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How Plants Get by When Hungry and in The Dark
A low-resource environment triggers the expression of MRF genes, which help adjust protein synthesis so that plants can survive and recover when conditions improve.
Plants often find themselves in stressful situations (like heat, cold, darkness, and dehydration) that they must endure because they cannot move to a better location. In response, plants have evolved ways of minimizing energy use to basic levels needed for survival when the environment is low in resources. Plants can alter the amount of protein synthesis (an energy-intense process) to correspond with energy availability, a balancing act also found in animals. However, unlike in animals, scientists know far less about how plants actually do this.
To investigate this question, researchers from Yonsei University in Seoul, headed by Dr. Hyun-Sook Pai, performed several experiments on Arabidopsis thaliana, a plant commonly used in genetic research. The team looked into the cellular functions of four homologs or variants of a protein (MRF1–4), all of which contain four MA-3 domains. They exposed Arabidopsis to darkness in conditions that lacked glucose—which plants use for energy—and found that the genes encoding MRF were highly expressed. Additionally, they found that mutants in which the MRF genes are “silenced,” or turned off, recovered from darkness and starvation much slower than non-mutant plants. Mutants that express these genes more than normal, on the other hand, recovered faster than non-mutants.
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The team was also interested in getting a better idea of exactly what MRF proteins were doing in the plant. Through a series of tests, Dr. Pai and her colleagues demonstrated that MRFs bind to eIF4A, a protein involved in the initiation of translation, and co-fractionate with ribosomes. MRF silencing decreases translation activity, while MRF1 overexpression increases it, accompanied by altered ribosome patterns, particularly in the dark and starvation (DS). Furthermore, MRF deficiency in DS causes altered distribution of mRNAs in sucrose gradient fractions, and accelerates rRNA degradation.
“Plants probably need MRFs to efficiently translate specific target proteins under energy-deficient conditions,” Dr. Pai says. “Our data also suggest that these MRF genes are under the control of the TOR signalling pathway that dynamically adjusts cell functions in response to environmental cues, like whether it’s too dark or too dry.”
The scientists linked MRF genes to TOR by using genetically modified plants in which a key TOR protein is silenced by chemical treatment. When this TOR protein was silenced, changes seen in MRF gene expression under darkness and starvation were disrupted, and phosphorylation and ribosome association of MRFs were inhibited.
“The TOR pathway is found in all eukaryotes and performs essential functions for cell growth and survival, but when it comes to plants, we don’t know much about the pathway or how their signalling components interact,” Dr. Pai adds. “Prior to this study, there wasn’t much known about MRF action, or how protein translation is controlled under unfavourable conditions in plants. Certainly, there was no data showing a link to TOR signalling.”
These findings provide some key pieces to the puzzle of TOR signalling in plants, and bring us closer to understanding how plants control protein synthesis in a changing environment.
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