Study Notes on Genetics of Plastids (With Diagram)

The below mentioned article provides a study note on Genetics of Plastids.

In the cytoplasm of the plant cells are found many small cytoplasmic bodies, called plastids. These plastids are of several types, such as, chloroplast, leucoplast, chromoplast and so on. Plastids arise from smaller particles, the proplastids, found in the egg cytoplasm. They reproduce themselves independently of other cell parts by process of division or self-duplication.

Plastid characteristics are inherited in the majority of the cases from the female parent but in some cases a few characteristics are transmitted from the male parent also. One of the common examples of plastid inheritance is the variegation found in plants. In variegated condition, some organs of the plant, say the leaves, may show green and white patches or green, yellow, white or pink spots.

These spots of varying colours, show differences in the kinds of plastid. Although in many plants variegations are known to depend on ordinary Mendelian genes (i.e., they are governed by nuclear genes), yet there are a few known cases which show a largely maternal type of inheritance. Inheritance in these cases depends on the nature of the plastids.

Carl Correns (1909) described a case of maternal transmission in mirabilis jalapa (four-O’clock plant). The blotchy or spotted leaves of these variegated plants show patches of green and white tissue but some branches carry only green leaves and others only white leaves. Flowers develop on all types of branches and may be cross-pollinated in different combinations. From these experiments two surprising features came to light:

(i) There was a difference in the results of reciprocal crosses; as for example, Green leaved ♀x white leaved ♂ gave green leaved progeny and white ♀x leaved green leaved ♂ gave white leaved progeny. The differences observed in the results of reciprocal crosses were quite contrary to Mendel’s findings.

In fact, in conventional genetics, differences in the results of reciprocal crosses are expected and encountered in the case of sex-linked genes. However, the results of the crosses of mirabilis jalapa cannot be explained by sex linkage.

(ii) The phenotype of female parent determined the phenotype of all progeny (uniparental transmission) and the phenotype of male parent did not contribute anything to the progeny. This phenomenon is referred to as maternal inheritance.

In subsequent generations, the maternal inheritance has been found to be effective in the same pattern as those observed in original crosses. The different leaf colours arise due to the presence of green or colourless plastids in the cells.

The inheritance of different leaf colours in mirabilis jalapa might be explained if the plastids are somehow genetically autonomous and are never transmitted through male parent. For an organelle to be genetically autonomous, it must be provided with its own genetic determinants that are responsible for its phenotype.

In this case, the plastids carry their own genetic .determinants that is responsible for their colour. Since the bulk of cytoplasm of zygote is contributed by the egg and the male gamete contributes negligible amount of cytoplasm, plastids present in the cytoplasm of egg are responsible for the appearance of maternal colour in the progeny and the failure of pollen parent to transmit its colour to progeny is reasonable.

The variegated branches produce three kinds of eggs; some with colourless plastids (leucoplasts), some contain only chloroplasts, and some are with both chloroplasts and leucoplasts. The egg containing green and colourless plastids produces a zygote that also contains both kinds of plastids.

In the course of development, some form of cytoplasmic division occurs during mitosis that segregates chloroplasts and leucoplasts into pure cell lines which produce variegation in the leaves of the progeny (Fig. 18.2).

This process of segregation of plastids during cytokinesis of mitosis is described as mitotic segregation. This process represents the segregation and recombination of organelle genotype. Therefore, it is also called cytoplasmic segregation and recombination (CSAR).

Inheritance of lojap Trait in Maize:

‘lojap’ is a condition which is found in maize. The name ‘iojap’ was derived from “Iowa” State (USA), the source of com strain and japanica, the name of a striped variety.

In corn the iojap gene is a recessive chromosomal gene which in homozygous condition produces green and white striped leaves on the plant. When a normal plant with green leaves (as female parent) is crossed to iojap (as pollen parent) the offspring are green leaved.

In reciprocal cross between normal green (ò parent) and iojap plant there appear offsprings of three different types as shown below:

In iojap plants, green and white striped condition is inherited from female parent (maternal inheritance). It seems that in iojap plants, there occur two types of plastids; normal green and abnormal iojap plastids. During differentiation of female gametes plastids are randomly distributed in the egg cells.

If the egg receives normal green plastids it will give rise to green leaved progeny irrespective of which plant acted as pollen parent. If the egg receives abnormal colourless plastids, it after fertilization, will produce white leaved offspring.

If the egg receives both green and white plastids then after fertilization it will produce offspring with green and white striped leaves. Probably a nuclear gene controls the development of abnormal plastids in the cytoplasm.

If striped leaved F₁, iojap (Ij/ij) female parent is crossed to normal green leaved (Ij/Ij) male parent, the following types of offspring are obtained:

This back-cross experiment shows that green males have no effect upon progeny.

If plastids are self-duplicating structures possessing their own hereditary factors, it should be possible to isolate them and demonstrate that they contain the complete genetic system. Researches in number of different laboratories have demonstrated that the chloroplasts contain DNA, RNA and ribosomes.

DNA found in the chloroplast is different from the nuclear DNA, as the plastid DNA has low molecular weight in comparison to nuclear DNA. Their base composition is also different from that of nuclear DNA. The lighter DNA has been called ‘satellite DNA’.

DNAs of different types of plastids are self-perpetuating and arise from pre-existing ones. They are not produced by nuclear DNA which would otherwise be capable of producing only one kind of plastid DNA. Evidence for this has been obtained by Gibor and Granic (1962) from the green alga Euglena.

They irradiated the cytoplasm with ultraviolet (UV) light. When the ultraviolet light was directed at the cytoplasm, the irradiation caused permanent disappearance of the plastid colour and thus the irradiated Euglena in successive generations never developed chloroplast.

This indicates that the chloroplasts are highly mutable. Only irradiation of cytoplasm caused irreversible mutation of the plastids which then grew and divided as tiny colourless proplastids in different generations.

Here, if it is reasoned that ultraviolet sensitive DNA units of the cytoplasm are formed by the nuclear DNA then the new cytoplasmic DNA units formed by the nuclear DNA should replace the damaged DNA of the cytoplasm and prevent bleaching.

Since this did not happen, it is concluded that DNA of plastid is formed from pre-existing DNA. Replication of plastid DNA has been suggested by auto-radiographic studies also. In all cases, there are multiple copies of plastid DNA in each chloroplast.

The transcription of RNA from plastid DNA has also been experimentally proved in the isolated chloroplasts. Smillie (1967) demonstrated that the chloroplast ribosomal RNA of euglena engages in complementary base pairing with euglena chloroplast DNA (hybridization technique) indicating that information necessary to make chlororibosomes resides in the chloroplast’s own chromosome.

This conclusion has been confirmed for tobacco chloroplasts by Tewari and Wildmann. Their estimation indicated that nearly four sets of ribosomal genes were present in the chloroplast. Their data also indicated that the nuclear DNA contains genes for chlororibosomes as well as for cytoplasmic ribosomes.

RNA-protein translation (protein synthesis) system has been demonstrated in isolated chloroplasts. Although protein synthesis in isolated plastids is suggested by incorporation of aminoacids, no net synthesis of a specific protein of the plastids has been demonstrated as yet.