Plants somehow respond to environmental cues and dangers, especially virulent pathogens, despite a lack of eyes or ears.
How is that possible? It's thanks to hundreds of membrane proteins that can sense microbes or other stresses, but only a small portion of these sensing proteins have been studied through classical genetics, and knowledge on how these sensors function by forming complexes with one another is scarce.
An international team has created the first network map for 200 of these proteins. The map shows how a few key proteins act as master nodes critical for network integrity, and the map also reveals unknown interactions. The novel comprehensive interaction network map focused on one of the most important classes of these sensing proteins -- the leucine-rich repeat receptor kinases, or LRR-receptor kinases, which are structurally similar to human toll-like receptors.
The LRR-receptor kinases are a family of proteins in both plants and animals that are largely responsible for sensing the environment. In plants, they have an extracellular domain of the protein, extending beyond the cell membrane, which can recognize chemical signals, such as growth hormones or portions of proteins from pathogens. The receptor kinases then initiate responses to these signals inside the cell, using an intracellular domain of the protein.
The model plant Arabidopsis thaliana contains more than 600 different receptor kinases -- 50 times more than humans -- that are critical for plant growth, development, immunity and stress response. Until now, only a handful had known functions, and little was known about how the receptors might interact with each to coordinate responses to often-conflicting signals.
For the study, scientists tested interactions between extracellular domains of the receptors in a pairwise manner, working with more than 400 extracellular domains of the LRR-receptor kinases and performing 40,000 interaction tests. Positive interactions were used to produce an interaction map displaying how those receptor kinases interact with one another, in a total of 567 high-confidence interactions.
Colleagues analyzed the receptor interaction map using algorithms to generate diverse hypotheses and then validated them separately.
372 intracellular domains of the LRR-receptor kinases whose extracellular domains had shown high-confidence interactions were tested to see if the intracellular domains also showed strong interactions. More than half did, suggesting that the formation of these receptor complexes is required for signal perception and downstream signal transduction. This also indicates a validation of the biological significance of the extracellular domain interactions. They cloned nearly all of the intracellular domains of the LRR-receptor kinases of Arabidopsis.
Notable was that LRR-receptor kinases that have small extracellular domains interacted with other LRR-receptor kinases more often than those that have large domains. This suggests that the small receptor kinases evolved to coordinate actions of the other receptors. Second, researchers identified several unknown LRR-receptor kinases that appear critical for network integrity.
The most important one, dubbed APEX, was predicted to cause severe disruptions to the rest of the network if removed. Researchers found that removal of APEX, and several other known LRR-receptor kinases, indeed did impair plant development and immune responses, even though those responses are controlled by receptor kinases several network steps away from the APEX node.