Chloroplast clustering around the nucleus is a plant defence mechanism under both biotic and abiotic stresses. However, a team of scientists revealed a surprising twist: potato spindle tuber viroid (PSTVd) infections thrive when chloroplasts cluster. PSTVd appears to manipulate this defence, raising questions about the complexity of plant-pathogen interactions.
In the ongoing battle between plants and harmful invaders, there’s a remarkable defence move that plants pull off – it involves the gathering of chloroplasts close to the cell nucleus. You can think of chloroplasts as the powerhouses of plant cells; they’re the ones responsible for capturing sunlight and transforming it into energy through photosynthesis. What’s really cool is that these chloroplasts can actually shift position and huddle around the nucleus when a plant senses danger. When the plant’s immune system kicks into gear, these chloroplasts become the source of various protective signals, including something called reactive oxygen species (ROS), which can be thought of as tiny reactive molecules. What’s even more intriguing is that recent research has shown that chloroplasts physically connect with the nucleus by forming clusters around it and extending something called stromules. This phenomenon often happens when the plant’s immune response, specifically something called effector-triggered immunity, is activated. For a long time, scientists thought that this clustering of chloroplasts around the nucleus was a standard defence strategy – like assembling an army to protect a fortress. But there’s more to this story than meets the eye!
Viroids are an enigmatic group of plant pathogens that plant scientists have been investigating. These tiny, renegade agents in the plant world differ from bacteria and viruses because they consist solely of genetic material, lacking the protective protein coat found in viruses. Within this group, there’s one notorious troublemaker known as potato spindle tuber viroid, or PSTVd for short. PSTVd is a listed quarantine pathogen in the EU, causing stunted growth, malformation, and cracking of potato tubers. While potatoes are its primary host, this disease also affects tomatoes and solanaceous ornamental plants. Scientists have been working diligently to unravel how plants defend against PSTVd.
A recent study revealed an intriguing twist in the interaction between PSTVd infection and chloroplast clustering. Surprisingly, while PSTVd itself didn’t induce chloroplast clustering, it thrived in the presence of clustered chloroplasts in Nicotiana benthamiana plants caused by the overexpression of chloroplast outer membrane protein 24 (NbOMP24). It seems that PSTVd has developed a clever strategy, using the physical connection between chloroplasts and the nucleus to facilitate its entry and exit. This is particularly significant as the nucleus is where PSTVd replication occurs. The current challenge is to uncover the possible channels that clustered chloroplasts establish for PSTVd to access and exit the nucleus. This discovery challenges our understanding of plant-pathogen dynamics, resembling a scenario where the adversary not only breaches the fort’s defences but ingeniously exploits them to its advantage. This revelation raises more questions than answers, underscoring the intricate complexity of the ongoing battle between plants and pathogens.
Overexpression of NbOMP24 facilitated PSTVd infection. (a) Overexpression of NbOMP24 resulted in perinuclear chloroplast clustering from 2 to 10 days post infection (dpi). (b) Accumulation of PSTVd RNA was increased in both local and systemic leaves of plants with NbOMP24 overexpression compared to control plants (pGUS).
Kelei Han, Zhaoxing Jia, Yuhong Zhang, Huijie Zhou, Shan Bu, Jianping Chen, Dankan Yan, Rende Qi, Fei Yan and Jian Wu published this study in Molecular Plant Pathology Journal:
TITLE IMAGE: Graphical Abstract illustrating that overexpression of outer membrane protein 24 (NbOMP24) in Nicotiana benthamiana induces chloroplast clustering around the nucleus to enhance potato spindle tuber viroid (PSTVd) infection.