Induction of systemic resistance: a main mechanism of action of biopesticides based on antagonistic fungi.

Numerous biocontrol agents, such as Trichoderma spp. fungal antagonists, have beneficial effects on plants because they can not only directly inhibit the pathogens, but also colonize root surfaces, exchange compounds that affect plant metabolism, enhance plant growth and development, increase nutrient availability, improve crop production and induce disease resistance (Harman et al., 2004; Vinale et al., 2008). The stimulation of plant defences by Trichoderma was associated with enhanced levels of production of pathogenesis-related (PR) proteins and/or accumulation of phytoalexins in plants (Howell et al., 2000; Harman et al., 2004). The induced systemic-resistance activated by Trichoderma is analogous to that observed with nonpathogenic rhizobacteria, whereby the plant defense response is triggered only when the presence of the pathogen is detected by the plant. During the Trichoderma-plant interaction, different classes of metabolites can act as resistance inducers. These include avirulence (avr)-like proteins, enzymes such as xylanase, or peptides with elicitor activity that induce the biosynthesis of terpenoid phytoalexins and peroxidase activity. We have found evidence that Trichoderma produce avr-like gene products that can induce defence mechanisms in plants. The effect is analogous to that caused by the products of avr4 and avr9 genes of the pathogen Cladosporium fulvum when it is applied to tomato cultivars with the corresponding Cf4 and Cf9 resistance genes. Another class of resistance inducer includes low-molecular-weight degradation products released from fungal or plant host cell walls due to the activities of secreted Trichoderma hydrolytic enzymes. These compounds consist of short oligosaccharides with two types of monomers, with or without a amino acid residue. The isolated saccharides elicited a hypersensitive reaction by the plant when applied to leaves or when injected into root or leaf tissues and were also able to stimulate growth and development of Trichoderma, as well as to increase the expression of genes involved in the mycoparasitic process (Woo et al., 2006). Recently, we investigated the potential of secondary metabolites produced in Trichoderma culture filtrates to induce systemic resistance in the plant. A reduction in disease symptoms was observed when tomato and canola seedlings were pretreated with the purified metabolites, then inoculated with the pathogens Botrytis cinerea or Leptosphaeria maculans, respectively. This effect was particularly evident when the Trichoderma compounds 6-pentyl-±-pyrone (6PP), harzianopyridone and harzianolide were used (Vinale et al., 2008). RT-PCR analysis of plant tissues treated with the isolated metabolites indicated that there was over-expression of several genes that are normally induced in plant defence. Moreover, analysis by Real Time-PCR of tomato plants sprayed with 6PP indicated a considerable increase in PR 1 and PR 4 gene expression, when compared to water-treated controls (48 h after treatment). Finally, we have found that the hydrophobin Hytra1 secreted by Trichoderma triggers plant defence reactions, probably by inducing an oxidative burst and PR1 and PR4 gene expression.