RNA Content in Chromoplasts of Carrot Roots: Biochemical Composition Analysis

RNA in carrot root chromoplasts accounts for roughly 0.08–0.375 % of the dry matter of the plastids and varies through the growing season, peaking when the root reaches about 7–12 mm in diameter. W. Straus studied the biochemical composition of chromoplasts in carrot roots and put their RNA content at 3.3–5.9 % relative to the dry matter of the plastids.

Comparable studies of other objects had not been carried out. There is, however, evidence of changes in the amount of RNA in the chloroplasts of the photosynthesising tissues of higher plants.

The RNA content of amyloplasts in potato tubers measured 0.4 ± 0.012 × 10^-14 g per organelle. The carrot as an object for studying the biochemical composition of root chromoplasts.

How much RNA do carrot root chromoplasts and amyloplasts contain?

V. P. Lobov and I. A. Petrov measured the RNA content of the chromoplasts and amyloplasts of red and white carrot roots, expressing the result per dry matter of the plastids, per plastid protein and per single organelle (Table 1). The figures below show how the value shifts across development.

Table 1. RNA content of chromoplasts in the roots of the red carrot varieties Kharkovskaya and Nantskaya, and of amyloplasts in the roots of the white carrot variety Belaya Zelenogolovaya.

In the chromoplasts of the Kharkovskaya and Nantskaya carrot varieties, RNA changed over the course of the growing season, ranging overall from 0.080 to 0.375 % of plastid dry matter, from 0.16 to 0.66 % of plastid protein mass, and from 0.24 to 0.74 × 10^-14 g per chromoplast. The RNA content of the amyloplasts likewise varied — from 0.068 to 0.157 % per organelle dry matter, from 0.07 to 0.81 % per plastid protein, and from 0.20 to 0.65 × 10^-14 g per amyloplast.

Overall, the changes in RNA content of the chromoplasts and amyloplasts paralleled the changes in DNA content of these same organelles. RNA was 2–3 times higher than DNA for both plastid types at every developmental stage of the white and red carrot roots.

Can carrot chromoplasts synthesise their own RNA?

Carrot chromoplasts synthesise RNA on their own template throughout the plant's entire growing season, much as the chloroplasts of green plants and the amyloplasts of potato tubers do. In young leaves this process runs more intensively than in the chloroplasts of mature and old leaves.

The rate at which a labelled precursor is incorporated into the amyloplasts of potato tubers depends on the physiological state of the storage organ. Amyloplasts from young tubers incorporated ³H-UTP into the acid-insoluble material of the organelles most actively. As tubers grew larger, incorporation of the label into the acid-insoluble material fell markedly and almost ceased entirely in the amyloplasts of mature tubers.

Plastids from dormant tubers and from tubers that had just emerged from dormancy did not incorporate the labelled precursor of RNA biosynthesis, whereas experiments with amyloplasts from sprouting tubers showed a slight rise in the radioactivity of the acid-insoluble fraction.

A system of isolated plastids is currently the most convenient way to study synthesis. The RNA synthesised in such a system represents the products of expression of the genome of the organelles under study, because contamination by products of nuclear-genome expression is excluded.

The work was based on the incubation medium proposed for studying RNA synthesis in a system of isolated chloroplasts. Mannitol served as the osmotic agent. In addition, monovalent cations were introduced into the incubation medium as 0.03 M KCl, which is needed to stabilise transcription on the plastid ribosomes.

Table 2. Incorporation of ³H-UTP into the acid-insoluble material of chromoplasts and amyloplasts (counts per minute) over the incubation period.

Experiments on the incorporation of ³H-UTP into the acid-insoluble material of red-carrot chromoplasts established that these organelles do take up labelled precursors. Incorporation of the label into chromoplasts isolated from roots in mid-season rose nearly twofold compared with chromoplasts from forming roots (3–5 mm in diameter).

Towards the end of the growing season a certain decline in the incorporation of ³H-UTP into the RNA of these organelles was observed.

The results therefore indicated that chromoplasts are able to synthesise RNA throughout the entire growing season of red-carrot plants (Table 2).

Amyloplasts isolated from the forming roots of the white carrot variety Belaya Zelenogolovaya also incorporated appreciable amounts of ³H-UTP into the acid-insoluble material of the plastids, and the intensity of this incorporation differed little from that of the chromoplasts of forming red-carrot roots (see Table 2).

Already by mid-season, however, when the roots measured 8–10 mm in diameter, a marked drop in the incorporation of the labelled precursor by the amyloplasts was seen; and by the end of the season (root diameter 16–20 mm) the incorporation of ³H-UTP into the RNA of these organelles fell almost tenfold compared with the incorporation by the amyloplasts of forming roots.

The study of ³H-UTP incorporation into the acid-insoluble material of the organelles thus showed that the intensity of RNA biosynthesis in chromoplasts and amyloplasts depends in a definite way on the physiological state of the roots. Chromoplasts retained a high RNA-synthesising capacity throughout the plant's entire growing season.

Amyloplasts, by contrast, synthesised RNA intensively in the forming roots; as the roots increased in size, the RNA-synthesising capacity of the amyloplasts dropped sharply.