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

Abstract

The causal agent of bacterial wilt, Ralstonia solanacearum, is a soilborne pathogen that invades plants through their roots, traversing many tissue layers until it reaches the xylem, where it multiplies and causes plant collapse. The effects of R. solanacearum infection are devastating, and no effective approach to fight the disease is so far available. The early steps of infection, essential for colonization, as well as the early plant defense responses remain mostly unknown. Here, we have set up a simple, in vitro Arabidopsis thaliana–R. solanacearum pathosystem that has allowed us to identify three clear root phenotypes specifically associated to the early stages of infection: root-growth inhibition, root-hair formation, and root-tip cell death. Using this method, we have been able to differentiate, on Arabidopsis plants, the phenotypes caused by mutants in the key bacterial virulence regulators hrpB and hrpG, which remained indistinguishable using the classical soil-drench inoculation pathogenicity assays. In addition, we have revealed the previously unknown involvement of auxins in the root rearrangements caused by R. solanacearum infection. Our system provides an easy-to-use, high-throughput tool to study R. solanacearum aggressiveness. Furthermore, the observed phenotypes may allow the identification of bacterial virulence determinants and could even be used to screen for novel forms of early plant resistance to bacterial wilt.

Deep Sequencing Reveals Early Reprogramming of Arabidopsis Root Transcriptomes Upon Ralstonia solanacearum Infection

“…Auxin signaling and transport genes were also up-regulated at the PC and LR stages. In line with our data, the expression of DR5, a marker gene of the auxin signaling pathway, was strongly induced in root vascular after R. solanacearum GMI1000 infection (Lu et al 2018). Moreover, the dgl1-1 tomato mutant with disordered auxin transport was highly resistant to R. solanacearum (French et al 2018).…”

“…This sequential activation and inactivation of JA signaling was also found in plant root response to Plasmodiophora brassicae (Gravot et al 2012;Irani et al 2018;Lemarié et al 2015). The jai3-1, jar1-1, and dde2 mutants with disabled JA biosynthesis or signaling showed similar root architectures as wild-type plants in response to this pathogen (Lu et al 2018), indicating that JA may be involved in plant response to the bacteria but not in the root morphogenesis changes caused by R. solanacearum.…”

“…As previously reported, Arabidopsis seedling roots exhibited primary root growth retardation, de novo root-hair formation, and cell death appearance around the root tip at 9 days after GMI1000 treatment (Lu et al 2018). To define appearance time of the three root phenotypes, we investigated the root structure changes of Arabidopsis seedling after infection with GMI1000 over time.…”

“…Cell death was observed on root tips at the PC stage of R. solanacearum infection. Interestingly, the occurrence of R. solanacearum-mediated cell death at the root tip was dependent on the presence of a functional T3SS (Lu et al 2018), indicating that this cell death occurs via effector recognition. Thus, cell death after infection probably results from ETI.…”

The Ralstonia solanacearum csp22 peptide, but not flagellin‐derived peptides, is perceived by plants from the Solanaceae family

“…Visual scoring of disease symptoms according to a scale ranging from '0' (no symptoms) to '4' (complete wilting) was performed as described previously (Vailleau et al, 2007). For internal growth curve assays (IGC), 8day-old Arabidopsis seedlings were grown on MS plates and inoculated with 10 5 cfu/mL GMI1000 bacterial suspension, as described in Lu et al (2018), with minor modifications: IGC was performed by collecting aerial parts of Arabidopsis seedlings at 2 dpi, followed by washing in sterile water. Colony-forming units were counted by spreading serial dilutions on solid B medium.…”