Abstract
Root endophytes can confer resistance against plant pathogens by direct antagonism or via the host by triggering induced resistance. The latter response typically relies on jasmonic acid (JA)/ethylene (ET)-depended signaling pathways, but can also be triggered via salicylic acid (SA)-dependent signaling pathways. Here, we set out to determine if endophyte-mediated resistance (EMR), conferred by the Fusarium endophyte Fo47, against Fusarium wilt disease in tomato is mediated via SA, ET or JA. To test the contribution of SA, ET, and JA in EMR we performed bioassays with Fo47 and Fusarium oxysporum f. sp. lycopersici in tomato plants impaired in SA accumulation (NahG), JA biosynthesis (def1) or ET-production (ACD) and -sensing (Nr). We observed that the colonization pattern of Fo47 in stems of wildtype plants was indistinguishable from that of the hormone mutants. Surprisingly, EMR was not compromised in the lines affected in JA, ET, or SA signaling. The independence of EMR on SA, JA, and ET implies that this induced resistance-response against Fusarium wilt disease is distinct from the classical Induced Systemic Resistance (ISR) response, providing exciting possibilities for control of wilt diseases independent of conventional defense pathways.
Highlights
Endophytes are microorganisms that can inhabit plant tissues without causing disease
Upon co-inoculation, Isochorismate synthase (ICS), Phenylalanine ammonia-lyase (PAL), and Pathogenesis-related 1a (PR1a) levels remained largely unaffected (Figure 1C), which coincides with the lack of disease symptoms in these plants (Figures 1A,B and Supplementary Figure S1A) (in the second experiment, PAL was found to be modestly induced upon co-inoculation (Supplementary Figure S1C))
We demonstrate that Fo47 can confer endophyte-mediated resistance (EMR) independently of SA, ET, or jasmonic acid (JA) – the major hormones that have commonly been associated with Induced Systemic Resistance (ISR) or Systemic Acquired Resistance response (SAR) (Pieterse et al, 2014)
Summary
Endophytes are microorganisms that can inhabit plant tissues without causing disease. There are many examples of endophytes that decrease disease susceptibility of their host upon pathogen infection. This property makes endophytes attractive agents for organic farming. Indirect antagonism describes an effect of the endophyte on the pathogen that is mediated via the host plant, for instance by triggering an Induced Systemic Resistance (ISR) response in the latter. ISR is normally triggered upon endophytic colonization of the roots, and primes the plant immune system for resistance against future pathogen attacks. Endophytes trigger ISR via the phytohormones jasmonic acid (JA) and ethylene (ET), resulting in a faster and stronger immune response following pathogen attack. Some endophytes are able to Endophyte-Mediated Resistance a Hormone Perspective trigger a Systemic Acquired Resistance response (SAR) that is salicylic-acid (SA) dependent, and results in primed host defenses (Sequeira, 1983; Fu and Dong, 2013; Pieterse et al, 2014)
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