Planta (2009) 229:415–426 DOI 10.1007/s00425-008-0840-z 123 ORIGINAL ARTICLE Plant-endophyte interplay protects tomato against a virulent Verticillium Hakeem O. Shittu · Danve C. M. Castroverde · Ross N. Nazar · Jane Robb Received: 7 October 2008 / Accepted: 10 October 2008 / Published online: 1 November 2008  Springer-Verlag 2008 Abstract Endophytes, bacterial, fungal or viral, colonize plants often without causing visible symptoms. More important, they may beneWt 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 quantita- tive 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 Wrst, Dvd-E6 can compete on an equal basis. Previ- ous studies suggested that Dvd-E6 suppresses symptom- related genes, raising the possibility that Dvd-E6 simulta- neously 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 inhibi- tory to the other. Dvd-E6 does not outgrow or inhibit Vd1, in vitro. The protective eVect apparently requires interplay between Dvd-E6 and the plant. Expression analyses of tomato genes involved in resistance and defence support this interpretation. Keywords Cross-protection · Endophyte · Lycopersicon · Tolerance · Verticillium Abbreviations BP Base pair CDK Cyclin dependent kinase CS cv Craigella susceptible Dpi Days post-inoculation E6 V. dahliae isolate Dvd-E6 ITS Internal transcribed spacer PR Pathogen-related PCR Polymerase chain reaction RT Reverse transcription SDS Sodium dodecylsulphate Vd1 Verticillium dahliae race 1 Introduction When Verticillium spp. infect a host three diVerent types of relationships are possible. In susceptible plants symptoms and fungal levels are high while in resistant plants both are low. The third possibility is a tolerant condition in which disease symptoms are minimal although the plants are colo- nized substantially. Historically, most research of a funda- mental or an applied nature has focused on resistance and susceptibility (Mace et al. 1981; Pegg and Brady 2002; Fra- din and Thomma 2006); but little is known about tolerance (Robb 2007). Often when the tolerant state is induced the colonizing organism is referred to as endophytic. An endophyte is usu- ally a bacterium or fungus that lives within the plant while H. O. Shittu · D. C. M. Castroverde · R. N. Nazar · J. Robb (&) Department of Molecular and Cellular Biology, University of Guelph, Guelph, ON N1G 2W1, Canada e-mail: jrobb@uoguelph.ca R. N. Nazar e-mail: rnnazar@uoguelph.ca