August 17, 2015

The chatter in a cell

How communication between organelles helps in stressful situations
By Josephine Lee

 

 

This summer, a Summer Undergraduate Research Fellowship award from the American Society of Plant Biologists gave me an awesome opportunity to stay in St. Louis and gain research experience in the Haswell Lab. After juggling research and coursework during the Spring and Fall semesters, it’s been a nice change to focus only on research. I’ve also had another change and am working on something different this summer – talking organelles!

But before zooming in to what organelles are talking to each other about, it helps to contextualize what an entire plant experiences. As its surroundings change, a plant needs to adapt to conditions such as high temperature, changes in light intensity or drought. Unlike animals, plants can’t move to a cooler, shadier or wetter spot. Instead, they must optimize physiological processes such as photosynthesis and respiration to cope with these stressful situations. This is important since stress not only reduces the efficiency of these physiological processes, but can also result in the overproduction of reactive oxygen species (ROS) – small molecules that can seriously damage the cell.

But optimization has its own challenges.

Fig 1: Some organelles in a plant cell. Image from: https://confluence.crbs.ucsd.edu/display/CS/Animal+Cell+versus+Plant+Cell

Photosynthesis occurs in chloroplasts and respiration occurs in mitochondria, the infamous “powerhouse of the cell” (Figure 1). However, genetic instructions to build components of the photosynthetic and respiratory factory lines are located in both the nucleus and in the respective organelle. Although there are a few chloroplast genome-encoded proteins involved in photosynthesis, most are encoded in the nuclear genome, then produced in the cytosol, transported to the chloroplast, and assembled with other photosynthetic proteins in the chloroplast. Genes for most mitochondrial proteins are also housed in the nucleus.

Maintaining the appropriate amounts of photosynthetic and respiratory proteins is mostly the responsibility of information that travels from the nucleus to an organelle, otherwise known as anterograde signals (Figure 2). This is in line with the view of the nucleus as the “brain” of the cell, sending instructions to control the chloroplast and mitochondria.

Fig 2: Signaling between nucleus and organelles Credit: Woodson, Jesse D., and Joanne Chory. "Coordination of gene expression between organellar and nuclear genomes." Nature Reviews Genetics 9.5 (2008): 383-395.

But since the chloroplast and mitochondria are so sensitive to environmental changes, they can also provide valuable feedback to the nucleus about these changes, and induce other responses to cope with stress. In some cases, overproduction of ROS that can potentially damage the cell can also signal stress to the nucleus. This organelle-to-nucleus signaling is known as retrograde signaling (Figure 2).

This summer, I’ve been focusing on the retrograde part of this two-way signaling street, and I will continue to work on it in the fall. But both anterograde and retrograde signaling are critical: for a plant in changing surroundings, it takes healthy chatter within a cell to keep the whole plant alive and well.

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