Partial Resistance ofMedicago truncatulatoAphanomyces euteichesIs Associated with Protection of the Root Stele and Is Controlled by a Major QTL Rich in Proteasome-Related Genes

Abstract

A pathosystem between Aphanomyces euteiches, the causal agent of pea root rot disease, and the model legume Medicago truncatula was developed to gain insights into mechanisms involved in resistance to this oomycete. The F83005.5 French accession and the A17-Jemalong reference line, susceptible and partially resistant, respectively, to A. euteiches, were selected for further cytological and genetic analyses. Microscopy analyses of thin root sections revealed that a major difference between the two inoculated lines occurred in the root stele, which remained pathogen free in A17. Striking features were observed in A17 roots only, including i) frequent pericycle cell divisions, ii) lignin deposition around the pericycle, and iii) accumulation of soluble phenolic compounds. Genetic analysis of resistance was performed on an F7 population of 139 recombinant inbred lines and identified a major quantitative trait locus (QTL) near the top of chromosome 3. A second study, with near-isogenic line responses to A. euteiches confirmed the role of this QTL in expression of resistance. Fine-mapping allowed the identification of a 135-kb sequenced genomic DNA region rich in proteasome-related genes. Most of these genes were shown to be induced only in inoculated A17. Novel mechanisms possibly involved in the observed partial resistance are proposed.

Hydrogen peroxide scavenging mechanisms are components of Medicago truncatula partial resistance to Aphanomyces euteiches

“…This result is in concordance to our previous microscope observations of phloroglucinol stained root sections which showed that lignin-like polymers accumulate around the pericycle in the A17 infected roots at 6 dpi (Djébali et al 2009). Lignification is a well known strategy that have developed many plants to prevent pathogen attack (Bhuiyan et al 2009).…”

“…Thus, with the induced production of soluble phenolics in roots of the partially resistant line A17, presumably a reaction of oxidation by POX occurs which lead to the observed browning in A17 cortical root cells at 3 and 6 dpi. Browning of root tissues were observed in response to A. euteiches in three resistant lines DZA45.5, A17, and DZA315.16, whereas the roots tissues of the susceptible line F83005.5 showed a faint honey-gray colour (Djébali et al 2009). Resistance to several fungal plant pathogens has been ascribed to the accumulation of phenolic oxidation products that can inhibit pathogen development (Lebeda et al 2008).…”

“…M. truncatula seed germination took place as described by Djébali et al (2009). A. euteiches (strain ATCC201684) culture conditions and inoculum preparation were performed using the protocol described by Badreddine et al (2008).…”

“…For necrotrophic oomycete pathogens relatively little research has attempted to elucidate the role of ROS in plant defence, especially at later stages of the interaction. The availability of M. truncatula lines differing in degree of resistance to A. euteiches root infection, i.e., line F83005.5 (susceptible) and line A17 (partially resistant) (Djébali et al 2009 Gara et al 2003). In addition to this enzymatic system, phenolics (especially flavonols and hydroxycinnamic acids) can be oxidized by peroxidase, and a H 2 O 2 scavenging function for ascorbic acid/phenolics/peroxidase systems has been proposed (Takahama 2004).…”

Applied Genetics and Genomics in Alfalfa Breeding

“…A major gene (RCT1) conferring resistance to anthracnose in alfalfa has been cloned in M. truncatula [80]. Similarly, QTL for resistance to A. euteiches [81,82], spring black stem and leaf spot [83] have been mapped in M. truncatula, but no verification of resistance in alfalfa has been documented. In addition to these disease resistances, QTL for nitrogen nutrition [84], flowering time [85] and aerial morphological characteristics [86] have been identified.…”