A low molecular weight phytotoxin produced by Phoma tracheiphila, the cause of mal secco disease in citrus

Phoma tracheiphila (Petri) K & G, the causal organism of mal secco disease in citrus secretes a low molecular weight phytotoxin during growth on a synthetic medium. The toxin is capable of inciting interveinal chlorosis and necrosis in lemon leaves. It also induces electrolyte leakage from lemon leaves or carrot discs and causes uncoupling of electron transport in lettuce chloroplasts. A linear relation was established between toxin concentration and the uncoupling activity. The electrolyte leakage activity and uncoupling activity were reduced to varying extents by various organic solvents. The toxin causes reversible inhibition of proton uptake, inhibition of photophosphorylation and stimulation of light-dependent electron transport by lettuce chloroplasts. It also stimulates the initial rate of Mg ++ -ATPase in the dark but the stimulation is followed in time with inhibition. This established the present toxin as an uncoupler of electron transport in chloroplast preparations. The toxin was partially purified by gel filtration on Sepharose 6B, Bio-Gel P-2, Sephadex G-10 and descending paper chromatography. The specific activity of chloroplasts per mg toxin (μmol O 2 mg −1 chl h −1 mg −1 dry wt toxin) was increased approximately 8-fold by purification. The estimated molecular weight for the toxin as judged by gel filtration is between 350 and 700 daltons. The toxin is hydrophilic and heat-stable (100 °C, 15 min) compound. It is inactivated by either acid or base hydrolysis, but is insensitive to periodate oxidation.