is discussed with many examples. Superior

mitochondrial and chloroplast functions resulting due to genomic and intergenomic complementations are considered as essential components of

heterosis. Arguments in favor of organelle polymorphism, which may be transmitted biparentally, and its genetic association with heterosis and Complementation are presented. Recent genetic studies indicate that genetic differences in cell

size are due to additive-type genes, whereas genetic differences in cell division

rate are due to nonadditive genes (epistatic interaction). Since heterosis is the

phenomenon of superior growth and development, a possible molecular

mechanism of heterosis would involve complementation of proteins or polypeptide subunits encoded by “multigenomes” instead of unique nuclear genes. The

resulting products due to intergenomic interactions in organelles of hybrid organisms would then be expected to ensure enhanced structural and catalytic

functions of these organelles and also superior cell division rate. Several distinct

lines of evidence ranging from biochemical, physiological, ultrastructural, and

restriction endonuclease organelle DNA fragment analyses are available at present to show that all three genetic sources (genomic, plasmonic, and inter-



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genomic interaction) and not just one of them are at work during the manifestation of heterosis. There is evidence to suggest that DNA of hybrids possesses

high replicative activity, repetitions of ribosomal RNA loci (rDNA), and relatively greater repeated nucleotide sequences in the genome’s active sites. The

plausibility that interaction between the active sites of multigenomes creates

favorable conditions for epistatic genes pertaining to major functional loci in

heterozygotes and its subsequent role during heterotic expression is also considered. It is suggested, however, that the active principle of intergenomic interactions (cooperative interactions between nuclear and organelle genomes) for

vigorous organelle function and enhanced cell division rate lies either with a

messenger RNA molecule or the subunit polypeptide encoded by the nuclear

genes.

The field of transmission genetics of organelles in relation to crop yield improvement appears to be coming of age. Recent results suggest that both

mt-DNA and ct-DNA are deeply involved in various genetic phenomena including cytoplasmic male sterility in higher plants. The genetic control of the content,

amino acid composition, and quality characters of proteins by means of cytoplasmic organelle-nuclear gene interaction in cereal crops has also been reported.

Many studies from a number of different laboratories reveal that hybrids exhibiting a high level of grain yield heterosis also show heterosis for early growth and

development at young seedling stages. Organelle heterosis and complementation

are also significantly correlated with grain yield heterosis in many crops. The use

of mitochondrial complementation or chloroplast complementation as a tool to

preselect well-combining parents under laboratory conditions for prediction of

grain yield heterosis in crop plants is recommended. Both mitochondrial ADP : 0

and respiratory control ratios have been found to be highly correlated ( r = 0.94)

with average grain yield in wheat. Several experimental approaches to enhance

grain yield potential of crop plants based on mitochondrial and chloroplast efficiencies have been discussed. The improvement of crop yield in terms of

mitochondrial and chloroplast breeding, especially in cereal crops, is a real

challenge to modem plant geneticists, and the maturity of the field is suggestive

of the fact that complementation data of organelles in 1 : 1 parental mixture

should be preferred over time-consuming and expensive biometrical methods and

estimations, because apparently both plasmon and nuclear genes are involved.

For major innovations in crop yield improvement, plant breeders first need to

bridge the technology gap between molecular genetics and plant breeding and to

utilize new methods by which they can follow rates of photorespiration, net

photosynthesis, and mitochondrial and chloroplast activities. The future task

would be to explain the phenomena of heterosis and complementation by

mitochondria and chloroplasts at the molecular level, in terms of the behavior of

parental organelles and their DNA molecules in a predominantly maternal andor

biparental zygote. Another major question that remains to be examined relates to



INTERGENOMIC INTERACTION, HETEROSIS, AND CROP YIELD



185



the precise operational mechanism of intergenomic interaction (complementation

between nuclear and organelle genomes): does it operate through transcriptiontranslation systems, a protein-synthesizing system, or both? It is my belief that

recent developments with protoplast fusion of crop plants will lead us to develop

new germplasm resources as a result of recombination of both nuclear and

organelle genomes for use in orthodox breeding and eventually to ways of selecting improved high yielding varieties.

ACKNOWLEDGMENTS

Much of the planning and writing of this review was done while the author was asked to deliver a

series of lectures in a joint UNESCO/UNDP sponsored international course on “Genetic Hierarchy in

the Multigenomic Eukaryotic Cells” for a group of professors belonging to several South American

universities at the University of Valle. I wish to thank many colleagues from all over the world who

generously shared reprints, manuscripts, and unpublished observations. I only regret that space did

not permit a discussion of all their work. My own research is supported by Colciencias and Semilla

Valle, S. A. My gratitude also goes to Miss Alba M. Dominguez for patiently typing this manuscript.



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VI. Improvement of Crop Yield

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