Plant-endophyte interplay protects tomato against a virulent Verticillium

Abstract

Endophytes, bacterial, fungal or viral, colonize plants often without causing visible symptoms. More important, they may benefit host plants in many ways, most notably by preventing diseases caused by normally virulent pathogens. Craigella tomatoes (Lycopersicon esculentum Mill.) can be infected with Verticillium dahliae Kleb., either race 1 (Vd1) or a non-host isolate Dvd-E6 resulting in susceptibility or tolerance, respectively. The present study sought to determine whether Dvd-E6 is endophytic and can protect tomato against Vd1. The total amount of Verticillium in stems and roots was determined by quantitative PCR; the relative amounts of Vd1 and Dvd-E6 were assessed by restriction fragment polymorphism. When Dvd-E6 infects before or together with Vd1, Vd1 is excluded almost completely from the root but, when Vd1 infects first, Dvd-E6 can compete on an equal basis. Previous studies suggested that Dvd-E6 suppresses symptom-related genes, raising the possibility that Dvd-E6 simultaneously induces tolerance to Vd1. This does not seem to be entirely the case since the minimal symptoms following Vd1 infection of Dvd-E6 tolerant Craigella result, at least in part, from restricted Vd1 colonization. Furthermore, when Vd1 and Dvd-E6 are cultured on PDA plates alone or together, the growth rates are similar and neither is inhibitory to the other. Dvd-E6 does not outgrow or inhibit Vd1, in vitro. The protective effect apparently requires interplay between Dvd-E6 and the plant. Expression analyses of tomato genes involved in resistance and defence support this interpretation.

Endophyte-inducedVerticilliumprotection in tomato is range-restricted

“…for extraction and fungal DNA assays. 7,9 All experimental results are summarized in Figure 1. At 10 d.p.i., the disease scores for Dvd-E6-infected plants were approximately one-third that of the Vd1-or Vaa-infected plants.…”

“…Host plants tend to be taller and more robust than their uninfected counterparts 6 and colonization of Craigella by Dvd-E6 provides protection against its virulent Verticillium dahliae race 1 (Vd1) cousin, limiting both Vd1 colonization and symptom development during dual infections. 7 Such attributes normally are associated with plant/endophyte partnerships, strongly suggesting that under some conditions, Verticillium dahliae can assume an endophytic role. Apparently interplay between Dvd-E6 and the plant establishes protection against the virulent Vd1.…”

“…Apparently interplay between Dvd-E6 and the plant establishes protection against the virulent Vd1. 7 Many endophytic infections provide protection for the host against a broad range of pathogens. 1 To test the ability of Dvd-E6 infection to protect Craigella against a more distantly related Verticillium pathogen we have now examined dual interactions with Verticillium albo-atrum (Vaa).…”

“…Individual (E6, Vd1 and Vaa) and mixed (E6/Vd1 and E6/Vaa) infections were established as previously described. 7 Plants were scored (upper) at 5 (light gray bars) and 10 (dark gray bars) dpi for symptoms (i.e., disease scores ± SD) or assayed 7 for total levels of Verticillium DNA (i.e., ng/g plant tissue ± SD) in the stems (lower). In mixed infections levels for each fungus also were determined (black and white bars, respectively).…”

“…Susceptible Craigella 8 seedlings again were inoculated at the 4-leaf stage by dipping the roots in Vd1, Dvd-E6 or Vaa conidial suspensions (1 x 10 7 spores/ml in 0.5% gelatin solution) to establish homogeneous interactions (reviewed in ref. 7). For dual interactions, seedlings were inoculated with Dvd-E6 spores at the 3-leaf stage and reinoculated at the 4-leaf stage with either Vd1 or Vaa spore suspension to establish the mixed infections.…”