Cytokinin-like substances and accumulation of labelled metabolites at infection sites of Alternaria brassicicola

Green islands were observed on mustard leaves beneath the infection drops containing germinating conidia of Alternaria brassicicola, when the surrounding uninfected tissue had yellowed due to senescence. Green island formation has been correlated with the secretion of cytokinin-like substances by the pathogens. A. brassicicola secreted cytokinin-like substances in a liquid synthetic medium and their application to detached host leaves evoked the formation of green islands in the dark. ¹⁴C studies confirmed that green islands act as metabolic sinks in which photosynthates are retained or accumulated. Cytokinin-like substances appear to be actively involved in infection and pathogenesis of A. brassicicola.