The Chloroplast Nucleoid in Ochromonas Danica

Gene expression characterizes different nutritional strategies among three mixotrophic protists

Mixotrophic protists, i.e. protists that can carry out both phototrophy and heterotrophy, are a group of organisms with a wide range of nutritional strategies. The ecological and biogeochemical importance of these species has recently been recognized. In this study, we investigated and compared the gene expression of three mixotrophic protists, Prymnesium parvum, Dinobyron sp. and Ochromonas sp. under light and dark conditions in the presence of prey using RNA-Seq. Gene expression of the obligately phototrophic P. parvum and Dinobryon sp. changed significantly between light and dark treatments, while that of primarily heterotrophic Ochromonas sp. was largely unchanged. Gene expression of P. parvum and Dinobryon sp. shared many similarities, especially in the expression patterns of genes related to reproduction. However, key genes involved in central carbon metabolism and phagotrophy had different expression patterns between these two species, suggesting differences in prey consumption and heterotrophic nutrition in the dark. Transcriptomic data also offered clues to other physiological traits of these organisms such as preference of nitrogen sources and photo-oxidative stress. These results provide potential target genes for further exploration of the mechanisms of mixotrophic physiology and demonstrate the potential usefulness of molecular approaches in characterizing the nutritional modes of mixotrophic protists.

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.

The division of pleomorphic plastids with multiple FtsZ rings in tobacco BY-2 cells

Plastids, an essential group of plant cellular organelles, proliferate by division to maintain continuity through cell lineages in plants. In recent years, it was revealed that the bacterial cell division protein FtsZ is encoded in the nuclear genome of plant cells, and plays a major role in the plastid division process forming a ring along the center of plastids. Although the best-characterized type of plastid division so far is the division with a single FtsZ ring at the plastid midpoint, it was recently reported that in some plant organs and tissues, plastids are pleomorphic and form multiple FtsZ rings. However, the pleomorphic plastid division mechanism, such as the formation of multiple FtsZ rings, the constriction of plastids and the behavior of plastid (pt) nucleoids, remains totally unclear. To elucidate these points, we used the cultured cell line, tobacco (Nicotiana tabacum L.) Bright Yellow-2, in which plastids are pleomorphic and show dynamic morphological changes during culture. As a result, it was revealed that as the plastid elongates from an ellipsoid shape to a string shape after medium renewal, FtsZ rings are multiplied almost orderly and perpendicularly to the long axis of plastids. Active DNA synthesis of pt nucleoids is induced by medium transfer, and the division and the distribution of pt nucleoids occur along with plastid elongation. Although it was thought that the plastid divides with simultaneous multiple constrictions at all the FtsZ ring sites, giving rise to many small plastids, we found that the plastids generally divide constricting at only one FtsZ ring site. Moreover, using electron microscopy, we revealed that plastid-dividing (PD) rings are observed only at the constriction site, and not at swollen regions. These results indicate that in the pleomorphic plastid division with multiple FtsZ rings, the formation of PD rings occurs at a limited FtsZ ring site for one division. Multiplied FtsZ rings seem to localize in advance at the expected sites of division, and the formation of a PD ring at each FtsZ ring site occurs in a certain order, not simultaneously. Based on these results, a novel model for the pleomorphic plastid division with multiple FtsZ rings is proposed.

The partitioning of cytoplasmic organelles at cell division

When an organism has only one or two mitochondria or chloroplasts per cell, it is probable that their partitioning is always stringently controlled so that each daughter cell always receives half the organelles in the parent cell. When there are more copies of an organelle, the available data suggest that partitioning is stochastic but far from random, with a strong tendency toward equality. The molecular mechanisms that promote equal partitioning are not known in any case, but the great variety of organelle behavior suggests that many different mechanisms are involved in different organisms. As Wilson (1925) pointed out, the precision of partitioning of cytoplasmic organelles rarely if ever equals that of mitosis, but it is still an expression of selection for mechanisms that will ensure the hereditary continuity of the organelles. How cells compensate for unequal partitioning by controlling organelle replication is known for only one case. But when one considers that Tetrahymena and Paramecium use different methods to compensate for unequal partitioning of macronuclear DNA, it would not be surprising if organisms use a variety of different compensating replication modes for organelles as well. What is surprising is that so little attention has been paid to these problems. Nothing could be simpler than counting organelles in dividing cells, but this has been done on a large scale in only two systems. Quantitative techniques in cell biology have been developed to the point where such studies could be done even on cells that have too many organelles for direct counting. Molecular mechanisms of partitioning have scarcely been touched on. Much more has been done on the role of the cytoskeleton in determining cell shape, and some observations have been made on its role in positioning organelles in interphase cells, but these kinds of studies have not been extended to dividing cells. Some experiments and observations have been made on the role of microtubules and microfilaments in moving cytoplasmic organelles around the cell during interphase, but again nothing has been done on their possible role in partitioning organelles at cytokinesis. The major lesson of this article is how little has been done, and how much can be done. The partitioning of cytoplasmic organelles at cell division is a wide-open field for future research, and one of great importance for both genetics and cell biology.