Perception of Fungal Sterols in Plants (Subnanomolar Concentrations of Ergosterol Elicit Extracellular Alkalinization in Tomato Cells)

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Abstract
Suspension-cultured cells of tomato (Lycopersicon esculentum Mill.) reacted to spores and spore exudates of the pathogen Cladosporium fulvum with a rapid, transient alkalinization of their growth medium that resembled the previously described alkalinization response elicited by chitin fragments (G. Felix, M. Regenass, T. Boller [1993] Plant J 4: 307–316) and was likewise inhibited by the protein kinase inhibitor K-252a. However, the spore factor recognized by the cells differed from chitin fragments in that it was butanol soluble and active in cells refractory to stimulation by chitin fragments. The spore factor was purified and identified as ergosterol, the main sterol of most higher fungi. With pure ergosterol, half-maximal induction was reached at about 10 pm. After treatment with ergosterol, tomato cells became refractory to a subsequent stimulation by C. fulvum and vice versa, indicating that ergosterol was the principal component of the spores recognized by the plant cells. Most other sterols were inactive, including cholesterol, a range of animal steroid hormones, and all natural plant sterols tested, except for stigmasterol, which was about 106 times less active than ergosterol. Our data demonstrate that tomato cells perceive ergosterol with a selectivity and sensitivity that resembles the perception of steroid hormones in animals.