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III. Organelle Involvement in Genetic Phenomena

III. Organelle Involvement in Genetic Phenomena

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INTERGENOMIC INTERACTION, HETEROSIS. AND CROP YIELD



131



organelle genes (Hanna er al., 1978), the need for extending these notions to

breeding aspects of crop plants becomes imperative. Jinks (1964) has stated “if

differences between reciprocal crosses are to be used as a criterion of extrachromosomal heredity, we must specify that the difference persists through the

successive generations that may be derived from them. Using such a criterion,

Ressler and Emery (1978) found reciprocal differences in growth habit expression of the F, , F2,F3, and BCIs, generations from reciprocal crosses of peanut

lines with reported differences in plasmons and genomes, respectively. Plastome

mutants producing chlorophyll-deficient seedlings are generally used as markers

for studying gene action, mapping chromosomes, determining the effects of

mutagens, and other fundamental studies on higher plants. Plant breeders are

beginning to pay attention to the “genome-plasmon” models for plant development and the potential use of plasmon in the screening procedure (Ashry, 1976).

The marked variation in mt-DNA and ct-DNA among maize cytoplasms (Pring

and Levings, 1978) provides additional evidence that organelles may be involved

in many genetic phenomena, including male sterility and disease susceptibility.





A. MITOCHONDRIAL

HETEROSIS



The involvement of mitochondria in heterosis as judged by increased respiratory function and higher enzyme activities in the hybrids has been demonstrated

(Srivastava, 1972). An operational basis for heterosis with regard to mitochondrial metabolism was first advanced in maize involving comparative studies of

oxidative metabolism and oxidative phosphorylation from hybrid and inbred

lines (McDaniel and Sarkissian, 1966; Sarkissian and Srivastava, 1967). Further

studies in hybrids of wheat have provided evidence for the hypothesis that enhanced mitochondria1 activity could be a general physicobiochemical mechanism for the expression of heterosis (Srivastava, 1974). Mitochondria in

wheat hybrids were found to possess more efficient systems for channeling

electrons to their final acceptor cytochrome oxidase for the production of ATP as

judged by relatively greater specific activities of the enzymes NADHcytochrome reductase and cytochrome c oxidase. The oligomycin-sensitive

adenosine triphosphatase (ATPase) system of mitochondria from a wheat hybrid

and its parents has also been studied and the results show correlation of ATPase

activity with heterosis (Srivastava, 1975). The hybrid therefore seems to possess

both a higher rate of synthesis of ATP (ADP : 0 ratios) and higher rate of release

of ATP energy (ATPase efficiency) than its parents, and this coupled mechanism

may be operating in eliciting the heterotic expressions of the hybrids.

Apart from the efficient systems for the conservation of ATP energy in hybrid

mitochondria, several distinct lines of evidence suggest that the quality and

quantity of mitochondria present seem to play an important role in determining



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the degree of vigor exhibited by a given genotype. The increased quantity of

mitochondrial protein of seedlings present in heterotic (heavy) seeds has been

shown to be indicative of a higher respiratory rate and greater amount of ATP

energy conservation (Kittock and Low, 1968; McDaniel, 1969). The hybrid

mitochondria possess relatively greater amounts of lipid and phospholipid, and

there is some marked difference in fatty acid composition of two important

phospholipid fractions-lecithin and cephalin-from the mitochondria of the

hybrid and its parents (Srivastava and Sarkissian, 1972). Evidence showing

hybrid mitochondria to possess abundance of linoleic acid in their fatty acid

fraction (Lahib and Kader, 1977) and a greater amount of internal “bound”

water (Khokhlova et a l . , 1975) is indicative of the physicobiochemical changes

in the biochemical systems of the hybrid during heterotic expressions. Some

significant quantitative differences in the amount of cytochromes a, b, and c

between a wheat hybrid mitochondria and its parental mitochondria have also

been observed (Sarkissian and Srivastava, 1971). The protein content of seed has

been clearly shown to be related to seedling vigor, as measured by seedling dry

weight (Ayers, et a f . , 1976), and to grain yield (Lopez and Grabe, 1973).

Significant variation in protein content, size, and seedling vigor with position of

seed in heads of wheat plant has been observed (Ries et a l . , 1976), suggesting

that larger seeds may contain more protein than smaller seeds as a result of size

or concentration. Recent results further indicate that mitochondrial ADP : 0

ratios in wheat seedling are correlated to seed metabolism (Flavell and Barratt,

1977), which is in turn determined by the metabolic activity of the plant during

formation and maturation of the seeds. Positive correlation between seed and oil

yield and mitochondrial activities in oil palm (Elaeis quineensis) has recently

been observed (Kouame, 1978). A relationship between mitochondrial metabolism and heterosis in Asparagus has also been demonstrated (A. Berville, personal communication). The more vigorous varieties of maize and soybean have

more tightly coupled mitochondria than the less vigorous varieties (Hanson et

a l . , 1975). All these observations are in concordance with the view that mitochondria play a significant role in the manifestation of heterosis.

The basic mechanism underlying growth and development is protein synthesis

(Chen, 1971), and any changes in morphology must ultimately result from

changes in transcription or translation of the DNA and RNA of the multiple

genomes. An association between heterosis and contents of nucleic acids and of

amino acids in generative organs of maize has been established (Scarascia,

1977). Significant differences in terms of ribosomal RNA cistron amplification

between total and nuclear DNA of maize hybrids and their inbred lines during the

phase of cell elongation have been recorded (Gilyazetdinov et al., 1977).

Mitochondria, by way of ATP energy supply, considerably affect the nuclear

genome transcription (Wolf and Rempan, 1977) as well as the crucial stages of

chloroplast biosynthesis (Schiff, 1975) in higher plants.



INTERGENOMIC INTERACTION, HETEROSIS, AND CROP YIELD



133



The functions of mitochondria are important not only in ATP energy production but also in cellular metabolic regulation, including nuclear DNA and RNA

syntheses. A correlation between mitochondrial heterosis and grain yield in

cereals was interpreted by McDaniel ( 1 973) and Sage and Hobson ( 1 973) to

indicate that mitochondrial activity is rate-limiting for yield and increased

mitochondrial efficiency is the physicobiochemical basis of heterosis. Another

interpretation that mitochondrial efficiency may not be a limiting factor for grain

yield in view of the marked increased vigor and yield following additional

nitrogen application in cereals (Ries and Emerson, 1973; Bingham, 1972) has

been put forward by Barratt and Flavell (1977). The basis of their hypothesis is

that many parts of cell metabolism, including mitochondrial and chloroplast

activities, are coordinately regulated in many biochemical systems. It predicts (i)

that heterosis due to a wide range of causes is associated with greater organellular

activities; ( i i ) that the genetic control of heterosis is not confined directly to

genes concerned with mitochondria and chloroplasts; and (iii) that mitochondria

and yield are not under common gene control, but are under the control of a

common general regulation of metabolism. There are no direct experimental

findings to establish these hypotheses, however, a properly designed genetical

experiment should be camed out in which the biometrical evidence from segregating lines would be crucial.

It is safe to consider that mitochondrial genomes in higher plants are present in

many copies per cell and that they are packaged in one to many mitochondria,

and within each mitochondrion, in one to many nucleoids. The segregating units

must therefore be groups of mt-DNA molecules, perhaps corresponding to the

nucleoids observed by Williamson et al. (1978) or to whole mitochondria or

segments thereof. Some evidence that the input ratio of parental mt-DNA

molecules in a yeast zygote affects the output ratio of mitochondrial genes in the

progeny has been shown (Boker et a/., 1976). Gametes in Chlamydomonas

possess one large and sometimes one to four small mitochondria, but these break

up into many small mitochondria in the zygote (Grobe and Arnold, 1975). These

changes in the number of mitochondria per cell suggest fusion and fission of mitochondria, and can be inferred as equal to the occurrence of genetic recombination.

Molecular evidence for mt-DNA recombination in animal cells has also been

obtained (Horak et al., 1974; Wallace et a / . , 1976). These findings support the

notion that heterotic hybrids contain a population of genetically heterogenous

mitochondria and that the recombination between mt-DNA molecules at the time

of fusion and fission of biparental mitochondria leads to mitochondrial heterosis.

The possibility that recombination between mt-DNA molecules even within the

same mitochondrion occurs in those organisms where the real fusion between

two mitochondria cannot be detected in crosses has been mentioned (Birky,

1978). Also, there is no sign of mitochondrial gene recombination or of sharing

of mt-DNA molecules between mitochondria in an exceptional organism



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H. K . SRIVASTAVA



Paramecium aurelia (Adoutte, 1977), and one would therefore expect the or-



ganelle as a whole to be the unit of segregation in this case.

B . MITOCHONDRIAL

COMPLEMENTATION



The biochemical basis for heterosis is fundamentally one of complementation,

usually either between proteins or between protein subunits. Genetic complementation involves interaction of gene products to produce a normal phenotype,

whereas wild-type recombinants produce a normal phenotype because a normal

sequence of nucleotides is actually created along a genome via genetic exchange

(Demrec and Hartman, 1959). The ability to generate a normal phenotype is

genetically transmitted from parent to offspring when chromosomes arise by

recombination, but progeny produced as a result of complementation (interallelic

interaction) remain individually defective in genotype. While recombination can

occur within a gene, complementation occurs only between one gene and another

belonging to either one genome or multiple genomes in the cell. The concept of

complementation at organellular level to explain the operational mechanism of

heterosis is attractive and could interpret various events at the cellular and subcelMar levels leading to heterotic expressions in higher plants. Recent observations

on the potential genetic capacity of mitochondrial genome (Pring and Levings,

1978), chloroplast genome (Coen et a l . , 1977), and intergenomic interactions

between organelles and nucleus (Srivastava, 1981) provide insights for the

existence of complementation at different levels of structural and functional

organization of a plant cell.

Mitochondria1 complementation (the enhanced oxidative phosphorylation efficiency of artificially mixed mitochondria of certain inbreds) has been reported as

correlated with seedling heterosis in maize (McDaniel and Sarkissian, 1966,

1968; Sarkissian and Srivastava, 1967), wheat (Sarkissian and Srivastava, 1969,

1971; Srivastava, 1974), barley (McDaniel, 1969, 1972), and alfalfa (Schneiter

er a l . , 1976). Hobson (1971) observed some complementation effects for grain

yield heterosis in wheat cultivars. A small but significant complementation effect

for sugarbeet (Beta vulgaris, L) root mitochondria, which seemed associated

with root yield heterosis, has been demonstrated (Doney et al., 1972). From

these findings, the general conclusion was drawn that measurement of

mitochondrial complementation, especially for ADP : 0 ratio, is a useful tool to

predict combining ability for yield in higher plants (Sarkissian, 1972; McDaniel,

1973; Srivastava, 1972, 1974).

McDaniel (1971) and Sage and Hobson (1973) made a rather strong claim,

based on the results of mitochondrial activities coupled with field data, that grain

yield heterosis in cereal hybrids was positively correlated with a higher activity

of mitochondrial complementing mixtures of the parents. The findings on the



INTERGENOMIC INTERACTION, HETEROSIS, AND CROP YIELD



135



relationship between mitochondrial complementation to yield heterosis in field

crops and their practical utilities in evaluating combining ability for yield potential of individual genotype in breeding programs, however, have not met with all

expectations. Zoble et al. (1972) were not able to show mitochondrial complementation i n winter wheat, and Ellis er al. (1973) could not duplicate

McDaniel’s (1971) results using the same varieties of barley. The small complementation effects, as observed in sugarbeet by Doney et al. (1976), made it

difficult to detect differences in complementation, and thus difficult to predict

root yield heterosis. Recent extensive studies of mitochondrial complementation

and grain yield in hybrid wheat have given new clues to the detectable correlation

between F, yield heterosis and mitochondrial efficiency (Lupton, 1976; Sagi et al.,

1976; Barratt and Flavell, 1977). These workers favor using mitochondrial complementation as a biochemical tool over agronomic selection criteria covering a

number of different traits in plant breeding programs for a rapid first screening of

desirable varieties. It is too early, therefore, to set aside the results of mitochondrial

complementation and its prospective use as a selection criterion during young

seedling stages of crop plants for breeding purposes. The results of “marginal”

complementation of a weak correlation between mitochondrial complementation

and yield heterosis as observed by some workers could be a reflection of either

mitochondrial preparations per se or mixing of I : 1 parental mitochondria in

virro to form complementing mixture of mitochondria for comparative studies on

mitochondrial complementation and yield heterosis.

The results on heterosis and complementation with regard to some key components of mitochondrial functions in many crops are summarized in Table 11. In

addition to the efficient enzyme systems of hybrid mitochondria and 1 : 1 complementing mixture of parental mitochondria, respectively, electron microscopic

studies of mitochondria isolated from heterotic cotton hybrids (Arslanova, 1973)

show highly developed cristae and respiratory assemblies reflecting the structural

superiority of these mitochondria. The mitochondria from the parental lines, on

the contrary, exhibit relatively less developed inner-membrane cristae and

smaller intramitochondrial respiratory assemblies. The hybrid mitochondria were

also found to be dividing at a faster rate than those of the parents. Some inter- and

intraspecific cotton hybrids, differing in their degree of yield heterosis, have

been shown to synthesize and accumulate bound fractions of DNA and RNA

more rapidly than their parents (Rakhmankulov er al., 1976). Complementation

in terms of ribosomal RNA cistron amplification during the cell elongation phase

using both total and nuclear DNA of the cell in the parents of heterotic maize

hybrids has been observed (Gilyazetdinov et af., 1977). A relationship between

complementation in the artificially made mixtures of parental mitochondria using

three enzyme assays (cytochrome c oxidase, succinic dehydrogenase, and

glycerine-1-phosphate dehydrogenase) and morphological heterosis in terms of

weight gains of the F, animals has also been demonstrated (Dzapo et al., 1974).



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H. K. SRIVASTAVA



TABLE I1

Heterosis and Complementation with Regard to Mitochondria1 Activities

Organisms



Parameters



Maize (Zea mays)



Amos and Scholl ( 1 977)



Maize (Zea mays)

Wheat (Triricum aesfivum)



Berville e f al. (1976, 1977)

Srivastava and Sarkissian

(1969); Srivastava (1972, 1974);

Sage (1973); Lupton (1976);

Barratt and Flavell (1977);

Flavell and Barratt (1977)

Sarkissian and Srivastava (1967);

McDaniel and Sarkissian (1970);

Sage (1973); Berville et al.

(1976, 1977)

McDaniel (1975)

Hanson e f al. (1975)

Israelstam and Fukumato (1977)

Schneiter er al. (1976)

Grimwood (1972)

Sarkissian and Srivastava (1971)

Srivastava and Sarkissian (1972)

Lahib and Kader (1977)

Kouame ( 1978)

Knyaseva and Romanova (1977)

Arlslanova (1973)



Maize (Zea mays)

Beans (Phaseolus vulgaris)

Triticum-Agropyron hybrid

Wheat (Triricum aesrivum)

Maize (Zeu mays)



Adenosine triphosphatase



Malate dehydrogenase

Succinic dehydrogenase

Isocitrate dehydrogenase

Pyruvate dehydrogenase

Fumarate dehydrogenase

Isocitrate lyase



Rye (Secale cereale)

Cotton (Gossypium hirsufum)

Wheat (Trificumaesfivum)

Maize (Zea mays)

Wheat (Triricum aesfivum)

Maize (Zea mays)

Wheat (Trificumaesfivum)

Barley (Hordeum vulgare)

Maize (Zea mays)

Maize (Zea mays)

Cotton (Gossypium hirsutum)



Srivastava and Sarkissian

(1969); Srivastava (1974);

Lupton (1976); Sagi e f al.

(1976); Barratt and Flavell

( 1977)

Srivastava (1972)

Muresan et al. (1976)

Karamanenko (1976)

Srivastava (1975)

Srivastava (1972); Barratt and

Flavell ( 1977)

Vecher et al. (1975)

Rakhmankulov (1975)

Srivastava (1972)

Srivastava (1972)

Srivastava (1972)

McDaniel and Sarkissian (1968)

Srivastava (1972)

McDaniel (1975)

McDaniel and Sarkissian (1968)

Roos and Sarkissian (1968)

Scholl (1974)



Wheat (Trificumaesfivum)



Cytochrome oxidase



f



Nitrate reductase

Glutamine synthetase

NADH-glutamate dehydrogenase

Oxidative phosphorylation

(ADP:O ratio)



t



Respiratory control index

(state 3 :state 4 oxidation ratio)



Cytochrome b, c, and a contents

Lipid-phospholipid content

Linoleic acid content

Oil yield (total)

Protein content

Ultrastructure of hybrid mitochondria



References



Maize (Zea mays)



Barley (Hordeum vulgare)

Soybean (Glycine max)

Pea (Pisum safivum)

Alfalfa (Medicago saliva)

Cucurbits (Cucurbifa maxima)

Wheat (Triricum aestivum)

Wheat (Triricum aestivum)

Wheat (Triricum aesfivum)

Oil palm (Elaeis quineensis)

Potato (Solanum tuberosum)

Cotton (Gossypium hirsutum)



INTERGENOMIC INTERACTION, HETEROSIS, AND CROP YIELD



137



It is of interest to note that all these studies on mitochondrial complementation

have been confined to the hybrids and their respective progenitors that were

already known for a number of heterotic or nonheterotic phenotypic measurements including yield. There is no doubt that the heterotic hybrids are endowed with a more balanced metabolism than the purebred parents. Since growth

and economic yield are the results of a series of metabolic reactions involving

multiple-enzyme complexes, it is reasonable to suppose that heterosis as a genetic phenomenon may be dependent on mitochondrial complementation. A working hypothesis based on the conception of nucleus-mitochondria-chloroplast

cooperative interaction in the development of the whole plant leading to heterotic

phenotypic expression is therefore suggested. The detailed discussion on the

implication of intergenomic interaction is presented elsewhere in this article.

How should the data on mitochondrial complementation be interpreted in

genetic terms? Considering that the origin of mitochondria from the preexisting

one is under nuclear genome control and that the differential contribution of

mitochondria to the hybrid individual is derived through the biparental transmission, the occurrence of polymorphic mitochondria possessing well-organized

ultrastructures as well as efficient enzyme systems in the hybrids, resulting from

both organellular recombination and mutation, is conceivable. A similar interpretation of the results on mitochondrial complementation was also given by

Wagner (1972). The fact that part of the mitochondrial protein is encoded in the

nuclear DNA can not be denied. Therefore, the protein or enzyme of the artificially mixed parental mitochondria could have been highly efficient as a result of

the heterozygosity of their chromosomal genes. A nuclear genome heterogeneity

could have produced the complementation results in terms of ADP : 0 ratio, R.C.

ratio, and oxygen uptake in 1 : 1 mixture of mitochondria from the two inbred

parents. However, whether the heterogeneity of mitochondria in 1 : 1 parental

mixture in vitro or in the hybrids in vivo is a reflection of heterozygosity of chromosomal genes or extrachromosomal genes still remains to be resolved. There

was also a demonstration (Srivastava and Sarkissian, 1970) that the mitochondria

of allohexaploid wheats were more vigorous than those of allotetraploid wheats,

which in turn exceeded those of diploid wheats. The data were interpreted as

evidence that the allohexaploids contained the most polymorphic types and the

diploids relatively uniform or unmixed mitochondrial types. Again, the data

could reflect the more heterogenous types of proteins supplied to the polymorphic

mitochondria by the chromosomal genes of the allohexaploids, as compared to

those supplied by the allotetraploids and diploids in wheat polyploid series.

Although mitochondrial heterosis appears to be the result of complementation

between parent mitochondria that may be present in the hybrid (Srivastava and

Sarkissian, 1972), little is known about the precise mechanism by which complementation is accomplished. It was suggested that particle-particle contact

between mitochondria is a physical basis of complementation (McDaniel and



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H. K . SRIVASTAVA



Sarkissian, 1970). The present evidence indicates that the modification of sulfhydryl groups (SH) when p-chloromercuribenzoate reacts with mitochondria

results in a complete loss of the kinetic expression of cooperative interaction of

the enzymes in the mixture of parental mitochondria (Sarkissian and Srivastava,

1973). Since SH groups are intrinsic to almost all mitochondrial functions

(Sanadi et af., 1968), it is a reasonable inference that SH groups of key

mitochondrial enzymes may be involved in internal hydrogen bonding to maintain a proper conformation of the active site(s) of the enzymes in the complementing mixture of mitochondria. Mitochondria1complementation and heterosis

may therefore be regulated through the conformation of membrane-bound enzymes. Such desired enzyme conformation for efficient metabolic function in the

complementing mixture of mitochondria could be produced by exchange of

intermediates between contrasting mitochondria from the parents. Some speculation that the control of ADP:O ratios in vitro is mediated via small molecules

either bound to or surrounding mitochondria has been made (Barratt and Flavell,

1977). The effective concentration of these would presumably alter the rate of

metabolism and tissue growth. If they were in different concentrations in

mitochondrial preparations from different varieties, then it is possible that a

mixture of the mitochondria would have a different ADP:O ratio from the

average of the two “parental” preparations, owing to a new effective concentration of regulatory molecules. The correlation between mitochondrial

complementation-heterosis and hybrid vigor would then be a consequence of a

similarity between the concentration of regulatory molecules derived from

polymorphic mitochondria in the parental mixture and in the hybrid.

The mitochondrial efficiency in terms of ADP:O ratios of 5-day alfalfa

seedlings has been positively correlated to forage yields of similar lines grown in

the field (Schneiter et al., 1974, 1976). The technique of the measurement of

mitochondrial efficiency used for selection of genotypes is limited because alfalfa is a very heterogenous crop and an evaluation of individual genotypes can

not be made using the large number of seedlings required for adequate sample. A

question one is tempted to ask is what relationship exists between mitochondrial

efficiency (ADP : 0 ratio) and plant productivity? Calculation of substrate utilization revealed that in higher plants with a relative growth rate of 0.1 g-’ day-’

(which is approximately the growth rate of young maize plants), an increase of

ADP :0 from 2 to 3 causes an increase in the efficiency of dry matter production

of 8% (Penning De Vries et al., 1974). The substantial differences in ADP: 0

ratio in testing plant material would have significant influence on dry-matter

production and yield. Growth analysis of maize hybrids and inbred lines demonstrated that hybrid vigor correlates with the rate of embryo development and the

utilization of reserve material during germination (Donaldson and Blackman,

1974). After emergence, hybrid vigor undoubtedly is related to net assimilation

rate-to-leaf-area ratio, but interactions between stage of development, environ-



INTERGENOMIC INTERACTION, HETEROSIS, AND CROP YIELD



139



mental conditions, and, above all, the genotype (together with plasmon) finally

determine the path towards heterosis. Recent results indicate that mitochondrial

ADP : 0 ratios in germinating seeds are correlated to seed metabolism, which in

turn is determined by the metabolic activity of the plant during formation and

maturation of the seeds (Barratt and Flavell, 1977). This provides an explanation

for the correlation between mitochondrial efficiency in germinating seedlings and

grain yield heterosis in the crops so far tested. In view of the results discussed

thus far, the employment of the mitochondrial complementation technique in

some limited crops as a selection criterion for performing preliminary screening

of potential inbred parents that are likely to produce heterotic hybrids in crossbreeding experiments under field conditions is recommended. The findings further establish the relative importance of mitochondrial efficiency as a means of

initial increased physicobiochemical superiority in eliciting heterosis. It is

suggested that to a certain extent heterosis is the result of mitochondrial complementation that occurs between polymorphic mitochondria of the hybrid.

C. CHLOROPLAST

HETEROSIS

A N D COMPLEMENTATION



It has already been emphasized that heterosis in plants often results in efficient

conservation of energy. Therefore, the utility of this phenomenon if observed in

both mitochondria and chloroplasts might be of greater importance for increasing

crop yield through the manipulation of organelle genomes. Chloroplast heterosis

and complementation with regard to several parameters in many economically

important crops have been demonstrated (Table 111). The term “chloroplast

complementation” is generally used to indicate the greater activity of 1 : 1 parental mixture of isolated chloroplasts when compared to the midparental values.

Higher photosynthetic rates in isolated chloroplasts of hybrids of many crop

species have been observed in the seedling stages (Sarkissian and Huffaker,

1962; Nagy et al., 1972; Heichel and Musgrave, 1969), and there is a consensus

for the occurrence of chloroplast heterosis in crop plants (Sinha and Khanna,

1975). Recent electron microscopic studies have provided evidence that hybrids

possess more highly developed chloroplast structure than their respective parents

(Hraska, 1978), and the increase in the size of the lamellae and thylakoid membrane structure in the chloroplasts of the hybrids was directly correlated with

their chlorophyll contents (Rakhmankulov et al., 1976). In addition to the enhanced activities of the key enzymes of the Calvin cycle in hybrid chloroplasts,

heterosis in chlorophyll content in maize and sorghum has also been reported

(Nosberger, 1970; Khanna, 1974; Fleming and Palmer, 1975; Planchon, 1976).

Results of chloroplast complementation based on Hill’s reaction and cyclic phosphorylation in maize showed 25-60% increased activity, and the enhanced activities due to chloroplast complementation were found to be closely associated with



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H. K. SRIVASTAVA



Table I11

Heterosis and Complementation with Regard to Chloroplast Activities

Parameters

Ribulose biphosphate

carboxylase/oxygenase

Photophosphorylation

Hill reaction

Chlorophyll content



Chlorophyll a / b ratio

Protein content



DNA and RNA contents



Dry matter of chloroplasts

Ultrastructure of hybrid

chloroplasts



Organisms

Maize (Zea mays); sorghum

(Sorghum vulgare); and

barley (Hordeurn vulgare)

Maize (Zen mays)

Cotton (Gossypium hirsurum)

Maize (Zea mays)

Cotton (Gossypium hirsutum)

Soybean (Glycine max)

Wheat (Triticum aesrivum)

Maize (Zea mays)

Cotton (Gossypiurn hirsutum)

Pea (Pisum sarivum)

Maize (Zea mays); cotton

(Gossypium hirsurum)

Maize (Zea mays)

Rye (Secale rereale)

Barley (Hordeurn vulgare)

Cotton (Gossypium hirsutum)

Tomato (Lycopersicum esculentum)



References

Sinha and Khanna (1975)



Berville ( I 977)

lmamaliev et a / . (1975)

Smirnova et al. (1975)

Rakhamankulov et al. (1976)

Starnes and Hadley (1965)

Planchon (1976)

Giles (1974); Fleming and

Palmer (1 975)

Rakhmankulov et al. (1976)

Vershinin et al. (1976)

Andregeva (1976)

Rakhmankulov et al. (1976);

Konarev (1976)

Vecher er a / . (1977)

Horak and Zalik (1975)

Rakhmankulov et al. (1976)

Kazim (1974)



the degree of grain yield heterosis (Ovchinnikova and Yakovlev, 1978). Some

evidence for a developmental complementation in amylase activity when kernels

of parental types were germinated together in a 1 : 1 ratio has also been recorded

in sorghum (Ghose et al., 1974). These observations on chloroplast heterosis and

complementation, like mitochondria1 complementation, could be interpreted to

mean that hybrids are endowed with more efficient energy processing systems in

their organelles.

Photosynthesis is limited by photorespiration in major crop species, including

wheat, rice, soybean, potato, peanut, barley, sugarbeet, cassava, and banana.

These are called C3 plants .because their primary photosynthetic products are

three-carbon compounds (3-phosphoglyceraldehyde)and they have only the reductive pentose phosphate (FU'P) cycle for COP fixation and reduction. The net

rates of COz fixation in C, plants are usually about half those of C4plants such as

maize, sorghum, pearl millet, sugarcane, and other tropical grasses. The more

efficient C4 plants characteristically have, in addition to the RPP cycle, another

Cot fixation cycle (Hatch and Slack, 1966; Hatch, 1976). In this cycle C 0 2 is

fixed by carboxylation of phosphoenolpyruvate (PEP) to give a four-carbon acid,

oxaloacetate or aspartate. Plants with this cycle are called C4 plants because the



INTERGENOMIC INTERACTION, HETEROSIS, AND CROP YIELD



141



first compounds formed after COz incorporation are four-carbon acids. In all

higher plants, irrespective of the type of primary fixation, CO, reduction to

carbohydrates occurs in the RPP cycle. The most important feature of C4 plants is

the presence in their leaves of two types of chloroplasts which differ in both their

ultrastructure and functions. In palisade mesophyll cells the chloroplasts have a

pronounced granal structure, while in bundlesheath parenchyma cells they are

usually paucigranal (Laetsch, 1974). These chloroplasts differ greatly in their

photochemical properties: photosystem I1 is active in mesophyll cells, while photosystem I is active in bundlesheath chloroplasts (Bazzaz and Govindjee, 1973;

Bishop et ul., 1977; K u et ul., 1974). Although the precise genetic nature of

dimorphic chloroplasts in C4 plants remains to be worked out, it is likely to

presume that each type results from the expression of distinct genes. Complementation of dimorphic chloroplasts in C4 plants is thus expected to lead to a

concentration of COz in the bundlesheath cells, where it decreases to a certain

extent the limitation of photosynthesis by the diffusive COPresistance and therefore aids in increasing the efficiency of the process. This is evidenced by the data

on photosynthetic efficiency of C4 plants in which photorespiration is absent

(Lorimer and Andrews, 1973). This situation could serve as one of the arguments

proving that photorespiration as a process of C02 release or loss in the light has

no great physiological significance.

Another important component of photosynthesis is photophosphorylation,

which is responsible for generating energy in terms of ATP and NADPHz for

productive processes in the chloroplasts. Significant variation in cyclic photophosphorylation and Hill reaction activities in isolated chloroplasts from hybrids

and their parents was demonstrated by Miflin and Hageman (1966). In these

studies the hybrids were either intermediate or followed the activity of one of the

parents, indicating either partial or full dominance. Khanna and Sinha (1975)

assayed chloroplast activity for cyclic and noncyclic photophosphorylation of

maize leaves in I-month-old seedlings. In the hybrids CM 400 X CM 300 and

CM 103 x CM 104, the noncyclic photophosphorylation activity was intermediate between that of their parents. In cyclic photophosphorylation the hybrid

CM 400 x CM 300 followed the more active parent, but CM 103 x CM 104

followed the less active one. These results suggest a biparental transmission of

genetically diverse chloroplasts in these hybrids. Both complementation between

parental chloroplasts and intergenomic interactions in heterotic hybrids might

lead to such results for traits like cyclic and noncyclic phosphorylations, where

the ultimate efficiency of the process is dependent on the formation of effective

hybrid enzyme molecules. There are also several reports showing heterotic activities of ribulose- 1,5-biphosphate carboxylase/oxygenase (RBPCase) and PEP

carboxylase in barley and sorghum hybrids (Khanna and Sinha, 1974, 1975).

Systematic studies have also shown that the activity of RBPCase is differentially

influenced during leaf growth and senescence in plants having the C4 pathway



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III. Organelle Involvement in Genetic Phenomena

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