Effects of selective toxin from Helminthosporium carbonum on chlorophyll synthesis in maize

Abstract

Etiolated maize leaves exposed to selective toxin (HC toxin) from Helminthosporium carbonum were placed in light for greening. After 6 h in light, significant inhibition of chlorophyll synthesis by toxin (<1·0 g ml −1 ) was evident, by comparison with synthesis in control leaves without toxin. Inhibition increased linearly with increases in toxin concentration over five orders of magnitude (0·1 to 1000 g ml −1 ). Chlorophyll synthesis in H. carbonum -resistant leaves was affected similarly, provided toxin concentrations were 100-fold higher than was required for inhibition in susceptible leaves. Toxin-induced inhibition of chlorophyll synthesis was prevented by δ-aminolevulinic acid (ALA), the first committed intermediate in the chlorophyll synthesis pathway. The effect on chlorophyll synthesis is the earliest inhibitory effect of HG toxin observed to date. The results may give clues to the mode of toxic action, and are a possible basis for a more rapid bioassay.