February 11, 2016

Cell Biology: Talking at Different Levels

Plant organelles, cells and tissues are constantly receiving and sending feedback like an interconnected series of Russian dolls.
by Josephine Lee, WUSTL Undergrad and 2015 ASPB Summer Undergraduate Research Fellow




Many choices in development

The prototypical animal cell is roughly spherical, with a nucleus and various other membrane-bound organelles. But the human body, for instance, contains many different cell types that aren’t quite like a prototypical animal cell: nerve cells with long axons, saucer-shaped red blood cells without nuclei, and thin muscle cells full of fibers, to name a few (Figure 1). A similar case is seen in plant cells. The prototypical plant cell is roughly box-shaped, with chloroplasts and a large vacuole. But there are many other types of plant cells, such puzzle-shaped epidermal cells, rigid sclerenchyma cells and elongated phloem cells with porous walls for transport but no nucleus, ribosomes or vacuoles.

Figure 1: Types of cells in a human

In different plant cells, there are also various types of plastids that differentiate from proplastids (Figure 2). Chloroplasts are the classic ones, and have an internal system of membrane stacks with chlorophyll that turn them green and enable photosynthesis. Chromoplasts have bright red, orange and yellow pigments and give orchid petals, ripe tomato fruits and carrot roots their characteristic color. Leucoplasts are colorless, and are filled with starch, lipids or proteins for storage and/or secretion.

Figure 2: Types of plastids

Coordinating change

All animal cells begin as stem cells that then differentiate into the appropriate type of cell. In mammals, this initial differentiation typically is terminal – a nerve cell is forever a nerve cell. However, plant cells and the plastids they contain are more plastic. Plant cells initially begin as undifferentiated meristem cells, which contain undifferentiated plastids known as proplastids. After their initial differentiation, both plant cells and plastids can interconvert between initial types.

For a plastid, the decision to differentiate or re-differentiate is influenced by environmental signals, such as light intensity and quality, and by the developmental status of the host cell. For instance, when exposed to light, amyloplasts in potato tubers re-differentiate into chloroplasts that color new potato sprouts green. As tomatoes ripen, chloroplasts re-differentiate into chromoplasts to give the fruit a bright red color. This means rewiring of genetic switches, biochemical pathways and cellular structures to implement major structural changes in plastid morphology. For instance, the amyloplast to chloroplast transition requires restructuring membrane systems from large, starch-filled granules to stacks filled with chlorophyll.

On an organellar level, differentiating and re-differentiating requires communication with the nucleus to coordinate regulation of plastidic and nuclear genes. On a larger level, this also means that the plastid, the cell containing the plastid, the rest of the cells in the plant and the environment surrounding the plant are constantly receiving and sending feedback like an interconnected series of Russian dolls. Plant development and response to the environment is a delicate task, and coordination within and between all levels of a plant is crucial.


Jarvis, Paul, and Enrique López-Juez. "Biogenesis and homeostasis of chloroplasts and other plastids." Nature Reviews Molecular Cell Biology14.12 (2013): 787-802. PubMed Journal

Solymosi, Katalin, and Áron Keresztes. "Plastid structure, diversification and interconversions II. Land plants." Current Chemical Biology 6.3 (2012): 187-204. Author Reprint