Root Barriers: Walking the Line between Protection and Exchange

The root endodermis in plants has all the features of an animal polarised epithelium, but evolved independently, thus providing a unique opportunity to study independent evolution of epithelial polarity, diffusion barrier establishment and directional transcellular transport. Through forward and reverse genetic screens, we discovered transmembrane proteins that localise to and define the Casparian strip membrane domain and identified numerous factors involved in their localisation and function. Casparian strips are ring-like impregnations of cell walls with lignin, analogous to animal tight junctions. Since the endodermis is central to our models of root function, the ability to visualise and manipulate endodermal development with great precision turned out to be extremely useful for advancing our understanding of root nutrient uptake and stress physiology. Recently, our group has investigated the importance of Casparian strips in restricting nutrient leakage to the rhizosphere and control of microbial colonization. Plant roots use exudates to assemble microbiomes that in turn influence the function and stress resilience of plants. We demonstrate that Casparian strips restrict nutrient leakage into the rhizosphere, coinciding with and controlling spatial colonisation patterns of rhizobacteria. We find that vasculature-derived glutamine is a major bacterial chemoattractant and enhancer of proliferation, defining a novel leakage pathway for root exudate formation. Bacteria defective in amino acid chemoperception display reduced attraction towards leakage sites, and roots with Casparian strip defects display bacterial over-proliferation, dependent on bacterial capacity for amino acid metabolisation. An associated, chronic immune stimulation suggests that endodermal nutrient restriction is crucial for beneficial microbial colonisation.