Necrogenic Activity of a Fusarium Elicitor Toward in Vitro Carnation Cells

Abstract

Cell and tissue death is an usual symptom in plant diseases caused by pathogenic fungi; in incompatible plant-fungus interactions the death of one or few cells generally is associated with plant resistance and the growth restricted of the potential pathogen. Moreover, while in incompatible interactions between plants and obligate biotrophs cell death explains host resistance, in the case of all other fungi host cell death cannot be a limiting factor of the growth of pathogen, but it could be a biological signal for the elicitation of effective defense mechanisms in plants. It was reported that some substances of fungal origin show necrogenous activity correlated to the elicitation of defense responses in plants; in opposite, there are other plantfungus systems where no correlation has been found between host cell death and disease resistance. In our previous papers we showed that thermostable substance(s) (elicitor) have been extracted from mycelial cell walls of Fusarium oxvsporum f.sp. dianthi race 2 (Fod2), which induce(s) synthesis of Phenylalanine Ammonia-Lyase, organic-soluble compounds and phytoalexins in callus cultures derived from carnation cultivars resistant to Fod2, but not in callus cultures derived from susceptible cvs. In the present work we show that the elicitor induces cell death of in vitro carnation cells derived from the cultivar Pulcino, resistant, but not in those from the cultivar Corrida, susceptible. About the 38 % of cells of Pulcino die just 8 hours after eliciting treatment - about the 55% after 12 hours -, when assessing cell viability by fluorescein acetate method. A cell line from Pulcino cells has been selected without mutagenesis, which does not die following eliciting treatment and it has been called sensitive-minus (Pulcino s.m.). Explants of callus of Pulcino s.m., when inoculated with Fod2, are extensively colonized as well as explants of callus of Corrida. To date, we cannot be sure whether and how cell death is involved in activing the mechanisms which retard the fungal colonization of callus cultures. But the fact that the sensitive-minus cell line from Pulcino contemporaneously is altered both in elicited cell death response and in the colonization by Fod2, suggests to us that cell death could be one of the first signals between carnation cells and hyphae for triggering further biochemical defense responses.