In vitro reaction of sunflower (Helianthus annuus L.) to the toxin(s) produced by Alternaria alternata, the casual agent of brown leaf spot

A bioassay system was developed for studying the in vitro reaction of sunflower (Helianthus annuus L. cv. 'Nanus') against the toxin produced by the virulent pathotype IMI 366417 (1) of the pathogenic fungus Alternaria alternata. Cotyledons from 2-week-old seedlings were cultured on a MS (Murashige and Skoog) medium supplemented with 0.3 μM NAA (α-napthylacetic acid) and 1.3 μM BA (6-benzyladenine). Exponentially growing calli were transferred to selective media containing toxin solutions at various concentrations. The fresh weight of the cultured calli was reduced as the toxin concentration increased, although the viability of the cells, expressed as callus dehydrogenase activity, increased. Selection for toxin-resistant genotypes was attempted at 30% toxin concentration, which causes a 90% reduction in callus growth. After one month in culture, 18% of the calli demonstrated resistance to the toxin. However, no plants could be regenerated from those calli after transfer onto a MS medium supplemented with 5.4 μM NAA and 4.4 μM BA. The effect of the toxin purification method on toxin yield and biological activity, as well as its possible mode of cellular action are discussed. The results of these experiments may contribute to a better understanding of the disease mechanism and help establish an efficient selection method of resistant sunflower genotypes.