Role of root border cells in the protection of the root tip from fungal infection

Abstract

Root microbe interactions are initiated generally in the region of elongation while the root tip remains infection free. In this dissertation, the hypothesis that root border cells play a role in protecting the root tip from fungal infections was examined. When radicles of pea seedlings were inoculated with Nectria haematococca , the zone of elongation became infected in the majority of the samples (ca. 90%) but the root tips appeared infection free and continued to grow essentially normally. However, microscopic evaluation revealed that N. haematococca propagules germinated and colonized border cells, leading to the formation of "mantles" that ensheathed the root tip. The tip tissue, nevertheless, remained infection free. Removing border cells prior to inoculation with the fungus or retarding their separation increased tip infections. Additionally, exudates from fungus inoculated root tips inhibited fungal growth in vitro when compared to those from uninoculated seedlings. When root tip infection occurred, a strong correlation between tip infection and cessation of further root growth was observed. In response to root tip infection by N. haematococca, a concomitant induction of border cell separation and defense gene expression in the tip was observed, suggesting that border cell separation may be a previously unrecognized alternative defense strategy of the pea root tip. The results are consistent with the following model: The separation of border cells, together with the presence of an inhibitory compound in the root exudates, minimizes contact between the fungus and the root tip thus protecting the tip from infection. Under conditions when tip infection does occur, the active induction of renewed border cell separation serves to remove infected tissue from the apical meristem area. Plant mutants altered in border cell separation can be used in the future studies to definitively establish the role of border cells in plant health.