Chloroplasts play an unexpected role in plant immunity

A team of scientists led by the University of California, Davis, has recently conducted a study revealing the critical role of chloroplasts in plant immunity against viral and bacterial pathogens, expanding their known function of converting sunlight into plant nourishment.

The role of stromules 

Chloroplasts are predominantly spherical structures but can occasionally transform to extend tube-like projections known as “stromules.” Despite being observed for over a century, the precise biological function of stromules has been elusive.

Previous research has indicated that the production of stromules increases when plants detect an infection, suggesting a potential role in plant immunity. The stromules appear to gather chloroplasts around the cell’s nucleus and serve as channels to transport pro-defense signals. 

However, until now, the specific role of stromules in immunity and the genes involved in their formation remained largely unknown.

Focus of the study

In this groundbreaking study, the researchers have identified a key protein associated with the formation of stromules during a plant’s immune response. Their findings have been published in the journal Science Advances on October 25.

To fully understand the role of stromules in plant immunity, the researchers sought a method to deactivate them and observe the resultant effects on plant cells when exposed to pathogens. This task was complicated by the lack of knowledge regarding the specific genes responsible for stromule creation.

The experts focused their attention on kinesins, motor proteins that facilitate intracellular movement, often involving the cell’s cytoskeleton composed of microtubules and actin filaments. They were particularly interested in a unique plant kinesin capable of binding both of these cytoskeletal fibers.

What the researchers learned 

Their experiments demonstrated that overexpressing the KIS1 kinesin induced stromule formation even in the absence of a pathogen. Conversely, plants modified to lack KIS1 were unable to produce stromules, leaving them vulnerable to pathogens.

Further exploration into the roles of microtubules and actin revealed that KIS1 must bind to microtubules to initiate stromule formation. However, for chloroplasts to migrate toward the nucleus, KIS1 also needed to bind to actin.

The study also delved into the broader implications of stromules in plant immunity. The researchers found that stromule formation is contingent on molecular signaling and requires an intact immune signaling system.

Study implications 

Senior author Savithramma Dinesh-Kumar, a plant biologist at UC Davis, emphasized the importance of these discoveries. “If we can better understand at the cellular level how organelles like chloroplasts help cells to defend themselves, we could help to engineer resistance to the pathogen,” he explained. 

Thus, this research not only elucidates the enigmatic function of stromules but also opens new avenues for enhancing plant resistance at the cellular level.

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