Dissection of Bacterial Wilt on Medicago  truncatula Revealed Two Type III Secretion System Effectors Acting on Root Infection Process and Disease Development    

Abstract

Ralstonia solanacearum is the causal agent of the devastating bacterial wilt disease, which colonizes susceptible Medicago truncatula via the intact root tip. Infection involves four steps: appearance of root tip symptoms, root tip cortical cell invasion, vessel colonization, and foliar wilting. We examined this pathosystem by in vitro inoculation of intact roots of susceptible or resistant M. truncatula with the pathogenic strain GMI1000. The infection process was type III secretion system dependent and required two type III effectors, Gala7 and AvrA, which were shown to be involved at different stages of infection. Both effectors were involved in development of root tip symptoms, and Gala7 was the main determinant for bacterial invasion of cortical cells. Vessel invasion depended on the host genetic background and was never observed in the resistant line. The invasion of the root tip vasculature in the susceptible line caused foliar wilting. The avrA mutant showed reduced aggressiveness in all steps of the infection process, suggesting a global role in R. solanacearum pathogenicity. The roles of these two effectors in subsequent stages were studied using an assay that bypassed the penetration step; with this assay, the avrA mutant showed no effect compared with the GMI1000 strain, indicating that AvrA is important in early stages of infection. However, later disease symptoms were reduced in the gala7 mutant, indicating a key role in later stages of infection.

Type III Secretion–Dependent and –Independent Phenotypes Caused by Ralstonia solanacearum in Arabidopsis Roots

“…Root-growth inhibition or delayed elongation has been previously observed as a result of R. solanacearum infection, when using gnotobiotic systems (Digonnet et al 2012;Turner et al 2009;Vailleau et al 2007;Vasse et al 1995;Zolobowska and Van Gijsegem 2006). One could hypothesize that root-growth inhibition is the direct cause of the massive cell death observed after infection in the root cortex of Arabidopsis (Digonnet et al 2012; this work) or other species (Turner et al 2009;Vasse et al 1995).…”

“…The use of in vitro pathosystems to study the interactions between the vascular pathogen R. solanacearum and some of its plant hosts has emerged as a very powerful technique to understand the early stages of infection (Digonnet et al 2012;Turner et al 2009;Vailleau et al 2007;Vasse et al 1995Vasse et al , 2000Zolobowska and Van Gijsegem 2006). In this work, we have used in vitro-grown Arabidopsis as the model host to deepen our knowledge on the first steps of R. solanacearum root invasion.…”

The awr Gene Family Encodes a Novel Class of Ralstonia solanacearum Type III Effectors Displaying Virulence and Avirulence Activities

“…In addition, AvrA and PopP1 effectors are also known to trigger plant defense responses. AvrA was shown to be an avirulence determinant of various R. solanacearum strains in Nicotiana tabacum (Carney and Denny 1990;Robertson et al 2004) and recent reports have also suggested a role in pathogenicity (Macho et al 2010;Turner et al 2009). PopP1 was described as a host-specificity factor that behaves as a typical avirulence gene in Petunia spp.…”

Gene-for-Gene Tolerance to Bacterial Wilt in Arabidopsis

“…solanacearum is a soilborne bacterium that enters hosts via root wounds or sites of secondary root emergence (Genin 2010). The bacteria spread toward the xylem vessels and colonize the root cortex and vascular parenchyma (Turner et al 2009). R. solanacearum secretes copious amounts of extracellular products, resulting in cell damage.…”