Root exudates: the hidden part of plant defense

Abstract

The significance of root exudates as belowground defense substances has long been underestimated, presumably due to being buried out of sight. Nevertheless, this chapter of root biology has been progressively addressed within the past decade through the characterization of novel constitutively secreted and inducible phytochemicals that directly repel, inhibit, or kill pathogenic microorganisms in the rhizosphere. In addition, the complex transport machinery involved in their export has been considerably unraveled. It has become evident that the profile of defense root exudates is not only diverse in its composition, but also strikingly dynamic. In this review, we discuss current knowledge of the nature and regulation of root-secreted defense compounds and the role of transport proteins in modulating their release. DOI: https://doi.org/10.1016/j.tplants. 2013.11.006 Posted at the Zurich Open Repository and Archive, University of Zurich ZORA URL: https://doi.org/10.5167/uzh-106507 Accepted Version Originally published at: Baetz, Ulrike; Martinoia, Enrico (2014 The significance of root exudates as belowground defense substances has long been underestimated, presumably due to being buried out of sight. Nevertheless, this chapter of root biology has been progressively addressed within the last decade through the characterization of novel constitutively secreted and inducible phytochemicals that directly repel, inhibit or kill pathogenic microorganisms in the rhizosphere. In addition, the complex transport machinery involved in their export has been considerably unraveled. It became evident that the profile of defense root exudates is not just diverse in its composition, but is also strikingly dynamic. In this review we will discuss the current knowledge about the identity and regulation of rootsecreted defense compounds and the role of transport proteins in modulating their release. Several reviews on root exudates have been published during the last years, but they addressed different topics such as aluminum tolerance, phosphate and iron nutrition or the beneficial interaction with microorganism. However, to our knowledge there is no review focusing on the root exudates that are involved in plant defense belowground.This may be due to the fact that this topic includes a very broad range of compounds, which ranges from volatiles to low molecular compounds and enzymes. Therefore, we think that reviewing this topic could help the scientific community to get informed efficiently about this topic where the publications are quite dispersed and are not easy to be assembled.We hope that the referees will also see the importance of such a review and like it but at the same time we are looking forward for getting their comments to improve the manuscript. Taken together, the penetration of physiologically independent border cells, the root cap and 10 border cell exudation of the mucilage layer including proteins and exDNA that immobilize 11 pathogens, as well as the secretion of, for instance, antimicrobial enzyme...

Bacterial Biosensors for in Vivo Spatiotemporal Mapping of Root Secretion

“…Heavy colonization at the root elongation zone is to be expected, as this is an area of actively growing root where many metabolites are secreted and exuded. Low Lux signal from the root cap is probably due to the reduced colonization of this area, which generally secretes antimicrobial phytochemicals (Baetz and Martinoia, 2014). The level of Lux signal detected in the rhizosphere was constant until 11 dpi and then was reduced, possibly due to a general decrease in root Table S8.…”

SENSITIVE TO PROTON RHIZOTOXICITY1, CALMODULIN BINDING TRANSCRIPTION ACTIVATOR2, and Other Transcription Factors Are Involved inALUMINUM-ACTIVATED MALATE TRANSPORTER1Expression

“…Organic acid (OA) excretion from the roots plays beneficial roles in stress adaptation processes of plants (Baetz and Martinoia, 2014). The root-exuded OAs detoxify rhizotoxic ions, such as aluminum (Al) and copper (Kochian et al, 2004) and improve availability of phosphorus (Neumann et al, 1999) and iron (Kobayashi and Nishizawa, 2012).…”

Plant Innate Immune Response: Qualitative and Quantitative Resistance

“…Mevalonate pathway: Acetyl-CoA leads to the production of terpenes, consisting of isoprene units forming mono-di-, tri-, and sesquiterpenes (Tholl, 2015). Fatty acid pathway: Acetyl-CoA produces malonyl CoA which leads to the production of fatty acids and lipids (Ahuja et al, 2012;Baetz and Martinoia, 2014;Grosjean et al, 2015). Polyamine pathway: The biosynthesis of arginine leads to the production of putrescine, spermidine, and spermine (Dong et al, 2015).…”