Differential root zone secretions and the role of root border cells in rhizosphere manipulation

Root tissues are broadly divided into mature tissue, elongation zone (developing tissue), root tip, and border cells. While each zone contributes individually to the overall root secretion profile, border cells are emphasized in this review due to their specialized secretory functions. Border cells are often overlooked in plant root focused studies, thus excluding an important component of root functionality. Border cells are a specialized cell type surrounding the root apical meristematic region of most plant species, with the exception of the Brassicaceae family that possess border-like cells. Both cell types share the commonality of complete detachment from the root tip and reliance on internal starch reserves to perform metabolic processes. However, border cells release from the root tip as single/individual cells whereas border-like cells separate as cohesive sheets. Furthermore, border cells, but not border-like cells, secrete a complex matrix consisting of mucilage, proteins, DNA, and metabolites. Many of these secreted compounds are bioactive (e.g. secreted mucilage supports microbial growth and DNA physically entangles pathogens) thus mediating symbiosis and pathogen defense. We are interested in metabolites secreted from individual root regions, with a heavy emphasis on those specifically arising from border cells. Border cell metabolite secretion is in need of further investigation, as current research indicates they secrete symbiosis-inducing, allelopathic, and defense oriented metabolites. This review will summarize current literature regarding metabolite secretions by specific root cell types and regions. In particular, it will focus on border cell contributions to the rhizosphere chemistry relative to other root tissue types.