Growth-defense gene pulls many strings in plant cell factories

Leaf in the dark with water droplets

 

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The Brandizzi lab has demonstrated how one master gene fine-tunes how massive protein factories in plant cells allocate resources to growth and defense functions.

The results have been published in The Plant Journal.

 

Grow or defend?

One of the biggest decisions plants constantly make is whether to grow or defend, because they cannot do both optimally and at the same time. So they have evolved strategies that allocate resources toward one or the other function depending on their needs.

“These types of balancing acts happen at many levels in a plant, including one my lab is interested in: the  endoplasmic reticulum, a massive factory found in any eukaryotic cell, producing one third of that cell’s  proteins.” says Cristina Ruberti, a post-doc in the Brandizzi lab and co-author of the published paper.

The published study focused on a master gene called CPR5 (constitutive expressor of pathogenesis-related genes-5) that has been previously observed to affect both growth and defense functions in the endoplasmic reticulum.

For example, when under attack from bacteria or pathogens, plants rely on the  hormone salicylic acid for protection, which is a common defense strategy. (fun fact: salicylic acid is used to treat acne, dandruff, and other skin conditions). During these threatening times, CPR5 fine tunes the hormonal system in order to ensure growth functions don’t fully shut down.

On the other side of the growth-defense spectrum, previous research had shown CPR5 helps the endoplasmic reticulum control plant cell development, including how big cells can grow or how the protective  cell wall is built.

“In that sense, the CPR5 gene is a master controller of crucial growth and defense processes, but we did not know how it worked in such balance.”

 

The endoplasmic reticulum connects the nucleus, the mastermind, to the cell's extremities.
By AJC ajcann.wordpress.com, CC BY-SA 2.0

 

Protecting the endoplasmic reticulum

The intersection of CPR5 with the other processes was found in the endoplamic reticulum’s quality control mechanism for protein production. Known as the unfolded protein response, this quality control kicks in during stressful times – such as extreme heat or growth – when the endoplasmic reticulum is prone to produce defective proteins.

Specifically, the unfolded protein response mechanism is activated through two “arms”, or light switches, that turn the quality control function on. And the connections between CPR5, the defense hormone, and the unfolded protein response play out at the site of these two light switches.

The researchers observed that when the plant is under pathogen attack, the salicylic acid defense hormone operates in the endoplasmic reticulum through the unfolded protein response’s light switches, simultaneously inhibiting growth. On its end, the CPR5 gene tempers the growth inhibition triggered by the hormone so that growth functions do not shut down.

In addition to controlling the defense hormone's interactions with the quality control mechanism, Cristina found that CPR5 directly monitors the quality control itself in times of extreme stress.

“Under severe environmental conditions, CPR5 directly represses both light switches, allowing some of the plant’s energy to go towards growth instead of exclusively prioritizing defense. In that role, CPR5 ensures the endoplasmic reticulum doesn’t overdose on its own quality control, which if extreme, causes a preemptive programmed cell death.”

Cristina adds that the study further demonstrates how CPR5 is truly a master gene with its hands on a number of unrelated processes.

“On a larger scale, it is becoming clear that plant components talk to each other in more complex ways than previously observed. In this case, one gene is helping to control two seemingly distinct protection mechanisms – the defense hormone and the quality control system – to ensure that the endoplasmic reticulum, one of the plant’s most important components, remains healthy.”