Are chloroplasts polyploid?

Inverted repeats in chloroplast DNA from higher plants

The circular chloroplast DNAs from spinach, lettuce, and corn plants have been examined by electron microscopy and shown to contain a large sequence repeated one time in reverse polarity. The inverted sequence in spinach and lettuce chloroplast DNA has been found to be 24,400 base pairs long. The inverted sequence in the corn chloroplast DNA is 22,500 base pairs long. Denaturation mapping studies have shown that the structure of the inverted sequence is highly conserved in these three plants. Pea chloroplast DNA does not contain an inverted repeat. All of the circular dimers of pea chloroplast DNA are found to be in a head-to-tail confirmation. Circular dimers of spinach and lettuce were also found to have head-to-tail conformation. However, approximately 70-80% of the circular dimers in preparations of lettuce and spinach chloroplast DNA were found to be in a head-to-head conformation. We propose that the head-to-head circular dimers are formed by a recombination event between two circular monomers in the inverted sequence.

Variable amounts of DNA related to the size of chloroplasts III. Biochemical determinations of DNA amounts per organelle

Plastid genomes (plastomes) are part of the integrated compartmentalised genetic system of photoautotrophic eukaryotes. They are highly redundant and generally dispersed in several regions (nucleoids) within organelles. DNA quantities and number of DNA-containing regions per plastid vary and are developmentally regulated in a way not yet understood. Reliable quantitative data describing these patterns are scarce. We present a protocol to isolate fractions of pure plastids with varying average sizes from leaflets (8 microm average diameter, corresponding from approximately a dozen to 330 genome equivalents per organelle and on average four to seven copies per nucleoid. The ratio of plastid/nuclear DNA changed continuously during leaf development from as little as 0.4% to about 20% in fully developed leaves. On the other hand, mesophyll cells of mature leaves differing in ploidy (di-, tri- and tetraploid) appeared to maintain a relatively constant nuclear genome/plastome ratio, equivalent to about 1,700 copies per C-value.

Multiple amounts of DNA related to the size of chloroplasts : I. An autoradiographic study

Plants containing genetically small or large plastids appear in some euploid and trisomic types of Beta vulgaris.Small tissue samples of the first 6-10 leaves of nearly 30 euploid plants from 7 different generations were incubated in a solution containing (3)H-thymidine in dark/light cycles, for up to 72 hr. For semiquantitative autoradiography the chloroplasts were then prepared on the slides with various kinds of isolation media, and fixed with OsO4, glutaraldehyde, formaldehyde, or ethanol. The specifity of incorporation was tested by observing the extractability of label after differential treatment with acid or nucleases.Chloroplasts in leaves 2-11 cm long preferentially incorporate (3)H-thymidine. The silver grains over plastids appear to be in clusters (centres). A relationship between the number of grains and also between the number of centres on the one hand, and the chloroplast size on the other could be found.It is concluded that chloroplasts occur in various degrees of polyenergide organization, as has been described, for example, for blue-green algae. Regarding the presence and degree of polyploidy-the other form of genetic "polyvalency" -the experiments provided no information. A remarkable variation in chloroplast size (and number of labelled centres) was observed, not only between different plants or between different leaves of a plants, but also within small tissue samples.