Trichoderma spp. are ubiquitous free-living soil fungi which act as biocontrol agents against several fungal phytopathogens. They are commercially applied as biopesticides, thus limiting the abuse of chemical fungicides [1,2]. The antagonist activity of Trichoderma depends on multiple synergistic mechanisms, including a direct interaction with the pathogenic partner (mycoparasitism), as well as indirect mechanisms based on competition for space and nutrients [3,4]. Trichoderma strains are rhizosphere competent, i.e. able to grow in association with plant roots, and can actually penetrate the first few layers of plant tissues [5,6]. The effects of Trichoderma colonization on plants include an improvement of plant growth and metabolism, as well as the induction of systemic and localized resistance to phytopathogenic fungi, bacteria and viruses (reviewed by [4]). Even though the physiological changes concerning the plant as a whole and induced by Trichoderma spp. have been relatively well investigated, there are only few reports on the mechanisms through which plant cells perceive fungal metabolites secreted during biocontrol. These fungal molecules, which include proteins, peptides, oligosaccharides and antibiotics, act naturally in mixtures. The presence in the Trichoderma exudates of many classes of chemical components potentially acting as elicitors may explain the ability of this fungus to activate induced systemic resistance (ISR) virtually on any plant variety [7].
During plant-fungal interactions an extensive exchange of molecular messages occurs. Variation in cytosolic free Ca2+ concentration ([Ca2+]cyt) is a well-known early component of signal transduction pathways involved in plant-pathogen interactions [8,9]. Plants respond to pathogen attack through a rapidly induced [Ca2+]cyt elevation, which in turn initiates a cascade of reactions leading to activation of defense responses. No information is still available on the possible involvement of Ca2+ as second messenger in the mechanism of Trichoderma perception by plants.
In this paper we investigated plant cell responses, including intracellular Ca2+ variations, to Trichoderma metabolites released in the culture media of the fungus grown alone or in direct antagonism with a Botrytis cinerea strain susceptible to mycoparasitic attack by T. atroviride P1. In addition, we compared the effect of metabolite mixtures from both T. atroviride strain P1 wild type and a knock-out mutant of it, defective in the production of an endochitinase found to be important for biocontrol [10]. Our results indicate that plant cells are able to selectively perceive through Ca2+ messages macromolecule components of the fungal culture filtrates, released in the different experimental conditions. Specific [Ca2+]cyt changes and levels of intracellular accumulation of reactive oxygen species (ROS), reduction in cell viability and occurrence of programmed cell death (PCD)/necrosis were detected.
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