Correlations, Regeneration and Compensatory Divisions
By studying the replication of plastids and the number of chloroplasts we deliberately confine ourselves to one of many aspects of the plastidome. This isolation of one aspect would not be desirable if we were able to study all aspects at the same time. Plastids depend on the nucleus and on the internal and external environment, but the nucleus and the internal environment also depend on plastids. An old example of such a feedback effect was given by Klebahn (1891) for Closterium and Cosrnarium. The zygotes of these species normally contain two chloroplasts. During meiosis, two of the four nuclei produced die, and each of the two resulting cells receives one nucleus and one chloroplast. A zygote, however, may contain one or three chloroplasts instead of two. If this occurs, one or three nuclei survive, respectively. Nuclear survival in these zygotes is controlled by the number of chloroplasts present. A recent example of the dependence of a nucleus on the activity of plastids or mitochondria was found in Acetabularia. In this green alga, protein synthesis at 70 S ribosomes is involved in the regulation of deoxythymidyl kinase used in the nucleus. Hence chloroplasts or mitochondria increase the production of an enzyme that is closely connected to the replication of nucDNA and the formation of secondary nuclei (Bannwarth, Ikehara, and Schweiger 1977).
KeywordsSugar Beet Guard Cell Mesophyll Cell Spongy Parenchyma Chloroplast Division
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