Analysis of the morphology of Citrus lemon seedlings inoculated with Trichoderma harzianum and Alternaria alternata
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Lemmon seedlings were inoculated with conidia from Trichoderma harzianum or Alternaria alternata, and were observed by optical and electron microscopy. Observations were made at time periods corresponding to maximal induction of the phenylpropanoid pathways, measured as the activity of phenylalanine ammonia-lyase (PAL). The induction of this pathway leads to the synthesis of defense metabolites, umbelliferone and scoparone, which are part of the hypersensitive response of lemon seedlings against inoculation by these fungi. Seedlings inoculated with T. harzianum showed degradation of cell walls. an observation that can be explained due to the cellulotyc activity of this fungus. On the other hand, A. alternata produces loss of cell structure without obvius disruption of cell walls, which correlates with its pectinolytic activity. Results showing the king of damage observed in lemmon seedlings, is related to the major enzyme secreted by each fungus. Therefore, the complete destruction of the plant cell wall is not necessary to induce the hypersensitive reaction, since both T. harzianum and A. alternata are able to elicit this response.