SPAIN – The main function of mitochondria and chloroplasts in cellular energy production is well known.  So is the fact that these organelles are endowed with genetic material (as a result of endosymbiotic origin, the primitive symbiosis of bacteria within a cell). Such genes allow them to send signals to the nucleus to inform of its state, which is known as retrograde signaling. Mitochondria and chloroplasts use this mechanism to apply the core proteins required and thus exercise their function properly as power producers.

Now, a study conducted by a team of Center for Research in Agricultural Genomics (CRAG) describes the effects of retrograde signaling in plants go far beyond what had been described so far. It has been found that also has the ability to modulate the overall development of the plant and may even be imposed hierarchically to the core. “Now we know that, like signaling from the mitochondria to the nucleus regulates key processes in animals [such as cell division], the chloroplast also regulates plant growth by a mechanism that we could describe at the molecular level,” said the study’s lead researcher, Elena Monte.

The chloroplast takes the lead

The authors used Arabidopsis thaliana small seedlings under development guided by the light (photomorphogenesis) to investigate the molecular mechanism mentioned.

It was known that the nuclear GLK1 gene plays a key role in the process of photomorphogenesis. This is a gene that is regulated by retrograde signaling and proteins called PIF (phytochrome interacting factors), which are sensitive to light. Under normal conditions, when the seedling is in the dark (when has not yet emerged from the earth), the PIF proteins are abundant and prevent GLK1 action;  but when springs and the light comes, the PIF proteins are degraded, allowing photomorphogenesis where GLK1 promote development process in which the plant leaves and acquires more chlorophyll.

Furthermore, in the present study has shown that when the chloroplast is damaged (for example, when applying a drug) or detects that environmental conditions are stressful (for example, by subjecting the plant to excessive illumination), the expression of GLK1 decreases in response to the backward signals sent by the chloroplast, through a mechanism independent of PIF. As a result, plant growth slows and the plant prevents photooxidative damage, leaving the impression that the conditions again become favorable for growth. In summary, the chloroplast functions as a stress sensor antenna capable of temporarily taking the direction of the cell to the nucleus to change the development of the plant and protect it.

For Elena Monte, “This work helps to understand how organelles in eukaryotic endosymbiont can change the overall development of the organism. This advance can help find solutions for plants to cope with the increased radiation, and therefore the light stress as a result of climate change.”

The study has been published in Nature Communications.