Ia. Dubery et al., PURIFICATION AND CHARACTERIZATION OF CACTOREIN, A PHYTOTOXIN SECRETEDBY PHYTOPHTHORA-CACTORUM, Phytochemistry, 35(2), 1994, pp. 307-312
An extracellular, hydrophilic, thermostable and non-dialysable phytoto
xin was purified to homogeneity from culture fluids of Phytophthora ca
ctorum with a yield of 2.7 mg l(-1). The toxin is an acidic protein wi
th a M(r) of 10 000 and a pI of 3.65. The amino acid composition was d
etermined and resembles that of characterized members of the elicitin
family of Phytophthora phytotoxins. Toxicity was evaluated with a cell
viability assay using tobacco cell suspensions, and a LD(50) value an
d a lethal dose of 38 and > 100 nM respectively, were determined for t
he purified toxin. When applied to tobacco plantlets, concentrations a
s low as 100 nM resulted in wilting and desiccation. Mature leaf tissu
e reacted hypersensitively to the toxin, with localized necrosis; and
produced pathogenesis-related proteins in response to the toxin. The t
oxin induced rapid extracellular alkalinization in cultured tobacco ce
lls and elicited active defence responses with concentration dependent
induction of sesquiterpenoid phytoalexins. Rishitin, phytuberol and c
apsidiol accumulated in the medium to concentrations of 25, 14, and 63
nmol g(-1) cells, 24 hr after exposure to a toxin concentration of 40
nM. Higher concentrations of toxin resulted in a loss of cell viabili
ty and eventual cell death.