A Proteinaceous Elicitor Sm1 from the Beneficial FungusTrichoderma virensIs Required for Induced Systemic Resistance in Maize

Abstract

We have previously shown that the beneficial filamentous fungus Trichoderma virens secretes the highly effective hydrophobinlike elicitor Sm1 that induces systemic disease resistance in the dicot cotton (Gossypium hirsutum). In this study we tested whether colonization of roots by T. virens can induce systemic protection against a foliar pathogen in the monocot maize (Zea mays), and we further demonstrated the importance of Sm1 during maize-fungal interactions using a functional genomics approach. Maize seedlings were inoculated with T. virens Gv29-8 wild type and transformants in which SM1 was disrupted or constitutively overexpressed in a hydroponic system or in soil-grown maize seedlings challenged with the pathogen Colletotrichum graminicola. We show that similar to dicot plants, colonization of maize roots by T. virens induces systemic protection of the leaves inoculated with C. graminicola. This protection was associated with notable induction of jasmonic acidand green leaf volatile-biosynthetic genes. Neither deletion nor overexpression of SM1 affected normal growth or development of T. virens, conidial germination, production of gliotoxin, hyphal coiling, hydrophobicity, or the ability to colonize maize roots. Plant bioassays showed that maize grown with SM1-deletion strains exhibited the same levels of systemic protection as nonTrichoderma-treated plants. Moreover, deletion and overexpression of SM1 resulted in significantly reduced and enhanced levels of disease protection, respectively, compared to the wild type. These data together indicate that T. virens is able to effectively activate systemic disease protection in maize and that the functional Sm1 elicitor is required for this activity.

Functional analysis of non-ribosomal peptide synthetases (NRPSs) in Trichoderma virens reveals a polyketide synthase (PKS)/NRPS hybrid enzyme involved in the induced systemic resistance response in maize

“…Fungal mutualists confer a wide range of benefits to their hosts (Porras-Alfaro & Bayman, 2011), including enhanced host disease resistance (Djonović et al, 2007;Pozo & Azcó nAguilar, 2007;Viterbo et al, 2007), drought tolerance (Hause & Fester, 2005), improved nutrient uptake and Trichoderma NRPSs growth (Balestrini & Lanfranco, 2006;Martin et al, 2007;Chacó n et al, 2007), and protection against invertebrate and vertebrate herbivory (Clay & Schardl, 2002). The expression of induced defences is mediated by complex signalling networks in which the plant hormones JA and SA play key roles (Smith et al, 2009).…”

“…2(a, c). Both of these strategies have been successful in previous functional studies of T. virens genes (Mukherjee et al, 2003b;Mukherjee & Kenerley, 2010;Djonović et al, 2007). Disruption of tex2 was by single cross-over with selection for hygromycin B-resistant colonies (Mukherjee et al, 2011).…”

“…During PGPR-mediated induced systemic resistance (ISR), the plants are 'sensitized/primed' (no major metabolic changes occur prior to challenge inoculation by a pathogen) in response to the presence of the bacteria in the root zone (van Wees et al, 1999). In contrast, root colonization by Trichoderma per se induces both priming effects and transcriptomic/metabolic changes in plants, including the triggering of defence response genes encoding enzymes such as phenylalanine ammonia lyase (pal), hydroperoxide lyase (hpl) and allene oxide synthase (aos) (Shoresh & Harman, 2008;Djonović et al, 2007;Vargas et al, 2008;Viterbo et al, 2005;Shoresh et al, 2010;Mastouri et al, 2010). PGPR and Trichoderma-mediated ISR, is different from pathogentriggered systemic acquired resistance (SAR), but both ISR and SAR require NPR1 (non-expressor of pathogenesisrelated genes; Van der Ent et al, 2008).…”

“…also induces other components of the ISR mechanism, namely the genes hpl, aos and peroxidase (prx) (Viterbo et al, 2005(Viterbo et al, , 2007Vargas et al, 2008). T. virens induces ISR, but not SAR responses in maize; mutants of T. virens lacking the elicitor protein SM1 failed to induce ISR responses (Djonović et al, 2007).…”

“…Trichoderma-secreted xylanase, cellulase and swollenin induce localized plant reactions and necrosis (see Shoresh et al, 2010). SM1 is a hydrophobic, small, cysteine-rich, secreted protein and one of the most well-characterized elicitor-like proteins secreted by T. virens (Djonović et al, 2006(Djonović et al, , 2007Vargas et al, 2008;Buensanteai et al, 2010). Two other well-characterized elicitors are the non-ribosomal peptides alamethicin (T. viride) and the 18-residue peptaibols Tex1 (T. virens) (Leitgeb et al, 2007;Viterbo et al, 2007).…”