Submitted to: Protoplasma
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/2/2004
Publication Date: 12/23/2004
Citation: Paolillo, D.J., Jr., Garvin, D.F., Parthasarathy, M.V. 2004. The chromoplasts of Or mutants of cauliflower (Brassica oleracea L. var. botrytis). Protoplasma. 224:245-253. Interpretive Summary: In plant cells, structures called plastids are present that have several essential roles. Some compounds in plastids also are important in human nutrition. For instance, plastids contain vitamin A in the form of beta-carotene. The amount of beta-carotene in plastids is highly controlled by the plant, and understanding how this control occurs may allow plants to be altered so that they accumulate more beta-carotene. The Or mutant of cauliflower has plastids that accumulate so much beta-carotene that parts of the plant are orange. Therefore, it is a useful model to study beta-carotene accumulation. The objective of this research was to characterize the cellular changes in the plastids of these mutant plants that allow them to accumulate so much beta-carotene. Through microscopic analysis, data were obtained that reveal that the beta-carotene accumulates in plastid structures similar to those in which beta-carotene accumulates in carrot roots. Further, the plastids in Or mutants become programmed to accumulate beta-carotene almost immediately upon formation, and this is associated with a reduction in how frequently these plastids replicate themselves. This work will be useful to scientists as they seek to increase vitamin A levels in crops. Vitamin A deficiency affects several hundred million people worldwide, and increasing beta-carotene levels in crops will have a profound beneficial impact on human health and well-being.
Technical Abstract: The Or mutation in cauliflower leads to abnormal accumulations of beta-carotene in orange chromoplasts, in tissues where leucoplasts are characteristic of wild type plants. Or chromoplasts were investigated using light microscopy of fresh materials and electron microscopy of glutaraladehyde- and potassium permanganate-fixed materials. Carotenoid inclusions in Or chromoplasts resemble those found in carrot root chromoplasts in their optical activity and angular shape. The electron microscope revealed that the inclusions are made up of parallel, membrane-bound compartments. These stacks of membranes are variously rolled and folded into three-dimensional objects. We classify Or chromoplasts as membranous chromoplasts. The Or mutation also limits plastid replication so that a single chromoplast constitutes the plastidome in most of the affected cells. There are one to two chromoplasts in the cells of a shoot apex. The ability of differentiated chromoplasts to divide in the apical meristems of Or mutant plants resembles the ability of proplastids to maintain plastid continuity from cell to cell in meristems of Arabidopsis mutants in which plastid replication is drastically limited. The findings are used to discuss the number of levels of regulation involved in plastid regulation.