Seminars

　Cell-to-cell communication plays critical roles in specifying cell fate and coordinating development in multi-cellular organisms. A new paradigm for such communication in plants is the selective trafficking of transcription factors through plasmodesmata (PDs), channels that traverse the cell wall and connect the majority of plant cells. We have taken an unbiased genetic strategy to dissect the mechanism of PD trafficking.
　The maize KNOTTED1 (KN1) homeodomain protein, an ortholog of SHOOTMERISTEMLESS, was the first plant protein found to selectively traffic through PD, and its¹ trafficking appears to be important for its function in stem cell maintenance. A gain-of-function trafficking assay in Arabidopsis was developed to demonstrate that the C-terminal region of KN1 is necessary and sufficient for trafficking in vivo. This system provides a simple and tractable model to understand how proteins traffic and to isolate mutants defective in trafficking.
　As a proof of concept for our strategy, a mutant with attenuated KN1 trafficking has been identified, and was found to be defective in a chaperonin gene. This chaperonin appears essential for PD trafficking of some but all non-cell-autonomous proteins, and biochemical evidence suggests a physical association between chaperonin and KN1. Proteins are thought to undergo partial unfolding during PD translocation, which makes the discovery of this chaperonin particularly exciting. A functional characterization of the role of chaperonins in will further our understanding of developmental regulation and mechanisms of selective cell-to-cell trafficking. In addition, it may give mechanistic insights into this elaborate protein folding machinery, which at a molecular level is not well understood in any system.