Helminthosporal (H-al) is a phytotoxic sesquiterpenoid dialdehyde produced by the common root rot fungus,Bipolaris sorokiniana(Sacc. in Sorok.) Shoemaker. In addition to its phytotoxic properties, H-al can stimulate the synthesis of amylase and other proteins by embryoless barley seeds, having an optimum effect at a concentration of 0.1 mM. The monoacid derivative formed by air oxidation of H-al was only one-fourth as active as the dialdehyde. Helminthosporal and, particularly, the monoacid, can also inhibit the gibberellic acid (GA3) induced synthesis of amylase in barley half-seeds. The extent of inhibition was reduced when these compounds were added to half-seeds preincubated with GA3. The results of studies on the effect of H-al and its monoacid on the respiration of various plant tissues, including barley half-seeds, and on protein synthesis both in vivo and in vitro, have suggested that these substances inhibit amylase synthesis by blocking the supply of energy (ATP) in cells of the aleurone layer. Part of the inhibition may also involve the binding of the monoacid to a receptor site concerned in the mechanism of amylase induction by GA3. The hypothesis was tested that some type of relationship might exist between the inhibition of specific mitochondrial reactions and the induction of amylase synthesis. Among a wide variety of mitochondrial inhibitors only atractyligenin, amytal, and the lactone derivative of H-al could promote amylase synthesis. Amytal had high activity. The role of the mitochondrion in GA3-induced processes in the barley aleurone cell appears to warrant further investigation.
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